Experimental Factors Concerning X-Ray Residual Stress Measurements in High-Strength Aluminum Alloys

X-ray diffraction as a means of determining stresses has found increasing application in the last few years. This is primarily because it is the only technique by which stresses can be determined without making measurements on the specimen or structure in the unstressed condition and, consequently, it is the only truly nondestructive technique for determining residual stresses. The principles of determining macrostresses on surfaces with commercially available equipment is quite well known and employs either the X-ray diffractometer or back-reflect ion camera techniques. The diffractometer technique was selected for this investigation because of its accuracy and because it allows both macrostresses and microstra in to be analyzed from the change in position and shape of the diffraction peaks. The X-ray analysis actually consisted of two separate phases. The first dealt with the X-ray determination of the elastic constants (Young's modulus and Poisson 's ratio) for several aluminum alloys, including 5083. These values were compared with the theoretical or published values as determined by standard tensile tests and used later in stress calculations. For these tests, a unique stress stage was used which allowed the specimen to be stressed while positioned in the diffractometer, and also have angular rotation about the diffractometer axis that is independent of the rotation of the counter and receiving slit system. The second phase consisted of analyzing different groups of 5083—aluminum alloy specimens which had been subjected to various degrees of cold working by rolling. This analysis consisted not only of the computation of macrostresses, but also of microstrain and change in particle size as a function of percentage reduction in thickness. The final portion of this phase dealt with electro polishing successive layers from the surface of each sample and relating the measured relaxation to the thickness of the layers removed. In this way, stress distribution in depth was obtainable as a function of cold working.