The effects of axial length on the fracture and fragmentation of expanding rings

Rings of Ti-6Al-4V with aspect ratios (wall thickness:axial length) of 1 : 1, 1 : 2 and 1 : 4 have been expanded to failure at radial strain rates εr ∼ 1 × 104 s−1 using 4340 (EN24T) steel and Cu-ETP cylindrical drivers containing a column of RDX. Expansion velocity was measured using VISAR enabling calculation of the stress-strain history of the ring alongside fragment recovery with up to 98% original ring mass recovered. Using the recovered samples average fragment length and mass and final strain have been measured along with analysis of the fracture sites to determine the active failure mechanisms. Perfect rings (aspect ratio 1 : 1) were found to undergo necking before failure, whereas the longer rings failed though ductile tensile cracking at 45∘ to the radius. This data is then compared with finite element analysis results

Impact response of titanium alloys at elevated temperatures

The paper presents results of investigations of temperature effect on dynamic yield stress and spall strength of high-purity titanium, commercial grade titaniurn, and $\alpha+\beta$ altoy Ti-6Al-2Sn-2Zr-2Cr-2Mo-Si at submicrosecond durations of shock-wave loading. The high-purity titanium exhibited anomalous growth of the yield stress whereas the behaviors of commercial titanium and titanium alloy are similar to that under normal conditions. In comparison with the yield stress the spall strength of the studied materials is much less sensitive to their composition and the test temperature. The measurements confirmed the polymorphous transformation in high-purity titanium, although both the transformation pressure and it temperature dependence differ from known data

Experimental investigations and modelling of strain rate and temperature effects on the flow behaviour of 1045 steel

If structures have to be designed to sustain low and high loading rates, the appropriate constitutive equations are essential for the accurate modelling of the structural response. However, a lack of the required data is observed very often, especially, if a very wide range of strain rates has to be covered. Therefore, the flow behaviour of the steel 1045 (C45E) was investigated with defined chemical composition and microstructure in a very wide range of strain rates between 10$^{-4}$ and 10$^{5}$ 1/s and different temperatures. The rate-dependent thermomechanical behaviour was determined using low strain rate (10$^{-4}$ to 10$^{0}$ 1/s) servo-hydraulic compression testing, high strain rate (~10$^{2}$ 1/s and 10$^{3}$ 1/s) compression drop weight testing and split Hopkinson pressure bar testing, and for very high strain rates (10$^{5}$ 1/s) the plate-impact test. Additionally, strain rate jump tests at relatively low strain rates and different temperatures were performed to determine the activation volume at constant temperature and deformation. The measured flow stresses as well as the strain and strain rate hardening behaviour as a function of strain rate and test temperature are discussed in terms of the microstructural deformation processes. The theory of thermally activated flow is applied and compared to the widely used models like the Johnson-Cook model and a dislocation drag model. The occurrence of possible dislocation drag effects is discussed in conjunction with the measured data. Our results show, that the strain rate dependence of the flow stress of 1045 steel can be described completely by the theory of thermal activation up to strain rates of 10$^{5}$ 1/s

Investigations of the Dynamic Strength Variations in Metals

A line-imaging laser Doppler velocimeter has been employed to investigate the uniformity of strength properties of materials at load duration of ≈10-7 sec or less. The shock load puises were generated by a pulsed high-power proton beam facility. More or less noticeable spatial variations of the spall strength were found in such different materials as coarse-grain cast magnesium and molybdenum single crystals. In the case of magnesium, the variations of the tensile strength are governed by grain boundaries. In the single crystals, small reductions of the spall strength were accompanied by a decreasing acceleration of the fracture process.Un vélocimetre laser à résolution spatiale monodimensionelle a été utilisé pour l'étude des variations locales des propriétés mécaniques des matériaux auxquels sont appliquées des charges d'une durée de 10-7 s ou moins. Les charges impulsives sont générées par un faisceau de protons puisé à haute puissance. Des différences plus ou moins notables de la résistance mécanique ont été constatées sur des échantillons aussi différents que du magnésium moulé à gros grains et du molybdène monocristallin. Dans le cas du magnésium, les variations de la résistance sont gouvernées par les joints de grains. Dans les monocristaux, de petites réductions de la résistance furent accompagnées d'une évolution initiale des fractures plus lente