Jean-François Ferrero

Torsion of thin-walled composite beams with midplane symmetry

Abstract The optimization of thin-walled beams can lead to the design of structures whose walls have significant differences in stiffness. The stress field resulting from a twisting moment is not correctly modeled by classical analytical theories, so numerical modeling is therefore essential. The analytical theory presented in this study provides us with a simple way of determining stress and stiffness in this type of structure, without having to resort to complex methods of calculation. The walls are made of isotropic or composite materials with midplane symmetry and one orthotropic axis is the longitudinal axis. The section may be open or closed and the constrained warping effect can be taken into account. The results obtained correlate accurately with those obtained from 3D FE modeling.

Study of the deformation of a sandwich shield subjected to bird impact: A behaviour analysis tool using vector decomposition:

In this work, a numerical finite element model of a 1.82 kg bird impacting a sandwich shield at 175 m/s is developed. Different shield designs are simulated and it appears that very different sandwich behaviours can occur, depending on the design. A new tool to analyse the deformation of the sandwich during impact is presented and is used to study the behaviour of a shield. As this tool makes it possible to easily compare the behaviour of different shields, it is used in a screening study to identify the more influential sandwich design parameters. If all design parameters are considered to be independent, the core out-of-plane plastic plateaus appear to be the most important. The core in-plane properties, elastic modulus and density and the back skin thickness have much less influence on the sandwich deformation under impact.

Charpy test investigation of the influence of fabric weave and fibre nature on impact properties of PEEK-reinforced composites

The aim of the work is to use Charpy impact test for quick evaluations of different Polyether-ether-ketone (PEEK)-reinforced composites to be used for impact protection. In the first part, the influence of weave pattern was first analysed by comparing the impact behaviour of three PEEK composites reinforced with plies of unidirectional (UD) tapes, 5H satin fabrics and 2 × 2 twill fabrics made of high-strength carbon fibres. In the second part, the influence of fibre nature was investigated for the same weave pattern. The impact behaviour of five 2 × 2 twill fabrics made from inorganic fibre (carbon, glass and basalt) and organic fibre (aramid and poly(p-phenylene-2,6-benzobisoxazole) (PBO)) has been compared. Two main types of failure modes were identified: a brittle behaviour mode with high failure strength and a highly deformable behaviour mode in which energy absorption is more important. The balance between brittle behaviour and highly deformable behaviour results from competition between the yarn crimp, weave pattern and fibre properties of the composite. Slight yarn crimp and small ply thickness increase the stiffness of the composite and induce brittle behaviour characterized by fibre failure in tension and a steep peak on the loading curves. This behaviour is observed in UD and 5H satin carbon-reinforced composites or 2 × 2 twill glass and basalt fabric-reinforced composites. In contrast, aramid and PBO 2 × 2 twill fabric composites exhibit high shear strength. The highly deformable behaviour of the specimens during the Charpy impact led, in the case of organic fibres, to a non-breakage of the fibres and consequently to a high level of energy absorption. This behaviour is necessarily interesting in armour applications.

Sandwich shield subjected to bird impact: Use of surrogate models for influencing parameter analysis and shield behaviour understanding

In this work, the behaviour of a sandwich shield subjected to a 1.82 kg bird impact at 175 m/s is studied using a finite element model. The 6 most influential design parameters are varied and their effects on the shield behaviour and on the target protection are assessed. First, we try to establish an engineers visualization by varying parameters 2 × 2 using three 5-levels full-factorial design of experiments. These three 2D design of experiments enable us to visualize precisely the different effects of each parameter. Then a full sensitivity analysis (6D) is performed using a Latin Hypercube sampling to assess the possible interactions between parameters. Surrogate models are constructed using the Gaussian Process framework to follow the variation of the outputs in the 6D design space. These surrogate models are finally studied using two statistical methods: the Sobol′ method and the Morris method. The methodology developed in this study enables to improve the understanding of the behaviour of a shield under a soft body impact, as a first step towards a shield design tool.

Experimental results of medium velocity impact tests for reinforced foam core braided composite structures

Absorbing impact energy at subsystem level is an attractive idea that is emphasized by new composite reinforcement techniques such as stitching or pinning. This paper reports experimental results of medium velocity impact tests carried out on several arrangements of reinforced foam/braided composite structures. The tests consisted of a steel ball shot at a velocity of 110 m/s from a gas gun impacting the structures on their leading edge. Post-mortem tomography analysis delivered very rich information which shed light on the damage mechanisms that the composite structures underwent. In addition, two fast-speed cameras were used to derive the energy absorption during the impact. Absorption capabilities were also compared with those of dynamic crushing tests (reported in a companion paper) and some designs clearly exhibited promising behavior as shock absorbers.

Mode I Delamination Modelling in a Composite Plate using the Plate Behavior of a New Multilayered Solid Element

The simulation of the delamination process in composite structures is quite complex and requires advanced FE modelling techniques. These techniques are difficult to implement when the modelled structures are industrial-size like helicopter blades and sine-wave crash absorber beams. In order to simplify the damage modelling in complex structure, the development of a solid hexahedron element for composite delamination analysis has been carried out. The 8-node solid is derived from a 20-node hexahedron element and it can be transformed into 2 physically independent 4-node shell elements. This separation into two shells which corresponds to the propagation of delamination process was controlled by a delamination beam criterion. This element simplifies present models and decreases the calculation time that is accepted only for 1D propagation of delamination. In this study a new way of modelling delamination growth is proposed. A delamination criterion based on the Ritz method and plate theory is used to govern the delamination propagation. Experimental tests are carried out on composite structures such as square composite panels in mode I. The crack propagates with an angle of 45 degrees relative to the panels edge. This test is modelled using the new element. The numerical simulation correlates well with the experimental results.