Measurement of microstructural damage in cylindrical components using guided waves

Abstract

In high temperature piping where internal pressure is present progressive material degradation due to creep damage is a significant limiting factor on the safe working life of components. With many plants approaching their original design life there is a desire to extend the safe working life to provide significant cost savings. There is also a demand for new non-destructive evaluation techniques for the detection of creep damage to improve upon the inspection speed and accuracy of current technologies. This thesis examines the potential application of guided wave testing to the detection of creep damage in piping. Initial field observations of ex-service creep damaged pipework found an unexpected sensitivity of guided wave testing to creep damage in the form of low amplitude reflected signals. The mechanisms behind these interactions were not well understood as the characteristic size of creep defects is significantly smaller than the wavelength used for guided wave testing. A series of ex-service creep damaged samples were analysed to investigate potential interactions in more detail. Geometric deviations from a simple pipe were present but these did not account for the guided wave indications seen. As direct numerical modelling of creep damage at the scales required for guided wave propagation is computationally prohibitive a homogenisation approach, which is applicable in the long wavelength regime, was used to model the interaction of creep damage with guided waves. A finite element homogenisation technique was used to calculate the effective linear elastic properties of creep damaged material. The microstructural creep damage was modelled as a series of voids whose properties were based on metallographic analysis of ex-service creep damaged samples. A significant reduction in the effective stiffness was predicted; for class three creep damage this was between (12±1) % and (22±2) % compared to the undamaged components of the stiffness tensor. The interactions of guided waves with creep damage, represented using the numerically homogenised material, were modelled using finite element techniques. Significant reflections were predicted from regions where there was a rapid axial change in damage severity, with an accompanying reduction in the wave speed through damaged regions. For class three creep damage a (4.8±0.2) % reduction in the T(0,1) wave speed was predicted. A reduction in the cut off frequency for the L(0,2) and L(0,3) modes of (4.0±0.4) kHz and (11.1±0.8) kHz respectively was also seen for class three creep damage.