Effect of stress on ultrasonic response in detection and sizing of cracks

In different NDT techniques huge developments have been achieved during the last few years with regard to crack detection and sizing. In a large range of materials, cracks are one of the most dangerous defect types. A crack is a planar reflector, which is sometimes extremely difficult to detect and to be sized. A crack can be totally open, partly closed or even totally closed because of compressive stresses. The effects of stresses cause problems, for instance, in in-service inspections of nuclear power plants in detection and sizing of closed cracks. This phenomenon causes similar effects in all kinds of plants and components. In this study some experimental inspections have been carried out as well some FEM calculations of stress field around the crack and compared to corresponding measurements in literature. The used method is dynamic loading, which gives during ultrasonic measurement clear evidence on the effect of the crack closure as well on the amplitude variation limits in ultrasonic testing. Materials used in the study are AISI 304, AISI 321 and ferritic piping steel. The load applied to the cracks was in form of different thermal cycles. The maximum temperature variations were from 20 °C to 600 °C depending on each dynamic loading cycle. Different types of ultrasonic methods were used in the measurements. The methods applied are normally used in the field in normal ultrasonic inspections like angle shear wave, creeping wave, TOFD and phased array. The results are presented and conclusions are drawn from the stress effect on the detection and sizing. Clearly the sizing methods have some differences when using manual sizing techniques, TOFD, phased array or SAFT technique. The effect of closure on the response of normal ultrasonic practical probes was recorded.