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There is a growing interest in material behaviour at strain rates greater than 104sec1, for instance in the design of aero-engine turbine blades.
It is necessary therefore, to develop material testing techniques that give well-defined information on mechanical behaviour in this very high
strain-rate regime. A number of techniques are available, including the expanding ring test1, a miniaturised compression Hopkinson bar technique
using direct impact and the double-notch shear test3 which has been described by Nicholas4 as "one of the most promising for future studies
in dynamic plasticity". However, although it is believed to be a good test for determining the flow stress at shear strain rates of 104sec and
above, the design of specimen used makes an accurate determination of strain extremely difficult while, in the later stages of the test the deformation
mode involves rotation as well as shear. If this technique is to be used, therefore, it is necessary to examine in detail the progressive
deformation and state of stress within the specimen during the impact process. An attempt can then be made to assess how far the data obtained
is a reliable measure of the specimen material response and the test can be calibrated. An extensive three stage analysis has been undertaken. In the first stage, reported in a previous paper5, the initial elastic behaviour was studied.
Dynamic photoelastic experiments were used to support linear elastic numerical results derived by the finite element method. Good qualitative
agreement was obtained between the photoelastic experiment and the numerical model and the principal source of error in the elastic region of
the double-notch shear test was identified as the assumption that all deformation of the specimen is concentrated in the two shear zones. For
the epoxy (photoelastic) specimen a calibration factor of 5.3 was determined. This factor represents the ratio between the defined (nominal) gauge length and the effective gauge length.
The second stage of the analysis of the double-notch shear (DNS) specimen is described in this paper. This consists of the use of ultra-high speed
photography to provide information on the plastic deformation behaviour of the specimen. Two different high speed cine cameras were used
for this work, a Hadland "Imacon" 792 electronic image converter camera and a Cordin 377 rotating mirror-drum optical camera. Implementation of the two cameras and photographic results are briefly compared and contrasted here.
Stage three of this work consists of an advanced numerical analysis of the elasto-plastic, strain rate dependent behaviour of the DNS specimen.
The principle intention of the authors was to use the physical data collected from high speed photographs for correlation with this work. Full
details of the numerical work are presented elsewhere6 but some salient results will be given here for completeness.
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