Study On High Cycle Fatigue Properties Of Metals Based On The Thermographic Method

Fatigue fracture is the main failure mode of engineering structures. The fatigue analysis plays an important role in determining the fatigue life of materials. For traditional method, the fatigue life is obtained by using a smooth specimen under constant amplitude stress conditions. The Wolher curve obtained by traditional method costs30days, and needs at least13specimens.As one of the most promising research directions of the thermograpic method, infared thermography has been widely applied to the scientific research based on the advantages of non-destructive, real-time and non-contact. Fatigue thermography is utilized for obtaining the fatigue limit of metals just in a short time, stirring up the strong interest from the scholars.In present study, the superficial temperature of443ferritic stainless steel,304austenitic stainless steel and AZ31B alloy specimen are measured by an infrared thermal scanner under fatigue loading. The difference of temperature changes and energy dissipations of the three materials has been discussed. Fatigue performance of the three materials have been achieved by fatigue thermograpy.When stresses above fatigue limit, results shows that the temperature evolution of443and304present four stages:phases Ⅰ-initial thermal increase, phase Ⅱ-temperature stabilization, phase Ⅲ-final rapid increase, phase Ⅳ-final drop, while the temperature evolution mainly undergoes five stages of AZ31B: phases Ⅰ-initial thermal increase, phase Ⅱ-temperature reduce, phase Ⅲ-temperature stabilization, phase Ⅳ-abrupt increase, phase Ⅴ-final drop. The differences in crystal structure of materials leads to a difference in their plastic deformation ability and energy dissipation under cyclic load. Based on the evolution of superficial temperature and energy approach, the downward gradient of energy dissipation in the fatigue process were304austenitic stainless steel,443ferritic stainless steel and AZ31B magnesium alloy respectively. This mainly relates to the dislocation slip of the three kinds of materials at room temperature. For face-centered cubic304stainless steel, there are12slip systems at room temperature. Due to more glide planes and larger density,304has the maximum energy dissipation during the fatigue process. For the body-centered cubic443ferritic stainless steel, there are also a total of12slip systems at room temperature. But because of the high stacking fault energy of443, oblique slip system prones to cross slip. The cross slip of the edge dislocation will lead to dislocation pile-up and work-hardening. So the energy dissipation is small. For close-packed hexagonal lattice of the AZ31B magnesium alloy, the energy dissipation is the lowest because there are only3slip systems at room temperature.Fatigue limit and S-N curves of the three materials were determined by fatigue thermograpy rapidly. The fatigue limit of443,304, AZ31B determined by traditional S-N curves were277.7MPa,246.7MPa,108MPa respectively. The fatigue limit of443,304, AZ31B determined by Loung method were310.5MPa,250.9MPa,110MPa. The relative error of the two methods were10.5%,1.7%,2.0%.