Mode I fatigue crack growth has been studied in notched specimens of 7017-T651 aluminium alloy subjected to fully compressive cyclic loads. The specimens were first subjected to a deliberate compressive preload which causes plastic deformation at the notch tip. On unloading, this region developed a residual tensile stress field and on subsequent compressive cyclic loading in laboratory air, a fatigue crack was nucleated at the notch and grew at a diminishing rate until it stopped. The final crack length increased with an increase in the value of the initial compressive preload and with an increase in the negative value of the applied cyclic mean load. To gain a better understanding of crack growth in residual stress fields, the magnitude and extent of residual stress induced from compressive preloads have been analysed. This was achieved when extending the notch by cutting while recording the change in the back face strain. From residual strain models it was found that the fatigue crack growth was confined to a region of tensile cyclic stress within the residual stress field. The effective stress intensity range was investigated at selected mean loads and amplitudes, for correlating purposes, using both the compliance technique and by invoking the crack growth rate behaviour of the alloy. Finally, a brief discussion of the fracture morphology of cracks subjected to cyclic compression is presented.
THE PURPOSE of this note is to draw attention to the apparently little-known fact that stage I1 type fatigue cracks can initiate and grow under compressive applied loads. The effect has been demonstrated in compact tension specimens, 25 mm thick [I], of normalised mild steel and En 8. First, the specimen with a machined notch was compressed for about 10 s by a load of 50.5 kN (corresponding to a stress intensity at the notch tip of 77.4 MN rnP3/'). Next, the specimen was subjected to a 25 Hz sinusoidal compressive load with a mean level of 14.1 kN and an amplitude of 11.7 kN (corresponding to a stress intensity range of 35.9 MN m-3/2). Growth of a crack from the notch tip was seen parallel to the plane of the notch. The length a of this crack on the surface of the specimen is plotted in Fig. 1 as a function of the number of compressive load cycles. It is clear that the crack grows at a decreasing rate until eventually it stops growing altogether. Once it was established that crack growth had stopped, the cyclic loading was ended and the specimen was subjected to a tensile "ramp" load, causing a brittle fracture. The fracture surface of such a specimen is shown in Fig. 2. The fatigue crack front has a marked concave shape in contrast to the slightly conveli shape shown in Fig. 3, which was obtained by subjecting an identical specimen to cyclic loads of the same magnitude in tension but without prior overload. Note too the smoother appearance of the fatigue fracture surface in this specimen.It is tentatively suggested that during the initial compressive overload, a plastic zone is formed near the tip of the notch with a shape that depends on the depth below the surface of
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