R. Cortés

The growth of microvoids under intense dynamic loading

Abstract This paper deals with the analytical study of the dynamic growth of microvoids. A previous model by Carroll and Holt (1972). Static and dynamic pore-collapse relations for ductile porous materials, J. Appl. Phys. 43 , 1626–1636, for porous perfectly plastic materials under the action of a purely hydrostatic stress is further developed to include the influence of material viscosity. strain hardening and thermal softening in the tensile fracture behaviour. The results of this analysis are discussed with the help of a numerical study of the void growth relationships derived; the conclusion is that both material viscosity and strain hardening may have an important influence on the tensile strength of ductile materials at high strain rates. It is also shown that, in the conditions of the analysis, thermal softening by itself has a negligible intluence on the dynamic tensile strength at high strain rates due to excessively localized heat generation near the surface of the voids.

Numerical modelling of normal impact on ceramic composite armours

Summary In this paper, the penetration of ceramic targets backed by thin metallic plates when impacted by cylindrical projectiles is studied. To achieve this, a two-dimensional axisymmetric numerical analysis of the normal impact problem is performed. The macroscopic material behaviour in the zone of finely pulverized ceramic ahead of the penetrator is modelled by means of a constitutive model taking into account internal friction and volumetric expansion. The amount of comminution at the computational cells is evaluated through a damage evolution equation, and the yield stress is assumed to be a function of the hydrostatic pressure, internal friction and amount of comminution. For the metallic materials involved, an elasto-plastic behaviour with a rupture condition was considered. Moreover, an erosion condition was included as a limit situation when the ruptured material limits its role in the penetration process to purely inertial effects. In this way, a detailed picture of the penetration process of the target by the impacting projectile was obtained. Then, the results of the numerical analysis were compared with the experimental observations of the projectile-target interaction, previously made by Reijer by using a flash X-ray technique. Under certain conditions, remarkable agreement between computations and experiments is encountered, thus suggesting the adequacy of the main assumptions made in the numerical approach to the physical situation.

Friction and wear behaviour of Kevlar fabrics

Experimental results of a number of tribological tests carried out on aramid woven fabrics are presented in this paper. Kevlar Ht, Kevlar 29 and Kevlar 49 aramid plain fabrics were employed in this work. The friction and wear phenomena of the fabrics were investigated, considering both fabric-fabric and metal-fabric interaction. From the experimental data, the evolution of parameters such as static and dynamic friction coefficients, dissipated energy, volume loss of the material, wear rate, specific wear and wear strength were studied. Moreover, values of the static force needed to pull out a single fibre from the woven fabric were measured. All these data are important for the numerical modelling of impact on such materials. In fact, experimental findings on yarn failure mechanisms show that apart from tensile rupture, failure modes such as cutting, shearing and fibre degradation take place in fabrics subjected to the ballistic impact of low-and medium-calibre ammunition.

Dynamic growth of microvoids under combined hydrostatic and deviatoric stresses

Abstract This paper deals with the analytical study of the dynamic plastic growth of microvoids under the combined action of hydrostatic and deviatoric stresses. The results of this analysis are discussed with the help of a numerical study of the void growth relationship derived, and applied to the case of spall fracture. The conclusion is that void expansion may be affected in different manners by the presence of a field of deviatoric (purely distortional) strain rates. If the deviatoric plastic strain rate is not large compared with the rate of volumetric expansion, then, for void growth controlled fracture, the spall strength of the material tends to decrease with respect to a purely hydrostatic stress. The quantitative loss of strength may be important, depending upon the loading conditions. When void growth initiates in a state of very large deviatoric strain rates then, under the conditions of the analysis, the volumetric expansion of the voids may require excessive large stresses, so as to become very difficult in practice. Then, in such a situation a different mechanism, such as void nucleation for instance, might control the fracture process rather than plastic void growth.

Some observations on the normal impact on ceramic faced armours backed by composite plates

Abstract This paper describes the results of experiments in which 7.62 NATO projectiles impacted on ceramic faced composite armours backed by a fibrous armour. The ceramics considered were AD-96 alumina and a mixture of boron nitride and silicon nitride. The backing was a composite plate made of either aramid fibres embedded in a vinylester matrix or polyethylene fibres embedded in a polyethylene resin matrix. An additional package of high tenacity aramid fabric layers was also included, separated from the composite plate by an air chamber. The design of the armours was based on an energy absorption model proposed by Hetherington and Rajagopalan for composite armours. Normal impact experiments performed for the selected configurations revealed that the model is able to predict fairly well the ballistic limit of composite armours backed by a fibrous armour.

Numerical study of the specimen size effect in the split Hopkinson pressure bar tests

A numerical and experimental assessment of the compression test in the split Hopkinson pressure bar (SHPB) has been made. The DYNA2D finite element code was employed in the numerical part. The aim of the work was to establish the influence of an important reduction in the specimen diameter on the results. To this end, several numerical experiments were carried out with different diameters. Experimental measurements using the SHPB technique were also performed. The material studied was the 7017 T73 aluminium alloy. In the simulations, stress histories were registered at different places in the incident and output bars, as well as in the test specimen. Numerical simulations show important three-dimensional effects in the SHPB, increasing for smaller diameters. Experiments show the same tendencies evinced by the numerical simulation. Care must be taken to minimize them to achieve the desirable uniaxial stress condition on the specimen.

Measurements of internal friction coefficient of SiC and Al2O3 powders

The frictional strength of the powder of two ballistic materials (SiC and Al2O3) under low confining pressures has been studied statically and dynamically. In the static tests, the influence of the ceramic fragment size upon the internal friction coefficient value was investigated. In the dynamic tests, the effect of the ceramic fragment size, the relative sliding velocity and the confinement load were studied. It was found in the experimental conditions, that the influence of the fragment size and of the relative sliding velocity upon the internal friction coefficient value of comminuted SiC and Al2O3 is important.

The plastic growth of a cavity nucleated at a shear band

Abstract In this paper, the dynamic expansion of a spherical cavity nucleated at a shear band is theoretically investigated. To this end, the voided solid is represented in the vicinity of the band by a model consisting of a rigid-plastic thick hollow sphere which experiences an expansion characterized by a radial velocity field with spherical symmetry plus an additional pure shear distortional field representing localized shear deformation within a thickness 2 h . The selected shear strain rate field enables us to cope with localized deformation within a band of arbitrary thickness, oriented parallel to the direction of the imposed macroscopic shear strain. Expressions for the macroscopic deviatoric and volumetric stresses required to deform the material with voids located at shear bands were then computed. Furthermore, the dynamic void growth equation was also derived. The formulation was applied to the dynamic fracture of a thermal softening material in discrete steps, and the influence of the model parameters on the fracture strength of the material was assessed.

Numerical modelling of ductile spall fracture

Abstract In this paper, a macroscopic constitutive equation for porous materials, which is based on a void growth model under the combined action of hydrostatic and deviatoric stresses, is presented. The formulation follows previous work by Gurson [Continuum theory of ductile rupture by void nucleation and growth: Part I—Yield criteria and flow rules for porous ductile media. J. Engng Mater. Tech. 99 , 2–15 (1977)] for perfectly plastic materials, which is here expanded to include a non-linear viscous term. Dynamic effects associated to void growth under a general stress state have also been incorporated in the model. In this manner, at each time increment of the numerical analysis, the rate of porosity increase may be computed and the local porosity values evaluated. The model is applied to spall experiments in aluminium. Spall fracture in aluminium was experimentally studied by loading aluminium plates by use of an explosive. The damage mechanism leading to fracture of the tested specimens was the nucleation and growth of microvoids. From the numerical analysis of the above experiments, the free surface velocity histories were compared with the corresponding experimental records, showing that in the cases studied the inclusion of a non-linear viscous term is essential to obtain good agreement between analysis and experiment.

Welding of dense alumina and aluminium by plastic deformation and diffusion

The development of a welding system which makes it possible to obtain a close joining of two very different materials is described: high density (low porosity) alumina and aluminium alloy. To determine the characteristics of the contact area, optical microscopy was used, whereas energy dispersive spectroscopic (EDX) microanalysis was employed in the study of the atom diffusion that may take place through the metal-ceramics interface. The Hopkinson bar was used to test the joint resistance in an impact test.