Thomas Jailin

This study investigates the assessment of stresses in a Thermoplastic Polyurethane (TPU) disk under diametral compression relying on full-field strain measurements by Localized Spectrum Analysis (LSA). A checkerboard pattern was first laser-engraved on the disk surface. The LSA processing of the checkerboard images captured in the undeformed (reference state) and deformed (at –8 kN) configurations allowed the determination of the two in-plane principal stretch ratios and the corresponding principal directions. An incompressible behavior was assumed to derive the out-of-plane stretch ratio. Using a neo-Hookean constitutive model, the maps of the Cauchy stress tensors and the First Piola-Kirchhoff stress tensors were deduced. This study demonstrates the potential of the LSA strain measurement technique for deriving stress maps in soft materials, which will be used in future work to feed into the Virtual Fields Method (VFM) for identifying interparticle contact forces in soft granular systems.

Cette thèse porte sur le comportement au fluage du tube de gainage en Zircaloy-4 en conditions représentatives de la phase post-DNB (Departure from Nucleate Boiling) d’un accident de réactivité (RIA). Ces conditions peuvent se résumer à une élévation brutale de la température de la gaine alors que celle-ci est intérieurement pressurisée, suivie de son ballonnement à haute température. La transformation de phase (α→β) du Zircaloy-4 a tout d’abord été étudiée à l’aide d’essais de dilatométrie pour des vitesses de chauffe comprises entre 50 et 2000°C/s. Une attention particulière a été portée aux moyens de mesure thermiques, particulièrement délicats à mettre en place sous transitoires rapides. La microstructure du matériau a également été analysée à l’aide d’essais trempés. Les résultats ont permis de caractériser un modèle de changement de phase valide de l’équilibre thermique jusqu’à des vitesses de 2000°C/s, basé sur une description de Leblond. Dans un deuxième temps, la plateforme d’essai thermomécanique ELLIE de l’INSA Lyon a été adaptée de façon à pouvoir simuler les conditions de chargement d’un post-DNB. Des essais de fluage en ballonnement ont été réalisés avec des vitesses de chauffe de 1200°C/s. Le domaine de température étudié s’étend de 840 à 1020°C. avec des pressions internes de 7 et 11 bar. Les champs de déplacements et de températures sont mesurés en surface de l’éprouvette, respectivement par stéréo-corrélation d’images numériques et thermographie proche infrarouge. Une procédure a été développée, afin d’obtenir ces champs à l’aide des deux mêmes caméras. Les essais ont mis en évidence un comportement au fluage complexe dans le domaine biphasé, qui semble étroitement lié à la proportion de phase dans le matériau. Un fort impact de la vitesse de chauffe sur la tenue mécanique de la gaine a également été observé, avec des vitesses de déformation bien plus importantes suite à un transitoire thermique rapide. Enfin, la méthode de recalage par éléments finis (FEMU) a été mise en place, afin d’identifier une loi de comportement en exploitant toute la richesse des essais. Le modèle éléments finis de l’éprouvette est basé sur une représentation 3D de la zone d’intérêt contrainte par une hypothèse de Love-Kirchhoff dans la section. Une loi de fluage, couplée à la fraction de phase, a été identifiée, et permet de représenter le comportement de la gaine pendant les 10 premières secondes des essais sur l’ensemble des conditions testées. Une ouverture qui vise à prendre en compte l’influence de la taille des grains sur le fluage est finalement proposée en vue de modéliser les essais sur un temps supérieur à 10 secondes.

Performing well‐controlled warm formability experiments on metal sheets is a topical industrial challenge as vehicle lightweighting turns to higher strength aluminium alloys. A number of difficulties with warm forming are tackled in this paper using both an experimental and a numerical approach. The sensitivity of the material behaviour to temperature and strain rate was first studied using tensile testing. It is shown that the strain rate does not have a significant impact on the material response below 180°C. At 180°C, the yield stress and the strain hardening are higher at faster strain rates. Nakazima experiments were then carried out on a forming press to study the formability of the material at different temperatures. The impact of several parameters on the results was investigated in order to conduct and analyse properly warm formability experiments (e.g., friction and filtering methods). A finite element model was also developed to support this sensitivity study. A comparison between the forming limit diagrams obtained experimentally and numerically is presented.

The checkerboard constitutes the best pattern for full‐field strain measurement because it maximizes image gradient. In the experimental mechanics community, employing this pattern is currently strongly limited because depositing it on the surface of specimens raises practical difficulties. A recent study shows, however, that it is technically possible by using a laser engraver. The present paper aims to push this solution forward by printing a checkerboard pattern on a thin polymeric film and then gluing the resulting laser‐engraved film on the specimen surface. The underlying idea is to separate the manufacturing process of this optical strain gauge on the one hand and its use on the other hand to help spread this strain measuring tool in the experimental mechanics community. The polymeric film employed here is the same as that used in the manufacturing process of classic electrical gauges, so one can rely on the know‐how of classic strain gauge bonding to glue this optical strain gauge on the specimen surface. The main difference between the proposed tool and classic electrical gauges is that the strain field beneath the polymeric support is measured instead of localized strain values. The paper is a proof of concept for this strain field measuring tool. The manufacturing and bonding processes are described in the paper. The localized spectrum analysis, a spectral technique developed for processing images of periodic patterns, is used to retrieve the strain fields from checkerboard images. Through two complementary examples, we show the ability of this new type of strain gauge to detect and quantify local details in the strain field beneath. A simplified 1D model is also proposed to assess the minimum width of the strain peak that can reliably be measured with this technique.

Background: Formed aluminium alloy sheet materials are increasingly adopted in production processes such as vehicle manufacturing, due to the potential for weight-saving and improved recyclability when compared to more traditional steel alloys. To maximise these benefits whilst maintaining sufficient mechanical properties, the link between formability and microstructure must be better understood. Virtual materials testing is a cost-effective strategy for generating microstructure-informed formability predictions. Methods: We developed an open-source hybrid framework, combining experimental and computational tasks, for generating reproducible formability predictions. Starting with experimental texture measurements and stress-strain curves, we calibrated crystal plasticity (CP) model parameters. The framework used these parameters to perform a large set of multiaxial full-field CP simulations, from which various anisotropic yield functions were fitted. With these anisotropy parameters, we then employed a Marciniak-Kuczyński finite-element model to predict forming limit curves, which we compared with those from experimental Nakazima tests. Results: We executed the workflow with the aluminium alloy Surfalex HF (AA6016A) as a case study material. The 18-parameter Barlat yield function provided the best fit, compared to six-parameter functions. Predicted forming limits depended strongly on the chosen hardening law, and good agreement with the experimental forming limit curve was found. All of the generated data have been uploaded to the Zenodo repository. A set of Jupyter notebooks to allow interactive inspection of our methods and data are also available. Conclusions: We demonstrated a robust methodology for replicable virtual materials testing, which enables cheaper and faster formability analyses. This complete workflow is encoded within a simple yet highly customisable computational pipeline that can be applied to any material. To maximise reproducibility, our approach takes care to ensure our methods and data — and the ways in which that data is processed — are unambiguously defined during all steps of the workflow.