Fabienne Touchard

Observation of the shock wave propagation induced by a high-power laser irradiation into an epoxy material

The propagation of laser-induced shock waves in a transparent epoxy sample is investigated by optical shadowgraphy. The shock waves are generated by a focused laser (3?ns pulse duration?1.2 to 3.4?TW?cm?2) producing pressure from 44 to 98.9?GPa. It is observed that the shock wave and the release wave created by the shock reverberation at the rear face are both followed by a dark zone in the pictures. This corresponds to the creation of a tensile zone resulting from the crossing on the loading axis of the release waves coming from the edge of the impact area (2D effects). After the laser shock experiment, the residual stresses in the targets are identified and quantified through a photoelasticimetry analysis of the recovered samples. This work results in a new set of original data which can be directly used to validate numerical models implemented to reproduce the behaviour of epoxy under extreme strain rate loading. The residual stresses observed prove that the high-pressure shocks can modify the pure epoxy properties, which could have an influence on the use made of these materials.

Effects of cultural conditions on the hemp (Cannabis sativa) phloem fibres: Biological development and mechanical properties

Natural fibres such as hemp are extensively used in new composite materials. This article deals with biological and mechanical aspects of hemp phloem fibres. The effects of different cultural parameters on some characteristics of hemp fibres were studied: variety, soil drought during plant cultivation and water-retting process. Results showed that there was no significant influence on studied properties of the hemp variety and of water-retting. Under water-limited conditions, results showed that plant growth was reduced and the maturation of phloem fibres was delayed. Secondary cell wall was synthesised later in comparison with water-controlled conditions and cell protoplast remains longer. These biological aspects can be related to the higher Young’s modulus and maximal stress values measured on fibre strands from plants grown under water-stressed conditions. These results are promising, by considering the extent and degree of drought conditions in the world.

Damage Mechanisms in Hemp-Fibre Woven Fabric Composite, and Comparison with Glass-Fibre Composite

This paper deals with hemp-fibre and glass-fibre woven fabric/epoxy composites. The aim is to analyse damage mechanisms in these composites by comparing three different methods: acoustic emission, microscopic observations and stiffness loss measurement. For acoustic emission (AE), an experimental multi-scale analysis was developed in order to identify the different damage mechanisms. Tensile tests were performed on single yarn, neat epoxy resin and composite materials to determine their AE amplitude signatures. A statistical analysis of AE amplitude signals was realised and correlated with microscopic observations. This study has enabled to identify the amplitude and to track, during tensile tests, three types of damage in these composites: matrix cracking, interfacial debonding and reinforcement fracture. It shows the different processes of damage development in natural fibre reinforced composites, in comparison with glass fibre composites. These results were compared to the measured stiffness loss of composites. It allows one to highlight the difficulty to link this mechanical damage parameter with physical AE events.

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.

Full-Field Measurement at the Weave Scale in Hemp/Epoxy Composite Using Digital Image Correlation

This paper presents the strain field analysis of a woven hemp composite. Two different stacking sequences were tested: [0/90]7 and [±45]7. Tensile tests were coupled with a digital image correlation method in order to measure in-plane longitudinal, transverse and shear strain fields. Strong heterogeneities were observed in both stacking sequences. The obtained spatial resolution was fine enough to allow the comparison of maximum and minimum deformation areas in strain fields with the corresponding weave architecture. It was observed that strain levels were directly linked with strand positions. Results gave, for the studied woven hemp composite, a complete analysis of the local deformation mechanisms.

Development of the laser shock wave adhesion test on bonded CFRP composite

Purpose – The purpose of this paper is to develop a laser shock adhesion test (LASAT) and evaluate its ability to reveal various bond qualities of stuck carbon fiber reinforced polymer (CFRP) industrial assemblies. Design/methodology/approach – Four grades of adhesion were prepared by release agent contamination of CFRP prior to assembly. Laser shots were performed at different intensities on these samples. Findings – To characterize and quantify the damage created by the propagation of shock waves in the bonded material, several diagnoses were used (confocal microscopy, ultra-sound inspection and cross-sections microscopy). These three post-mortem techniques are complementary and provide consistent results. Originality/value – The combination of these diagnoses along with the LASAT technique provides relevant information on the bond quality in agreement with GIC values measured by the University of Patras.

Mechanical and laser impact effects on woven composites with hemp or glass fibres

Purpose The purpose of this paper is to study the damage induced in “green” and synthetic composites under impact loading. Design/methodology/approach The study was focussed on epoxy-based composites reinforced with woven hemp or glass fibres. Six assessment techniques were employed in order to analyse and compare impact damages: eye observation, back face relief, terahertz spectroscopy, laser vibrometry, x-ray micro-tomography and microscopic observations. Findings Different damage detection thresholds for each material and technique were obtained. Damage induced by mechanical and laser impacts showed relevant differences, but the damage mechanisms are similar in both types of impact: matrix cracks, fibre failure, debonding at the fibres/matrix interface and delamination. Damage shape on back surfaces is similar after mechanical or laser impacts, but differences were detected inside samples. Originality/value The combination of these six diagnoses provides complementary information on the damage induced by mechanical or laser impacts in the studied green and synthetic composites.

Use of laser shock waves to test piezochromic coatings for impact detection

Abstract. The design of piezochromic pigments is a promising way to adjust smart painting and therefore to develop a “visual” impact detection coating. This paper deals with the possibility to use laser shock waves to test piezochromic coatings for impact detection. For that purpose, an experimental setup was developed in order to obtain compressive load in the coating thanks to the impedance mismatch between selected materials. An analytical modeling was used to validate the proposed method. The experimental investigation coupled with finite-element modeling on four smart coatings showed that these coatings can reveal impact location by a significant change of color if a relevant pressure threshold is reached. The results presented in this work are promising and demonstrate the ability of the proposed laser shock method for characterizing the pressure thresholds of piezochromic smart paintings. It opens the door for studying future smart paintings with different critical pressure levels, depending on the targeted application.

Numerical Determination and Experimental Validation of a Technological Specimen Representative of High-Pressure Hydrogen Storage Vessels

A technological specimen representative of type IV high-pressure hydrogen storage vessels is developed. An analytical model is used to compute fiber orientations in the specimen in order to be as representative as possible of the stress level reached in a tank during pressurization. A three-dimensional finite-element model is used to determine the best stacking sequence with these fiber orientations. A validation is done by performing tests with digital image correlation in order to measure displacements on the lateral side of the specimen. A comparison between the calculated and experimentally found strain fields is made. The results obtained highlight the influence of stacking sequence on the development of damage and the difficulty arising in designing representative specimens.

Non-Woven Flax Fibre Reinforced Polypropylene: Static and Low Velocity Impact Behaviour

Non-woven flax fibre reinforced new and recycled polypropylene were manufacturing by an industrial process of carding, batt, needle-punching and thermo-pressing. Polypropylene fibres were mixed with flax fibres in the carding process forming a thick multi-layered mat which was used for thermo-compression process. Effects of multi-layered non-woven manufacturing on mechanical properties were analyzed in different composites with three fibre areal weights. The results highlight the anisotropic behaviour of the obtained composites and a constant ratio of absorption energy to impact energy. In addition, cyclic tests and acoustic emission method coupled with image correlation measurement were specially employed in order to detect the damage development process in the composites and to qualify the deformation process in tensile test.