Crack Detection Using Acoustic Emission Methods – Fundamentals and Applications

This paper addresses the fundamentals of the acoustic emission effect associated with fatigue and stress corrosion cracking in metals. It considers the microstructure of cracks and the magnitude of the different types of physical event that can occur at the crack tip during plastic deformation and stable crack growth. Expressions are given for the threshold plastic zone size ‘Dl’ at which local fracture instability occurs and the stress-wave displacement amplitude as a function of distance ‘ui(r)’ for the different wave types ‘i’ produced during crack extension. Dispersion of the stress-wave and its convolution into an electrical burst signal at the sensor is considered together with the choice of appropriate sensing frequency. A methodology is described for correcting the measured signal amplitude for attenuation in the structure and for determining the maximum sensor spacing for the detection and location of events of a specified magnitude ‘Mae’ similar to the Richter scale. Case studies are presented to illustrate the extensive database now available on acoustic emission from crack growth in metallic structures and the technical and commercial benefits to be gained from an acoustic emission based inspection strategy. The applications considered are: • Fatigue crack growth in the node joints of offshore structures, • Stress corrosion cracking in platform flow lines.