The determination of residuad stresses in metals by X-ray dif- 

 fraction is based on the measurement of residual lattice strain of 

 favorably oriented crystallographic planes of suitable Miller in- 

 dices. Residual stresses develop in metal components and weldments 

 during fabrication as a result of thermal gradients and metallur- 

 gical reactions. The presence of residual stresses in metail struc- 

 tures may weaken their strength and shorten their service life. 

 As higher strength materials are required in the construction of 

 naval vessels, residual stresses become of greater concern, 

 especially in the presence of surface discontinuities as well as 

 alloy heterogeneities. High tensile residual stresses, for ex- 

 ample, in the presence of notches and cracks may lead to brittle 

 fracture at relatively low temperatures. Fatigue properties and 

 dimensional stability of the structure may also seriously suffer 

 from the residual stresses. 



X-ray diffraction is the only practical nondestructive method 

 for the determination of residusil stresses in metal structures. 

 X-ray stress analysis has been useful in many fundamentail and 

 applied research studies, including quality control and mainte- 

 nance of structural components in the aircraft, railroad and 

 ball-bearing industries. As already mentioned, residual stresses 

 are calculated from residual strains, assioming that the material 

 behaves elastically. When the material is a weld deposit, the 

 residual strain may not be entirely elastic, and, therefore, the 

 calculated stresses may contain considerable errors. It has been 

 shown^ that residual stresses in butt-welds determined by X-ray 

 diffraction agree, within experimental error, with the residual 

 stress values obtained by layer removal technique despite the 

 fact that some plastic deformation may have been present. On this 

 basis, plastic deformation in welds does not appear to be a serious 

 factor; therefore, no consideration was given to its possible 

 effects on the results of this investigation. Stress analysis by 

 X-rays is by no means an easy task, even when the material is 

 ideally suitable for the analysis. The analysis involves linear 

 measurements of the diffraction line position, and those measure- 

 ments require a reproducibility of at least a tenth of a milli- 

 meter if the method is to be useful for the determination of low 

 and intermediate residual stress values. Measurements of diffrac- 

 tion line shift and distance between film and specimen are most 

 important; they therefore must be effected with great precision. 



1. DiGiacomo, G. "Residual Stresses in High Strength Steel Weld- 

 ments and Their Dimensional Instability During Welding and 

 Stress Relieving"; MS Thesis (Physics) I968, Polytechnic Insti- 

 tute of Brooklyn. 



2. Ibid. 



297 



