The authors' previous studies on the crack growth under impact fatigue load for several metallic materials have revealed that the crack growth rate da/dN is higher in impact fatigue than in non-impact fatigue when compared on a da/dN-AK diagram and, in some cases, da/dN in impact fatigue shows its dependence on an applied stress range; the larger the applied stress range the higher the value of da/dN [1][2][3][4][5].Such features of impact fatigue crack growth suggest that the development of plasticity near the crack tip in impact fatigue is different from that in non-impact fatigue.In this study, microscopic deformation characteristics of material in the area including a crack propagated under impact load were observed by using fine meshes produced on a specimen surface by means of fine grating technique [6]. A discussion was made on the observed result by comparing it with the result obtained under non-impact load.High strength low alloy steel HT-60 supplied as was used in the experiment, whose ultimate tensile MPa.The specimen was center notched plate with a the width of 30mm.Impact fatigue tests were conducted by using an machine of Hopkinson bar type at a loading rate of shows an example of impact stress pattern obtained 3.2 mm thick plates strength is about 600 thickness of 2mm and impact fatigue testing 9Hz [4,5].Figure 1 by this testing machine. Crack growth tests in non-impact fatigue were made by an electro-hydraulic servo type fatigue testing machine at a loading rate of 30Hz, and stress ratio R was chosen as zero to coincide with the impact stress pattern shown in Fig. i. Relations between da/dN and AK in impact fatigue are plotted in Fig. 2 together with the crack growth curve in non-impact fatigue.No significant difference is observed between the impact and non-impact fatigue crack growth behaviors.Nevertheless, some differences were observed in microscopic crack opening behaviors. Figure 3 shows a i0 vm pitch grating pattern covering the crack tip area.The gauge length was taken as 30 ~m (3 pitchs).The displacement vertical to the crack was measured under static load condition by increasing the load step-wise from zero to the maximum applied stress, after the crack was propagated to attain the stress intensity factor range of about 15MPa/m under both load conditions. An example of the measured displacement with the increasing stress is shown in Fig. 4(a). Figure 4(b) illustrates the method how to determine the stress parameters a., ~ and • . lop o a which characterize the microscopic crack openlng behaviors, where u ~. is the stress needed to initiate the crack opening, a is the stress t~°~pen the crack completely and ~ is an intermediate s t r~s level U Int Journ of Fracture 29 (1985)
This paper describes the fatigue crack growth and crack opening behaviors in impact fatigue of two alloy steels; one is a high strength low alloy steel HT-60 and the other is an austenitic stainless steel SUS 316. Both of the alloy steels have almost the same ultimate tensile strength of about 600 MPa. Major attention was paid to the microscopic crack opening behavior under impact fatigue load measured by means of the fine grating method. The results of the crack growth tests indicated that the crack growth rate in impact fatigue is higher than that in non-impact fatigue for SUS 316 alloy steel, but not for HT-60 alloy steel. Such a difference in crack growth behavior between two steels was correlated to the following microscopic crack opening behavior of these steels: In the case of SUS 316 alloy steel, both of the effective stress intensity range at a crack tip and the effective crack opening displacement behind the crack tip are larger for the crack propagated under repeated impact load than those for the crack under non-impact load, but in the case of HT-60 alloy steel, the difference in those values measured under impact and non-impact loads is relatively small comparing to the case of SUS 316 alloy steel.
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