Neutron diffraction maps of stress concentration near notches under load at temperature

Recent advances in neutron detector technology and the ability of neutrons to penetrate through many millimetres of most engineering materials have made it feasible to investigate the effects of sub-surface stress-concentration in the vicinity of notches. Neutron-diffraction strain measurements are non-destructive and capable of tracking the development of the elastic strain field as a function of applied load in a single specimen. This paper presents two demonstrations of neutron-diffraction strain-scanning with high spatial resolution. First, the development of the strain field near a notch in an HY-100 steel bar is scanned as a three-point bending load is increased to the point of metal-tearing. Second, the stress-concentration effects of a blunt notch in a tensile test specimen are presented. The measurements are performed while the specimen is held at a temperature of 250 °C and while load is applied to maintain constant total strain. The neutron diffraction measurements reveal a gradual redistribution of the stress concentration over time. In both examples, the gradients of the strain distribution are substantial, but neutron diffraction has a sufficient spatial resolution (of the order 0.5 mm to 1.0 mm) to characterize the details of a stress concentration field.