Ultrasonic Investigation of Tensile Fatigue Overloads in Fatigue Cracks

The growth of a fatigue crack is generally modeled in terms of empirical rules such as the Paris law [1] which states that da/dN = A(ΔK)m where a is the crack length, N is the number of fatigue cycles at a particular stress intensity range ΔK and A and m are material constants. Recently, evidence has accumulated that establishes that the full excursion of the applied load or the full stress intensity range does not drive the crack tip forward due to a variety of phenomena often referred to as crack tip shielding [2,3]. This shielding arises as a consequence of the formation of contacting asperities between the surfaces of the fatigue crack. Since these contacting asperities bear a portion of the load, they modify the stress intensity at the tip, thereby altering the growth rate