Session P1: ME.3 Inelastic Deformation, Fracture, and Spall V

Gas Gun Driven Dynamic Fracture and Fragmentation of Ti-6Al-4V

The dynamic fracture and fragmentation of a material is a complex late stage phenomenon occurring in many shock loading scenarios. Improving our predictive capability depends upon exercising our current failure models against new loading schemes and data. We present a series of experiments creating axially symmetric high strain rate (10$^{4}$ s$^{-1})$ expansion of Ti-6Al-4V cylinders under controlled loading achieved using the ogive based gas gun technique. A steel ogive is located inside the cylinder, into which a polymer rod is launched. Deformation of the rod around the insert drives the cylinder into rapid expansion. This technique facilitates repeatable loading independent of the sample temperature and straightforward adjustment of radial strain rate. Expansion velocity was measured at multiple points along the cylinder outer wall using PDV. Two high speed imaging systems are used to track the overall expansion process to record strain at failure. Optical and SEM imaging is used to measure fragment size and mass distributions and examine the fracture surfaces to reveal the failure mechanism. For a peak radial strain rate of (1.1 $\pm$ 0.1) x 10$^{4}$ s$^{-1}$ strain localisation initiates on the outer surface at a radial strain of around 12{\%}, with cracks fully penetrating the cylinder wall at around 22{\%}. Hydrocode modelling has been completed with very strong agreement in predicting the expansion velocity and profile but further work is needed to develop an accurate representation of the fracture and fragmentation.   

Understanding the micro-mechanics of spall initiation in titanium alloys

Significant progress has been made in understanding the effects of microstructural and micro-textural features on the mechanical behavior of titanium alloys at low strain rates. However, very little information is available in the literature on the effects of microstructure and micro-texture on the behavior of titanium alloys at high strain rates, particularly in terms of important failure mechanisms such as spall. In this study, the micro-mechanics of spall initiation in the titanium alloy, Ti-6Al-4V, was investigated via Backscatter Electron (BE) microscopy and Electron Back-Scatter Diffraction (EBSD) analysis of a recovered specimen from a typical plate-impact experiment. The analysis shows that spall initiates due to the nucleation of voids at the grain boundaries between plastically hard/soft grains and then propagates via the highly localized coalescence of the nucleated voids.   

The influence of the admixture of the fullerene C$_{60}$ on strength properties of aluminum and copper under shock-wave loading

Hugoniot elastic limit and dynamic (spall) strength measurements of pressed aluminum and copper samples with admixture of the fullerene C$_{60}$ by 2-5 wt {\%} under shock-wave loading were carried out. The peak pressure in shock-wave is equal to 6 GPa. The measurements of elastic-plastic and strength properties were based on the recording and the following analysis of the sample free surface velocity histories, recorded with VISAR laser-Doppler velocimeter. It was found, that the admixture of 5 wt {\%} fullerene in aluminum samples leaded to increasing of Hugoniot elastic limit for aluminum samples by a factor of ten. The copper samples with admixture of 2 wt {\%} fullerene also demonstrated multiple increasing of the Hugoniot elastic limit in comparison with the commercial copper. The measured values of Hugoniot elastic limit were equal of 0.82-1.56 GPa for aluminum samples and 1.35-3.46 GPa for copper samples in dependence on their porosity. As it was expected, the spall strength of the samples with fullerene decreased about three times in comparison with the undoped samples as a result of influence of the solid fullerene particles which were concentrators of tension stresses in material in dynamic fracture process.   

Effect of severe plastic deformation on microstructure and mechanical properties of magnesium and aluminium alloys in wide range of strain rates

Results of researches testify that a grain size have a strong influence on the mechanical behavior of metals and alloys. Ultrafine grained HCP and FCC metal alloys present higher values of the spall strength than a corresponding coarse grained counterparts. In the present study we investigate the effect of grain size distribution on the flow stress and strength under dynamic compression and tension of aluminium and magnesium alloys. Microstructure and grain size distribution in alloys were varied by carrying out severe plastic deformation during the multiple-pass equal channel angular pressing, cyclic constrained groove pressing, and surface mechanical attrition treatment. Tests were performed using a VHS-Instron servo-hydraulic machine. Ultra high speed camera Phantom V710 was used for photo registration of deformation and fracture of specimens in range of strain rates from 0,01 to 1000 1/s. In dynamic regime UFG alloys exhibit a stronger decrease in ductility compared to the coarse grained material. The plastic flow of UFG alloys with a bimodal grain size distribution was highly localized. Shear bands and shear crack nucleation and growth were recorded using high speed photography.   

Temperature effects on the low strain rate behaviour and piezoelectric charge production of PZT

This presentation looks at the effect of varying strain rates and temperatures on the production of charge and fracture of the piezoceramic PZT. The samples are studied in the range of -80C to $+$200C; whilst a range of strain rates (10$^{\mathrm{-4}}$ s$^{\mathrm{-1}}$ to 10$^{\mathrm{+3}}$ s$^{\mathrm{-1}})$ is achieved using quasi-static Instron equipment, drop weight machines and compression Split Hopkinson Pressure Bars. Stress-strain data is obtained, and further analysis is made possible by the use of high-speed images.