It is important to investigate how the coefficient of friction affects stresses at critical locations, and how stress hardening may affect critical stresses. The left-hand side of Fig.4 shows some results for stresses at the critical location for different coefficients of friction. The trend shows that the residual stress has been reduced more than the stress concentration when the coefficient of friction is reduced from 0.8 to 0.4. Use of the low coefficient of friction will lead to a slightly less residual strain and a small increase in the stress concentration. The strain hardening may increase the local stress after the plastic yield so that the effect of friction becomes more severe. Some finite element stress results are shown on the left-hand side of Fig.4. In this figure, “P-plastic” denotes the elastic perfectly plastic assumption while “Hardening” denotes the strain hardening consideration. For the same friction condition, strain hardening increases the residual strain while the stress concentration is slightly reduced. This is due to a stronger friction resulted from the increased contact stresses due to the strain hardening. The stress range is increased even though stress ratio has been reduced. Plastic yield makes it necessary to analyse stresses for individual load sequence since there is no linear correlation between the load level and stress concentration. Three load levels are considered; MPa 80 , MPa 100 , and MPa 120 . The stress-strain results are shown on the right-hand side of Fig.4. There is very large plastic deformation at the remote stress of MPa 120 . The total strain becomes more than 3 % for the first load cycle. Even at the minimum load after the first fatigue cycle, the residual strain remains to be more than 1 %. It is interesting to find that at the stabilised state, the stress-strain response still appears to be linear even though the stress range and concentration are different. Similar stress-strain response exists even for a load level as low as MPa 80 . Still, there is a substantial plastic yield, indicating that non-linear stress analyses are required. The total strain is larger than 1 % at the first load cycle. At the stabilised state, the strain is slightly less than 1%. The low stress level seems to cause less residual stress, and the reverse yield occurs later. In addition to the stress concentration, fretting should be considered as well3, especially if the fretting area is close to the stress concentration4 5. Basically, the major fretting crack initiation mechanism is due to the repeated small stress cycles along the fretting boundary. The stick-slip boundary has been identified as a major crack initiation location6. According to the finite element analyses, the contact condition can be analysed. The left side of Fig.5 shows schematically a computational contact area between the sheet and splice for the lap joint. The finite element results indicate that the contact is not significant around the rivets for a neat fitted condition. The slip-stick boundary is located at the highest stress concentration location. At this location, fatigue cracks will be initiated much easier than at other locations. This location is also the actual crack initiation location observed in all the fatigue tests. ANALYSES OF CRACK INITIATION AND PROPAGATION Crack Al rivet Hi-lok bolt Fig.5: Contact boundary at the most frequent fatigue level, compared to experimental crack initiation locations. Load transfer
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