An experimental study of the dynamic response of notched bars
A survey is made of analytic, experimental and numerical techniques in the field of the dynamic initiation, and early propagation, of cracks. As no closed form analytic solutions exist for finite geometries, even in the elastic case, numerical and experimental techniques have to be developed. In instrumented impact tests plasticity often occurs. The specific problem of the Instrumented Charpy Test is discussed in detail by virtue of its technological significance and the extensive literature available for the test. Although a standard for the test has been proposed there are still outstanding questions to be answered, for which the techniques described above can be used. The problem of the dynamic calibration of various notched geometries is addressed in the original work of the thesis. The Charpy, Izod, Slender Cantilever and Double Notched Bar geometries are studied using dynamic photoelasticity and 8000 fps photography. It is shown that the response of the DNB is more straightforward than the Charpy geometry. Further photoelastic study of the latter two geometries, using epoxy model material and 106 fps photography, gives a quantitative measure of the growth of stress intensity factor at the notch tips and hence a dynamic calibration is deduced. An explicit finite difference code is used to supplement photoelastic data. Having achieved progress in the derivation of the dynamic calibration of the two selected geometries, corresponding instrumented impact tests are then undertaken. The Hopkinson Pressure Bar method of loading is used. It is concluded that the proposed standard for the Instrumented Charpy Test is valid within limits but that there is a requirement for a dynamic calibration. Such a calibration is complex in the case of the Charpy geometry whereas a simpler geometry, viz. DNB, could prove to be more amenable to analysis and hence be more practical from the technological point of view.