Etude expérimentale et numérique du comportement dynamique de composites aéronautiques sous choc laser. Optimisation du test d'adhérence par ondes de choc sur les assemblages composites collés

This work aims the development of a non-destructive technique to control the mechanical quality ofaeronautics adhesive bonds (ENCOMB project). Shocks were realized by use of laser sources or gas gun, anddifferent techniques were used to analyse the shocks such as: VISAR, PDV, Shadowgraphy, optical andconfocal microscopy, X-ray radiography, ultrasound testing…Epoxy resins and carbon/epoxy compositelaminates were first investigated. Monitored laser shocks, in addition to post-mortem analyses, enabled tobetter understand the shock phenomenon on these complex materials. The results obtained on bondedcomposite showed that the laser shock technique can be used to discriminate different adherence levels.The use of numerical models, developed thanks to the experimental data, enabled to analyze the shockpropagation in these complex assemblies. They also evidenced the need for optimization, in order to testonly the bond interface and not to break the composite parts on the assembly. Several optimizationsolutions are formulated such as the use of tuneable pulse duration, or double shock configurations. Someof these solutions have been experimentally validated, and the numerical optimization gives the shockparameters to use for the next experimental campaigns. Finally, this work provides original results on thedynamic behaviour of composite materials under laser shock and leads to the adaptation of the laser shockadhesion test to any kind of bonded composite assemblies.Ce travail vise le développement d’une méthode non destructive permettant de contrôler la qualitémécanique des joints collés aéronautiques, en utilisant les ondes de choc générées par impact laser (projetENCOMB). Des chocs ont été réalisés à l’aide de dispositifs tels que des sources laser ou des canons à gaz.Différents diagnostiques ont été utilisés : le VISAR, la VH, la visualisation transverse, la microscopie optiqueet confocale, la radiographie X, le contrôle ultrasons...Des échantillons de résine et des compositesstratifiés carbone/époxy ont d’abord été étudiés. Des chocs laser instrumentés, couplés à des analysespost-mortem, ont permis une meilleure compréhension des phénomènes de choc dans ces matériaux. Lesrésultats obtenus sur les assemblages composites collés montrent que la technique de choc laser permetde discriminer différents degrés d’adhérence. L’utilisation de modèles numériques, développés grâce auxdonnées expérimentales, a permis d’analyser la propagation du choc dans ces assemblages complexes. Cesrésultats ont démontré la nécessité d’optimiser la technique, afin de tester exclusivement l’adhérence dujoint collé, sans endommager les composites de l’assemblage. Plusieurs solutions d’optimisation sontproposées tels que l’utilisation d’une impulsion variable, ou celle de double chocs. Ces solutions ont étévalidées expérimentalement et l’optimisation numérique a fourni les paramètres de choc pour de futurstests. Finalement, ce travail fournit des résultats originaux sur le comportement dynamique de compositesstratifiés et permet de progresser vers l’adaptation du test d’adhérence par choc laser à différentsassemblages composites

Development of a laser shock adhesion test for the assessment of weak adhesive bonded CFRP structures

Adhesive bondin,g bas a great poœnlial far future ligbtweight bigb-loaded structures in the a.eronautic industiy. A preœquisite for sucb an application is dtat the bond quality of the adhesive joint can be assessed in a non-destructive way. However, the use of da.ssicaJ Non•DesiiUctive Techniques (NDT) does not aUow the evaluation of the adhesion stren,gt:h of an adhesive bond yet This paper pn!sents an investigation made on weak composite bonds in on!er to develop a laser shock wave adhesion test First, the procedure to produce controlled weak bonds is desaibed. CFRP bonded samples are pn!pared in a spedfic way and characterized by ultrasonic techniques to assess the absence of any detectable defect. 1ben, for sorne of the .samples, their bond streDgth is evaluated by mechanical destructive œsts and ether .samples are loaded by v.arious intensity lasers shocks. The obtained results help to understand the behavior of the composite bonds under Jaser shock loading:. thanks to two post-mortem techrùques. 1becorrelation between the laser parameterS and the induced damage is demon.strated, The potential of the laser shock. technique to dl.saiminate different bond quallties is shawn, and the need for the œst optinùzationlsdÛ(

Laser shock adhesion test numerical optimization for composite bonding assessment

The present work presents the latest development of laser shock adhesion test (LASAT) technology, targeting the weak bond detection in bonded aeronautic structures. This problematic is still holding back a wider use of bonding, which could however be a significant breakthrough in the way of assembling parts. By mechanically loading the bondline thanks to laser-induced shock waves, LASAT acts as proof test to reveal the presence of local weaknesses. In the present paper, focus is made on the optimization of the laser shock parameters regarding the assembly to test. Objective is to avoid loading too much the composite, thus avoiding damage, to increase the test performances. Numerical modelling is used, following a specific methodology, to understand the phenomena and identify the key parameters. The basic laser shock configuration was first investigated. Due to the stress distribution, this setting allows one to test a bond whose strength is equal or below 40% of composite inter-laminar strength. The effects of the laser focal spot on the stress distribution are also quantified. A 4 mm diameter shows good performances for the assembly to test. For the first time, three different optimizations are proposed: tunable pulse duration, double pulses on the front face and symmetrical laser shocks. They are first theoretically described. Numerical results then support these configurations’ performances. The double pulse solution makes it possible to test a bond strength equal or inferior to 80% of composite inter-laminar strength, when symmetrical pulses enable to reach 100% thanks to a sharp stress distribution. These results are validated by experimental evidence that is also presented. Finally, the present work offers helpful information for the development and deployment of LASAT for aeronautic bonded structures

Development of the symmetrical laser shock test for weak bond inspection

This study aims to assess the capability of the LAser Shock Adhesion Test to detect weak bonds in assemblies made of carbon fibre reinforced polymer laminates as well as understand the behaviour of different bonded composite structure under a shock load. A specific setup based on symmetrical laser shocks has been used. After each test, ultrasounds are used to determine if the bond has been damaged or not. At first, samples with two contaminants - de-icing fluid and finger prints - were studied. Then, the bond quality of two partially contaminated aircraft parts were investigated. These original results demonstrate the efficiency of the symmetrical laser shocks method as a Non-Destructive Test for bonded carbon fibre reinforced polymer assemblies

Laser shock a novel way to generate calibrated delamination in composites: concept and first results

Structural Health Monitoring (SHM) has been gaining importance in recent years. SHM aims at providing structures with similar functionality as the biological nervous system and it is organized into four main steps: detection, localization, assessment, and prognosis. This paper considers SHM assessment level and more particularly the estimation of the severity of delamination-type damage in Carbon Fiber Reinforced Polymer (CFRP) laminates. Prior to quantification algorithms implementation, it is critical to properly prepare the supports on which algorithms will be tested. Teflon inserts and conventional drop tower impacts are commonly used techniques in the SHM community to generate or simulate delaminations. However with such techniques it is difficult to generate controlled delaminationtype damage in a realistic manner. Conventional impacts do not necessarily induce uniquely delamination-type damage. Teflon inserts still remain very far from representing a realistic delamination. In the present paper we investigate Laser Shock Wave Technique (LSWT), a new way to generate controlled delaminations in composites. In particular, the symmetrical laser shock approach was applied to CFRP laminates in order to generate delamination-type damage in a calibrated and realistic way. A particular attention was paid to the effect of time delay and laser beams energies on damage position and severity respectively. Post-mortem analyses were performed to characterize the induced damage. Results show a high potential of LSWT for damage calibration in both size and location

Numerical modeling of laser-induced shock experiments for the development of the adhesion test for bonded composite materials

In this work, laser shock experiments on composite material are modeled. Focus is made on the development of a reliable numerical model to be used for the laser shock wave adhesion test of bonded composites. Technique principle is explained as well as the laser shock experiment procedure. Then, the numerical investigations are presented. A calibration method is given to set the model input parameters, and the modeling choices are detailed. Dynamic material parameters are identified thanks to experimental results, and validated through a complete campaign of laser shocks on various carbon fiber reinforced plastic (CFRP) materials (monolithic and bonded). Finally, numerical results for bonded composites are discussed. They enable to understand the stress distribution within the composite assembly during the wave propagation. This is a key step toward the development of a reliable and controlled laser shock adhesion test

Laser shock a novel way to generate calibrated delamination in composites: concept and first results

Structural Health Monitoring (SHM) has been gaining importance in recent years. SHM aims at providing structures with similar functionality as the biological nervous system and it is organized into four main steps: detection, localization, assessment, and prognosis. This paper considers SHM assessment level and more particularly the estimation of the severity of delamination-type damage in Carbon Fiber Reinforced Polymer (CFRP) laminates. Prior to quantification algorithms implementation, it is critical to properly prepare the supports on which algorithms will be tested. Teflon inserts and conventional drop tower impacts are commonly used techniques in the SHM community to generate or simulate delaminations. However with such techniques it is difficult to generate controlled delaminationtype damage in a realistic manner. Conventional impacts do not necessarily induce uniquely delamination-type damage. Teflon inserts still remain very far from representing a realistic delamination. In the present paper we investigate Laser Shock Wave Technique (LSWT), a new way to generate controlled delaminations in composites. In particular, the symmetrical laser shock approach was applied to CFRP laminates in order to generate delamination-type damage in a calibrated and realistic way. A particular attention was paid to the effect of time delay and laser beams energies on damage position and severity respectively. Post-mortem analyses were performed to characterize the induced damage. Results show a high potential of LSWT for damage calibration in both size and location

Experimental and numerical investigations of shock and shear wave propagation induced by femtosecond laser irradiation in epoxy resins

In this work, original shock experiments are presented. Laser-induced shock and shear wave propagations have been observed in an epoxy resin, in the case of femtosecond laser irradiation. A specific time-resolved shadowgraphy setup has been developed using the photoelasticimetry principle to enhance the shear wave observation. Shear waves have been observed in epoxy resin after laser irradiation. Their propagation has been quantified in comparison with the main shock propagation. A discussion, hinging on numerical results, is finally given to improve understanding of the phenomenon

Generation of controlled delaminations in composites using symmetrical laser shock configuration

Structural Health Monitoring (SHM) is defined as the process of implementing a damage identification strategy for aerospace, civil and mechanical engineering infrastructures. SHM can be organized into five main steps: detection, localization, classification, quantification and prognosis. Our work considers SHM quantification level and in particular the evaluation of the severity of delamination-type damage in CFRP composite laminates. Prior to quantification algorithms implementation, it is important to properly prepare the supports on which algorithms will be tested. Teflon inserts and conventional impacts are commonly used techniques to generate or simulate delaminations. However, with such rudimentary techniques it is difficult to generate controlled delamination-type damage in a realistic way. In the present work, we investigate symmetrical laser shock approach, as a new method to calibrate delaminations in composites. By tuning the time delay between the two laser beams and laser energy, through-thickness damage position and severity can respectively be adjusted. The effect of multiple contiguous laser impacts was also investigated. Post-mortem analyses using A-scan and C-scan testing as well as penetrant testing were performed in order to characterize laser impact induced damage. Results are encouraging and demonstrate the high potential of symmetrical laser shock for damage calibration in both size and location

Laser shock peening: toward the tse of pliable polid polymers for confinement

This paper presents the first extensive study of the performances of solid polymers used as confinement materials for laser shock applications such as laser shock peening (LSP) as opposed to the exclusively used water-confined regime up to now. The use of this new confinement approach allows the treatment of metal pieces needing fatigue behavior enhancement but located in areas which are sensitive to water. Accurate pressure determination in the polymer confinement regime was performed by coupling finite element simulation and experimental measurements of rear free-surface velocity using the velocity interferometer system for any reflector (VISAR). Pressure could reach 7.6 and 4.6 GPa for acrylate-based polymer and cross-linked polydimethylsiloxane (PDMS), respectively. At 7 and 4.7 GW/cm2, respectively, detrimental laser breakdown limited pressure for acrylate and PDMS. These results show that the pressures produced were also as high as in water confinement, attaining values allowing the treatment of all types of metals with LSP and laying the groundwork for future determination of the fatigue behavior exhibited by this type of treated materials