The behaviour of materials under dynamic shear loading

Part I This section is concerned with the propagation of stress waves in metals such that the material is at some stage in the plastic state. An account is given of previous work on the subject of plastic wave propagation whence two distinct theories emerge, the so-called strain-rate dependent and independent theories. The greatest difference between the two theories is in their prediction of the propagation speed of an incremental stress superimposed on an already plastic stress field. From criticisms of earlier attempts to decide which is the more applicable theory it is deduced that the most convincing way of answering that question is to apply torsional increments to thin-walled tubes already plastically strained in torsion. A brief outline of the two theories as they apply to the torsion of thin-walled tubes is provided in Chapter 3, during which the values of the predicted incremental wave speed are derived. Some of the problems encountered in designing apparatus to carry out the required experiments are discussed in Chapter 4, which includes a full description of the final design. This consists of a standard torsion machine in which is placed the specimen, fixed between two long bars. The relative strengths and sizes of the specimens and bars are such that the latter remain elastic even at large plastic strain in the specimen. A friction clamp fits round the input bar and is pressed on to it by tensioning a length of high tensile steel wire. By rotating the end of the input bar, the specimen can be prestrained whilst a clip at the clamp builds up a small friction torque. The friction torque is rapidly released by breaking the tensioning wire and the resulting stress waves in the system are detected by means of strain gauges on the input and output bars. THe time travel of an incremental stress through the specimen can then be calculated. The rise time if the stress wave is approximately 30 μsec. Following the description of the apparatus, details of the specimen are given. These were made from mild steel and commercially pure aluminium and copper, all in the annealed state. Four different sizes were made, three for use in the wave propagation tests, the forth was used in the same machine but to provide an indication of the strain rate sensitivity of copper and aluminium. The results of the tests on all the above types of specimen are laid out in Chapter 6. Prestrains of up to 25% were used in the wave propagation work and in all cases it was found that the propagation speed was equal to the torsional elastic wave speed as predicted by the strain-rate dependent theory. Also shown in this chapter are the variations in transmitted pulse shape between the various materials and prestrains. Part II The second part of the thesis is concerned with the process of punching or blanking and opens with an account of previous work on the subject. The following chapter describes the apparatus which enables the punch speed to be varied from 10-5 to 103in/sec. and allows the punch load to be measured at each instant of the deformation process. Three different machines are described, a standard Instron testing machine which provides the low punch speeds, a special hydraulic machine giving higher punch speeds and a drop-weight impact testing machine from which the highest speeds are obtained. Load-displacement curves were obtained over this range of punch speeds for six materials, mild steel, copper, aluminium (all annealed), soft and hard brass, and a high tensile steel. The load-displacement curves of the first three materials, which had very similar compositions to those materials used in Part I, were converted into sheer stress - shear form, and using a gauge length estimated from microhardness traverses of the sectioned specimens. In Chapter II the strain rate sensitivities obtained from the punch test are used to discuss the observed pulse magnitudes of the wave propagation tests, in conjunction with the predictions of the strain-rate dependent theory. It is shown that the predictions agree qualitatively with the observed trends. The second part of this chapter examines the effects of increasing the punch velocity in the blanking process on the quality of product and energy absorbed by the process. It is shown that the high speed punching process is most advantageous when using brittle materials.