Experiment Analysis Of Energy Dissipation During Fatigue And Its Application Study

Fatigue damage is the energy dissipation process. As the most important energy dissipation form during fatigue process, the evolution of thermal dissipation can reflect the evolvement process of the crack sprouting-expanding, and then material rupture. The evolution of temperature field due to thermal dissipation is a sign and measurement of the thermal effect during the material deforming process. So it's the experiment foundation of studying the energy dissipation phenomenon during fatigue process to measuring temperature exactly.In this paper, the temperature evolution of several metals during fatigue process was measured by an infrared photography instrument with high sensitivity and high resolution. The temperature evolution measured by infrared camera indicated three stages: the initial increase stage, the stabilization stage and abrupt increase before failure stage. The influence of the change of stress amplitude and the change of average stress are considered. Generally speaking, in a fatigue test without outside heat sources, the specimen temperature variation can be explained by three factors: the thermoelastic effect, the inelastic effect and the heat-transfer effect. It's also deemed that heat generated mainly by plastic deformations or viscoelasticity when the load is upper or lower than fatigue limit.In Chapter 3, from the theory of energy dissipation equilibrium during fatigue process and analyzing the various forms and mechanics during energy dissipation process, an energy dissipation theory model during fatigue process can be constituted. After simplified it appropriately, the theoretical relationship between temperature evolutions, power released as heat by the material in a unit volume, stored energy and input work.It's shown in much experimentation that the abrupt increase of temperature during fatigue process is related to fatigue limit relatively. So the fatigue limit value can be estimated according to this method. The fatigue limit values of 45 Steel, Q235 Steel and pure copper have been obtained by this method using the infrared thermographic technique. Compared to the fatigue limit value obtained by some conventional method, the test result is reliable no matter by one curve fit method or two curves fit method. Furthermore, this novel fatigue limit testing method can be more efficient and low cost. The energy variation and the thermal dissipation can reflect the dissimilar course of the fatigue behaviors of materials. So it's reasonable to explore the fatigue life prediction method based on thermal dissipation. In this paper, the relationship between fatigue life and the quantity of the thermal dissipation in a unit volume of material per cycle during the stabilization temperature stage was proposed. And the quantity can be derived from temperature drop curve when the load was stopped at the stabilization temperature stage of sample. Then it's possible to obtain the quantitative relation between fatigue life and thermal dissipation by experimentation.