Heat Production Analysis Of AZ31B Magnesium Alloy During Fatigue And Properties Prediction

Due to a number of advantageous properties such as low density, high specific strength, high specific rigidity, easy recovery, etc, magnesium alloys can be widely used in aerospace, railway,3C product, and become the third kind of material after steel material and aluminium alloy. Magnesium alloys are called green engineering materials of the21th century.Fatigue failure is a major failure mode in engineering components. In the fatigue process, evolution of internal micro-defects cause plastic deformation accompanied with energy dissipation, the temperature of specimen will change.An experimental study was carried out to investigate the temperature field evolution of the extruded AZ31B magnesium alloy specimen under high cyclic fatigue load using an Infra Tec IR imaging senor. In this paper the law of superficial temperature change for specimens was explored and the heat production mechanism was analyzed. Experimental static tensile testing was carried out to obtain the mechanical properties of the specimens in different phases during the fatigue process. Heat production during fatigue process was calculated quantitatively through thermodynamic equation. The fatigue strength has been predicted according to the law of superficial temperature change in fatigue procedure. Base on energy approach, the method of calculate the residual life was proposed.The thermal image reveal that, the maximum temperature evolution over a given specimen generally exhibits five stages when the cycle loading above the fatigue strength:initial temperature rise stage, the temperature steep drop stage, the temperature steady stage, rapid temperature rise stage before specimen fracturing and final temperature drop. Macro cracks are formed in the end of temperature stable stage. While the cycle loading below the fatigue strength, the temperature change is not obvious.Thermo-elastic effect, inelastic effect, heat-conduct effect are major factors affecting the temperature field during fatigue process. When R=0.1, elastic could lead to a slight drop in superficial temperature. Inelastic effect could cause most of the heat. Temperature distribution of specimens tends to uniformity because of heat-conduct effect.Though tensile tests, stress-stain curves of specimens in different fatigue stages were obtained. The result revealed that, in the initial fatigue stage, specimen experiences work-hardening, with the increase for the number of cycles, work-softening takes place. Work-hardening and work-softening have the same trend as temperature change. In the static tensile testing, through theory method and linear fitting method, the quantitative calculation expression were ΔT=-4.1×10-3σ and ΔT=-4.3×10-3σ, respectively.Characteristic parameter Φ was obtained equal to1.48×106(℃.m-3) base on the theory of energy approach. According to the special characteristics of superficial temperature variation, a formula was proposed to calculate the residual fatigue life using energy approach. Fatigue life was predicted using temperature data, draw the S-N curve, achieving good agreement with the experimental results.In this paper, through1×10(?)cycles, the fatigue strengths of magnesium alloy predicted by the infrared thermographic method (ΔσTM=102.3MPa) and the energy approach (ΔσΦ=99.3MPa) were compared with the fatigue strength (ΔσSN=99.8MPa) derived from traditional procedure and the percentage differences were1.5%,0.5%, respectively.