Cyclic stress measurement method using electrodeposited copper foil (Principal stress measurement using Debye ring of X-ray diffraction)

When a copper foil is adhered to a machine element subjected to repeated loads, grains grow in the foil. It has already been revealed that the maximum shear stress and principal stresses, which are important in evaluating the fatigue strength of the element, can be measured from density and crystal orientation of grown grains. EBSD or XRD method was used to analyze the crystal orientation. However, both methods require measurement of the foil inside each equipment, meaning that the principal stress measurement is not convenient. The portable X-ray residual stress analyzer can obtain the crystallographic features of the grown grains in situ as Debye ring. In this study, therefore, we proposed a method for measuring the principal stresses using Debye ring of grown grains. First, a plate-type specimen with a copper foil was subjected to cyclic loading under various biaxial stress conditions to grow the grains in the foil. The copper foil was then measured using a portable X-ray residual stress analyzer to obtain the Debye ring. As a result of various biaxial stress tests, the intensity peak of the Debye ring shifts from 0° to 180° as the biaxial stress ratio increases. This tendency can be explained by the out-of-plane shear stress and is in good agreement with the EBSD method, suggesting that the Debye ring can be used for principal stress measurement. A calibration equation was derived to measure the principal stresses by quantifying this tendency and the measurement accuracy was also examined.