Impact behavior of honeycombs under combined shear-compression. Part II: Analysis

AbstractIn this paper, a numerical virtual model of honeycomb specimen as a small structure is used to simulate its combined shear-compression behavior under impact loading. With ABAQUS/Explicit code, the response of such a structure made of shell elements is calculated under prescribed velocities as those measured in the combined shear-compression tests presented in Part I of this study.The simulated results agree well with the experimental ones in terms of overall pressure/crush curves and deformation modes. It allows for the determination of the separated normal behavior and shear behavior of honeycomb specimen under dynamic combined shear-compression. It is found that the normal strength of honeycombs decreases with increasing shearing load. Quasi-static calculations were also performed and a significant dynamic strength enhancement found in experiments was validated again in the numerical work. A crushing envelope in normal strength vs. shear strength plane was obtained on the basis of these simulations

La dynamique fait son cinéma : De l'apport de l'imagerie et des mesures de champs cinématiques pour l'analyse du comportement dynamique des matériaux

National audienceDepuis de nombreuses décennies, l'imagerie rapide a permis d'observer des phénomènes se produisant sur des échelles de temps très petites (de l'ordre de la milliseconde voire de la microseconde). Avec l'avènement plus récent des caméras numériques, de nouvelles applications sont possibles (p.ex. la tomographie rapide). L'utilisation quantitative d'images est également possible, notamment grâce aux techniques de corrélation et de stéréocorrélation d'images. Différentes applications seront présentées afin d'illustrer les apports pour l'analyse du comportement mécanique des matériaux sous sollicitations dynamiques

The instrumented dynamic perforation test applied to a composite shell

Perforation tests are commonly used on composites but give limited results. In this study, a single layer of a thermoplastic woven composite is tested at high velocity (45 m/s) by means of an instrumented perforation test. First, an inversed dynamic perforation tests is performed with a modified Hopkinson bar apparatus. This allows to measure accurately the perforation curve (force vs. displacement). Such a test is supplemented by the analysis of the strain displacement field measured by digital image correlation during the perforation process. This gives relevant information in case of inversed identification of several behavior parameters. This measurement technique allows to take into account the anisotropy of the material or an accurate estimation of the boundary conditions. A new experimental procedure is conducted for both quasi-static and dynamic tests. Results are presented concerning two points: the uncertainty of the measurements and the correction of boundary conditions from optical measurements

On the piercing force enhancement of aluminum foam sandwich plates under impact loading

International audienceThis article tends to explain the observed piercing force enhancement under impact loading of the sandwich panels, which is made of rate-insensitive aluminum skin sheet and Cymat foam core. Using an elastic–plastic damageable model for the skin and an isotropic foam constitutive model for the core, a numerical model is proposed and validated by the comparison with experimental results. Virtual tests allow revealing that the damage evolution in the skin sheet is strain rate dependent because of the wave propagation effects. Under impact loading, the damage area is larger and it leads to a larger deflection of the skin plate at its breaking. Therefore, the foam cores are more compressed under impact loading and induce the piercing force enchancement of the sandwich

Dynamic compression and recovery of cancellous bone for microstructural investigation

Knowledge of soft porous materials, such as cancellous bone, under dynamic loading requires accurate descriptions of high-rate mechanical responses. A novel modification of the standard Split Hopkinson Pressure Bar (SHPB) technique, that makes dynamic specimen recovery possible, is presented. Two impedance matched tubes, operating in tandem, are concentrically aligned with the incidence bar and placed in contact with a collar at the striker end. The collar transfers half of the incidence stress wave and most (>90%) of the reflected stress wave into the concentric tubes. In other words, the tubes act as sequential momentum traps and provide a single specimen loading event of predefined intensity and duration. This approach allows for routine testing without the need for initial “gap setting” , i.e. an accurate initial offset of the momentum trap with respect to the collar. Experimental results from a series of tests on cancellous bovine bone are presented. Furthermore, results from a microstructural investigation of the recovered specimens are presented and compared with quasi-statically loaded specimens

Measurement of deforming mode of lattice truss structures under impact loading

Lattice truss structures, which are used as a core material in sandwich panels, were widely investigated experimentally and theoretically. However, explanation of the deforming mechanism using reliable experimental results is almost rarely reported, particularly for the dynamic deforming mechanism. The present work aimed at the measurement of the deforming mode of lattice truss structures. Indeed, quasi-static and Split Hopkinson Pressure Bar (SHPB) tests have been performed on the tetrahedral truss cores structures made of Aluminum 3003-O. Global values such as crushing forces and displacements between the loading platens are obtained. However, in order to understand the deforming mechanism and to explain the observed impact strength enhancement observed in the experiments, images of the truss core element during the tests are recorded. A method based on the edge detection algorithm is developed and applied to these images. The deforming profiles of one beam are extracted and it allows for calculating the length of beam. It is found that these lengths diminish to a critical value (due to compression) and remain constant afterwards (because of significant bending). The comparison between quasi-static and impact tests shows that the beam were much more compressed under impact loading, which could be understood as the lateral inertia effect in dynamic bucking. Therefore, the impact strength enhancement of tetrahedral truss core sandwich panel can be explained by the delayed buckling of beam under impact (more compression reached), together with the strain hardening of base material

Evolution of the martensitic transformation in shape memory alloys under high strain rates

The specific properties of the shape memory alloys are mainly due to the martensitic transformation occuring in the material when mechanical or thermal loadings are applied. Here, the effect of strain rate on the transformation on an NiTi SMA is studied in tension. Different tests were performed at different strain rates in the range of 0,0001 /s to 15 /s. Two distinct methods were used to measure the extension rate of the martensitic phase region in the specimen: digital image correlation technique and infrared thermography (IR during quasi-static tensile tests only). For the dynamic tensile tests, a Split Hopkinson Tensile Bar set-up was used with a fast camera recording at 45’000 fps. A superimposition of DIC and IR measurements in time and space can be done during quasi-static tests and results show that the temperature peak, as expected, follows the transformation front. As a consequence of the former validation of the DIC procedure, the velocity of the transformation front at high strain rate is deduced from space-time figures. As a conclusion, in the range of strain rates investigated in this paper, no strain rate sensitivity is observed for dynamics of extension of the transformation region