Вариационный подход к определению динамической прочности материала

The problem of the determination of material strength properties through the Kolsky experimental technique is considered. Small size specimens of M1 copper alloy are tested on a split Hopkinson pressure bars equipment. The experimental data of tensile tests observed under both dynamic and quasi-static conditions are analysed within the framework of the incubation time criterion and the Sign-Perturbed Sums method. It is shown that the influence of a test performance error is considered in the data treatment procedure based on the developed method.

The Effect of Temperature Gradients on Elastic Wave Propagation in Split Hopkinson Pressure Bars

If it is desired to obtain high rate mechanical data of materials at non-ambient temperatures using the split Hopkinson (Kolsky) bar technique, it is necessary either to consider what effect a temperature gradient has on the propagation of elastic waves along a metallic rod or to design a mechanism that minimises the exposure of the Hopkinson bars to heating or cooling. Two main mechanical systems have been devised: the first where the bars are brought into contact with the specimen a short time (less than one second) before the specimen is dynamically loaded; the second where the specimen is moved into position just before it is dynamically loaded. As these mechanisms are complex to design and build, many researchers choose the simpler option of heating (or cooling) the ends of the bars as well as the specimen. This review summarises issues that should be considered if this option is taken.

Impact Perforation of aluminum Cymat Foam

The behavior of aluminum Cymat foam under impact perforation loading was studied using experiments and simulations. Measurements at 40 m/s were performed with an inverse perforation setup using a Split Hopkinson Pressure Bars system. Such measurement is missing in a classical free-flying penetrator–immobile–target scheme under impact loading and makes it possible to directly compare impact the perforation force–displacement curves with the static ones. Compared with quasi-static test perforation forces obtained under the same geometry and clamping system, a significantly enhanced perforation force was found under impact loading. Numerical simulations of the perforation test were developed using LS-DYNAfinite element code to provide the local information necessary to understand the unexpected enhancement in perforation force. The shock effect was found to be responsible for enhancement of the perforation force and revealed that the honeycomb model with appropriate tensile failure criteria was more suitable for model perforation of the foam than the Deshpande and Fleck model with volumetric failure strain criteria

Low energy shock response of a melt cast simulant material

To manufacture its insensitive munitions (MURAT MUnitions à Risque ATténué), NEXTER Munitions uses a melt cast explosives as an EIDS (Extremely Insensitive Detonating Substance). Unlike commonly used and well-documented EIDSs such as PBX, melt cast have a high volumetric matrix percentage. Moreover, in its life cycle, the ammunition can undergo severe loads, such as cannon firing, accidental shocks and terminal ballistics events. The objective of this paper is therefore to analyse, how these dynamic loads induce changes in the material (damage, cracking, de-cohesion), and then, to evaluate how these alterations influence the pyrotechnic properties of the melt cast explosive. To address the first point, we delimited the scope of the study in pressure and strain rate ranges which corresponds to the context of the ammunition. To safely explore this area, we have created an inert material that is morphologically and mechanically representative of the melt case explosive. It is used to setup and validate the experimental technic that will be applied to damage the melt cast explosive in the future. In this article the mechanical behaviour of the inert material is investigated under simple compression and passive confinement. This was done under the quasi-static and dynamic regimes thanks to a compression press and a Split Hopkinson Pressure Bars setup. Firstly, the results obtained show the emergence of a damage which increases with the loss of cohesion of the material during the test. This seems to be related to the extension strain. Then, for all tests, a strain rate dependent mechanical response is observed. Finally, the end of the test shows the material behaviour without cohesion. Then, a rate dependent ultimate shear criterion is deduced. To complete the interpretation these results, a model is proposed. It intends to be simple as it tries to describe the whole degradation of the material with a unique scalar parameter.

Flow Stress of beryllium: Attempt for a Bayesian Crossed-data Analysis from Hopkinson Bars to Rayleigh-Taylor Instabilities

In this paper, we demonstrate by a Bayesian approach the incapacity of the Preston-Tonks-Wallace (PTW) strength model to represent, with the same set of parameters, the flow stress of beryllium in both moder-ate and highly dynamic experiments, and suggest hypotheses explaining that limitation. Usual plasticity models such as Johnson-Cook (JC) and PTW are mostly adjusted onto quasi-static and dynamic uni-axial compression data acquired thanks to compression machines and split Hopkinson pressure bars. Nonetheless, they may be used beyond the range of mechanical loading in which they have been fitted. This is the case of the simulations of solid Rayleigh-Taylor instabilities (RTI) driven by high explosives. A recent work of Henry de Frahan et al. noticed the inability of various plasticity models to stand for the growth of beryllium RTI. Amongst them, the PTW model has been particularly examined through four different sets of parameters, each of them largely un-derestimates the growth of the experimental instability. Thus, this work is an attempt, regarding the plastic flow modeling of beryllium, to conciliate uni-axial compression tests (CT) and RTI by means of a crossed Bayesian analysis.

Effect of Material Composition on Impact Energy Absorbing Capability of Composite Laminates

A novel experimental method was proposed for characterizing the energy absorbing capability of composite materials during the progressive crushing process under impact loading. A split Hopkinson pressure bars system was employed to carry out the progressive crushing tests under impact loading. The stress wave control technique was used to avoid the inhomogeneity of dynamic stress field in the specimen. The progressive crushing behavior was successfully achieved by using a coupon specimen and anti-buckling fixtures. With increasing strain rate, the absorbed energy during the crushing process slightly decreased, whereas the volume of the damaged part clearly increased regardless of material type. Consequently, the energy absorbing capability decreased with increasing loading rate. The effects of material composition, such as fiber type, matrix type and fabric pattern, on energy absorbing capability were also investigated by using the proposed method.