Cyclic Deformation And Fatigue Crack Growth Behavior Of High Manganese Austenitic TWIP Steel

With the development of automobile industry, reducing vehicle weight and thus decreasing energy consumption and polluting emissions has become the development trend of modern cars. Increasing the strength of automotive steel is the effective ways to decrease the thickness of steel plate and thus reduce vehicle weight and energy consumption. New type high-manganese austenitic TWIP steel has high strength and plasticity, which is considered to be the most potential for development of automobile structure steel. As structural materials, TWIP steels will inevitably be subjected to alternating load, and finally lead to fatigue failure. Therefore, the investigations on the fatigue behavior of high-manganese austenitic TWIP steel not only have practical values, but also can provide a reliable theoretical basis for the fatigue resistant design and safety usage of structural components.In this dissertation, the high-manganese austenitic TWIP steel as the research object was studied. At room temperature, low-cycle fatigue behavior of TWIP steel was investigated, and the effects of strain range and stain rate on low-cycle fatigue behavior of TWIP steel were analyzed; the fatigue crack growth behavior was also investigated, and the effects of grain size, stress ratio and Al element on fatigue crack growth behavior were analyzed. The main results are as follow:The results of low-cycle fatigue tests indicated that the dynamic strain aging of the high-manganese austenitic 0Al steel is assoclated with the strain rate. At fixed strain range was 1.4%, no serration appeares on the stress-strain hysteresis loop at low strain rate(8×10-4 s^-1 and 2×10-3 s^-1); serration appeares on the stress-strain hysteresis loop at medium strain rate(8×10-3 s^-1), but the wave amplitude of serration decreases with the increase of cyclic number, and gradually disappears; the wave amplitude of serration at high strain rate(2×10-2 s^-1) is higher than that of medium strain rate, and also decreases with the increase of cyclic number, but the serration does not disappear, it accompanies the whole fatigue cycle.The cyclic hardening/softening behavior of high-manganese austenitic 0Al steel in the process of low-cycle fatigue deformation is closely related to the strain range and strain rate. At the strain rate of 8×10-3 s^-1, high-manganese austenitic 0Al steel showed the initial cyclic hardening followed by cyclic saturation and cyclic softening till failure at the strain range of 1.0% to 1.4%. However, at the strain range of 0.8%, the 0Al steel showed cyclic hardening followed by cyclic saturation and slight cyclic softening, and it also experienced a cyclic stable stage before the cyclic failure. Furhtermore, at constant strain range of 1.4%, the cyclic stress response behavior of 0Al steel was characterized by initial cyclic hardening followed by cyclic saturation and cyclic softening till failure under different strain rate.At the constant strain rate of 8×10-3 s^-1, the low-cycle fatigue life of 0Al steel decreases with increase of strain range, and obey the Coffin-Manson relationship. Thus, the low-cycle fatigue life of 0Al steel can be comparatively accurately predicted by utilizing the fatigue performance parameters of this paper. At the constant strain range of 1.4%, the low-cycle fatigue life of the 0Al steel decreases with increase of strain rate. This is because the cumulative plastic damage of each cycle in the process of cyclic deformation at high strain rate is slighter higher than that of low strain rate. And local inhomogeneous deformation and persistent slip bands（PSBs） produce in the process of cyclic deformation at high strain rate. These local inhomogeneous deformation and PSBs are conducive to crack nucleation, promote fatigue crack growth, and thus reducing the fatigue life.The fatigue crack growth resistance increased with decrease of stress ratio in both the coarse- and fine-grained high-manganese austenitic 0Al steel. At high and low stress ratios（0.6 and 0.1）, the fatigue crack growth threshold values of the coarse-grained 0Al steel are higher than that of fine-grained 0Al steel. In the Paris regime, the effects of grain size on the fatigue crack growth rate decrease with increase the stress intensity factor range. In addition, it also found that the fatigue crack growth resistance of 0Al steel increases with the decrease of the stress ratio. The influence of grain size on the fatigue crack growth threshod value is mainly caused by the different of crack closure and slip planarity. In addition, the crack branching can also consumes large mounts of energy in the process of fatigue crack growth, thereby reducing the crack-tip driving force and helping increase the fatigue crack growth threshold.Based on the traditional two-dimensional observation methods and three dimensional synchrotron radiation CT technique, the fatigue crack growth behaviors of 0Al and 3Al steels were investigated at the stress ratio of 0.1. The results show that the fatigue threshold value of 0Al steel is higher than that of 3Al steel. This is because the effects of crack closure on 0Al steel are higher than that of 3Al steel in the near threshold. And the addition of Al increases the stacking fault energy of 3Al steel, thereby reducing the slip planarity of material and resulting in the decrease of slip reversibility. In addition, the intersection of crack segments can also reduce the effective driving force of crack tips, thereby resulting in a low crack growth rate.