Understanding the response and damage evolution of structures subjected to multiple impact events is essential for designing resilient structures capable of withstanding complex loading scenarios, such as impacts from hail, gravel or foreign object debris. This article presents the development and characterization of a novel test bench, the “Compressed air multi-cannon”, designed specifically for studying the multi-impact behavior of composite materials. This test bench offers advantages over traditional impact testing methods by enabling controlled and adjustable impact parameters, including number of impacts, spatial and temporal lag, energy, angle and impactor dimensions. The primary objective of this work is to provide a detailed description of the test bench design, construction, and validation procedures. Key components such as the pressurized air system, projectile launch mechanism, target mounting arrangement, and data acquisition system are discussed. Experimental methodologies for assessing multi-impact response, specimen preparation, instrumentation, and data analysis techniques are outlined. Through a series of single-impact and multi-impact tests, distinctive damage mechanisms and energy absorption characteristics were observed in composite structures, revealing significant differences in how composites respond under single- and multi-impact conditions. It was found that the single-impact configuration remains particularly critical compared to multi-impact configurations with a high number of impacts. However, further testing is required to determine whether this result holds true under varying impact parameters, highlighting the unique value of this machine for exploring new, realistic questions in the literature.

Flax fiber-reinforced plastics have an innate eco-friendly nature due to the fiber reinforcement and reduced energy requirements in fabrication when compared to current fiber reinforced composite materials. They possess a complex time-dependent material behavior, which is investigated in the present paper. A composite material with flax fiber reinforcement on the load direction, embedded in an epoxy resin matrix, was studied. The procedures used were tensile tests, repeated loading-recovery, and creep-recovery tests, which were meant to expose the components of the response with respect to stress level and load duration. The results showed an elastic bi-linear behavior, a yield point at approximately 20% of the ultimate tensile stress, and tensile moduli of 35.9 GPa and 26.3 GPa, before and after yield. This is coupled with significant non-linear viscoelastic and, after yield, viscoplastic components, accounting for up to 14% of the strain response. The behavior is inherited from both the matrix and the fiber reinforcement and is attributed to the amorphous nature of the matrix combined with the microstructural re-organization of the fiber under load, which are partially reversible.

Previous studies on the photooxidation of natural fiber composites have focused on using short fibers and virgin matrices. In this work, unidirectional flax fiber composites with virgin and recycled polypropylene matrix were prepared by thermocompression to compare their accelerated aging. These composites were exposed to two accelerated xenon arc UV exposures where only the irradiance was modified. A multi-scale characterization was carried out to evaluate the consequences of the photooxidation on their mechanical and physicochemical properties with time. Results show that the photooxidation was limited to the surface. In addition, the photo-degradation of the surfaces of both composites was identified by the appearance of cracks, the variation of crystallinity, the decrease in weight average molar mass, and the appearance of new chemical products. These physicochemical variations are more evident with increasing irradiance. Despite these variations, the mechanical tensile properties of recycled matrix composites remain relatively indifferent to the two UV exposures compared to those with virgin polypropylene. It can be concluded that recycled polypropylene, if well selected, can replace virgin polypropylene in natural fiber composites for better mechanical resistance to photooxidation.

The aim of the present study is to develop a multifunctional sugarcane fibres reinforced epoxy porous composite combining good acoustic and mechanical properties.The study of the bagasse fibres geometry has shown that fibres' length and diameter distribution can be fitted by lognormal laws.Composites manufactured by thermo-compression process with an epoxy matrix were characterized using an experimental design whose parameters were the diameter of the fibres (between 0.5 and 4 mm) and their mass ratio (between 40 and 70 %).The study of acoustic properties showed that the sound absorption increases with the diameter of the fibres, while decreasing with their mass proportion, over a frequency range between 500 and 1000 Hz.Mechanical characterisation by bending tests, has shown a fragile behaviour, with deviations of stiffness and maximum stress around 36 %.The stereo-correlation image analysis confirmed the heterogeneity of the strain fields throughout the thickness, in relation to the fracture observation.

Although the research and development in plant fibre composites is growing rapidly, these materials still require specific considerations before being adopted by industry in structural applications. One of the main issue is related to their durability. The objective of this study is to investigate the influence of hydrothermal ageing on the fatigue behaviour of an unidirectional flax-epoxy laminated composite by implementing the fatigue tests in a water bath. Results show that while the quasi-static strength and rigidity are significantly affected by the ageing, the fatigue strength is however improved. The fatigue strength coefficient is more than two times lower after ageing, indicating a lower decrease of the maximum stress with the increasing number of cycles. After ageing, the maximum stress level for an expected lifetime of 5.3 10(6) cycles is approximately 90 MPa, which is comparable to the one of unaged specimens. It is also pointed out that the remaining quasi-static tensile properties and behaviour of the aged specimen are not significantly affected after 1.3 10(6) cycles at a stress level of 90 MPa.

In the middle of the 20th century, fiber reinforced composites became mainstream, with carbon and glass fibers at the forefront of this developing industry. However, they have certain disadvantages which have driven the research towards the discovery of novel materials. Today's demands go towards cheap and renewable materials. It has been proven that flax fiber possesses some of the best mechanical properties of bio-reinforcements, close to the ones of the glass fibers. These advantages, together with a lower density than glass and the possibility of recycling have made the flax fiber reinforcement a candidate for the next generation of composites. Along with these strong points, comes a behavior, which, for the composites mentioned earlier, can often be neglected: the time dependent behavior. The present work aims at studying this property and to propose a basic model for its evolution. The analyzed composite is an epoxy resin reinforced by long flax fibers, oriented at +/- 45 degrees, which show the shear behavior of the material. The experimental procedure consists of a tensile test to determine the mechanical properties of the composite on this direction and to establish the stress levels for the time dependent tests. Secondly, four levels of creep recovery tests are conducted with a creep of one hour and recovery of five hours. In the last part of the paper, linear modeling of this phenomenon is presented, based on the Zener and Burger models.

Bio materials have seen an increase of interest from the scientific community and the industry as a possible future generation of mass produced materials, some of the main arguments being their renewability, low production costs and recyclability.The current work is focused on the experimental data required for the viscoelastic characterization of a composite material. Similar work has been conducted on different types of composite materials by Tuttle and Brinson [1] who verified for a carbon epoxy laminate the possibility of long term predicament of creep. Nordin et al [2] studied paper impregnated with phenol-formaldehyde under compression. Muliana [3] conducted experiments on E-glass/vinyl ester materials. Behavior characterization was based on a model presented by Schapery [4]. The main objective of this work is to understand the mechanical behaviors of bio-laminates structures subjected to long and severe operating conditions.The studied material is a bio composite laminate consisting in long flax fibers embedded in an epoxy resin system. The laminates were obtained from pre-impregnated unidirectional fibers, which were cured though a thermo-compression cycle followed by a post curing cycle. Test specimens were cut down to sizes, with the help of an electric saw. The concerned fiber direction was 0° with sample dimensions of 250x25x2 mm.First, testing consisted in quasi static mechanical tests. Second, to characterize linear viscoelastic behavior of the bio-laminates, creep – recovery tests with multiple load levels have been performed for the chosen fiber direction.

The development of composites based on vegetal fibers requires a good control of manufacturing process. The aim of this work is to determine the key parameters to produce high grade flax/epoxy unidirectional laminated composite by thermocompression. So, many processing parameters have been tested and ranked according to their influence on mechanical properties. Since variability can be high for this kind of materials, statistical analyses have been used to determine if properties variations were significant or not. Among all studied parameters, the three which have been identified as first rank influence on mechanical properties are: fibers conditioning, curing pressure and exit plate temperature.

Weight reduction of truck cabs can be achieved by using structural parts made from composite materials and adhesively bonded with a flexible adhesive. This paper presents an experimental study on the behaviour of adhesively bonded joints of fibre reinforced thermoplastic composites. U-shaped glass-polyamide substrates are joined with a flexible polyurethane adhesive and the specimens, inspired from spot weld characterization, are tested in a device allowing loading in different directions.Effect of load orientation, temperature, loading speed and adhesive thickness on the stiffness, the strength and the energy absorption of the joints are presented and discussed. Results show that the assembly overall mechanical behaviour is mainly controlled by the polyurethane adhesive. Such an adhesive/substrates combination is able to sustain high dynamic loadings when loaded in shear direction. High temperatures have a detrimental effect on the performances of the adhesively bonded joints.

This paper reviews multi-scale computational homogenisation frameworks for the non-linear behaviour of heterogeneous thin planar shells. Based on a review of some of the currently available methods, a computational homogenisation scheme for shells is applied on representative volume elements for plain weave composites. The effect of flexural loading on the potential failure modes of such materials is analysed, focusing on the reinforcement-matrix delamination mechanism. The attention is next shifted toward failure localisation in masonry unit cells. Subsequently, a recently developed computational FE$^2$ solution scheme accounting for damage localisation at structural scales based on RVE computations is applied.

L’effet de massification automobile qui accompagne les besoins croissants de mobilite terrestre et l’offre de nouvelles infrastructures routieres qui soutient de nouvelles demandes rendent les modeles de mobilite actuels de plus en plus difficilement acceptables. En effet, les personnes souffrent de l’inconfort des villes engorgees (nuisances), de l’insecurite lors de transports du fait de defaillances techniques ou humaines, de la tres grande difficulte d’optimiser ses itineraires en fonction des horaires ou circonstances exceptionnelles (accident par exemple). Par ailleurs, la mobilite se heurte toujours aux contraintes liees aux rigidites des infrastructures composees de nœuds, et de contraintes permanentes qui etouffent nos villes. Pour repondre a ces enjeux, la transition vers la mobilite autonome est en marche et l’ecosysteme vehicules – infrastructures doit evoluer pour desengorger nos villes. Ces nouveaux systemes de transport sous-tendent une revolution technologique dans le domaine des transports et devront repondre a des preoccupations socio-economiques capitales en etant tout a la fois adaptables, resilients, et evolutifs, cooperatifs et acceptables des points de vue economiques, juridiques et sociaux.

La production actuelle de materiaux composites structuraux repose principalement sur des ressources fossiles non renouvelables. De nombreuses reglementations incitent aujourd’hui les industriels a s’orienter vers des solutions ecoresponsables. Dans ce contexte, les fibres vegetales apparaissent comme une alternative interessante aux fibres synthetiques en raison de leurs bonnes proprietes mecaniques et de leur faible densite. En revanche, l’emploi de telles fibres reste limite pour des composites structuraux du fait de leur caractere hydrophile qui impacte la fabrication, la tenue en service ainsi que la durabilite. L’objectif de ce travail est d’etudier l’effet d’un vieillissement hygrothermique, consistant en une absorption suivie d’une desorption, sur les proprietes mecaniques d’un materiau composite lin/epoxy. Parmi les parametres de fabrication, l’influence du conditionnement des fibres et de la pression de thermocompression ont ete etudies. Les resultats de cette etude montrent l’importance de maitriser les conditions de stockage des fibres afin de garantir un materiau reproductible pourvu de bonnes proprietes mecaniques. Une pression de fabrication de 3 bars a permis d’obtenir les proprietes optimales. L’absorption d’eau a induit une plastification du materiau et la desorption a montre la reversibilite des phenomenes de plastification, mais a egalement revele des phenomenes d’endommagement irreversibles affectant les proprietes ultimes.

Experimental and numerical methods to identify the linear viscoelastic properties of flax fibre reinforced epoxy (FFRE) composite are presented in this study. The method relies on the evolution of storage modulus and loss factor as observed through the frequency response. Free-free symmetrically guided beams were excited on the dynamic range of 10 Hz to 4 kHz with a swept sine excitation focused around their first modes. A fractional derivative Zener model has been identified to predict the complex moduli. A modified ply constitutive law has been then implemented in a classical laminates theory calculation (CLT) routine.