The effect of specimen dimensions on the propensity to adiabatic shear failure in Kolsky bar experiments

Abstract

The issue of adiabatic shearing is discussed in this work and a new interpretation is given to some failure phenomena which are usually termed as adiabatic shears. We propose that only a few materials undergo a truly inherent failure which is due to thermal softening at the shearing zone and that the interplay between microvoids, cracks and narrow shear bands should be taken into account through the temperature rise at the front of advancing cracks. Also, the size of the plastic zone ahead of a crack plays an important role in determining the brittleness of a given specimen and should be taken into account when specimens of different sizes are tested. Experimental results for several alloys in the Kolsky bar system support our approach.Keywords: Kolsky bar, SHPB, adiabatic shear banding. INTRODUCTIONAdiabatic shear banding is one of the major failure mechanisms of solids subjected to high rate loading. Extensive research on this subject has been conducted over the past 50 years, both experimentally and theoretically, much of which is summarized in BAI and DODD [1]. The most common explanation of this dynamic instability is based on the competing tendencies of the solid to strengthen at high strains (and strain rates) and, on the other hand, to soften with the temperature increase under adiabatic loading conditions. Although many materials seem to behave accordingly, the basic mechanism which causes this softening is still unresolved. Recent work on the subject is focused on the basic physics behind this process, as can be found in [2-5], for example. These works emphasize the link between adiabatic shearing and ductile fracture by the coalescence of microvoids, since the shear zones act as precursor sites for eventual failure by cracks (see [2]). GIOVANOLA [3,4] conducted a careful experimental study on 4340 steel specimens, in order to follow the evolution of strain localization and failure which he finds to proceed in two stages. He found that the first stage of localization (deformed bands) results in the formation of local perturbations for the strain field, such as machining marks. These perturbations grow as a result of the imbalance between strain hardening and thermal softening. In the second stage, softening is due to the nucleation and growth of microvoids, leading to the shear fracture within the band. GIOVANOLA [3,4] suggested that this process can take place under other conditions, including quasi-static loading. He also emphasized the role of the compressive stress which acts on the shear plane to reduce the microvoid nucleation process.FLOCKHART et al.[5] used numerical simulations to follow the shear band in dynamically compressed specimen, by analyzing the loci of velocity discontinuities within the specimen. They also claim that "the distinction between shear fracture and adiabatic shear failure is not clear", and that "material susceptibility to adiabatic shear alone does not guarantee that failure occurs by this mechanism, as shear fracture may intervene". This interplay between shear...

Specimen Geometry Effect on the Deformation Mechanisms of Polypropylene-Based Composites Under Impact Loading at Different Temperatures

“…4, 5). Similar results of the rapid decrease of the stress in thick specimens have been reported for an aluminum alloy [48]. The rapid decrease of the stress after the yield point in thick specimens may be attributed to the barreling effect [59].…”

“…Indeed, when polymers are subjected to high loading rates, the effects of strain hardening, strain rate and temperature sensitivity play important roles in the behavior of materials [47]. The dynamic behavior of materials is based on the opposing tendency of materials to strengthen at high strain rates and to soften with the temperature increase produced by adiabatic loading conditions [48,49]. The ratio b (the fraction of plastic work converted to heat for plastic materials during high strain rate deformation) ranges from 0.4 to 1 [50][51][52].…”

On thermal softening and adiabatic shear failure of dynamically compressed metallic specimens in the Kolsky bar system

“…Moreover, these strains are especially low (of the order of 0.2), in comparison with the other materials which we tested. We conclude that these alloys are quasi-brittle by nature and we relate their brittleness with the plastic zone ahead of a crack, r y , as we have done in [11]. The plastic zone ahead of a crack is defined by…”

“…In other tests with L/D larger than 1, we found that the specimens failed at high strains of about 0.7. This L/D effect, in dynamically compressed specimens, has been discussed in [11] and in the references therein.…”

“…where K Ic and Y are the fracture toughness and the strength of the specimen, respectively. As we discussed in [11], a specimen can behave either in a ductile or a brittle manner, depending on its size as compared with r y . If the specimen is much larger than r y , the material behaves in a quasi-brittle manner, and vice versa.…”