Microscopic Study of 5083-H321 Aluminium Alloy Under Fretting Fatigue Condition

In this investigation, fretting fatigue study is carried out using 5083-H321 aluminium alloy. The test rig was designed to apply the normal load on the thickness side of the surface of the specimen, which is presumed to be the active surface for the fretting process. Fretting fatigue experiments were designed with an emphasis to study the crack initiation behaviour in the fretted region using scanning electron microscope. In order to investigate the possible crack initiation mechanisms in fretting fatigue as well as for the associated fatigue two different types of tests were proposed along with fundamental pure plain fatigue tests. These tests are named as fretting fatigue test and interrupted fretting fatigue test. First, pure plain fatigue tests were carried out with three maximum stresses of 200 MPa, 220 MPa, and 230 MPa, and with a stress ratio R of 0.1 at a frequency of 20 Hz. At each stress level three specimens were tested. Further fretting fatigue tests were carried out for normal pressures of 15 MPa, 30 MPa, and 45MPa at each axial stress level of 200 MPa, 220 MPa, and 230 MPa, with a stress ratio, R of 0.1 at a frequency of 20 Hz and. It has been found that the fatigue life reduces by a factor of 2.55, 3.48, and 3.54 for specimens tested with normal pressures of 15 MPa, 30 MPa, and 45 MPa using a maximum axial stress of 200 MPa respectively. The life reduces by factors of 3.16, 2.07, and 4.54 for specimens tested with normal pressures of 15 MPa, 30 MPa, and 45 MPa respectively using a maximum axial stress of 220 MPa. On the other hand the fatigue life reduces by a factor of 2.24, 2.37, and 2.81 for normal pressures of 15 MPa, 30 MPa, and 45 MPa respectively using maximum axial stress of 230 MPa. In general we can say that the fatigue life reduces by a factor of 2 to 3 for all specimens tested at fatigue stress levels and the normal pressures applied to the specimens as indicated above due to fretting fatigue in this investigation. For the fretting fatigue specimen tested with maximum axial stress of 200MPa and a normal stress of 15 MPa the crack initiates from the locations along the pad edge boundary of the specimen with a mild river pattern and subsequently propagates to failure. For the fretting fatigue specimen tested with maximum axial stress of 200MPa and a normal stress of 30 MPa the crack appears to propagate in multi directions whereas for the case of maximum axial stress of 200MPa and a normal stress of 45 MPa the behaviour is somewhat similar to the cases of normal stress of 15 MPa and 30 MPa but the rate of crack propagation is relatively faster. The fracture surface of the specimens tested with normal stress of 15 MPa, 30 MPa, and 45MPa using maximum axial stresses of 220 MPa and 230 MPa indicate a similar pattern of cracking behaviour as indicated for maximum axial stress of 220. As far as the fracture morphology is concerned more tilt towards the fibrous nature along with increased crack propagation rate has been observed for specimens tested with more normal pressure/ stress as well as with increase in applied maximum axial stress. Also it is observed that the fretting fatigue failure often initiated at the boundary between the pad and specimen contact surface. As far as the damage is concerned the fretting damage increases with number of cycles and crack initiated after the specimens are tested to in the life range of 40% to 60% of life at maximum axial stress of 200MPa giving the damage threshold in basic fretting behaviour. But the crack initiation life is less for higher normal pressure/stress at the contact interface.

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