Effect of crack shape and variable amplitude loading on fatigue crack growth in PWR environments

Plate specimens (25 mm x 100 mm) have been prepared with 10 mm x 20 mm (a x 2c) part-through surface cracks located on the center line. Specimens have been fabricated from both clad and unclad pressure vessel and piping steels. Fatigue crack growth rate tests in air environments indicate that crack shape is highly dependent on test temperature. At high temperatures, the width-to-depth ratio is decreased significantly. For clad steels, the tensile stresses in the clad at low (room) temperature result in increased crack opening forces and a large width to depth ratio. The effect of variable amplitude loading has been studied through a program of test matrices of increasing complexity. Initial variable amplitude testing was in a dry helium gas environment in order to ascertain the effects of single and multiple overload and underload schemes without the complex effects of a PWR environment. Later tests employed more complex spectra, containing blocks of spectral components with load ratios simulating start-up/shut-downs, normal power loadings and unloadings, and simple reactor or turbine trips. After completion of testing in helium gas environments, the testing in PWR environments was initiated, with the result that crack extension in a block was essentially dependent on the test frequency of the spectral components