| Two sequences of tension-fatigue and fatigue-tension were performed on T2 copper and brass H62 sheet. For the angle between the two successive loading directions, two typical values Φ(0 and 45°)have been chosen. The effect of strain path change on subsequent initial work hardening rate and saturation stress in tension-fatigue, as well as the effect of strain path change on subsequent yield and flow behaviour in fatigue-tension have been investigated. The strain rate for the tension tests was 5×10-3s-1,while the fatigue tests were performed under constant plastic strain amplitude control with different values of amplitudes( εpl =2.0×10-4,5.0×10-4 and 1.0×10-3). Dislocation microstructure were observed by optical and transmission electron microscopy (TEM) after mechanical tests. Under these conditions, it was found that:For the tension-fatigue sequential loading mode, the amount of tension prestrain has obviously affects on the initial cyclic hardening rate of T2, while it has almost no effect on the saturation stress of subsequent fatigue behaviours, irrespective of the value of the angle between the two successive loading directions. For Φ=0°,at small tensile prestrain (less than or equal to 0.05),the preloading history has almost no effect on the saturation dislocation microstructures formed in the subsequent fatigue process while at relative high tensile prestrain (0.08),the dislocation cell structure formed in tension remains in some grains throughout the fatigue loading. For Φ=45°,the most prevailing dislocation structures in most samples are cell and parallel wall structures.For the fatigue-tension sequential loading mode, fatigue prestraining of T2 increases the reloading yield stress of subsequent tension markedly. However, the subsequent mechanical behaviour of samples seems only to be affected by the magnitude of strain path change (namely, the angle between the two successive loading directions), and not by the value of the plastic strain amplitude of the preceding fatigue tests. The dislocation structures formed in the first strain path are retained when the second tensile strain... |
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