Papers by Author: A.J. McEvily

Abstract: In this study, fatigue crack closure behavior was investigated in the aluminum alloy 6061-T6 and the carbon steel JIS. S25C. It was found that crack closure in the aluminum alloy 6061-T6 showed the characteristics of plasticity-induced fatigue crack closure (PIFCC), whereas the carbon steel JIS. S25C showed the characteristics of roughness-induced fatigue crack closure (RIFCC). The experiments included the determination of the crack-opening levels K_op as a function of stress intensity factor range ΔK and the effect of surface removal on the crack-opening level. In order to simulate the behaviors of the plasticity-and the roughness-induced fatigue crack closure, the finite element method was adopted. The results of FEM were in good agreement with the experimental results. It was cconcluded that at a given yield strength level , a low Youngs modulus and a low work-hardening coefficient will favor PIFCC, whereas a high Youngs modulus and a high work-hardening coefficient will favor RIFCC.

Abstract: Many of the recent advances in the understanding of the fatigue crack growth process have resulted from an improved realization of the importance of fatigue crack closure in the crack growth process. Two basic crack closure processes have been identified. One of which is known as plasticity-induced fatigue crack closure (PIFCC), and the other is roughness-induced fatigue crack closure (RIFCC). Both forms occur in all alloys, but PIFCC is a surface-related process which is dominant in aluminum alloys such as 2024-T3, whereas RIFCC is dominant in most steels and titanium alloys. A proposed basic equation governing fatigue crack growth is (1) where where Kmax is the maximum stress intensity factor in a loading cycle and Kop is the stress intensity factor at the crack opening level. is the range of the stress intensity factor at the threshold level which is taken to correspond to a crack growth rate of 10-11 m/cycle. The material constant A has units of (MPa)-2, and therefore Eq. 1 is dimensionally correct. Eq.1 has been successfully used in the analysis of both long and short cracks, but in the latter case modification is needed to account for elastic-plastic behavior, the development of crack closure, and the Kitagawa effect which shows that the fatigue strength rather than the threshold level is the controlling factor determining the rate of fatigue crack growth in the very short fatigue crack growth range. Eq. 1 is used to show that The non-propagating cracks observed by Frost and Dugdale resulted from crack closure. The behavior of cracks as short as 10 microns in length can be predicted. Fatigue notch sensitivity is related to crack closure. Very high cycle fatigue (VHCF) behavior is also associated with fatigue crack closure.

Abstract: The fatigue lifetime of the aluminum alloy 7075-6 depends upon the cyclic stress amplitude and the environment. At an R value of 0.05, and a maximum stress of 400 MPa the lifetime in vacuum is an order of magnitude greater than that in air, but interestingly at a maximum stress of 275 MPa, the lifetime in vacuum is only three times that in air. It was noted that well defined striations were observed in air, whereas striations were absent in vacuum, instead indications of ductile rupture were observed. The mechanism of fatigue crack initiation and propagation behavior as influenced by the environment is discussed.

Abstract: A study has been made of the striations and fissures developed in the aluminum alloy 2024-T3 during fatigue crack growth. Fissures were found to form on inclined facets. They were uniformly spaced as the result of a shielding process. Striation spacings were in accord with da/dN values at the higher levels of
K investigated, but at low
K levels striation spacings were larger than the corresponding da/dN values. The percentage of the fracture surface containing striations varied with the
K level, ranging from less than 1 % at low
K levels to 80 % at higher
K levels. The reason for the discrepancy between the spacing of striations and the corresponding da/dN values is discussed.

Abstract: Fatigue tests were performed in laboratory air using extruded Ti-6Al-4V to determine the effect of microstructure on S-N curve, and the crack initiation and crack propagation behavior of the alloy. A modified linear elastic fracture mechanics approach was used in the analysis of the short crack propagation behavior to predict the S-N curve and the crack propagation curve. The predictions agreed well with the experimental results.

Abstract: A modified linear-elastic fracture mechanics approach proposed by McEvily has been applied to predict the effects of small defects on the fatigue limit and the threshold level. In the analysis, three modifications were taken into account (1) the effect of elastic-plastic behavior of small cracks, (2) the Kitagawa effect where in the very small crack regime the required stress for propagation is controlled by the fatigue limit of a smooth specimen rather than by the long-crack threshold condition, and (3) the effect of crack closure development from zero up to the macroscopic level as a newly formed crack extends. Three steels, a brass and an Al alloy were investigated. Good agreement between predicted and experimental results has been obtained and a rational basis for the area parameter model was shown.

Abstract: The behavior of short fatigue cracks is a matter of importance not only because much of the fatigue lifetime is spent in propagating these cracks, but also because the boundary between propagation and non-propagation separates the safe from the potentially unsafe fatigue regimes. The method of analysis is based upon the following equation: