Ultrasonic stress measurements in a carbon steel beam under bending

Bending stresses occur frequently in nuclear pressure vessels walls and internals and must be accounted for when assessing their structural integrity. As a first step toward estimating the accuracy of ultrasonic measurements of the magnitude and direction of the principal stresses in metallic vessels under bending, the acoustic birefringence technique was employed in this work to determine the in-plane bending stresses in a simply supported carbon steel beam subjected to a locally distributed load. The experimentally determined stresses were compared with analytical values computed from the theory of strength of materials. Normal incident ultrasonic shear waves were employed and the fractional difference of the time of flight measurements in two orthogonal directions at the same point - the birefringence ratio - was determined. The ultrasonic waves were generated, acquired and digitalized by an especially developed ultrasonic system, and their times of flight calculated using mathematical processing. Selected regions of the beam were examined ultrasonically, before and during the mechanical loading. The comparison of the analytical and experimental results indicated that the ultrasonic birefringence technique can be useful in determining the magnitude and direction of the principal stresses in beams subjected to in-plane bending. (author)