MD Audrey Nicolas

MD Madeleine Aumar

Chantal Léa

Alice Tran

Rémi Tiret

Laure Duclaux-Loras

Florence Bridoux-Henno

Alexandre Campeotto

Anne Fabre

Jeanne Breton

Languepin

MD Tran

MD Alice Tiret

MD Rémi Duclaux-Loras

MD Laure Bridoux

MD Florence Campeotto

MD Alexandre Fabre

MD Anne Bretuon

MD Jeanne Languepin

Maéva Kyheng Bst

MD Jérôme Viala

Comparison of Endoscopic Dilatation and Heller's Myotomy for Treating Esophageal Achalasia in Children: A Multicenter Study

Keywords: Achalasia, Endoscopic dilatation, Heller's myotomy, Children

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Objective To compare the efficacy of, and complications from the two main treatments for achalasia are endoscopic dilatation and surgical cardiomyotomy (Heller's myotomy). Study design We retrospectively collected data on children treated for achalasia over an 11-year period from eight tertiary pediatric centers. A line of treatment was defined as performing either Heller's myotomy or 1-3 sessions of endoscopy dilatation over three months. Treatment success was a priori defined as clinical improvement and no need for new treatment.

Results 97 children (median age: 12 years, 57% boys) were included. Median time-to-diagnosis was 10.5 months, and the median follow-up period was 27 months. Thirty-seven children were treated by Heller's myotomy and 60 by endoscopy dilatation as the first-line treatment. After adjustment for potentially confounding factors, Heller's myotomy was significantly more successful than endoscopy dilatation (hazard ratio, HR 3.93 [1.74;8.88], p=0.001), with a median survival without failure of 49 and 7 months, respectively, and with no significant difference in the occurrence of complications (35.2% for Heller's myotomy , 29.7% for endoscopy dilatation, p=0.56). Hydrostatic dilatation was as successful as pneumatic dilatation (HR 1.35[0.56;3.23], p=0.50).

Conclusions

Heller's myotomy is more successful than endoscopy dilatation, with no significant difference in the occurrence of serious complications. This raises the potential role of peroral endoscopic myotomy as an alternative treatment to Heller's myotomy .

Achalasia is a rare motor disorder of the esophagus characterized by esophageal aperistalsis and defective relaxation of the lower esophageal sphincter (LES) [START_REF] Vaezi | ACG Clinical Guideline: Diagnosis and Management of Achalasia[END_REF]. There is currently no curative treatment, but two main techniques can limit excessive LES pressure: endoscopic dilatation and myotomy, either by surgical route (Heller's myotomy) or more recently by endoscopy (peroral endoscopic myotomy, POEM); however, there is no clear consensus on management. Due to the low incidence of achalasia in pediatrics, between 0.10 and 0.18 per 100,000 children/year [START_REF] Smits | Pediatric Achalasia in the Netherlands: Incidence, Clinical Course, and Quality of Life[END_REF][START_REF] Marlais | UK incidence of achalasia: an 11-year national epidemiological study[END_REF], comparisons of treatment efficacy and complications in this population are usually limited to retrospective, single-center, small sample studies. One systematic literature review found insufficient comparable data to recommend a first-line treatment (4). Thus, our aim herein was to compare success rates and complications with the two techniques (i.e., endoscopy dilatation and Heller's myotomy) currently used most often for first-line pediatric treatment.

Methods

Study Population

We retrospectively collected data on patients followed for achalasia, confirmed by esophageal manometry (aperistalsis of the esophageal body and increased LES tone). Subjects were diagnosed before the age of 18 years and treated for the first time, by either endoscopy dilatation or Heller's myotomy, over an 11-year period at one of the eight French tertiary referral centers.

Data were collected from electronic and paper records using data collection sheets.

The main study objective was to compare the success of endoscopy dilatation (pneumatic and hydrostatic, then pneumatic alone) and Heller's myotomy as first-line treatments, by analyzing median survival without recourse to a new treatment. A line of treatment was defined as either performing Heller's myotomy or performing 1-3 endoscopy dilatation sessions, with a maximum period of three months between the first and third sessions.

Treatment success was a priori defined as clinical improvement (improvement of clinical signs and no need for further treatment) and failure as the need for a new treatment (e.g. Heller's myotomy, endoscopy dilatation, or POEM) and/or enteral nutrition support (delivery of nutrition directly to the stomach or jejunum via feeding tube or gastrostomy). Patients who presented with symptom recurrence more than two months after 2-3 dilatations, and who therefore needed more than 3 endoscopy dilatations, were considered treatment failures and were candidates for new treatment (endoscopic dilatation, Heller's myotomy, POEM, or enteral nutrition). The choice between Heller's myotomy and endoscopy dilatation was at the clinician's discretion.

Secondary objectives were to compare complications from these therapeutic methods and to compare the success of pneumatic dilatation and hydrostatic dilatation. A complication was defined as any event occurring during either a procedure or follow-up, which needed an intervention (e.g., perforation or pneumomediastinum).

Statistical Analyses

Continuous variables were expressed as mean (standard deviation) when normally distributed, or as median (interquartile range). Categorical variables were expressed as value (percentage).

Distribution normality was assessed using histograms and the Shapiro-Wilk test. We estimated the risk of failure at different timepoints using the Kaplan-Meier method by censoring lost to follow-up at the last available follow-up. Failure risks were compared between treatments using a univariate Cox proportional hazard regression model and then adjusting for prespecified factors (age at diagnosis, delay-to-treatment, and triple A syndrome). The same approaches were used to identify baseline characteristics associated with risk of failure. The log-linearity assumption for continuous measures was checked by examining Martingale residual plots and using restricted cubic spline functions, and the proportional hazard assumption for each potential predictive factor was checked by examining the Schoenfeld residuals. Comparisons of adverse events and complications between first-line treatment groups were performed using generalized linear mixed models (with a binomial distribution and logit function) by including subject as a random effect (i.e., several treatments by subject). Baseline characteristics were compared between first-line treatment groups using Students t-test for Gaussian continuous variables, Mann-Whitney U test for non-Gaussian continuous variables, and Chi-square test (or Fishers exact probability test when expected cell frequency was <5) for categorical variables, as appropriate.

Statistical testing was conducted at the two-tailed -level of 0.05. Data were analyzed using SAS software version 9.4 (SAS Institute, Cary, NC, USA).

Ethics

The study was declared to the French Data Protection Authority (Commission nationale informatique & libertés). All data were anonymized. The study was approved by the Ethics Committees of the Francophone Group of Hepatology, Gastroenterology and Pediatric Nutrition.

Results

Study Sample

Between 2005 and 2016, 97 patients under the age of 18 years (55 boys) with achalasia, confirmed by esophageal manometry, were treated by first-line endoscopy dilatation or Heller's myotomy at one of the eight selected centers. Data were missing for manometry type for 18 patients, and for achalasia subtype for 5 patients who underwent high-resolution manometry (HRM); however, they all had manometric confirmation at diagnosis. Sample characteristics are shown in Table I.

Symptoms and Additional Investigations

The two main clinical signs at diagnosis were dysphagia and regurgitation. All patients had esophageal manometry at diagnosis, confirming achalasia. Clinical characteristics are reported in Table II. Twenty-seven patients (28%) underwent HRM, making it possible to define achalasia type according to the Chicago classification; 30% of these patients had achalasia type I, 48% type II, and 22% type III. The median LES relaxation pressure was 22.8 mmHg, and the median minimal LES pressure at rest was 28.8 mmHg. Manometric metrics are detailed in Table I.

Treatment Groups

Sixty patients (61.9%) were treated by first-line endoscopic dilatation, and 37 (38.1%) by Heller's myotomy (Figure 1). The two treatment groups were comparable (Table II p=0.50), with new treatment-free survival medians of 7 and 3 months, respectively (Figure 4; available at www.jpeds.com).

Treatment Complications

The overall complication frequency, early (before 48 hours) or late, and severe or not, including all treatment lines, did not differ significantly after endoscopy dilatation or Heller's myotomy (29.7% and 35.2%, respectively, p=0.56)(Table III; available at www.jpeds.com). The frequency of serious complications, early or late, including all treatment lines, did not differ after endoscopy dilatation or Heller's myotomy (2.6% and 9.6%, respectively, p=0.080).

Serious complications were discontinuation of the operation, conversion to laparotomy, inprocess transfusion, esophageal or gastric perforation, cervical emphysema, pneumomediastinum, peritonitis, esophageal fistula, and gastric volvulus with hiatal hernia. No death occurred among this sample. The frequency of early complications, whether serious or not, including all treatment lines, did not differ after endoscopy dilatation or Heller's myotomy (18.4% and 16.7%, respectively, p=0.79), nor did the frequency of late complications, whether serious or not, including all treatment lines (10.8% and 21.8%, respectively, p=0.092).

Gastroesophageal reflux disease (GERD) was suspected in 21.3% of patients after endoscopy dilatation and 20.6% after Heller's myotomy; this was confirmed by pH manometry in 0% and 8.8% of patients, respectively. GERD was considered a mild complication, as no patient needed fundoplication. Mucosal breach after endoscopy dilatation and Heller's myotomy was found in 8.9% and 13.0% of patients, respectively, and esophageal perforation in 1.6% and 3.7% of patients, respectively.

Discussion

Endoscopic dilatation and Heller's myotomy are both successful and safe treatments [START_REF] Franklin | Childhood achalasia: A comprehensive review of disease, diagnosis and therapeutic management[END_REF][START_REF] Nardo | Pneumatic balloon dilation in pediatric achalasia: efficacy and factors predicting outcome at a single tertiary pediatric gastroenterology center[END_REF]), yet the optimal treatment of achalasia in pediatric patients remains debated and findings are conflicting [START_REF] Smits | Pediatric Achalasia in the Netherlands: Incidence, Clinical Course, and Quality of Life[END_REF][START_REF] Sharp | Treatment of Idiopathic Achalasia in the Pediatric Population: A Systematic Review[END_REF][START_REF] Nardo | Pneumatic balloon dilation in pediatric achalasia: efficacy and factors predicting outcome at a single tertiary pediatric gastroenterology center[END_REF][START_REF] Saliakellis | Long-term outcomes of Heller's myotomy and balloon dilatation in childhood achalasia[END_REF]. A systematic literature review of seven pediatric studies showed a lack of comparable data to recommend a first-line treatment (4); two studies recommended Heller's myotomy as the first-line treatment [START_REF] Lee | Outcomes of Treatment of Childhood Achalasia[END_REF][START_REF] Zhang | Diagnosis and management of esophageal achalasia in children: analysis of 13 cases[END_REF], one recommended pneumatic dilatation [START_REF] Nakayama | Pneumatic dilatation and operative treatment of achalasia in children[END_REF], one found comparable efficacy between the two treatments [START_REF] Pastor | A single center 26-year experience with treatment of esophageal achalasia: is there an optimal method?[END_REF], and three recommended that therapeutic choice should be according to the age of the child [START_REF] Jung | Treatments for pediatric achalasia: Heller myotomy or pneumatic dilatation?[END_REF][START_REF] Azizkhan | Achalasia in childhood: a 20-year experience[END_REF][START_REF] Berquist | Achalasia: diagnosis, management, and clinical course in 16 children[END_REF]. Herein, we report children with achalasia whose characteristics are similar to the few published pediatric series [START_REF] Smits | Pediatric Achalasia in the Netherlands: Incidence, Clinical Course, and Quality of Life[END_REF][START_REF] Marlais | UK incidence of achalasia: an 11-year national epidemiological study[END_REF][START_REF] Sharp | Treatment of Idiopathic Achalasia in the Pediatric Population: A Systematic Review[END_REF][START_REF] Franklin | Childhood achalasia: A comprehensive review of disease, diagnosis and therapeutic management[END_REF][START_REF] Pandian | Laparoscopic esophagomyotomy for achalasia in children: A review[END_REF][START_REF] Pacilli | Results of Laparoscopic Heller's Myotomy for Achalasia in Children: A Systematic Review of the Literature[END_REF]. In our study, as in most studies, endoscopy dilatation was most often the first-line treatment [START_REF] Smits | Pediatric Achalasia in the Netherlands: Incidence, Clinical Course, and Quality of Life[END_REF][START_REF] Sharp | Treatment of Idiopathic Achalasia in the Pediatric Population: A Systematic Review[END_REF][START_REF] Zagory | Heller Myotomy Is Superior to Balloon Dilatation or Botulinum Injection in Children with Achalasia: A Two-Center Review[END_REF].

Our analyses indicate that Heller's myotomy as the first-line treatment may be more successful than endoscopy dilatation, whether using pneumatic alone or pneumatic and hydrostatic. Other studies have also found that Heller's myotomy is a superior first-line treatment [START_REF] Smits | Pediatric Achalasia in the Netherlands: Incidence, Clinical Course, and Quality of Life[END_REF][START_REF] Lee | Outcomes of Treatment of Childhood Achalasia[END_REF][START_REF] Pastor | A single center 26-year experience with treatment of esophageal achalasia: is there an optimal method?[END_REF][START_REF] Zagory | Heller Myotomy Is Superior to Balloon Dilatation or Botulinum Injection in Children with Achalasia: A Two-Center Review[END_REF], including retrospective studies with smaller samples [START_REF] Saliakellis | Long-term outcomes of Heller's myotomy and balloon dilatation in childhood achalasia[END_REF]. This is in contrast to a large randomized trial in adults showing that Heller's myotomy and endoscopy dilatation symptom-based outcomes are similar at posttreatment years one, two, and five [START_REF] Boeckxstaens | Pneumatic dilation versus laparoscopic Heller's myotomy for idiopathic achalasia[END_REF][START_REF] Moonen | Long-term results of the European achalasia trial: a multicentre randomised controlled trial comparing pneumatic dilation versus laparoscopic Heller myotomy[END_REF]. A major difference in time-to-relapse was found, of a few months after endoscopy dilatation and more than four years after Heller's myotomy. This superiority can be understood because there is a more complete dissection of the LES muscle fibers and therefore greater efficacy. We found no age-based difference in outcomes between techniques, indicating that age need not be a criterion for choosing one over the other.

Our study is consistent with data from adults showing that Heller's myotomy is more successful in young people than in older people. The success rate of endoscopy dilatation as a first-line treatment was relatively low in our study (20%) compared with previous studies (60-90%) [START_REF] Nardo | Pneumatic balloon dilation in pediatric achalasia: efficacy and factors predicting outcome at a single tertiary pediatric gastroenterology center[END_REF][START_REF] Nakayama | Pneumatic dilatation and operative treatment of achalasia in children[END_REF][START_REF] Jung | Treatments for pediatric achalasia: Heller myotomy or pneumatic dilatation?[END_REF], but higher than the study by Smits et al (10%) [START_REF] Smits | Pediatric Achalasia in the Netherlands: Incidence, Clinical Course, and Quality of Life[END_REF]. Our lower rate can be explained by different definitions of success and failure across studies, and that previous studies were monocentric or bicentric and included small samples. Hydrostatic dilatations have not been validated for the treatment of achalasia but are the only ones available for infants and small children. It did not seem appropriate to exclude them given their frequent use in pediatrics (a quarter of dilatations in our study). Because the minimal diameter of pneumatic balloons is 30 mm, it is unusable in patients younger than 6 years or those weighing <20 kg. Herein, patients treated by hydrostatic dilatation were significantly younger and had lower weight than those treated by pneumatic dilatation. Distinctions between hydrostatic and pneumatic dilatation in previous pediatric studies are often lacking, making comparisons with their results difficult. We did not find any difference in the efficacy of hydrostatic dilatations compared with pneumatic dilatations. This may be due to a true absence of difference, which may help validate their use in achalasia with confirmation by other studies, or a lack of statistical power. Our poorer endoscopy dilatation results might also be explained by differences in dilatation protocols and technical characteristics (e.g., balloon sizes, pressures, times, and number of dilatations per session). Our dilatation protocol is similar to that reported by Boeckxstaens et al, the standard in adults [START_REF] Boeckxstaens | Pneumatic dilation versus laparoscopic Heller's myotomy for idiopathic achalasia[END_REF]. The main difference herein regards the second part of their protocol, which deals with the management of patients whose symptoms recur during followup. Indeed, were we to adhere strictly to the Boeckxstaens' protocol, our success rate with balloon dilatations would have been much higher: 55%. Overall, the relatively low success rates of the two treatment types confirm the difficulties with achalasia management, with 60% of patients requiring a minimum of two lines of treatment.

The frequency of complications overall was similar in the two treatment groups, giving even greater importance to Heller's myotomy, which is often associated with more complications [START_REF] Smits | Pediatric Achalasia in the Netherlands: Incidence, Clinical Course, and Quality of Life[END_REF]. The complication rates seemed high for both treatment types (~30%), but mucosal breaches-often minor-and suspected GERD were most common. However, although nonsignificant, more serious complications might be associated with Heller's myotomy. Posttreatment GERD frequency is difficult to compare across studies, as its definition is not uniform. The difference in GERD frequency confirmed by pH measurement between the two groups was worse with surgery-nearly 9% vs. 0%-with anti-reflux surgery performed for 90% of Heller's myotomy. Performing systematic anti-reflux surgery during Heller's myotomy remains controversial, with some studies showing low rates of GERD without fundoplication. Routine fundoplication might have little benefit while placing the patient at risk of a too tight anti-reflux procedure resulting in dysphagia [START_REF] Pacilli | Results of Laparoscopic Heller's Myotomy for Achalasia in Children: A Systematic Review of the Literature[END_REF][START_REF] Corda | Laparoscopic oesophageal cardiomyotomy without fundoplication in children with achalasia: a 10-year experience[END_REF]. Also, dysphagia following Heller's myotomy with fundoplication makes it difficult to know whether it is a sign of recurring achalasia, and therefore a treatment failure, or if it is because of an antireflux device that is too tight. Thus, Heller's myotomy failures were probably overestimated, as any new treatment was considered a relapse. According to some teams, concomitant antireflux surgery should be determined on a case-by-case basis, depending on personal history and GERD risk factors.

A similar rate of esophageal perforation after endoscopy dilatation (1-2%) is found in most pediatric studies [START_REF] Smits | Pediatric Achalasia in the Netherlands: Incidence, Clinical Course, and Quality of Life[END_REF][START_REF] Lee | Outcomes of Treatment of Childhood Achalasia[END_REF], though this rate is 2-5% in adults [START_REF] Boeckxstaens | Pneumatic dilation versus laparoscopic Heller's myotomy for idiopathic achalasia[END_REF][START_REF] Moonen | Long-term results of the European achalasia trial: a multicentre randomised controlled trial comparing pneumatic dilation versus laparoscopic Heller myotomy[END_REF]. This difference can be explained by the use of smaller diameter balloons in pediatrics, as the perforation rate increases with balloon size [START_REF] Smits | Pediatric Achalasia in the Netherlands: Incidence, Clinical Course, and Quality of Life[END_REF][START_REF] Boeckxstaens | Pneumatic dilation versus laparoscopic Heller's myotomy for idiopathic achalasia[END_REF]. However, there may also be a modifying factor, as larger diameter balloons are also more effective in pediatrics [START_REF] Smits | Pediatric Achalasia in the Netherlands: Incidence, Clinical Course, and Quality of Life[END_REF]. Regardless, a low perforation rate is even more attractive following endoscopy dilatation, as surgery negates the benefit of the endoscopic procedure (4).

One strength of our study was its multicenter character. Our relatively large pediatric sample allowed us to evaluate achalasia treatments using multivariate analysis, whereas most previous studies have relied on cohorts of 10-40 patients [START_REF] Nardo | Pneumatic balloon dilation in pediatric achalasia: efficacy and factors predicting outcome at a single tertiary pediatric gastroenterology center[END_REF][START_REF] Lee | Outcomes of Treatment of Childhood Achalasia[END_REF][START_REF] Zhang | Diagnosis and management of esophageal achalasia in children: analysis of 13 cases[END_REF][START_REF] Nakayama | Pneumatic dilatation and operative treatment of achalasia in children[END_REF][START_REF] Pastor | A single center 26-year experience with treatment of esophageal achalasia: is there an optimal method?[END_REF][START_REF] Jung | Treatments for pediatric achalasia: Heller myotomy or pneumatic dilatation?[END_REF]. A main study limitation was its retrospective nature. The fact that dilation technique (pneumatic vs. hydrostatic) likely differed among institutions may otherwise have influenced the results. The choice between endoscopy dilatation and Heller's myotomy was not recorded herein. This may have led to selection and information biases, center-and/or doctor-dependent, with likely differences between surgeons and pediatric gastroenterologists. However, children treated by endoscopy dilatation or Heller's myotomy did not differ at diagnosis, especially according to weight loss and nutritional status, which are good surrogates for disease severity. Our manometric inclusion criteria are open to criticism: with associated aperistalsis and increased LES tone, but without LES cutoff values. This choice is justified by the manometric definition of complex achalasia in pediatrics, by diagnostic heterogeneity with conventional manometry and HRM comprising different thresholds of pathological LES relaxation pressure, by pathological thresholds of relaxation pressure varying according to the HRM system, and by missing data (LES pressure is not always recorded).

Type-II childhood achalasia was the most highly represented in our study, though at a lower rate than among adults (48% vs. 70% in adults) [START_REF] Roman | The Chicago classification for achalasia in a French multicentric cohort[END_REF]. Studies on adults show that therapeutic choice can be guided by achalasia type [START_REF] Moonen | Long-term results of the European achalasia trial: a multicentre randomised controlled trial comparing pneumatic dilation versus laparoscopic Heller myotomy[END_REF][START_REF] Rohof | Outcomes of treatment for achalasia depend on manometric subtype[END_REF] but given the low HRM use-onethird of patients-we could not correlate treatment success with achalasia type using the Chicago classification. Indeed, our results may have been impacted by the fact that achalasia treatment is centralized to specialized centers in the national French network CRACMO (Centre de Référence des Affections Chroniques et Malformatives de l'Oesophage).

Information bias may have been caused by missing data, leading to difficulties in interpreting the results and lower statistical power. Though the Eckardt score was not used as a primary endpoint, due to its low use in pediatric practice, we defined strict a priori criteria for failure. That our study showed the superiority of Heller's myotomy over endoscopy dilatation supports additional studies on POEM, which allows the same myotomy type but is less invasive [START_REF] Caldaro | Treatment of esophageal achalasia in children: Today and tomorrow[END_REF][START_REF] Miao | Peroral Endoscopic Myotomy in Children With Achalasia: A Relatively Long-term Single-center Study[END_REF][START_REF] Nabi | Outcome of peroral endoscopic myotomy in children with achalasia[END_REF][START_REF] Lee | Peroral endoscopic myotomy (POEM) for the treatment of pediatric achalasia: a systematic review and meta-analysis[END_REF][START_REF] Chone | Multicenter evaluation of clinical efficacy and safety of per-oral endoscopic myotomy in children[END_REF][START_REF] Werner | Endoscopic or surgical myotomy in patients with idiopathic achalasia[END_REF]. Only one patient herein was treated by POEM, after the failure of two pneumatic dilatation treatment lines. The efficacy of POEM in the medium-and long-term remains to be determined, compared with Heller's myotomy, as does its safety, because questions remain as to the long-term frequency of GERD [START_REF] Miao | Peroral Endoscopic Myotomy in Children With Achalasia: A Relatively Long-term Single-center Study[END_REF][START_REF] Chone | Multicenter evaluation of clinical efficacy and safety of per-oral endoscopic myotomy in children[END_REF][START_REF] Werner | Endoscopic or surgical myotomy in patients with idiopathic achalasia[END_REF].

Future studies should address safety and long-term outcomes using a prospective randomized trial design.

  ). Nine patients were treated by first-line hydrostatic dilatation and 44 by pneumatic dilatation (7 missing data). Patients treated by hydrostatic dilatation (n=45, 26.0%) were significantly younger than those treated by pneumatic dilatation (n=128, 84.0%) (8.6 [±4.7] years vs. 13.3 [±2.7] years, p<0.001). Treatment Efficacy As a first line of treatment, Heller's myotomy was significantly more successful than endoscopy dilatation (risk of failure with Heller's myotomy group as reference, hazard ratio [HR] 3.56[1.73;7.29], p=0.0005) and pneumatic dilatation alone (risk of failure with Heller's myotomy group as reference, HR 3.41[1.62;7.18], p=0.0013) (Figure 2 and 3; available at www.jpeds.com). Similar results were found after adjustment for age at diagnosis, delay-totreatment, and history of triple A syndrome (risk of failure with Heller's myotomy group as reference, adjusted HR 3.93[1.74;8.88], p=0.001 for comparison with endoscopy dilatation; adjusted HR 3.89[1.59;9.53], p=0.003 for comparison with pneumatic dilatation). Hydrostatic dilatation was as successful as pneumatic expansion within first-line treatment (risk of failure with Heller's myotomy group as reference, HR 1.35[0.56;3.23],

Acknowledgments

We thank Doctor Laurent Michard for his involvement in this work, Doctor Marie-Dominique Lamblin for her help, and the abdominal-thoracic malformations network (FIMATHO) for their travel funding that enabled this work. Both of them have no conflict of interest and no funding source or industry-relation to declare.


Julien Monniot

François Tessier

Matthieu Robert

Gabriel Antoniu

StorAlloc: A Simulator for Job Scheduling on Heterogeneous Storage Resources

The ability of large-scale infrastructures to store and retrieve a massive amount of data is now decisive to scale up scientific applications. However, there is an ever-widening gap between I/O and computing performance. A way to mitigate this consists of deploying new intermediate storage tiers (node-local storage, burst-buffers, ...) between the compute nodes and the traditional global shared parallel file-system. Unfortunately, without advanced techniques to allocate and size these resources, they remain underutilized. In this paper, we investigate how heterogeneous storage resources can be allocated on an HPC platform, in a similar way as compute resources. In that regard, we introduce StorAlloc, a simulator used as a testbed for assessing storage-aware job scheduling algorithms and evaluating various storage infrastructures.

Introduction

Running scientific applications at scale requires the power of a large infrastructure such as a High-Performance Computing (HPC) system. For years, HPC systems have been designed with the main objective of improving computing power. However, nowadays the corpus of compute-centric applications has evolved towards complex data-centric workflows across the domains of modeling, simulation, AI and data analytics. The data deluge engendered by these workloads has been observed in major supercomputing centers: the National Energy Research Scientific Computing Center, USA, noticed that the volume of data stored by applications has been multiplied by 41 over the past ten years while the annual growth rate is estimated to 30% [START_REF] Lockwood | Storage 2020: A Vision for the Future of HPC Storage[END_REF]. Yet, during the same period, we have observed a relative performance decrease of storage systems: a study of the top three supercomputers from the Top500 ranking between 2011 and 2021 shows that the ratio of I/O bandwidth to computing power has been divided by 9.6.

An attempt to mitigate this gap has led to the emergence of new tiers of intermediate storage, such as node-local disks or burst buffers [START_REF] Henseler | Architecture and design of Cray DataWarp[END_REF], backed by diverse technologies (Flash memory, NVDIMM, NVMeoF, ...), and placed between the compute nodes and the global shared parallel file-system. Although this storage disaggregation offers new alternatives to a centralized storage system, advanced techniques for sizing and allocating these resources have yet to be devised to fully leverage them.

Unfortunately, exploring methods for allocating storage resources on supercomputers suffers from several limitations such as a difficult access to the hardware with enough privileges or a panel of technologies reduced to those deployed on the studied system. Simulation is one way to overcome these constraints. At the cost of a loss of accuracy, ideally as moderate as possible, simulation offers much better flexibility for representing a wide variety of storage architectures and can be used to evaluate storage infrastructures before they are deployed.

In this paper, we propose to explore how storage resources can be allocated on HPC systems, i.e. with which method (scheduling algorithm) and with which efficiency (metric) a set of I/O intensive jobs can be scheduled on a pool of heterogeneous storage resources. To do so, we introduce StorAlloc, a Discrete-Event Simulation-based (DES) simulator of a batch scheduler able to play (or replay) the scheduling of I/O intensive jobs on intermediate storage resources. We first present the architecture of StorAlloc, then we evaluate the tool on a set of basic scheduling algorithms and on multiple models of infrastructures featuring heterogeneous storage resources. From our simulations, we can conclude on the right sizing of intermediate storage resources among a set of architectures or analyze the utilization rate of the underlying disks.

Context and Motivation

For many years, supercomputers have followed a hyper-centralized paradigm regarding storage: a unique global shared parallel file-system such as Lustre [START_REF]Lustre filesystem website[END_REF] or Spectrum Scale (formerly GPFS [START_REF] Schmuck | Gpfs: A shared-disk file system for large computing clusters[END_REF]), used as a staging area from which data is read or written by applications or workflow components. These file-systems, although increasingly powerful, suffer the drawbacks of any highly centralized system: contention and interference make them very prone to performance variability [START_REF] Jay | Managing variability in the io performance of petascale storage systems[END_REF]. In order to overcome this problem, we have seen the emergence of new storage systems, closer to the computing nodes. Node-local SSDs, burst buffers or dedicated storage nodes with network-attached storage technology (NVMeoF), to name a few, are all technologies that provide fast storage, albeit with limited capacity, various data lifetime, cost and performance, and different means of access.

This last point in particular makes the use of these resources complicated. To illustrate this, Table 2 presents the multiple ways of accessing resources for a subset of storage tiers that tend to become popular on large-scale systems. The usual scope of the storage space and the commonly deployed data manager, if any, are also listed.

This variety, which would require working on new levels of abstraction, also raises another problem: how to preempt all or part of these storage resources so as to make them available for the duration of an I/O-intensive job's execution, as we do for compute nodes? Allocation methods exist for storage tiers but they are numerous and not interoperable: storage allocated at the same time as the compute node, dedicated APIs integrated or not into the job scheduler, complex low-level configurations. Thus, while it is common on HPC systems to get access exclusively to compute nodes (usually though a job scheduler), the allocation of those intermediate levels of storage remains minor in practice and often limited to homogeneous resources. In order to use these new levels of storage to their full potential, new allocation techniques must be invented and deployed on supercomputers.

The development of such solutions would, however, require access to intermediate storage resources with enough rights to repurpose them, which is usually not possible on deployed infrastructures for various reasons such as security or maintenance efforts. In addition, such experimentation can easily disrupt other users' workloads on production systems. An alternative approach is to use simulations as a way to reproduce with a certain degree of accuracy the behavior of a system with a very low footprint. While experiments on real systems would be limited to the embedded technologies, a simulator can also evaluate new types of architectures combining existing and emerging storage tiers, for example to make decisions about their sizing or their design. Several simulators already exist for scheduling jobs on compute nodes or for optimizing I/O, yet very few has been done to model and allocate storage resources. Therefore, in this paper, we propose StorAlloc, a simulator of a storage-aware job scheduler whose main objective is to explore heterogeneous storage resource allocation on supercomputers.

Related work

To the best of our knowledge, there is no tool whose goal is to simulate the scheduling of jobs on heterogeneous storage resources of a supercomputer. Simulators allowing to play or replay the execution of parallel and distributed applications on HPC systems exist and have been studied for many years. However, it is the computational aspect that is essentially addressed. SimGrid [START_REF] Casanova | Versatile, scalable, and accurate simulation of distributed applications and platforms[END_REF], for example, is a powerful framework for simulating the scheduling and execution of a large number of applications on real or made-up infrastructure models. The I/O aspect is limited to simulating data movement but, although preliminary work was started a few years ago [START_REF] Lebre | Adding storage simulation capacities to the simgrid toolkit: Concepts, models, and api[END_REF], storage resource allocation is absent from the framework. A few SimGrid derived simulators also have job scheduling oriented approaches. This is the case of batsim [START_REF] Dutot | Batsim: A realistic languageindependent resources and jobs management systems simulator[END_REF] or Wrench [START_REF] Casanova | Developing Accurate and Scalable Simulators of Production Workflow Management Systems with WRENCH[END_REF] for example. However, the full support of heterogeneous storage levels as allocatable resources is not implemented (disk capacity is not modeled in batsim for example). Another difference between these solutions and StorAlloc concerns the design of the tool as described in Section 4. StorAlloc has all its components decoupled. Therefore, the servers can be distributed on multiple nodes while the simulator component can be disabled to turn StorAlloc into a real storage-aware job scheduler.

The world of Cloud Computing is more familiar with the allocation of storage tiers as well as compute or network resources. Work has been done to simulate the allocation of resources between different users [START_REF] Núñez | Icancloud: A flexible and scalable cloud infrastructure simulator[END_REF][START_REF] Calheiros | Cloudsim: a toolkit for modeling and simulation of cloud computing environments and evaluation of resource provisioning algorithms[END_REF] in virtualized environment but these works are outdated and have very limited storage support.

Finally, models for partitioning and sizing intermediate storage resources such as burst buffers have been studied [START_REF] Aupy | Sizing and Partitioning Strategies for Burst-Buffers to Reduce IO Contention[END_REF][START_REF] Ruiu | Automatic dynamic allocation of cloud storage for scientific applications[END_REF]. These techniques are the basis of storage-aware job scheduling algorithms that could be evaluated in our simulator.

Architecture

StorAlloc is a tool able to simulate the scheduling of I/O-intensive jobs on heterogeneous storage resources available on a HPC system. In this section, we present its design and discuss implementation choices.

The objective of StorAlloc is to provide a simple way to develop and evaluate storage-aware job scheduling algorithms targeting heterogeneous storage resources (any kind of disk-based storage can be described). Therefore, StorAlloc has been designed following the basic principles of a job scheduler, i.e. a middleware allowing clients to request resources available on a supercomputer. Extending from the original architecture, we added the ability to run it as a simulator, using a single code base. StorAlloc's design is based on the composability of several components, which can be run together and extended in order to provide the desired behavior. Figure 1 depicts the components already implemented and how they fit together. At the core, one or multiple server and client agents are communicating through a central orchestrator. The clients request storage allocations to the orchestrator and expect connection settings to the newly allocated storage space in return. The server components declare a pool of available resources under their responsibility to the orchestrator and perform the storage management operations when needed (partitioning, rights granting, exposure on the network, releasing). In between, the orchestrator handles routing messages between components, keeps track of running and pending allocations and hosts the scheduler process.

In addition to these core components, we have extended StorAlloc with two simulation units (client and server), a visualisation server for real time plotting during simulation and an external log aggregator. The architecture of the tool makes it possible to add additional elements if necessary. All of these components are interconnected using a message-based protocol we have defined. They can be deployed across a set of hosts, or run on a single machine. While the former case is intended to properly map clients and servers onto an actual HPC platform, the latter is sufficient for simulations. The current design only allows for one orchestrator component to be running at any time. This constraint creates a single point of failure when deployed as a middleware in a production setting, and will be addressed in further developments.

In the following sections, we detail design choices for StorAlloc. In particular, we explain the general functioning of the scheduler, a central component in our simulator. Then we describe the storage abstraction layer used to characterize the pool of resources. In 4.3, we present the simulation capability with a focus on the real-time collection of scheduling data. We end this section with some technical considerations about StorAlloc.

Scheduling of storage requests

We define a storage request as a triple consisting of a capacity in GB, an allocation time in minutes and a submission time in a datetime format. The scheduling of storage requests takes place in a scheduler sub-component of the orchestrator, as depicted in Figure 1. This sub-component receives requests through messages from clients and process them asynchronously in the receiving order. The scheduler has access to both the entire list of available storage resources and the list of currently allocated requests. Any algorithm can thus make a resource allocation decision backed by a full view of the platform state. So far, four naive algorithms have been implemented in StorAlloc as listed below:

random: storage resources are picked randomly with a chance of failure; round-robin: storage space is allocated in a round-robin manner; worst-fit: disks are filled until no more space is available; best-bandwidth: nodes and disks on nodes are selected according to the best remaining bandwidth, considering a permanent maximum I/O regime for the existing allocations.

At launch time, the scheduler chooses one of these algorithms through a userdefined parameter. The scheduling algorithms share a common interface which accepts a storage request and a list of available storage resources, and returns an identifier for the resource(s) on which the desired storage space will be allocated. A request can also be refused (no space left for instance). In this case, we assume that the job falls back to a traditional parallel file-system, instead of using the intermediate storage tiers available through StorAlloc.

The scheduling of storage requests can also be adjusted by leveraging two strategies presented in Table 2. They are meant to help allocate requests when resources are constrained. The impact of these strategies, independent of the scheduling algorithms, is evaluated in Section 5. Again, we make the assumption that in case of (possibly repeated) allocation failures, I/O will be performed on the global shared parallel file system.

Default setting Comment

Split

Threshold at 200GB Split requests with capacity over threshold and allocate the parts on multiples resources.

Requeued 5 retries, one every 5m Postpone starting time and retry a failed allocation.

Table 2. Optional scheduling strategies

Storage abstraction

Because the available storage tiers can be extremely heterogeneous, an abstraction layer is needed to allow scheduling algorithms to accommodate the variety of technologies without needing to know the technical details of each level. In StorAlloc, storage platforms are represented through a hierarchy of three objects: servers, nodes and disks. Servers are top-level StorAlloc components which act as an interface between the orchestrator and one or many storage nodes. Nodes embed at least one disk. Nodes and disks may be of heterogeneous nature (number of disks, disk capacity, read and write bandwidth, node's network bandwidth). Whenever required by a parent server, a node should be able to setup and expose a specific partition of their storage resources, whose ownership will be transferred to a client. In simulation mode, servers passively accept requests without taking any action, but we still ensure that any allocation would be legal in terms of available resources. It has to be noted that when defining a storage layout, we consider the network to be flat. This is motivated by the fact that dynamic routing policies are unpredictable, either because the vendor does not provide enough details (such as on the Cray XC40 Theta platform which provided the input data used in Section 5 [START_REF] Chunduri | Performance evaluation of adaptive routing on dragonfly-based production systems[END_REF]) or because there are too many factors involved in packet routing decisions to be accurately modeled. Hence we only define the bandwidth at the node and disk levels and let the scheduling algorithm model the impact of concurrent allocations on these resources.

Simulation

A longer-term goal of StorAlloc is to provide a single code base for a storageaware job scheduler and its simulator. Therefore, we have designed our simulation server with a "component in the middle" approach. The core components run as if they were actually deployed on a real system except that, if the simulation mode is enabled, the requests are rerouted to the simulation server which stacks them until a specific message triggers the actual execution of the simulation. Then, the simulation is unrolled and go through the scheduler, using a discrete event simulation (DES) model [START_REF] Fishman | Principles of discrete event simulation[END_REF]. During that phase, data measuring the impact of scheduling is collected and feeds a visualization server in real-time. In particular, we measure the following indicators:

-Total allocated (and deallocated) volume.

-Mean and max number of simultaneously allocated requests (global, per node and per disk). -Mean and max percentage of non-free disk space for each disk over the simulation. -Number of requeued requests and total delay time during the simulation.

-Number of split requests if any. -Request's status: allocated or refused.

Implementation details

The proof of concept presented in this paper is implemented using Python3. Our messaging protocol relies on ZeroMQ, while the DES model used for the simulation comes from the SimPy library1 . The source code of StorAlloc can be found at https://github.com/hephtaicie/storalloc.

Evaluation

In this section, we evaluate the benefits of our simulator to assess storage-aware job scheduling algorithms on heterogeneous resources. To do so, we run multiple configurations and show their impact on the storage tiers thanks to metrics we have defined.

Simulation setup

To simulate storage requests from clients representative of real applications, we used a dataset composed of one year of a Darshan 2 logs on Theta, a 11.7 PFlops Cray XC40 supercomputer at Argonne National Laboratory3 . We extracted from these traces jobs spending at least 10% of their run time doing IO, and reading or writing at least 10 GB of data. It resulted in about 24 000 jobs out of approximately 624 000 jobs, each one translating into a storage request in StorAlloc: the requested capacity is based on the maximum of either read or write volume while the allocation time uses the initial job duration.

In order to have a good overview of what can be observed with our simulator, we have run 192 different simulation setups based on the settings presented in Table 3. The average simulation time is around 25m48s per run, in a range of [5m40s; 1h29m57s] on a single core of a Intel Core i7-1185G7 processor. This variability is due to the difference in complexity of the algorithms and the activation or not of the requeuing and splitting systems.

Settings Tested values Comment

Algorithm Random, round-robin worst-fit, best-bandwidth See Section 4.1

Total capacity 8TB, 16TB, 64TB Disk sizes are 1, 2 and 8TB respectively.

Storage Layout

Single node, single disk (1N1D) Single node, multi disks (1NnD) Multi nodes, single disk (nN1D) Multi nodes, multi disks (nNnD)

1N1D serves as baseline.

Requeued

Enabled or disabled When enabled, new attemps every 5m, until a 60m delay.

Split 200 GB or disabled

When disabled, some requests will be too large for any of the disks.

Table 3. Simulation settings

Analysis

We present here results plotted from StorAlloc simulation data. From these figures, we can conclude on an approximation of a right sizing of the platform and we can compare the efficiency of the tested scheduling algorithms. For this analysis, platforms and algorithms have been chosen to reflect a variety of behavior.

Platform sizing In our dataset, the sum of all the storage capacities requested by clients, called sum_cap, reaches 1.6 PB. In Figure 2, we plot the percentage of this value achieved by each of the 192 runs of our simulation according to storage layouts and algorithms, grouped by platform capacity and split strategy. On the top row (no request split), only the 1N1D layout at 64TB capacity reaches 100% of sum_cap. However this layout is merely a baseline which shouldn't be used, as it leads to a high concurrency and consequently a very low node bandwidth. From this result, we can also conclude that never more than 64TB are needed at the same time in our dataset. This information must be balanced by the fact that we exclude from Theta's traces several hundreds of thousands of jobs that we do not consider I/O intensive. The best results with other layouts peak slightly above 60%, which hints towards an underprovisioning of storage resources. The bottom row depicts the same analysis with requests split in chunks of 200GB. We see that all layouts reach a 100% of sum_cap at least once for 64TB. More generally, the splitting of requests allows a better use of resources and requires less storage space (the 16TB platform reaches 90% of sum_cap for half of the runs). These results give little information, however, about the use of the disks composing the modeled platform.

Figure 3 proposes to study this. Here, we plot the maximum disk utilization, called max_disk_use, for both 16TB and 64TB infrastructures (excluding 1N1D layout). As expected, the disk utilization rate correlates with the ability to absorb split requests for storage space (Figure 2). Nevertheless it is possible to quantify a potential underutilization, as seen for the 64TB platform where no more than 65% of disk capacity is ever used. The worst-fit algorithm is specifically intended for maximising the use of a single disk from a single node, which explains that it reaches 100% of max_disk_use for several disks. This first analysis shows that a platform slightly larger than 16TB can handle all the I/O intensive jobs in our dataset, as long as the requests are split into 200GB blocks. In that case, the targeted disks are mostly used at their full capacity at least once, leaving little flexibility in case of a sudden overload, while the average disk utilization rate is however very low (2.82%), which is explained by the sparsity of the jobs studied spread over a whole year. Finally, the different layouts tested (1NnD, nN1D, nNnD) behave in much the same way. Nevertheless, they have an impact on the available aggregated bandwidth as long as the scheduling algorithms can efficiently take advantage of the storage disaggregation, as shown in the rest of this paper.

Scheduling algorithms comparison

We have implemented four different storage-aware job scheduling algorithms in StorAlloc, as described in 4.1. To evaluate their efficiency, we propose to define a fairness metric that looks at the maximum and average number of concurrent allocations per disk allocated by each algorithm. This metric provides information on the balancing of the distribution of requests (split or not) and consequently on the potential bandwidth available for the allocations: in a permanent maximum I/O regime hypothesis (all jobs with continuous I/O operations), the less allocations are concurrent on resources, the more bandwidth will be available.

Figure 4 depicts this fairness for our four algorithms. First, we can see that the general variability (standard deviation) in both the mean and max numbers of allocations per disk are lower for round-robin and best-bandwidth than for random and worst-fit. As expected, worst-fit stands out, as its design clearly goes against fairness. We also observe that round-robin and best-bandwidth have quite similar fairness, with a slight advantage to best-bandwidth. This latter is the most advanced algorithm as it takes into account existing allocations on disks to make a decision. In terms of maximum number of allocations per disk, best-bandwidth is the most stable, and also usually leads to the smallest maximums. In other words, this algorithm can be expected to provide the best average bandwidth to jobs in the permanent regime case. Best-bandwidth behaves better than round-robin which, under the same conditions, tends to show more irregularities. Finally, we can see the impact of a queuing system on the number of allocations, i.e. fewer jobs are refused and have to fall back on the parallel file system.

Conclusion

In this paper, we have introduced StorAlloc, a DES-based simulator used to explore the scheduling of I/O intensive jobs on heterogeneous storage resources distributed across a HPC system. We have detailed its extensible design and configuration settings for modeling storage infrastructures and implementing various scheduling strategies. Our evaluation demonstrated how StorAlloc can ingest a large number of allocation requests generated from production traces and output storage-related metrics which provide valuable insights for storage platform sizing and scheduling algorithms evaluation. Building upon this preliminary work, we plan to extend this experimental campaign to more metrics, infrastructures and storage-aware scheduling algorithms. Another direction we want to take is to evaluate the benefits we could get from simulation frameworks such as Wrench [START_REF] Casanova | Developing Accurate and Scalable Simulators of Production Workflow Management Systems with WRENCH[END_REF] for the implementation of our simulation component. Finally, a longer term goal will be to explore how to combine computing and storage resources within the same request and provide suitable scheduling algorithms.

Fig. 1 .

 1 Fig. 1. StorAlloc Architecture

Fig. 3 .

 3 Fig. 3. Maximum disk capacity utilisation (% of capacity), for 16TB and 64TB platforms with request split threshold at 200G. The 1N1D layout has been removed.

Fig. 4 .

 4 Fig. 4. Mean (top) and max (bottom) number of allocations per disk, grouped by algorithms, for 16TB platform and split strategy. Storage layout 1N1D excluded. Dots plot the mean and max number of allocations of each disk separately.

Table 1 .

 1 Type of access, scope and default data management system on a subset of storage resources that tend to be democratized on large-scale systems.

		Access	Scope	Data manager
	Global storage system	Mount point System-wide Parallel file-system
	Node-local disk	Mount point Node	File-system
	NVDIMM -FSDAX	Mount point Node	DAX-enabled file-system
	NVDIMM -DEVDAX	Direct access Node	Raw persistent memory
	Burst buffer	Middleware Job	(Parallel) file-system
	Network-attached storage API	Node(s)	Raw storage space

  Percentage of sum_cap (sum of the requested capacities in the entire dataset) per simulation run, grouped by capacity and split strategy.

	Storage layout Multi node, multi disk Single node, multi disk Multi node, single disk Single node, single disk	Algorithm Random choice Worst-Fit	Round-robin Best bandwidth
	Fig. 2.

Resp. https://zeromq.org/ and https://simpy.readthedocs.io/en/latest/

Darshan is a popular I/O monitoring tool. https://www.mcs.anl.gov/research/projects/darshan/

This data was generated from resources of the Argonne Leadership Computing Facility, which is a DOE Office of Science User Facility supported under Contract DE-AC02-06CH11357.


Westwood

Hybosorus Roei Westwood

Hybosorus Carolinus Leconte

Frank-Thorsten Krell

Aleš Bezděk
email: bezdek@entu.cas.cz

Andrey Frolov
email: frolov@scarabaeoidea.com

Jean-Bernard Huchet
email: jean-bernard.huchet@mnhn.fr

Denis Keith
email: denis.keith@orange.fr

José Ignacio López-Colón
email: lopezicolon@gmail.com

Eric Matthews
email: eric.matthews@samuseum.sa.gov.au

Federico Ocampo
email: federico.ocampo@gmail.com

Antonio Rey

Fernando Z Vaz-De-Mello
email: vazdemello@gmail.com

Insecta, Coleoptera): proposed conservation by giving it precedence over Hybosorus pinguis

Keywords: Nomenclature, taxonomy, Coleoptera, Hybosoridae, Hybosorus illigeri, Hybosorus pinguis, Hybosorus roei, Hybosorus carolinus

The purpose of this application, under Article 23.9.3 of the Code, is to conserve the widely used specific name Hybosorus illigeri Reiche, 1853 (Coleoptera: Scarabaeoidea, Hybosoridae), a globally widespread and common scarab beetle. The name is threatened by the very rarely used senior subjective synonyms Hybosorus pinguis Westwood, 1845, Hybosorus roei Westwood, 1845, and Hybosorus carolinus LeConte, 1847. Precedence of the name Hybosorus illigeri is proposed to maintain stability of nomenclature.

1. Hybosorus illigeri Reiche (1853: 87) (Coleoptera, Scarabaeoidea, Hybosoridae, Hybosorinae) is a widespread common beetle, occurring through most of Africa (including Madagascar and the Mascarene islands), the Mediterranean and most of temperate and tropical Asia as far as China. This species is prone to passive transportation and has been introduced into the U.S.A., Central America (Cuba, Haiti, Bahamas, Jamaica, Nicaragua and Mexico) and Venezuela [START_REF] Ocampo | Hybosorids of the United States and expanding distribution of the introduced species Hybosorus illigeri (Coleoptera: Scarabaeoidea: Hybosoridae)[END_REF][START_REF] Ocampo | Phylogenetic analysis of the scarab family Hybosoridae and monographic revision of the New World subfamily Anaidinae (Coleoptera: Scarabaeoidea)[END_REF]Peck, 2005). It is one of the very few scarabaeoidea known to be a predator [START_REF] Rozas | Observación de hábitos depredadores en Hybosorus[END_REF] and in some areas has been reported as a nuisance pest of golf courses [START_REF] Buss | Flight activity and relative abundance of phytophagous scarabs (Coleoptera: Scarabaeoidea) from two locations in Florida[END_REF].

2. The widely used name Hybosorus illigeri Reiche, 1853 is a replacement name for Scarabaeus arator Illiger (1803: 210) due to homonymy with Scarabaeus arator Fabricius (1775: 18) (now Heteronychus arator; scarabaeidae, dynastinae). The original type of Scarabaeus arator Illiger, 1803 is lost and a neotype was designated by [START_REF] Landin | The identity of "Scarabaeus arator Fabricius, 1775" (Col. Lamellicornia) with the designation of neotypes of arator Fabr. and arator Illig. and a list of the insects from the Cape Colony described by Fabricius 1775[END_REF]. It is deposited in the collection of the Entomological Institute of the University of Lund, Sweden.

3. Being a widespread species, it is not surprising that Hybosorus illigeri has several synonyms. Among them there are three senior names, namely Hybosorus pinguis Westwood (1845: 440) (first synonymized by [START_REF] Endrődi | Hybosorinae (Coleoptera, Lamellicornia, Scarabaeidae). Parc National de l'Upemba. I[END_REF], Hybosorus roei Westwood (1845: 440) (first synonymized by [START_REF] Arrow | Scarabaeidae: Pachypodinae, Pleocominae, Aclopinae, Glaphyrinae, Ochodaeinae, Orphninae, Idiostominae, Hybosorinae, Dynamopinae, Acanthocerinae, Troginae[END_REF], and Hybosorus carolinus LeConte (1847: 84) (first synonymized by LeConte, 1862) that have been treated as junior synonyms of H. illigeri (i.e., listed as synonyms of a valid H. illigeri). This happened perhaps because some authors (e.g., [START_REF] Arrow | Scarabaeidae: Pachypodinae, Pleocominae, Aclopinae, Glaphyrinae, Ochodaeinae, Orphninae, Idiostominae, Hybosorinae, Dynamopinae, Acanthocerinae, Troginae[END_REF]Schmidt, 1913), while dealing with this species, simply copied the list of junior synonyms elaborated when the valid name was still H. arator [START_REF] Illiger | Verzeichniss der in Portugall einheimischen Käfer[END_REF], without taking into consideration that the name had changed for a much younger one. According to Article 23.3 of the Code Hybosorus pinguis, Hybosorus roei, and Hybosorus carolinus strictly should take precedence over Hybosorus illigeri.

4. [START_REF] Kuijten | Revision of the genus Hybosorus Macleay (Coleoptera: Scarabaeidae, Hybosorinae)[END_REF] published a revision of the genus Hybosorus, which after 35 years still represents the state of the art for the taxonomy of this genus and is unanimously followed by all authors involved in hybosorine taxonomy (including the authors of this application, who comprise almost all the students currently publishing on hybosorine taxonomy). In that revision Kuijten studied the type material and designated lectotypes for, amongst others, H. pinguis, H. roei, and H. carolinus, solved the taxonomy of Hybosorus illigeri and allied species, but did not resolve the aforementioned nomenclatural problem and listed the three senior synonyms as if they were junior synonyms of Hybosorus illigeri. With Kuijten's work, we now have a clean and accepted taxonomy but still a problematic nomenclature for this species.

5. In December 2006, Allsopp & Branco tried to solve the nomenclatural problem by sending an application to the Commission, which was later published as Case 3400 [START_REF] Allsopp | listed 42 post-1899 usages, up to 2006[END_REF]. The authors asked the Commission for a reversal of precedence of the names Hybosorus roei Westwood, 1845 and Hybosorus illigeri Reiche, 1853 to save the name H. illigeri since Art. 23.9.2 was not applicable due to two usages of the name H. roei post 1899 (see below). They also claimed that the other two names (H. pinguis Westwood, 1845 and H. carolinus LeConte, 1847) were nomina oblita. The application was eventually rejected in Opinion 2230 (ICZN, 2009), without placing any names on Official Lists or Indexes and leaving the issue open for making new proposals to the Commission.

6. According to the Commissioners who commented on the Case, the reasons for the rejection were the perceived lack of a sound taxonomic base for the involved taxa and the lack of evidence that the names H. pinguis and H. carolinus were nomina oblita. The lack of a sound taxonomic framework, as we outlined above, was a misperception as Kuijten's revision fulfils this purpose. Conversely, the Commissioners who had doubts about the nomina oblita were correct. A careful review of the literature revealed that both H. pinguis and H. carolinus had rare post-1899 usages as valid names.

7. Hybosorus carolinus has been used once by [START_REF] Endrődi | Hybosorinae (Coleoptera, Lamellicornia, Scarabaeidae). Parc National de l'Upemba. I[END_REF] as a subspecies of H. illigeri, quoting Hybosorus arator (sic!) ssp. carolinus as a doubtful geographical subspecies. Hybosorus pinguis had been used as a valid name by Arrow (1912: 36) and then by Schmidt (1913: 45). Finally, as already stated by [START_REF] Allsopp | listed 42 post-1899 usages, up to 2006[END_REF], H. roei has been used twice by Ádám (1994: 9, 2003: 132), an author known for finding older unused synonyms and disregarding prevailing usage. We can add three more usages of roei, i.e., by [START_REF] Endrődi | Hybosorinae (Coleoptera, Lamellicornia, Scarabaeidae). Parc National de l'Upemba. I[END_REF] as a doubtful geographical subspecies of H. arator, by [START_REF] Chikatunov | Catalogue of the beetles (Coleoptera) in Israel and adjacent areas: 1. Scarabaeoidea[END_REF], and by Chikatunov et al. (1999: 80). It must also be added that, due to the rejection of the precedence of H. illigeri in Opinion 2230, there have been same year in the same paper and no first reviser has yet stated which of them should have priority. This means that in case this application is rejected, then the correct name to apply to the taxon currently known as H. illigeri would remain unresolved between pinguis and roei.

11. Since we have a name used over 170 times by at least 230 authors and three competing senior synonyms used very few times, we suggest that, in order to fix the nomenclature once for all, precedence should be given to the most widespread name, i.e., H. illigeri under Art. 23.9.3, in order to maintain universality and to avoid confusion.

12. The International Commission on Zoological Nomenclature is accordingly asked:

(1) to use its plenary power to give the specific name illigeri Reiche, 1853, as published in the binomen Hybosorus illigeri, precedence over the names: (a) roei Westwood, 1845, as published in the binomen Hybosorus roei, whenever the two are considered to be synonyms; (b) pinguis Westwood, 1845, as published in the binomen Hybosorus pinguis, whenever the two are considered to be synonyms; (c) carolinus Le Conte, 1847, as published in the binomen Hybosorus carolinus, whenever the two are considered to be synonyms;

(2) to place on the Official List of Specific Names in Zoology the following names:

(a) illigeri Reiche, 1853, as published in the binomen Hybosorus illigeri, with the endorsement that it is to be given precedence over the following names: (i) roei Westwood, 1845, as published in the binomen Hybosorus roei, whenever the two are considered to be synonyms; (ii) pinguis Westwood, 1845, as published in the binomen Hybosorus pinguis, whenever the two are considered to be synonyms; (iii) carolinus Le Conte, 1847, as published in the binomen Hybosorus carolinus, whenever the two are considered to be synonyms; (b) roei Westwood, 1845, as published in the binomen Hybosorus roei, with the endorsement that it is not to be given priority over the name illigeri Reiche, 1853, as published in the binomen Hybosorus illigeri, whenever the two are considered to be synonyms; (c) pinguis Westwood, 1845, as published in the binomen Hybosorus pinguis, with the endorsement that it is not to be given priority over the name illigeri Reiche, 1853, as published in the binomen Hybosorus illigeri, whenever the two are considered to be synonyms; and (d) carolinus Le Conte, 1847, as published in the binomen Hybosorus carolinus, with the endorsement that it is not to be given priority over the name illigeri Reiche, 1853, as published in the binomen Hybosorus illigeri, whenever the two are considered to be synonyms.

Acknowledgement of receipt of this application was published in BZN 75: 3.

Comments on this case are invited for publication (subject to editing) in the Bulletin; they should be sent to the Secretariat, ICZN, Lee Kong Chian Natural History Museum, 2 Conservatory Drive, Singapore 117377, Republic of Singapore (e-mail: iczn@nus.edu.sg).

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Yann Landon
email: yann.landon@univ-tlse3.fr

Alexandra Lacombe

Landry Arnaud

Kamgaing Souop

Alain Daidié

Manuel Paredes

Audrey Benaben

Clément Chirol

Manuel Pa

Correlations between the hole surface integrity and fatigue life for drilled 2024-T351 aluminum alloy

Keywords: Surface integrity, Drilling of aluminum alloys, Fatigue strength

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Introduction

In the aeronautical filed, the final assembly process requires the drilling of a large number of fastening holes. The drilling operation is particularly critical because it is performed on high-value parts during the last steps of the aircraft assembly process. Moreover, drilled holes are stress concentration zones, and thus are critical according to the assembly fatigue strength. But different drilling parameters can generate different surface integrities, that could lead to different fatigue strength of the drilled part [START_REF] Ralph | Effect of various aircraft production drilling procedures on hole quality[END_REF][START_REF] Ralph | Fatigue performance of production-quality aircraft fastener holes[END_REF][START_REF] Elajrami | Effect of various drilling procedures on the fatigue life of rivet holes[END_REF].

The orbital drilling process (also called helical milling) consists on milling a hole using an endmill, with a smaller diameter than the hole, driven along an helical trajectory. It has been more and more considered in aerospace industries in the last years [START_REF] Whinnem | Development of Orbital Drilling for the Boeing 787[END_REF], especially due to a better hole quality (in terms of diameter, roughness and burr) in comparison to axial drilling for hybrid stack drilling [START_REF] Olvera | Hole Making Using Ball Helical Milling on Titanium Alloys[END_REF][START_REF] He | Helical Milling of CFRP/Ti-6Al-4V Stacks with Varying Machining Parameters[END_REF][START_REF] Pereira | A review of helical milling process[END_REF]. Though, it was reported by Deitert that orbital drilling also generates less compressive residual stress, leading to a reduced fatigue life for aluminum drilled parts [START_REF] Deitert | Orbital Drilling[END_REF]. The opposite conclusion was stated by another study on the same material (AA2024-T351), showing that orbital drilling could permit to obtain an improved fatigue strength [START_REF] Sun | Holemaking processes and their impacts on the microstructure and fatigue response of aircraft alloys[END_REF]. This question remains thus open for this alloy. The influence of the different drilling processes in terms of global hole surface integrity, and the impact on the fatigue life, must be studied further.

The surface integrity can be defined as a set of geometrical (surface topology), microstructural (microstructure, hardness) and mechanical (residual stress) parameters. Its global characterization requires specific methods, and appears to be a challenge for drilled aluminum parts. Indeed, the affected layer thickness after drilling is particularly thin in this alloy, which makes difficult any surface analysis. In order to assess the thickness of the affected layer after cutting, a preliminary FEM model was set up, based on the work of Atlati [START_REF] Atlati | Développement d'une nouvelle approche hybride pour la modélisation des échanges thermiques à l'interface outil-copeau[END_REF]. To represent the cutting involved in drilling that impacts the hole surface, the lateral cutting was considered as orthogonal cutting. The simulated depth of the affected layer was between 20 and 40 µm, largely lower than the depth usually encountered in the literature (over 100µm) [START_REF] Elajrami | Effect of various drilling procedures on the fatigue life of rivet holes[END_REF][START_REF] Deitert | Orbital Drilling[END_REF][START_REF] Sun | Holemaking processes and their impacts on the microstructure and fatigue response of aircraft alloys[END_REF][START_REF] Wang | Investigation on fatigue performance of cold expansion holes of 6061-T6 aluminum alloy[END_REF]. The correlations between machining processes, surface integrity and related fatigue strength were widely studied in the literature [START_REF] Wang | Investigation on fatigue performance of cold expansion holes of 6061-T6 aluminum alloy[END_REF][START_REF] Pramanik | Fatigue life of machined components[END_REF][START_REF] Maximov | Effect of slide burnishing basic parameters on fatigue performance of 2024-Т3 high-strength aluminium alloy[END_REF][START_REF] Sasahara | The effect on fatigue life of residual stress and surface hardness resulting from different cutting conditions of 0.45%C steel[END_REF]. From these works the following conclusions can be established:

• A lower surface roughness leads to a better fatigue life. It is explained by the micro-topology of the surface. The micro-grooves and scratches act as critical spots for fatigue crack initiation. • A greater compressive residual stress leads to a better fatigue life. It is explained by the effect of crack closure that limits the propagation of the crack.

A tensile stress acts at the opposite, with an opening effect.

• A significant material hardening leads to a better fatigue life. It is explained by the local increase of the yield strength. Thus, the impact of each surface integrity parameter was studied independently. But first, strong interactions exist between these parameters (e.g. between residual stress and material hardening or microstructure). Also, surface integrity has to be considered globally (e.g. high compressive stress associated with important roughness). And finally, the influence of a parameter is not linear (e.g. surface roughness impacts the fatigue life only when the depth of the scratches exceeds a given level) [START_REF] Siebel | Influence of surface roughness on the fatigue strength of steels and non-ferrous alloys[END_REF].

These issues, associated to the difficulty encountered for the characterization of the surface, make the determination of the impact of the surface integrity on the fatigue strength for drilled 2024-T351 aluminum parts a challenge. This paper presents the study of the correlations observed between the hole surface integrity parameters and the associated fatigue life. For this purpose, two drilling processes were considered (axial drilling and orbital drilling), in order to obtain different combinations of surface integrity parameters. On the drilled holes, surface characterization methods were carried out to analyze surface roughness, microstructure and hardening, and residual stress state. Samples were also drilled in the same conditions to perform open-hole (without fastener) and filled-hole (with fastener installed) fatigue tests. S-N curves and Airbus Fatigue Index (AFI) were reported and analyzed. Correlations with the surface integrity are finally set.

2

Materials and methods

Drilling processes

Two drilling processes were considered in this study: axial drilling and orbital drilling.

The axial drilling tests were conducted on a CNC machine DMG DMU 85 eVo. The orbital drilling tests were carried out using a specific orbital spindle PRECISE France -ORBIBOT. All tests were performed under external MQL lubrication. Two hole diameters were tested: 6.35mm (4/16") and 9.52mm (6/16"). The used cutting tools and cutting conditions corresponded to industrial applications (Table 1).

Fatigue test samples

The fatigue test samples were T-Type specimens (Fig. 2), with a width equal to 3 times the hole diameter (D) and a thickness equal to 6.35mm for the hole diameter D=6.35mm and 10mm for the hole diameter D=9.

Surface integrity characterization

The surface integrity was characterized through surface roughness measurements, hardness measurement, microstructure analysis, and residual stress evaluation.

For the surface roughness characterization, a profilometer (measurement of Ra, Rq and Rz parameters with a 0.8mm cut-off) and an optical microscope (ALICONA In-finiteFocus to measure Sa, Sq, Sp, Sv, Sz, Sdq, Ssk and Sku parameters with a vertical resolution of 0.2µm) were both used.

For hardness characterization, Vickers microhardness measurements and nano-indentation tests were conducted. After cutting the samples along the working section, Vickers microhardness was measured using a 1kgf loading applied during 15s. The obtained results were corrected according to [ASTM E92-82] to consider the cylindrical shape of the surface. For nano-indentation tests, the samples were cut at mid-thickness and polished (with emery paper, diamond paste, and OP-S suspension). The tests were carried out using a Berkovich diamond indenter under a 5mN force. According to the results of the preliminary FEM simulation, a tightened matrix of indentation was set to 5 rows of 20 indents spaced 5µm apart. The first indents were located 5µm distant to the hole edge (Fig. 3). The Oliver&Pharr method was used to analyze the results in [START_REF] Pharr | Measurement of mechanical properties by ultra-low load indentation[END_REF]. For the analysis of the material hardening, Electron Back-Scatter Diffraction technique (EBSD) was carried out with a Feg Jeol JSM 7100F Scanning Electron Microscope equipped with an Oxford Instruments Nordlys Nano-detector (done at the Centre de microcaractérisation Raimond Castaing, CNRS UMS3623, Toulouse -France). The plastic strain was studied through the observation of the local changes in the crystal orientation using the GROD criteria [START_REF] Kamaya | Measurement of plastic strain of polycrystalline material by electron backscatter diffraction[END_REF]. The analyzed surface of 600x200µm 2 (scanning pitch of 0.5µm) was the section located at mid-thickness of the samples around the hole, polished with a cross-section polisher. All measurements were conducted with a 20kV voltage and a tilt of the sample of 70°.

For the evaluation of the residual stress state, X-ray diffraction and incremental hole drilling techniques were firstly used. Due to the shape and dimension of the hole and the large grain size of the aluminum alloy, X-ray diffraction did not give valuable results. The incremental hole drilling technique did not manage to give better results, due to the low depth of affected layer. To overcome these obstacles, a novel technique was developed, inspired by crack opening methods as the splitting method [START_REF] Astm E | Practice for Estimating the Approximate Residual Circumferential Stress in Straight Thin-walled Tubing[END_REF]. The proposed Hole Opening Comparative Technique (HOCT) consists in drilling a hole in a specimen, measuring a profile along the specimen edge, then opening the hole (by Electrical-Discharge Machining) and finally measuring again the profile on the specimen edge to deduce the specimen deformation (Fig. 4a). This deformation is the consequence of the release of the residual stress. Thus, a large deformation is obtained for samples presenting a high residual stress level around the hole. The direction of the deformation also informs about the nature of residual stress. A compressive (respectively tensile) stress leads to a (concave (respectively convex) deformation (Fig. 4b). It has to be noted that the material considered is treated T351. It implies a stress release stage (under tension load) after a solution heat treatment. Thus, the material was assumed free of stress before being drilled.

Fatigue tests

Open-hole fatigue tests were carried out at room temperature on a servo-hydraulic machine equipped with a 100kN load cell. A sinusoidal tensile-tensile load (R 0.1) was applied with a frequency of 20Hz. S-N curves (normalized stress S in the specimen working section reported as a function of the number of cycles N) were plotted thanks to different stress levels tested (from 100MPa to 280MPa at the maximum in the working section).

For filled-hole fatigue tests, a double-lap configuration was considered (Fig. 5). The fasteners used were bolts in titanium with steel nuts. Again, a sinusoidal tensile-tensile load (R 0.1) was applied. The maximal stress (in the working section) was set to 59.65MPa. This value corresponds to a real maximal stress level equivalent to the one of open-hole tests at 150MPa. To study the impact of the hole surface integrity, the following points were considered:

• All assemblies presented a clearance fit (avoiding interference interactions).

• The tightening torque applied on the fastener was set at the minimum (4Nm) to avoid any load transfer through the fastener.

• PTFE washers were used to limit interactions (Fig. 5). For all fatigue tests, the fracture surface was observed and showed the same failure mode for all configurations, with crack initiation in the area of the hole edge.

Results and discussion

The results for open-hole fatigue tests are presented Fig. 6. In this paper, the AFI results are presented as a ratio to the maximal value (obtained for axial drilling at D=9.52mm).

The results (S-N curves and AFI ratios) obtained for D=6.35mm showed no significant difference between the 2 drilling processes. The results for D=9.52mm are different. A significant decrease in fatigue life is observed on S-N curves with the orbital drilling process, especially for low stress levels. This leads to a loss of 15% in AFI ratio.

Filled-hole fatigue tests confirmed these observations as results were in accordance with the ones of open-hole fatigue tests. For D=6.35mm, a slight increase in fatigue life is observed with orbital drilling. But at the opposite, for D=9.52mm, axial drilling brought a significantly better result. The results show that the drilling process can impact significantly the fatigue life of the drilled parts. The possible correlations between this impact and the hole surface integrity have to be studied.

In this study, it seemed that surface roughness has no significant impact on fatigue life. The results of arithmetic roughness (Ra) are presented in Table 2, as a percentage of the Ra specification. For all configurations, the aeronautical specification is largely respected (percentage less than 100%). For D=6.35mm, orbital drilling leads to a largely better roughness Ra than axial drilling. But no difference was noted in fatigue life for this diameter. For D=9.52mm, orbital drilling gives again a better roughness Ra but led to a worst fatigue life. The same observations were made for all other surface roughness parameters (2D and 3D).

It can be concluded that there is no correlation between surface roughness and fatigue life in this study. In this work, small defect sizes were obtained. It confirms that surface roughness could impact the fatigue life only when it exceeds a given level and that the fatigue crack initiation seems to be mainly due to microstructural changes [START_REF] Siebel | Influence of surface roughness on the fatigue strength of steels and non-ferrous alloys[END_REF]. The results in terms of Vickers micro-hardness are presented through normalized values in Fig. 7. For D=6.35mm, similar micro-hardness levels are noted for both processes, whereas a significant difference of 29% is observed for D=9.52mm. A clear correlation with fatigue life observations can be set. The loss of fatigue strength with orbital drilling could be explained by a significant decrease of the surface micro-hardness for this configuration. To confirm this observation, nano-indentation test results were analyzed. They are presented in Fig. 8 in terms of evolution of the nano-hardness in relation to the distance to the hole edge. For D=6.35mm, a similar depth of affected layer (around 15µm) can be observed for the two drilling configurations. For D=9.52mm, this depth is around 5µm for orbital drilling whereas it is around 40µm for axial drilling. The difference is significant and the results confirm the thinness of the affected layer predicted with the preliminary FEM simulations. During drilling 2024-T351 aluminum alloy, the depth of affected layer is particularly small. The same observation can be made from the maximal nano-hardness measured. It is in accordance with the micro-hardness results.

It has to be noted that, for D=9.52mm and axial drilling configuration, indents close to the hole edge were irregular and no nano-hardness value could be stated. It probably expresses a high value of nano-hardness on the hole edge vicinity.

It can be concluded that the hole surface hardness could have a significant influence on fatigue life in this study. To go further, the microstructural cause of hole surface hardening was analyzed through EBSD observations, using the GROD criterion evolution in the hole edge area (Fig. 9). For D=9.52mm and axial drilling configuration, grain misorientations can be clearly observed. A layer affected by strain hardening was noted, of around 40µm deep. This configuration generates a significant strain hardening of the hole surface. For other configurations, no clear grain misorientation can be noted. These observations confirm the previous hardness results.

Finally, residual stress was analyzed with the help of the HOCT novel technique developed. The results in terms of measured deformations are presented Fig. 10. For D=9.52mm and axial drilling configuration, a significant deformation can be noted, whereas no deformation is noticed for the other configurations. It expresses the presence of higher residual stress in the hole edge area with this configuration in comparison to the others. Again, a clear correlation can be set with the fatigue test observations. Moreover, the high residual stress is in association with an important hole surface hardening. These 2 aspects seem correlated, as residual stress relates to the strain hardened material layer.

Conclusions

In this paper, correlations between the hole surface integrity parameters and the fatigue life were studied for drilled 2024-T351 aluminum parts. Open-and filled-hole fatigue tests were performed. The parameters describing the hole surface integrity were characterized. For residual stress assessment, a novel technique was developed (HOCT). From this study, the following conclusions can be made:

• Due to small defect sizes, the hole surface roughness levels were low and were not correlated to fatigue strength results in this study; • The hole surface hardness and the residual stress fields in the hole edge area seem to have a significant impact on fatigue life; • The increase in hole hardness is mainly caused by a strain hardening of material in the subsurface, which slows crack initiation; • The residual stress state seems to be associated with the surface hardness. In future works, the impact of the drilling process (tool geometry, cutting parame-ters…) will have to be studied in more details to explain the obtained surface integrities. Also, a numerical model will be developed to quantify the residual stress from the HOCT deformation measurements.

Fig. 1 .

 1 Fig. 1. Simulation of drilling lateral cutting. (a) Data and chip formation. (b) Plastic strain. (c) Residual stress.

  52mm. It permitted to obtain a stress concentration factor equal to 3.5 and thus to ensure a crack initiation at the hole edge. The length of the specimens was set equal to 200mm for open-hole fatigue tests, and to 25 times the hole diameter for filled-hole fatigue tests. An overlap of 2 times 2.D was considered for filled-hole fatigue samples.The samples were drilled and deburred before performing the fatigue tests.

Fig. 2 .

 2 Fig. 2. Geometry of the samples for (a) open-hole fatigue tests and (b) filled-hole fatigue tests.

Fig. 3 .

 3 Fig. 3. (a) Nanoindentation device. (b) Matrix of indents obtained.

Fig. 4 .

 4 Fig. 4. HOCT novel method for the analysis of residual stress.

Fig. 5 .

 5 Fig. 5. Set-up for filled-hole fatigue tests.

  Fig.6a-b present the obtained S-N curves. Fig.6c-d present the Airbus Fatigue Index (AFI) results for each configuration. The AFI represents the stress value S leading to a fatigue life of 100 000 cycles.

Fig. 6 .

 6 Fig. 6. Open-hole fatigue test results.

Fig. 7 .

 7 Fig. 7. Normalized microhardness results (as a ratio of the maximum value).

Fig. 8 .

 8 Fig. 8. Normalized nanohardness results (as a ratio of the maximum value).

Fig. 9 .

 9 Fig. 9. GROD mapping in the hole edge area.

Fig. 10 .

 10 Fig. 10. Specimen deformation profiles obtained with the HOCT.

Table 1 .

 1 Drilling tools and associated cutting parameters.

		D=6.35mm	D=9.52mm
		Axial drilling	Orbital drilling	Axial drilling	Orbital drilling
	Tool	Carbide heli-	Carbide end mill	Carbide heli-	Carbide end mill
		coidal drill	4 teeth	coidal drill	4 teeth
	Tool diameter	6.35mm	4.85mm	9.52mm	8.00mm
	Cutting speed	180	610	120	1005
	(m/min.)
	Axial feed	0.1	0.0015	0.1	0.0015
	(mm/rev.)
	Orbital pitch	-	0.04mm	-	0.04mm

Table 2 .

 2 Normalized Ra results (as a % of the Ra specification).

		D=6.35mm	D=9.52mm
		Axial drilling	Orbital drilling	Axial drilling	Orbital drilling
	Ra (%)	57.5	31.6	16.5	12.2

Acknowledgements

The authors would like to thank Airbus France for its technical and financial support.