疲劳分析

Copyright © 1999 MacNeal Schwendler Corporation
Lec 16-6
Stress Life Analysis
PAT 318 Course Notes
应力幅值
Introduction to MSC/FATIGUE
Stress Amplitude
光滑试样
Unnotched Shaft
• 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)
临界位置的真实应力
Actual Stress at Critical location
• 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
Stress Life Analysis
PAT 318 Course Notes
Introduction to MSC/FATIGUE
S-N曲线的离散性
Scatter in S-N curves
Copyright © 1999 MacNeal Schwendler Corporation
Lec 16-13
• 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。
Introduction to MSC/FATIGUE
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
PAT 318 Course Notes
Introduction to MSC/FATIGUE
Stress-Life (S-N) Theory
Copyright © 1999 MacNeal Schwendler Corporation
Lec 16-1
Stress Life Analysis
PAT 318 Course Notes
测试的名义应力
Measured nominal stresses
Copyright © 1999 MacNeal Schwendler Corporation
Lec 16-4
Stress Life Analysis
PAT 318 Course Notes
Introduction to MSC/FATIGUE
Notched Shaft
Log (fatigue life)
缺口试样
Some of Wohler‟s data for rotating bending tests
Copyright © 1999 MacNeal Schwendler Corporation
Lec 16-7
Stress Life Analysis
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
• 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
PAT 318 Course Notes
Introduction to MSC/FATIGUE
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.
Introduction to MSC/FATIGUE
Material S-N Curves
Log(Stress)
Steel or Ti
Al alloy or steel in seawater
铝合金或钢在海水中
Log(Life)
Copyright © 1999 MacNeal Schwendler Corporation
• 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.
• 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 subassemblies, or for complete structures
Copyright © 1999 MacNeal Schwendler Corporation
Copyright © 1999 MacNeal Schwendler Corporation
Lec 16-2
Stress Life Analysis
PAT 318 Course Notes
Introduction to MSC/FATIGUE
Some Definitions
应力幅值
平均应力
应力比R=
Copyright © 1999 MacNeal Schwendler Corporation
Lec 16-3
Stress Life Analysis
PAT 318 Course Notes
Introduction to MSC/FATIGUE
SN Analysis
• 输入应力循环
Input is cycles of STRESS
Copyright © 1999 MacNeal Schwendler Corporation
Lec 16-14
Stresቤተ መጻሕፍቲ ባይዱ Life Analysis
PAT 318 Course Notes
Introduction to MSC/FATIGUE
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
Copyright © 1999 MacNeal Schwendler Corporation
PAT 318 Course Notes
Introduction to MSC/FATIGUE
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
Stress Life Analysis
PAT 318 Course Notes
Introduction to MSC/FATIGUE
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 material that there is little point in applying corrections to a material S-N curve
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
• 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) though local stresses at the critical location will be plastic
• 铝合金无明确的水平区段
Aluminum alloys do not seem to exhibit no such limit
Copyright © 1999 MacNeal Schwendler Corporation
Lec 16-11
Stress Life Analysis
PAT 318 Course Notes
Lec 16-8
Stress Life Analysis
PAT 318 Course Notes
Introduction to MSC/FATIGUE
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
S-N Curve
Copyright © 1999 MacNeal Schwendler Corporation
Lec 16-5
Stress Life Analysis
PAT 318 Course Notes
Introduction to MSC/FATIGUE
沃勒疲劳实验机
Wohler‘s Railway Component Test Rig (1852 to 1870)
Lec 16-9
Stress Life Analysis
PAT 318 Course Notes
Introduction to MSC/FATIGUE
S-N Curves
Copyright © 1999 MacNeal Schwendler Corporation
Lec 16-10
Stress Life Analysis