Quantitative analysis by micro-CT of damage during tensile test in a woven hemp/epoxy composite after water ageing

International audienceThe influence of water ageing on the evolution of damage during tensile loading in woven hemp/epoxy composites is studied. First, the water uptakes of several types of samples are compared. Micro-CT observations show that, from the beginning of desorption but not during the water ageing, damage appears in the composites. At macroscopic scale, tensile tests reveal a significant modification of mechanical properties of the composite after water ageing. Then, an original test, combining tensile loading and X-ray microtomography, is performed on a ±45 hemp/epoxy composite material. The volume of damage inside two specimens (with and without water ageing) at several steps during a tensile test can be quantified. Results highlight significant differences in the evolution of the volume of damage between the two samples. This work shows how the mechanisms of deformation and damage of plant fibre composites are governed by their moisture exposure history

A study of composite material damage induced by laser shock waves

A laser shock wave technique has been used to study the damage tolerance of T800/M21 CFRP (Carbon Fiber Reinforced Polymer) composite material with different lay_ups. Different levels of damage have been created according to various laser irradiation conditions. Several characterization methods such as Optical Microscopy, X-ray Radiography, or Interferometric Confocal Microscopy have been used to quantify these defects. The nature of the defects induced by the shock wave propagation has been studied. The sensitivity of the composite material damage to the shock conditions has been shown and quantified. Moreover, the experimental results gathered with each technique have been compared to each other and it leads to a better understanding of the CFRP behavior under high dynamic loading. These original results have enabled the definition of a specific damage criterion for CFRP under dynamic loading

Study of plant fibre composites with damage induced by laser and mechanical impacts

Polymer composite materials provide good strength to weight ratio and tailored mechanical properties thanks to the reinforcing fibres. Until recently, the need for taking into account the whole life cycle of a composite structure was neglected and only the service aspects were important. Today, the designers of a new composite structure have to take into account the environmental aspects from the sustainability of raw materials to the management of end life products. There are recycling issues related to the most popular composites. A solution for the recycling issue can be sought in green composites with reinforcing fibre originating from plants. The behaviour of eco-composites, when subjected to laser or mechanical impact loadings, is not well known yet. Short fibre composites were made with spruce fibres. Another set of samples was made of flax fibres. Also a woven hemp fabric-based eco-composite was investigated. A fully synthetic woven composite was used for comparison with green composites. Mechanical impacts were performed by means of a falling dart impact testing machine. Laser impacts were made with high power laser source. Four assessment techniques were employed in order to analyse and compare impact damage. Damage detection thresholds for each material and technique were obtained

Thermo-oxidation behaviour of organic matrix composite materials at high temperatures

The present paper is a review of the main activities carried out within the context of the COMPTINN‟ program, a joint research project founded by a FUI program (Fonds Unifiés Interministériels) in which four research teams focused on the thermo-oxidation behaviour of HTS-TACTIX carbon-epoxy composite at „high‟ temperatures (120°C-180°C). The scientific aim of the COMPTINN‟ program was to better identify, with a multi-scale approach, the link between the physico-chemical mechanisms involved in thermo-oxidation phenomena, and to provide theoretical and numerical tools for predicting the mechanical behaviour of aged composite materials including damage onset and development

Influence of Hygrothermal Aging on Mechanical Properties and Damage Mechanisms of Hemp-Reinforced Biocomposites

International audienceBiocomposites are being developed for structural applications, particularly in the automotive industry. The use of plant fibers as reinforcement is a first step toward the development of eco-materials, but it is necessary to use also a more eco-friendly matrix. In this paper, woven hemp composites impregnated with two different eco-polymers are compared: hemp/Greenpoxy and hemp/Elium. Greenpoxy is a partially bio-based thermoset resin, and Elium is a recyclable thermoplastic polymer. The influence of various hygrothermal aging conditions on the mechanical properties of these composites is studied, and results are compared with the behavior of samples stored at ambient temperature and humidity. Water immersion was carried out at three different temperatures: 21°C, 60°C and 70°C. A Fick’s law was used to model the water uptake. Modulated DSC was performed to measure the evolution of the glass transition temperatures. Repeated progressive tensile loading tests were carried out and instrumented with acoustic emission monitoring. The evolution of the damage factor, the residual strains and the acoustic events was analyzed. Fracture surface analysis and microtomography observations were also performed. Damage mechanisms are discussed. Results show that mechanical properties are highly affected by the hygrothermal aging whatever the matrix, but the increase in temperature affects the two composites differently

Heterogeneous tensile behavior of impact-modified polypropylene plates processed by injection moulding

International audienceDue to the fast cooling of polymer on the mould surface, the injection moulding process is known to be responsible of property gradients through thickness. Nevertheless, the mechanical behaviour is usually characterized by considering an homogeneous polymer, even when tensile test specimens processed by injection moulding are used. In order to characterize the local three-dimensional behaviour, a new set up based on digital image correlation has been developed and applied to specimens machined from injection moulded plates. This experimental set up allows to measure the strain fields on both front and lateral sides of specimens with a good accuracy. In this study, two impact modified polypropylenes have been analyzed under tensile loading. These polymers are made of ethylene-propylene rubber nodules and lamellar talc particles embedded in a polypropylene matrix. They differ from the volumetric amount of talc (4% in material A, 0,5% in material B). Unlike neat polypropylene, these materials do not exhibit a necking phenomenon in tension. The axial strain and transverse strain through specimen width remain homogeneous along the gauge length during the tensile test. On the other hand, the transverse strains through thickness differ for both polymers. Polymer B exhibits large gradients of εzz strain between the centre and the plate surfaces while polymer A presents an homogeneous transverse deformation. As confirmed by microstructure investigation, these results reveal a large volumetric dilatation due to void growth for both polymers, but for polymer B, the large transverse gradient measured is due to a high volumetric dilatation gradient through plate thickness. The good thermal conductivity of talc could explain the different behaviour between the two polymers and the origin of volumetric dilatation gradient. In polymer B, the different cooling rates between plate core and surfaces induce large residual stresses which could create damage and initiate voids in plate core

Heterogeneous tensile behavior of impact-modified polypropylene plates processed by injection moulding

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Analyse couplée par micro-tomographie et émission acoustique des endommagements par fatigue d'un composite tissé chanvre/époxy : effet de l'humidité.

International audienceThe purpose of this work is to characterise the influence of moisture on the fatigue behaviour of a woven hemp fibre reinforced epoxy composite. At first, the water uptake of this composite has been studied, then the mechanical behaviour is analysed under two different conditionings. The so-called “non-aged samples” have been stored ands ubmitted to fatigue tests at ambient temperature and hygrometry, and the so-called “water-aged” samples have been tested in 95% of relative humidity atmosphere after water immersion until saturation. Results show that the fatigue behaviour is affected by the moisture conditions: for a given fatigue stress level, the fatigue lifetime is shorter for the “water-aged” samples. Fatigue damage mechanisms have been analysed by combining different techniques: in-situ acoustic emission monitoring (AE), post-mortem scanning electronmicroscopy (SEM) and X-ray micro-tomography. For the “water-aged” specimens,damage appears earlier during the fatigue testsand is more extended than for the “non-aged” ones.Ce travail vise à caractériser l’influence de l’humidité sur le comportement en fatigue de composites tissés chanvre/époxy. Dans une première partie,l’absorption d’eau du composite est étudiée, avant d’analyser son comportement mécanique en fatigue.Deux conditionnements sont étudiés:pour le conditionnement nommé «Ambiant»les éprouvettes ont été stockées et testées à l’ambiante, alors que pour celui nommé «Eau» les éprouvettes ont été immergées dans l’eau jusqu’à saturation et testées en fatigue dans une enceinte climatique maintenue en humidité. Les résultats montrent que la tenue en fatigue est influencée par le conditionnement hydrique: pour un niveau de contrainte donné, la durée de vie en fatigue est plus faible pour les éprouvettes «Eau»que pour les éprouvettes «Ambiant». Les mécanismes d’endommagement ont été analysés durant l’essai par émission acoustique, et post mortem par MEB-FEG et micro-tomographie. Les micro-tomographies et les émissions acoustiques combinées mettent en évidence des mécanismes d’endommagement distincts en fonction du conditionnement. Les éprouvettes «Eau» ont subi un endommagement plus important en volume et plus précoce au cours de l’essai par rapport aux éprouvettes «Ambiant

Damage tolerance assessment of the interface strength gradation in thermoplastic composites

The newly developed hybridization approach for thermoplastic composites, based on the gradation of the interlaminar interface strength (IGIS design), was used to prepare polypropylene/glass fibre composites by properly alternating layers of reinforcing fabric with compatibilized and not compatibilized polymeric films. Maleated polypropylene was used to improve the matrix/fibre interface strength. The flexural and low-velocity impact characterizations showed that the use of the coupling agent improved the quasi-static flexural properties through the strengthening of the matrix/fibre interface, but considerably lowered the low velocity impact resistance of the laminate. The use of the IGIS design, which grades the interface strength through the laminate thickness, enabled the fabrication of composites with a favourable combination of flexural properties and impact resistance and proved to be effective in better preserving the integrity of the fibres. The damage after impact has been assessed by means of micro-computed tomography, which elucidated the improving effect of the matrix hybridization on the impact resistance, and acoustic emission analysis, which allowed the identification of the mechanisms responsible for impact energy absorption. The role played by the matrix hybridization sequence was pointed out along with the effective role of IGIS design in the overall mechanical performance of laminates

Surface Modification of Flax Yarns by Enzymatic Treatment and Their Interfacial Adhesion with Thermoset Matrices

The aim of this study was to assess the effects of commercially available and relatively inexpensive enzyme preparations based on endo 1,4-β-xylanase, pectinase and xyloglucanase on the thermal (TGA), morphological (SEM), chemical (FT-IR) and mechanical (single yarn tensile tests) properties of flax yarns. The preparation based on pectinase and xyloglucanase provided the best results, resulting in the effective removal of hydrophilic components such as hemicellulose and pectin, the individualization of yarns and increased thermal stability at the expense of a reduction in mechanical properties, depending on the treatment parameters. Single yarn fragmentation tests pointed out an improved interfacial adhesion after enzymatic treatment, with reduced debonding length values of 18% for an epoxy matrix and up to 36% for a vinylester resin compared to untreated flax yarns