STRESS ANALYSIS USING BREMSSTRAHLUNG RADIATION

Diffraction methods are the most reliable nondestructive m ethods of measuring residual stress. X-ray diffraction is restricted to penetration depths of some 10 µm, whereas neutron diffraction can provide information at significantly higher depths, but the cost is very high and the availability of facilities is much lower. A sensitive nondestructive probe for detecting defects and measuring stresses in thick materials (tens of cm) does not exist. The first highly penetrating system to measure stress/strain in thick materials based on using bremsstrahlung beams from small electron accelerators (3-6 MeV Linacs), is presented in this paper. These bremsstrahlung beams, which exhibit excellent penetration, create positrons inside the materials via pair production. The positrons annihilate with the material electrons emitting 511 keV radiation, which is influenced by the momentum distributions of the atomic electrons. Because of the fact of positron trapping at defects, positron annihilation is very sensitive to any change in the microstructure and can probe nano-void defects. Open volume defects, voids or dislocations are reflected in the line width of the 511 keV peak. Tensile stresses and resultant strains have been measured in an engineering alloy using this technique. This technique can be used to infer residual stress and to detect defects in crystals, polymers, metals and alloys up to tens of gm/cm 2 for material science and engineering applications. The low cost and compact size of small electron accelerators, and the high penetrability of MeV bremsstrahlung beams allow the development of portable systems for industrial applications.