Study On Fatigue Crack Propagation Behavior Of 4003 Ferritic Stainless Steel And Its Welded Jonts Based On Thermographic Method

Due to the excellent corrosion resistance, high mechanical properties and low price, 4003 ferritic stainless steel is widely used in industries, which contains it subjected to alternating loads, and its welded joints subject to fatigue loading has become the weakness to limit its widespread use. Fatigue crack propagation can reflect the prevent crack propagation capacity of the materials. The current study about crack propagation is mainly based on fracture mechanics, and the stress intensity factor is used to describe the crack propagation rate. Thermographic method is applied to study the crack propagation behavior due to its advantages of real-time and rapid, and from the perspective of energy dissipation material crack growth analysis of the nature. Therefore the use of infrared thermography to study the crack propagation behavior is of great scientific value and significance.Infrared thermography is used to study the temperature evolution of 4003 ferritic stainless steel and its MIG(Metal Inert-gas Welding) welded joints during fatigue crack propagation. The results showed that: when the initial ΔK(stress intensity factor range) is small, the temperature evolution during fatigue crack propagation test can be divided into five stages: initial temperature drop stage Ⅰ, temperature equilibrium stage Ⅱ, slow temperature rise stage Ⅲ, rapid temperature rise stage Ⅳ, cooling stage Ⅴafter failure. While the heat mechanism of specimen during fatigue process was discussed, and the cyclic plastic zone around crack tip is the moving heat source during crack propagation process; and the relationship between the temperature change and the stress intensity factor ranges has been established, and the temperature is mainly affected by the stress intensity factor ranges and the crack propagation rate.The scanning electron microscopy is used to study fatigue crack fracture morphology; the results showed that: at the initial propagation stage the fatigue fracture mechanisms of base metal, weld, HAZ, are respectively quasi-cleavage fracture, cleavage fracture, quasi-cleavage fracture, which is brittle fracture mechanism; while at the stable propagation stage the fatigue fracture mechanisms of all the above are fatigue striations morphology, which is plastic fracture mechanism.When the microscopic fracture mechanisms are cleavage or quasi-cleavage fracture which is brittle fracture mechanisms, the crack would be located in the initial slow propagation stage, the macroscopic temperature did not change significantly; when the microscopic fracture mechanisms are fatigue striations morphology which is plastic fracture mechanisms, the crack would be located in stable propagation stage, the macroscopic temperature would rise rapidly; at the transition of temperature variation, the microscopic fracture mechanisms are quasi-cleavage fracture mixed with plastic fatigue striations. The temperature variation and the fracture mechanism variation confirm each other, this indicates that the temperature variation is an effective way to study crack propagation behavior.