Samuel Rivallant

Combined shear/compression structural testing of asymmetric sandwich structures

Asymmetric sandwich technology can be applied in the design of lightweight, non-pressurized aeronautical structures such as those of helicopters. A test rig of asymmetric sandwich structures subjected to compression/shear loads was designed, validated, and set up. It conforms to the standard certification procedure for composite aeronautical structures set out in the “test pyramid”, a multiscale approach. The static tests until failure showed asymmetric sandwich structures to be extremely resistant, which, in the case of the tested specimen shape, were characterized by the absence of buckling and failure compressive strains up to 10,000 μ strains. Specimens impacted with perforation damage were also tested, enabling the original phenomenon of crack propagation to be observed step-by-step. The results of the completed tests thus enable the concept to be validated, and justify the possibility of creating a much larger machine to overcome the drawbacks linked to the use of small specimens.

Discrete Impact Modeling of Inter- and Intra-laminar Failure in Composites

The goal of this study is to initiate a “test-calculation dialogue” on low velocity/low energy impact tests in laminated composites. The different types of impact damage developing during an impact test, i.e. matrix cracking, fiber failure, interface delamination and permanent indentation, are simulated. The bibliography shows a general lack of detailed validation of impact modeling and the originality of this work is to use refined and complementary experimental data to build and validate a numerical model. The good correlation between the model and this refined experimental database gave us relative confidence in the model, despite a few non-standard material parameters.

Permanent indentation characterization for low-velocity impact modelling using three-point bending test

This paper deals with the origin of permanent indentation in composite laminates subjected to low-velocity impact. The three-point bending test is used to exhibit a non-closure of matrix crack which is assumed as a cause of permanent indentation. According to the observation at microscopic level, this non-closure of crack is produced by the blocking of debris inside matrix cracking and the formation of cusps where mixed-mode delamination occurs. A simple physically-based law of permanent indentation, “pseudo-plasticity”, is proposed. This law is qualitatively tested by three-point bending finite element model and is lastly applied in low-velocity impact finite element model in order to predict the permanent indentation. A comparison between low-velocity impact experiments and simulations is presented.

Modeling Impact on Aluminium Sandwich including Velocity Effects in Honeycomb Core

A numerical model has been developed on metallic sandwich structures as an armor for aeronautical applications. Several combinations of AA5086-H111 aluminium skins and aluminium honeycomb core have been studied, considering medium-velocity and high-energy impacts. The aim is to establish links between the sandwich performances and the material and geometrical parameters. An elasto-plastic, strain-rate dependent behavior has been implemented to represent the skins and the core. The sandwich model has been calibrated and validated from the experimental data. Dynamic effects, as well as strong couplings between the skins and the core appear to have a significant effect on the target performance.

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.

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.