| Due to the material characteristics and energy advantage, lightweight alloy such as magnesium and aluminum alloy are widely used in aerospace and other traffic transport structures. These structures under cyclic loading are prone to fatigue failure. Fatigue failure is low stress fracture that is no obvious plastic deformation. Once the fatigue accidents appeared, it is a serious threat for people lives and property. The infrared thermography as a real-time, intuitive display, non-destructive method to investigate the failure process and fatigue property of material has become popular. The thermal radiation ability was affected by the different thermal conductivity between the magnesium alloy and aluminum alloy. In this paper, infrared thermal image method is used to analysis fatigue life and crack propagation characteristics of magnesium alloy and aluminum alloy. This study broadened the application of the infrared thermography, which has important theoretical significance and practical value.The superficial temperature of AZ31B magnesium alloy and6061-T6aluminum alloy under high-cycle fatigue loading were measured by the thermographic method. An experimental approach is developed to predict fatigue life by the thermographic technique and based on the initial gradient of the temperature evolution at the initial stage of fatigue test. Based on the thermal images during the fatigue fracture process, the fatigue crack propagation was monitored in real time and the plastic zone size on the fatigue crack tip was measured, which recorded the propagation process of fatigue crack for AZ31B magnesium alloy and6061-T6aluminum alloy.When the loading is greater than the fatigue strength, the temperature evolution of AZ31B magnesium alloy undergoes five stages:initial rapid increase stage, steep decrease stage, steady-state stage, abrupt increase stage, and final drop stage. When the load is lower than the fatigue strength, temperature variation on the surface of sample is not obvious and temperature change is lower than2℃. The temperature evolution of6061-T6aluminum alloy undergoes four stages:initial increase stage, slowly decline stage, slowly rising stage and rapid increase stage.Due to the difference in fatigue fracture process and theoretical mode of predicting fatigue life based on the law of the thermodynamics, the theoretical approach depends on the slope of the temperature evolution curve. The proposed model of predicting fatigue life for the AZ31B magnesium alloy is NAZ31BR=707.07300; the proposed model of predicting fatigue life for the6061-T6aluminum alloy is N6061R=13.30036. The theoretical model was consistent with the experimental result of fatigue life.According to the thermal images, the plastic zone at the fatigue crack tip could be measured. The results showed that the crack tip plastic zone of theoretical formula for AZ31B magnesium alloy is3.7mm2, the crack tip plastic zone measured by infrared thermography is4.1mm2, two methods of the result error is9.7%; the crack tip plastic zone of theoretical formula for6061-T6aluminum alloy is3.3mm2, the crack tip plastic zone measured by infrared thermography is3.5mm2, two methods of the result error is5.7%. |
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