J. Phys. IV France 10 (2000) Pr9-215-Pr9-219
A photodiode optical deformation-measurement system for high strain-rate tensile testsM.S. Mirza and D.C. Barton
School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, U.K.
This paper describes a novel approach developed at Leeds University to measure deformation during strain-rate tensile tests using a Photodiode Optical Deformation-measurement System (PODS). The System is developed to obtain the stress-strain data from the dynamic tensile testing rig known as the "flying wedge". PODS consist of an array of photodiode cells which is placed on one side of a cylindrical specimen whilst a collimated light beam is placed on the other side. As the specimen deforms, the voltage output from each element of the photodiode array changes. This change is directly related to the change in diameter of the specimen. Each element of the photodiode array is connected to a purpose built low noise high frequency amplifier. Output from each element is recorded simultaneously using multi-channel digital oscilloscopes and a complete profile of the deforming specimen then constructed by post-processing the recorded signals. The paper describes the careful calibration of the system and its validation for dynamic tests using the "flying wedge" tensile test machine in conjunction with high-speed video recordings of the deforming specimen. Typical results are presented in terms of true stress-strain curves for thermoplastic polymers (ultra high molecular weight polyethylene) measured using the above system at high strain rates. This new system can be used to study the effect of strain-rate and stress triaxiality and to derive constitutive and fracture models for different materials. Not only that PODS cost significantly less than a high speed video camera but it also allows for the complete deformation history up to fracture to be recorded.
© EDP Sciences 2000