The temperature evolution in an AZ31B magnesium alloy plate was measured during static and cyclic loading via infrared thermography. The relationship between loading process and temperature evolution was established. The yield limits during static and cyclic loading were predicted. The temperature variation on the specimen surface was closely related with the applied load. The initial decrease in temperature during tension was caused by the thermoelastic effect, and the minimum temperature corresponded to the yield limit. During cyclic loading, the thermoelastic effect, viscous effect and plastic work had an effect on the temperature evolution. The cyclic yield limit was <1/6 of the yield limit obtained in the tension test.
Based on the infrared thermography method, experiments are carried out to investigate the evolution of temperature field of the extruded AZ31B magnesium alloy specimens under high cyclic fatigue load. The experimental results show that the superficial temperature of specimen under cyclic fatigue load changes with the number of cycles. According to the characteristics of surface temperature change, we propose a formula to calculate the residual fatigue life using energy approach. The proposed formula to assess the fatigue parameters (fatigue limit, residual fatigue life, fatigue life and S–N curve) achieves good results for AZ31B magnesium alloy. Furthermore, the fatigue limits (Δ σeSN = 90·3 MPa) derived from the traditional method through 107 cycles were compared with the values predicted by the infrared thermographic method (Δ σeTM = 87·3 MPa) and the energy approach (Δ σeΦ = 86·2 MPa), and the comparison results of percentage differences are 3·3 and 4·5 respectively.
A B S T R A C T Fatigue crack propagation (FCP) behaviour of 4003 ferritic stainless steel was investigated using infrared thermography. Four stages of superficial temperature evolution were observed during the FCP tests: an initial temperature decrease stage, a temperature equilibrium stage, a slow temperature increase stage and an abrupt temperature increase stage; a thermal model is developed to explain the observed temperature evolution. The experimental results indicate that: when the range of stress intensity factor (ΔK) is at a low level where the crack is located in slow propagation region, thermoelastic effect will be in dominant status; when the ΔK is at a high level where the crack is located in stable propagation region, the temperature rise can be used to describe FCP rate. The fatigue fracture surfaces were examined using scanning electron microscope (SEM) in order to understand the effect of the fatigue mechanisms on temperature variation.Keywords fatigue crack propagation; fatigue fracture mechanisms; infrared thermography; stress intensity factor. N O M E N C L A T U R Ea crack length da/dN crack propagation rate ΔK the range of stress intensity factor ΔK th the threshold value of ΔK for crack starting to increase ΔK th,temp the threshold value of ΔK where the temperature begin to increase sharply C Paris coefficient m Paris exponent T the highest superficial temperature value around the crack tip T 0 room temperature ΔT maximum temperature increment R loading ratio f the frequency of loading Q the energy conversion rate per unit volume q the dissipated power per unit length of crack front k the thermal conductivity of the material r c the radius of the cyclic plastic zone v the crack propagation velocity Cp the heat capacity at a constant pressure α l the coefficient of linear expansion ρ the density Δσ the principal stress ξ the dissipated energy per unit length of crack front during one cycle η a material-dependent proportionality factor σ y the cyclic yield stress of the material
The work-hardening/softening behaviour of AZ31B magnesium alloy during high cycle fatigue was investigated. The superficial temperature evolution during fatigue tests was used as a criterion for the different levels of work-hardening/softening. The microstructures under different cycles were observed by transmission electron microscope. Tensile test (with post-fatigue) was conducted to quantify the work-hardening/softening behaviour which showed that high dislocation density after cyclic loading lead to high tensile strength. The temperature evolution of the specimens with different levels of work-hardening/softening during tensile tests is related to the microstructures; the results indicated that the temperature rise of the specimen with high density dislocation was lower. Microstructures after tensile tests showed that high dislocation density after cyclic loading would lead to high twinning density.
Fracture toughness of AZ31B magnesium alloy subjected to quasi-static loading was investigated by infrared thermography. The results showed that temperature evolution around the crack propagation path during fracture underwent three stages: initial steady stage, monotonic increase stage and final steady stage. The temperature increase at the beginning of stage II is nearly corresponding to the initiation of unstable crack propagation. And based on this phenomenon, a method applying infrared thermography to estimate fracture toughness of AZ31B magnesium alloy was proposed. Fracture toughness was calculated through infrared thermography, which was in good agreement with the result determined by traditional standard method. Finally, the fracture mechanism was investigated.
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