The influence of load ratio on fatigue crack growth in 7090-t6 and in9021-t4 p/m aluminum alloys

On the Distinction Between Plasticity- and Roughness-Induced Fatigue Crack Closure

“…[5] This type of crack closure is referred to as plasticity-induced fatigue crack closure (PIFCC), and Elber proposed that PIFCC was caused by the residual stretch of material in the wake of the crack tip. Minakawa et al [6] and Budiansky and Hutchinson [7] provided a mathematical analysis of plasticity-induced closure that is consistent with Elber's interpretation. However, higher strength aluminum alloys might not exhibit PIFCC, e.g., 7090-T6 (YS 650 MPa) and IN9021-T4 (YS 530 MPa).…”

“…The fact that the rates of fatigue crack propagation can be expressed as in Eqs. [6] and [7] suggests that the CTOD plays an important role in the crack growth process. All alloys will undergo both PIFCC as well as RIFCC.…”

“…In this type of crack closure, the crack-opening level in the macrocrack range is constant and independent of the DK level, in contrast to PIFCC. An example of this type of RIFCC behavior is shown in Figure 2 [6] for a 6-mm-thick 9Cr-1Mo steel (YS 530 MPa). Here, the K op level is approximately 2.5 MPaÖm.…”

“…2-K op vs DK plot for 9Cr-1Mo steel for two specimen thicknesses. [6] Fig. 3-K op vs DK plot for C1018 steel for two specimen thicknesses.…”

On Crack Closure and Crack Tip Shielding During Fatigue Crack Growth

“…They explained this behavior by the increased 00 due to the larger plastic zone associated with the overload. This explanation has been challenged by McEvily and co-workers [15][16][17][18]where he demonstrates that there are two crack opening loads and that the upper one is only associated with the plane stress region near the surface of the sample. When the surface layer of material is removed from a specimen much thicker then the plastic zone the retardation effect in the plane strain region is almost totally removed.…”