Application of the Photograting Technique to Analyze the Creep Strain State of a Notched Specimen

Abstract

A new technique of photograting method to measure the plastic strain produced at high temperature was proposed by the authors previously. The method is applied here in order to clarify the creep strain state produced in aluminum notched specimen at 200°C. Frequently the creep strength of the notched specimen is higher than the smooth specimen. In fact, the creep rupture time of the notched specimen used here is twice as long as the smooth specimen at the same stress level. In order to precisely and exactly determine the cause of this phenomenon, it is very important and valuable to clarify the state of creep deformation of the notched specimen.The new technique has given us clear interference fringe patterns to measure the strain distributions produced in the notched specimens. From direct observation of the strain distributions, it has become clear that, if the elongation of the specimen is decided within the gauge length, the strain distribution is the same, regardless of the applied stress and the elapsed time.Approximate figures of stress distributions were obtained, using the experimental values of strain. It can be concluded that the average strain in the minimum cross section of a notched specimen produced at a definite elapsed time is less than the strain produced in a smooth specimen when the average applied stress for the notched specimen is the same as the applied stress for the smooth specimen.