Nondestructive methods for assessment of creep and fatigue damage

Abstract

It is well known that components and structures subjected to service environment such as exposure to high temperature, cyclic loading etc. undergo degradation due to microstructural changes, creep and fatigue damage. Since the conventional microscopy techniques do not give global information about the whole volume of the components, it is essential to develop Non-Destructive Evaluation (NDE) techniques for assessment of damage and estimation of remaining life of the component. For more than two decades, many NDE techniques were developed for the assessment of crep and fatigue damage. NDE techniques like In-situ Metallography, Acoustic Emission (AE), Ultrasonic Attenuation and Velocity Measurements, Electrical Resistivity, Resonant Frequency Measurement, Magnetic Bardhausen Noise, Acoustic Bardhausen Noise, Laser Interferometry, Positron Annihilation, X-ray Diffraction, Small Angle Neutron Scattering (SANS) etc. are used for studying creep and fatigue damage. Acoustic Emission Technique (AET) is widely used to determine the fatigue crack propagation rate which in turn estimates the severity of flaws. Different stages of fatigue crack growth can be identified using AET. Changes in Ultrosonic absorption and velocity have been correlated with changes in dislocation substructure, dislocation density, microrack formation etc. Electromanetic Non-Destructive Testing (NDT) techniques can estimate the fatigue crack depth, void density, microstructural variations, creep strain etc. Recently developed pulsed laser induced ultrasound technique is offering an unique non-contact NDE method to determine creep and fatigue damage by characterising the damage through absorption coefficient. Techniques like Acoustic Microscopy, Positron Annihilation, Neutron Scattering etc. are capable of quantifying the damages produced by creep and fatigue phenomena. In addition, miniature specimen technology is emerging as a strong possibility to evaluate creep and fatigue damage in components. Miniature specimens are taken out of the component in such a way not as to evaluate creep and fatigue damage in components. Miniature specimens are taken out of the component in such a way not as to impair the performance of the component. In this paper, an overview of applications of various NDE techniques for assessment of creep and fatigue damages and for life prediction is presented with emphases on the work being carried out in the authors' laboratory.