This paper describes an alternating-current electrical-potential system for measuring crack growth that is more sensitive than a corresponding direct-current system and which is capable of a discrimination i i i crach length of approximately 20 pni, I t shows how the positions of the current supply and potential sensing leads can be optimized using twodimensional finite-element and boundary integral equation methods. Both methods give calibrations which are in good agreement with optical nieasurements of crack growth and with nieasurenients made using a graphitized electrical analogue paper technique. The boundary integral equation approach is shown to be the more efficient to use for a range of geometries, since it is only necessary to specify the shape of the boundary and redundant information at internal nodes need not be obtained. Proc. Royal Soc. 1963 275, Ser. A. RIZZO, F. J. 'Boundary integral equation methods. A modern computational procedure in applied mechanics ', Proc. Boundary Integral Equation Methods Computational Applications in Applied Mechnaics 1975, ASME A M D 11. BREBBIA, C. A. and DOMINGIEZ, J. 'The boundary integral equation method in potential problems',

Two boundary integral equations (BIE) methods have been applied to obtain stress intensity factor K calibrations for cracked C-shaped, split-ring (SR) and ring specimens. It is shown that good agreement is achieved with experimental determinations and other information in the literature. Also fatigue crack growth studies have demonstrated that all the shapes are suitable for establishing the fatique properties in the radial direction of thick-wailed tubing. The ring specimen is shown to be particularly appropriate for determining fatigue crack growth behaviour in tubes which have been autofrettaged

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