Significance of the small crack growth law and its practical application

The effects of microstructure and specimen size on the fatigue crack growth rate of an annealed 0.42 C steel were investigated under uniaxial fatigue loading in air. Although a dramatic fluctuation of crack growth rate was fo und in the propagation process of microstructurally small cracks, the mean value of crack growth rate can be evaluated by a simple mechanical paramete r, sigma(a)(n)l (l, crack length; n, constant), under high stress levels wh ere small-scale yielding conditions are exceeded. This parameter is also ef fective for cracks larger than 1 to 2 mm under high stress levels, as long as the finite boundary effect of a specimen on the driving force of crack p ropagation is considered. The crack growth rate of the alloy was described as a function of stress amplitude and crack length in terms of two mechanic al parameters, sigma(a)(n)l and Delta K. The applicable conditions of the t wo parameters were discussed and manifested.