金属材料-疲劳裂纹中文教程.pptVIP

Stress-Life (S-N) Theory S-N方法并不严格区分裂纹产生和裂纹扩展，而是给出结构发生突然失效前的全寿命估计 The S-N approach estimates total life without distinguishing crack initiation from crack propagation 通常需要材料S-N曲线 It usually requires that the test data relate to the geometry of the structure under assessment (structure S-N curves) 材料S-N曲线可由光滑试样测试得到，并可根据真实结构如凹槽、表面处理条件等因素的影响对之进行修正。 Material S-N curves can also be generated from smooth specimen test data; they are subsequently modified to reflect the effects of notches, surface conditions, etc. of the real structure Some Definitions SN Analysis S-N Approach S-N方法用名义应力（弹性范围）作为疲劳载荷 The S-N approach uses the (assumed elastic) nominal stress range (S) as a measure of the severity of fatigue loading 疲劳寿命基于实验数据 Life to failure (two pieces) is recorded in experiments 通过测试不同应力水平描绘S-N曲线 Tests at several levels of stress range characterise the S-N curve 该曲线可基于光滑试样、零部件或结构 Such a curve can be derived for smooth specimens, for individual components, for sub-assemblies, or for complete structures S-N Approach S-N方法用途包括： The uses of the S-N approach include: 破坏概率（设计准则） establishing a well defined fatigue curve for the purposes of design 设计寿命下的疲劳强度 determination of a fatigue strength at a specified life 通过改变材料或表面处理提高抗疲劳能力 demonstration of improved fatigue resistance from a material or surface treatment 找出与加工相适应的材料 acceptance of material for manufacturing purposes 解决服役期失效问题 answering questions posed by a service failure S-N Curves S-N Curves 结构钢和钛合金钢的S-N曲线水平区段的最大应力为疲劳极限——应力低于该值不会发生应力破坏 Steels tested with constant amplitude loading normally exhibit a fatigue limit - a stress below which no fatigue damage appears to occur. 疲劳极限与裂纹萌生相关联。它在大负荷或腐蚀介质等情况下会降低。 The fatigue limit is associated with the difficulty a crack has in getting past the first grain boundary, or dominant microstructural barrier. It can be reduced or eliminated after e.g. a few large loads, or in corrosive environment, etc. 铝合金无明确的水平区段 Aluminum alloys do not seem to exhibit no such limit Mate