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1、Lec 16-1,Stress-Life (S-N) Theory,Stress Life Analysis,Lec 16-2,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 geom
2、etry 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,S
3、tress Life Analysis,Lec 16-3,Some Definitions,应力幅值,平均应力,应力比R=,Stress Life Analysis,Lec 16-4,输入应力循环 Input is cycles of STRESS 即高周疲劳或名义应力疲劳法 Also known as “High Cycle Fatigue” or “Nominal Stress Approach” 尽管在临界区域的局部应力会是塑性的但名义应力循环必须为弹性应力(因此为高周疲劳） Nominal stress cycles must be elastic (hence high cycle)
4、 though local stresses at the critical location will be plastic 在MSC/FATIGUE SN分析中有限元弹性计算结果可直接利用（无塑性修正） In MSC/FATIGUE SN analysis, elastic FE results are used directly (no plasticity correction),测试的名义应力 Measured nominal stresses,临界位置的真实应力 Actual Stress at Critical location,SN Analysis,Stress Life A
5、nalysis,Lec 16-6,沃勒疲劳实验机 Wohlers Railway Component Test Rig (1852 to 1870),Stress Life Analysis,Lec 16-7,Some of Wohlers data for rotating bending tests,应力幅值,光滑试样,缺口试样,Stress Life Analysis,Lec 16-8,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 (
6、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, fo
7、r sub-assemblies, or for complete structures,Stress Life Analysis,Lec 16-9,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
8、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,Stress Life Analysis,Lec 16-10,S-N Curves,Stress Life Analysis,Lec 16-11,S-N Curves,结构钢和钛合金钢的S-N
9、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 bou
10、ndary, 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,Material S-N Curves,Log(Stress),Log(Life),Steel or Ti,Al alloy or steel in seawater,铝合金或钢在海水中,S
11、tress Life Analysis,Lec 16-13,S-N曲线的离散性 Scatter in S-N curves,Stress Life Analysis,Lec 16-14,部件S-N曲线 Component S-N curves,在某些情形如结构或特征存在点焊等需要修正材料S-N曲线（但通常是较困难的） For some components or features, especially structural joints such as welds, there are so many things modifying the behaviour of the base ma
12、terial that there is little point in applying corrections to a material S-N curve 在此情形下最好采用适用于该部件或特征的名义应力-寿命曲线 In cases like this it is best to use a nominal stress-life curve which applies particularly to that component or feature,部件S-N曲线可以使用远端的名义应力进行疲劳分析Component S-N Curves may use remote or nomin
13、al stress,P,P,Nominal Stress,P,_,A,A,CLASS F WELD DETAIL (BS7608),Stress Life Analysis,Lec 16-16,BS7608焊接件的S-N曲线 BS7608 Weld S-N Curves,静应力强度,清洁空气下，常幅值载荷的情况,不同幅值载荷作用下的疲劳曲线,Stress Life Analysis,Lec 16-17,S-N Method - Similitude,The life of this . . . . . . . . . . . . . . . . is the same as the life
14、of this . . . . . if both are subject to the same nominal stress,如果他们承受相同的名义应力，那么他们的疲劳寿命一致。,Stress Life Analysis,Lec 16-18,S-N方法假定零件或部件的寿命在同一名义应力下与测试试样一致 The S-N method assumes that the life of a component or structure is the same as that of a laboratory test specimen if both are subject to the same
15、 nominal stresses. 如果在结构中存在与实验条件不一致，需要进行修正（如平均应力、环境、表面处理等因素） If the conditions in the test are different to those in the structure, similitude breaks down, and we need to make corrections for factors such as mean stress, environment, surface finish, etc.,S-N Method - Similitude,Stress Life Analysis,
16、Lec 16-19,对变幅载荷的处理-迈因纳线性累积损伤法则和雨流记数 Handling Variable Amplitude Loads - Miners Rule and Rainflow Counting,Stress Life Analysis,Lec 16-20,名义应力有限寿命设计法是一种较早使用的有限寿命设计法，它是无限寿命设计法的直接发展，两者的基本设计参数都是名义应力。其设计思想也大体相似，都是从材料的S-N曲线出发，再考虑各种影响因素，得到零件的S-N曲线，并根据零件的S-N曲线进行疲劳设计。 所不同的是，无限寿命设计法使用的是S-N曲线的右段-水平部分，亦即疲劳极限；而有限寿命设计法使用的是S-N曲线的左段-斜线部分，亦即有限寿命部分。而有限寿命设计的设计应力一般都高于疲劳极限，这时就不能只考虑最高应力，而需要按照一定的累积损伤理论估算总的疲劳损伤。,名义应力有限寿命设计,Stress Life Analysis,Lec 16-21
