New Stress Measurement Method of Thin Plate Using Transmission Diffracted X-Rays

Abstract

Residual stresses which occur in a thin plate and a film of both MEMS and an electric device have become a direct cause of a deformation and a destruction of them. To evaluate the residual stress is therefore very important for improving their mechanical reliability. A strain gauge is often incapable of using to measure the strain of the thin plate and the film, while X-ray diffraction method is effective in the nondestructive measurement of residual stress of them. If a diffracted X-ray transmitted through the thin plate is available, spacing of the lattice plane orthogonal to the direction along the surface could be measured directly. It suggests potentiality for the strain measurement of thin plate with high precision and efficiency. This paper presents a new measurement method of the stress in a polycrystalline thin plate by using transmission diffracted X-rays. This method is able to determine the principal stress and its direction in the thin plate from the measurement of the lattice spacing with random three directions within thin plate and the vertical direction of thin plate surface. The method was verified using a pure aluminum thin plate of 50μm thickness which was loaded with tension using compact tensile loading equipment. The X-ray elastic modulus of Al(311) lattice plane which was measured at the start of the experiment was derived as Young's modulus of 61.7GPa and Poisson's ratio of 0.33. As a result, principal stress and its direction in the aluminum thin plate could be evaluated with errors of less than about 9MPa and 3degree, respectively. It was confirmed that this method is effective for the stress measurement of thin plate.