Strain Rate History Research Articles

Strain rate history effects for polycrystalline aluminium and theory of intersections

T his work presents an application of the dislocation theory of intersections to the experimental results obtained at changed strain rates under conditions of pure shear for polycrystalline aluminium. Simultaneously, strain-rate history effects have been shown, and discussed in terms of dislocation behaviour.

Some experiments with the split hopkinson pressure bar∗

A n application of the split Hopkinson pressure bar to the dynamic testing of materials is given whereby continuous records of the strain vs. time, strain rate vs. time, stress vs. time, and stress vs. strain may be simultaneously recorded. For many materials a nearly constant true plastic strain rate can be maintained for loading durations on the order of 100 μsec. The assumptions made in applying the method are checked experimentally. Three annealed face centred cubic metals—lead, aluminium, and copper, are tested both dynamically and at lower strain rates to determine their rate sensitivity. The results indicate that a single thermally activated mechanism is predominant. A possible mechanism of this type is the intersection of glide and forest dislocations. In aluminium, the previous strain rate history of loading is shown to affect the material's dynamic response only. A rapid annealing effect is also evidenced. Finally, the method is shown to be applicable to some non-metallic materials.

Transient Stresses in Nonhomogeneous Viscoelastic (Maxwell) Materials

This paper determines the transient-stress distribution, due to imposed strain rates, which exists in bars made of a linear nonhomogeneous viscoelastic (Maxwell) material. The cases of constant and exponentially decreasing strain-rate histories are solved. The particular nonhomogeneity is an exponential variation of the fluidity in the thickness coordinate. I t is shown that this fluidity variation can be the result of a steady linear temperature gradient. One-dimensional strength of materials assumptions are made for the problems of axial extension and bending. I t is further assumed that all of the initial stresses due to heating have vanished prior to load application. I t is found in the case of constant strain rate that the stress distribution approaches the configuration associated with a purely viscous material after one relaxation time of the cold face. In addition, an approximate solution to the problem of constant load is given in Appendix A.

The yield stress of pure lead in compression

The variation of the yield stress of pure lead in uniaxial compression with temperature, strain and strain-rate is described together with experiments designed to test the accuracy of the results. The average coefficient of friction between the platens and specimen has been measured for lubricated and unlubricated compression, and its dependence on strain rate demonstrated. The effect of previous strain-rate history on the yield stress is also discussed.