材料力学课堂教学软件（英文版）-范钦珊-Strength of Materials－11.pptVIP

Putting the data of Smax－N test on the lgS—lgN coordinate system,we can get the curve of stress vs longevity ,which consists of two straight lines. There are two regions :infinite longevity region and limited longevity region as shown in figure. Infinite longevity region Limited longevity region In the lgS—lgN coordinate system the sideling line can be described by Chapter 11 Fatigue behavior of materials and design of fatigue life of structures ? Design based on limited life and unlimited life ? Basic concepts In engineering,the longevity of structures include the periods of occurring and extending of cracks . For some materials, when the crack occurred it will failure at once .But other materials can subject to loads ，when crack occurred ,for this case ,it is allowable that there are some cracks in elements or parts. That is the basis of the design of fatigue based on limited life. Chapter 11 Fatigue behavior of materials and design of fatigue life of structures ? Design based on limited life and unlimited life ? Basic concepts Chapter 11 Fatigue behavior of materials and design of fatigue life of structures ? Design based on limited life and unlimited life ? Introduction to unlimited life design If the amplitude of stress-cycle will not be changed ,it will be called alternative stress with equal amplitude. In engineering ,it is convenient to use the safety-factor method for the design of the fatigue strength. In this method, the criterion of design will be Chapter 11 Fatigue behavior of materials and design of fatigue life of structures ? Design based on limited life and unlimited life ? Introduction to unlimited life design n – Safety-factor in work of elements or parts [n] – Prescribed safety factor where When the material is very homogeneous,and the calculations of loads as well as the stresses are very accurate,we can take [n]＝1.3. The ma