The Influence of Stress on the Detectability of Fatigue Cracks using Ultrasonics

Abstract

Under favourable conditions fatigue cracks can be readily detected by a variety of ultrasonic techniques. There are indications, however, that compressive stresses can render the detection of these cracks very difficult, if not impossible. The factors which influence the detection of both stressed and unstressed cracks such as, probe angle and frequency, crack size and type, scanning position and material have been investigated. Compressive bending stresses were found to have a considerable influence on the ease of crack detection in steel and aluminium alloy. "Complete closure’ of certain crack configurations was found to occur under elastic compressive stresses when using certain ultrasonic techniques. Detailed work has been carried out to examine the interaction of ultrasound with fatigue cracks in the weld metal of QT35-steel, mainly to study the influence of elastic compressive stresses and related factors. These results indicate that fatigue cracks can best be located using 45 probes. Heat treatment prior to fatigue cracking allows, fatigue cracks to be grown-which are more readily detected and which are less prone to crack closure under compressive stresses. These findings and others are interpreted in terms of the topography of the fatigue cracks. This work also investigates the effectiveness of shear wave inspection of fatigue cracks in the HAZ of austenitic stainless steel weldments which have always been considered difficult to inspect using ultrasonics. However, there is now some experimental evidence to suggest that shear wave inspection techniques can be applied in certain circumstances. The limitations of these techniques are demonstrated and possible solutions are proposed.