Cyclic Stress Measurement Using XRD Analysis of Grains Grown in Electrodeposited Copper Foil

BackgroundMeasurement of local stress amplitude at the stress concentration point is a key component of mechanical design from the viewpoint of preventing metal fatigue. Copper electroplating method, which is a stress measurement method utilizing grains grown in a copper foil by cyclic loading, is suitable for such microscopic stress measurement. Reports have shown that the maximum shear stress and principal stress, which are important components in the evaluation of fatigue strength, can be measured by examining the density and crystallographic features of grown grains.ObjectiveElectron backscatter diffraction (EBSD) analysis of grown grains can be utilized to understand the crystallographic features of the grains, but EBSD equipment requires technical skill to operate, which makes it inconvenient. In this paper, we explore the feasibility of stress measurement by analyzing the crystallographic features of grown grains using the X-ray diffraction (XRD) method, which is more versatile than the EBSD method.MethodsCyclic loading tests were conducted using smooth specimens with copper foil adhered under various biaxial stress conditions of bending and torsion. The surfaces of the grains-grown copper foil were then analyzed by the XRD method.ResultsThe peak of diffraction intensity tended to shift from the (220) plane to the (111) plane as the biaxial stress ratio increased. We quantified this tendency using the Lotgering factor and developed an empirical formula for determining the relationship between the Lotgering factor and the biaxial stress ratio.ConclusionsOur proposed empirical formula enables principal stress measurement within a biaxial stress ratio C ranging from -0.45 to 0, and the measurement accuracy is comparable to that of the conventional EBSD method.