Ultrasonic Characterization of Tensile Overload Effects and Stress Intensity Shielding

Abstract

The growth of a fatigue crack is modified by the development of contacts between the crack faces^{1, 2}, creating shielding and thus canceling a portion of the applied load. These contacts develop through a number of mechanisms, including plastic deformation, sliding of the faces with respect to each other and the creation and collection of debris such as oxide particles³. Compressive stresses are created on either side of the partially contacting crack faces resulting in opening loads that must be overcome in order to apply a driving force to the crack tip for growth. In this way, the crack tip is shielded from a portion of the applied load, thus creating the need for modification¹ of the applied stress intensity range from ΔK = K_Imax − K_Imin to ΔK = K_Imax − K_Ish. Determination of the contact size and density in the region of closure from ultrasonic transmission and diffraction experiments⁴ has allowed estimation of the magnitude of K_Ish on a crack grown under constant ΔK conditions. The calculation has since⁵ been extended to fatigue cracks grown with a tensile overload block. The calculation was also successful in predicting the growth rate of the crack after reinitiation had occurred. This paper reports the results of attempts to define the amount of retardation remaining before reinitiation of crack growth in terms of the parameters used by the distributed spring model. In addition, this paper reports on the acoustic response of a crack grown using a decreasing ΔK and the subsequent implications for determination of the threshold stress intensity factor.