残余应力分析.pdf

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1、A world of solutionsAUTOMATED RESIDUALSTRESS ANALYSISAN INTRODUCTIONX-ray Diffraction Residual Stress MeasurementAN INTRODUCTIONWhat is Residual Stress?Definition Residual stress is defined as“the stress resident inside a component or structure after all applied forces have been removed”.Compressive

2、 residual stress acts by pushing the material together,while tensile residual stress pulls the material apart.Mathematically,compressive stress is negative and tensile stress is positive.Stresses can also be characterized as normal stresses that act perpendicular to the face of a material and shear

3、stresses that act parallel to the face of a material.There are a total of 6 independent stresses at any point inside a material(3 normal and 3 shear stresses).Units of Stress SI unit for stress is the Mega Pascal(MPa).US Customary unit for stress is kilo pound-force per square inch(ksi).6.895 MPa=1

4、ksiWhat Causes Residual Stress?Residual stresses are generated,upon equilibrium of material,after plastic deformation that is caused by applied mechanical loads,thermal loads or phase changes.Mechanical and thermal processes applied to a component during service may also alter its residual stress st

5、ate.What is The Total Stress in a Component?The total stress experienced by the material at a given location within a component is equal to the residual stress plus the applied stress.TOTAL STRESS =RESIDUAL STRESS +APPLIED STRESSIf a material with a residual stress of a-400 MPa is subjected to an ap

6、plied load of+500 MPa.The total stress experienced by the material is the summation of the two stresses,or+100 MPa.Therefore,knowledge of the residual stress state is important to determine the actual loads experienced by a component.In general,compressive residual stress in the surface of a compone

7、nt is beneficial.It tends to increase fatigue strength and fatigue life,slow crack propagation,and increase resistance to environmentally assisted cracking such as stress corrosion cracking and hydrogen induced cracking.Tensile residual stress in the surface of the component is generally undesirable

8、 as it decreases fatigue strength and fatigue life,increases crack propagation and lowers resistance to environmentally assisted cracking.Types of Residual StressResidual stresses can be characterized by the scale at which they exist within a material.Stresses that occur over long distances within a

9、 material are referred to as macro-stresses.Stresses that exist only locally(either between grains or inside a grain)are called micro-stresses.The total residual stress at a given location inside a material is the sum of all 3 types of stresses.Type I Stresses:Macro-stresses occurring over distances

10、 that involve many grains within a material.Type II Stresses:Micro-stresses caused by differences in the microstructure of a material and occur over distances comparable to the size of the grain in the material.Can occur in single-phase materials due to the anisotropic behaviour of individual grains

11、,or can occur in multi-phase material due to the presence of different phases.Type III Stresses:Exist inside a grain as a result of crystal imperfections within the grain.Tensile residual stressopens crack and increases crack propagationCompressive residual stress closes crack and slows crack propag

12、ationStress components inside a materialX-ray Diffraction Residual Stress MeasurementImportance of Residual StressResidual stress affects:Low Cycle and High Cycle Fatigue performance Distortion Peen forming(controlled distortion)Fretting Stress Corrosion Cracking(SCC)and Hydrogen Initiated Cracking(

13、HIC)Crack initiation and propagation.(Damage Tolerance)Residual Stress distribution is rarely as assumed in FE models and or Fracture Mechanics;real data is necessary to improve the accuracy and effectiveness of the modeling.The Benefits of Measuring and Monitoring Residual Stresses Optimize process

14、 parameters,such as measuring the effectiveness of peening on a part at critical locations.Provide a quantitative metric to enable specifications and Go/No-Go decisions.Improve product quality,substantiate supplier quality,Engineering Source Approval(ESA)Improve safety and reduce catastrophic failur

15、es.Extend component or structure life by ensuring sufficient compressive residual stress is present.Validate repair area has been“restored”to original specifications.More accurate replacement part requirements by tracking residual stress degradation;thus,enabling retirement for quantitative cause.Re

16、sidual stress information can improve the P.O.D.of other NDE techniques.Residual Stress ManagementThere are many ways to introduce and manage residual stresses,among them:cold working techniques,such as shot peening,laser shock peening,ultrasonic peening,planishing,hammering,burnishing,low plasticit

17、y burnishing,rolling,coining and split sleeve expanding,can generate compressive residual stresses.Hot working techniques,such as heat treatment,controlled cooling and localized heating are often used to minimize or reduce the magnitude of residual stress in components.Coupons or Almen strips are of

18、ten used to control the process;unfortunately,they do not provide information about the resultant residual stress in the actual component.Peening the Almen strip for example gives details about the peening process,but not about resultant residual stress on the part.Additionally,the Almen strip does

19、not account for upstream processing,complex geometry or phase changes.X-Ray Stress Measurement MethodsX-ray diffraction can be used to measure residual stress using the distance between crystallographic planes,i.e.,d-spacing,as a strain gage.When the material is in tension,the d-spacing increases an

20、d,when under compression the d-spacing decreases.Stresses can be determined from the measured d-spacings.X-rays diffract from crystalline materials at known angles 2 according to Braggs Law:n=2dsinwhere:n=order of diffraction =wavelength of the x-ray beam d=distance between lattice planes inside the

21、 material =angle of the diffracted beamincidentx-ray beamdiffractionconePolycrystalline Samplediffraction peakson detectorsd=0 0d+dChanges in stress cause changes in atomic lattice spacing“d”and an angular shift of the diffraction peak.X-ray Diffraction Residual Stress MeasurementX-ray tubeLXRD Labo

22、ratory Residual Stress Measurement S1-800-965-8378USAProto Manufacturing Inc.1980 East Michigan AvenueYpsilanti,Michigan48198-6010Tel(313)965-2900CANADAProto Manufacturing Ltd.2175 Solar CrescentOldcastle,Ontario N0R 1L0Tel(519)737-6330 Copyright 2006-Rev.A061101 X-RAY DIFFRACTION RESIDUAL STRESS ME

23、ASUREMENTWhy X-ray Diffraction is the Preferred Method to Characterize Residual Stress A number of different methods,are available to measure residual stress;however,only a few are actually quantitative.Only x-ray diffraction has the appropriate spatial and volumetric resolution to fully and adequat

24、ely characterize the residual stress distributions often found in the areas of critical importance.Works on all crystalline materials,such as metals and ceramics High spatial resolution Capable of measuring both surface and sub-surface stresses Very fast.Can be used in line Nondestructive Quantitati

25、ve results.Easily validated Lowest cost per measurement Can measure dislocation density Capable of measuring specific phases PortableSurface and Subsurface Residual Stress1.Fatigue cracks normally initiate at the surface of a component,therefore,surface residual stresses are crucial to the potential

26、 for crack initiation and initial propagation.Subsurface crack initiation only occurs if a flaw or inclusion is present subsurface.2.Surface stresses can also strongly influence subsurface crack propagation.3.Surface and Subsurface stress gradients are required to fully characterize the effects of s

27、hot peening,machining or other surface modification processes.4.In specific cases where machining and other surface modifications are adequately controlled,surface residual stress may be used as an indicator of process quality.Residual Stress and Damage Tolerance In fracture mechanics design and lif

28、ing paradigms,the assumption of cracks already existing can lead to overly heavy designs(as opposed to LCF based approaches)as well as an undue emphasis on stress at depth without consideration of surface and near surface stress distributions.Practical experience and accumulated data have shown surf

29、ace,near surface and sub-surface stresses are very important to the growth of fatigue cracks and all need to be considered with the assumption of existing cracks.Standards and Guidelines SAE J784a RS measurement ASTM E915 RS measurement ASTM E1426 4 pt bend XEC ASM Handbook Vol.11 General guidelinesResidual Stress vs.Depth-1200-1000-800-600-400-20002000.00.20.40.60.81.0Depth(mm)Residual Stress(MPa)low intensity peenhigh intensity peenMeasuring Residual Stress on an Airframe with an iXRD Portable SystemResidual Stress vs.Depth for Different Peening Conditions