Abstract
Strain gages made of copper plating foil are devised for measuring the elastic surface stress of machine parts in operation. The elastic stress is measured by observing slip bands in the bonded foil. Calibration studied by cyclic tension test and cyclic torsion test with various stress ratios and different stress waveforms are performed on round steel bars with plating foil. It is verified that the relation between the threshold stress for the first appearance of slip bands and the number of cycles is not affected by the stress ratios and stress waveforms. Using a computer image-processing system, the density of slip bands in a microscopic image of the bonded foil is analyzed automatically and quantitatively. Under various stress ratios, the relation between the stress amplitude and the density of slip bands for a constant number of cycles is examined. In order to examine the accuracy of this method, the peak stresses in grooved shafts under cyclic tension test are obtained using the present method. The results are compared with those derived from the design formulas by Roark and Young. It is confirmed that this method provides an accurate calibration value for measuring the cyclic stress.
