On the Distinction Between Plasticity- and Roughness-Induced Fatigue Crack Closure
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TRODUCTION
ACCORDING to ASTM E647, the Standard Test Method for Measurement of Fatigue Crack Growth Rates, the term crack closure refers to the phenomenon whereby the fracture surfaces of a fatigue crack come into contact during the unloading portion of a force cycle and force is transferred across the crack. In many materials, crack closure can occur while the force is above the minimum force in the cycle even when the minimum force is tensile. After reloading from the minimum force, some increment of tensile loading must be applied before the crack is fully open. It is also noted that ASTM Standard Test Procedures 982[1] and 1343[2] provide excellent overviews of the subject of fatigue crack closure. In the 1970s, Elber[3,4] introduced the important topic of fatigue crack closure into the literature and showed by means of experiments with the aluminum alloy 2024T3 (yield strength [YS] = 350 MPa) that at a given R value, the crack-opening level increased with DK. Similar results have been obtained with other lowand medium-strength aluminum alloys, e.g., 6061-T6 (YS 276 MPa) (Figure 1).[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 SOTOMI ISHIHARA, Professor, and YUYA SUGAI, Graduate Student, are with the Department of Mechanical Engineering, University of Toyama, Toyama 930-8885, Japan. Contact e-mail: [email protected] ARTHUR J. McEVILY, Professor Emeritus, is with the Department of Chemical, Materials, Biomolecular Engineering, University of Connecticut, Storrs, CT 06269. Manuscript submitted July 4, 2011. Article published online March 23, 2012 3086—VOLUME 43A, SEPTEMBER 2012
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). Soon after Elber’s initial studies, other forms of crack closure were found, including a type of material-related crack closure now known as roughness-induced fatigue crack closure (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 Kop level is approximately 2.5 MPam. It is also to be noted in Figure 2 that at a thickness of 0.3 mm, PIFCC was observed. However, for a 1018 steel (YS 260 MPa), only RIFCC at a higher Kop of 4.3 MPam was observed for both thicknesses (Figure 3).[5] The variations in the Kop level for RIFCC most likely are caused by the variations in the grain size that influence the degree of roughness, in which the grain size of the 1018 steel was coarser than that of the 9Cr-1Mo steel. It is noted also that the effect of crack closure on the rate of fatigue crack g
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