文本說(shuō)明案例rubr-l03testing

1、Lesson content:Modes of DeformationUniaxial tensionPlanar tensionUniaxial compressionEquibiaxial tensionConfined compressionLoading HistoryTesting at temperatureTest SpecimensTest Data GuidelinesTesting for Time-Dependent PropertiesWorkshop PreliminariesWorkshop 1: Axial Deflection of a Rubber Bushi

2、ng (IA)Workshop 1: Axial Deflection of a Rubber Bushing (KW)Lesson 3: Mechanical Testing2 hoursBoth interactive (IA) and keywords (KW) versions of the workshop are provided. Complete only one.Modes of Deformation (1/18)What do we mean by modes of deformation and why will we talk so much about them?I

3、nitially (1930s 1950s) all focus was on uniaxial tests and fitting coefficients to uniaxial data.Researchers observed that these uniaxial fits (for phenomenological models) did not correlate with data taken from other types of tests.Need to perform other tests and use this data for fitting too.These

4、 other tests are from strain states other than uniaxial thus the phrase “modes of deformation.”Modes of Deformation (2/18)Strain states, testing and curve fittingNeeds:Simple test to perform, simple specimen to prepare.Homogeneous state of strain/stress in the gage length. Analytical expressions for

5、 the stress as a function of strain for the strain-energy density functions (for curve fitting).In general, one wants to perform several types of tests (modes of deformations) and curve fit a material model using multiple test data sets. Remember we are defining a function. The more information we g

6、ather, the better the definition will be.Modes of Deformation (3/18)The common tests for solid rubberUniaxial tension (simple tension)Uniaxial compressionPlanar tension (shear)Equibiaxial tensionConfined compressionNote order of stiffness:Equibiaxial Planar Uniaxial Modes of Deformation (4/18)Uniaxi

7、al tensionWhat is uniaxial tension?Uniaxial loadingFree of lateral constraintAlso called simple tensionModes of Deformation (5/18)Uniaxial tension (contd)Measure strain only in the region where a uniform state of strain existsDo not use crosshead travel to measure strain! Use non-contact measurments

8、:Laser ExtensometerVideo ExtensometerModes of Deformation (6/18)Uniaxial tension (contd)For an pressible material, the uniaxial tension test can be plotted in the l1-l2 plane.The test gives us information on the U surface above this particular curve.l1 = uniaxial stretchl2 = l1-1/2 (l3 = l2)Modes of

9、 Deformation (7/18)Planar tensionWhat is planar tension?Uniaxial loadingPerfect lateral constraintAll thinning occurs in one directionEquivalent to pure shearModes of Deformation (8/18)Planar tension (contd)Strain measurement is particularly criticalSome material flows from the gripsThe effective he

10、ight is smaller than starting height so 10:1 width:height is neededModes of Deformation (9/18)Planar tension (contd)For an pressible material, the planar tension test can be plotted in the l1-l2 plane. The test gives us more information on the U surface.l1 = planar tension stretchl2 = l1-1 (l3 = 1 )

11、Note that the curves are close together.Modes of Deformation (10/18)Uniaxial compressionRequirements:Uniaxial loadingNo lateral constraintAlso called simple compressionModes of Deformation (11/18)Uniaxial compression (contd)It is experimentally difficult to minimize lateral constraint due to frictio

12、n at the specimen-platen interfaceFriction effects alter the stress-strain curvesThe friction is not known and cannot be accurately correctedEven very small friction levels have an effect at very small strainsEasy to prepare specimen, but difficult to achieve friction-free test, thus difficult to ac

13、hieve state of pure compressionPreferable to perform equibiaxial tension test instead!Uniaxial compression tests can be very useful for foams, especially when the Poissons ratio is effectively 0.Modes of Deformation (12/18)Equibiaxial tensionWhy?Same strain state as compressionCannot do pure compres

14、sionCan do pure equibiaxialModes of Deformation (13/18)Equibiaxial tension (contd)Analysis of the specimen justifies geometryTabs may fail for TPE materialsMembrane inflation may be used instead (good for fatigue testing)Equibiaxial tension (contd)Modes of Deformation (14/18)Modes of Deformation (15

15、/18)Equibiaxial tension (contd)For an pressible material, the equibiaxial tension test can be plotted in the l1-l2 plane. The test gives us yet more information on the U surface which is above.l1 = equibiaxial tension stretchl2 = l1 (l3 = 1/l12 )Note that curves are far apart.Confined compressionDir

16、ect measure of the stress required to change the volume of an elastomerRequires resolute displacement measurement at the fixtureValid for nearly pressible materials only!Modes of Deformation (16/18)Modes of Deformation (17/18)Confined compression (contd) We make use of the fact that for solid rubber

17、 the bulk modulus is much higher than the shear modulus. At very low stress the material shears to fill the rigid container, and the response is dominated by the bulk (volumetric) properties of the material. This test makes no sense for foam materials. True volumetric testing of foams is possible bu

18、t rarely done.Modes of Deformation (18/18)Confined compression (contd) Initial slope = bulk modulusHighly constrained applications require an accurate measure of the volumetric response.Some compressibility can be numerically beneficial in Abaqus/Standard even if there is low confinement.Abaqus/Expl

19、icit cannot handle an pressible material.Loading History (1/5)Initial monotonic loading, typical of data from existing standardsLoading History (2/5)Repeated loading produces complicated stress/strain curve.Loading History (3/5)ConsistencyBe careful that the real component and the test specimen shar

20、e the same load history and pre-conditioning.Is the analysis for a 1st load condition (installation for instance)?Then test the virgin material under monotonic loading.Is the analysis for a repeated use condition? Then test the specimen after pre-conditioning (repeated loading).Be careful to test at

21、 consistent strain rates for each deformation mode.Test at strain rates consistent with the real component use situation.Loading History (4/5)Some common elastomers exhibit dramatic strain amplitude and cycling effects at moderate strain levelsConclusions:Test to realistic strain levelsUse applicati

22、on-specific loadings to generate material dataLoading History (5/5)Testing at temperatureElastomers have temperature-dependent material properties.Conclusion: Test materials at the temperature required by the simulation.Environment chambers are used to bring specimen to the required temperature.Test

23、ing at temperature is more expensive.Testing may have to be done at a range of temperatures rather than just at the extreme temperatures if the extremes are far apart.Test Specimens (1/2)Verify that the tested material is the same as the partProcessingColor Cure History All are the same compound!Tes

24、t Specimens (2/2)Consistent within the experimental data setCut all specimens from the same slabBiaxialTensileVolumetricPlanar tensionTest Data Guidelines (1/6)Test availabilityThe availability of sufficient and accurate test data is the most significant factor in choosing a rubber material model.Th

25、erefore, collect data from as many modes of deformation as possible. Uniaxial tension Planar tension Equibiaxial tensionShear (foams)If compressibility is important, volumetric data must be collectedFor example, highly confined applications such as o-rings under significant compression. Poissons rat

26、io cannot be measured directly for nearly pressible materials. It can be measured for foams.Test Data Guidelines (2/6)DamageIt is not mon for elastomers to exhibit elasticity damage and hysteresis during the cyclic loading. As shown in the figure, a few cycles of loading result in a decrease in stif

27、fnessthis is termed Mullins effect.Abaqus provides a material model to capture Mullins effect.To calibrate the model, supply data from several loading cycles for analyses of components that are in repeated use conditions.If you do not wish to model Mullins effect, pre-condition the test specimen pri

28、or to testing.Test virgin specimens for a 1st-use component analysis (installation).Test Data Guidelines (3/6)Test data variationsThe properties of elastomers are known to change from batch to batch. All tests done to characterize a given material should be performed on the same batch. Taking all th

29、e test specimens from the same physical slab of material is highly mended.It may be necessary to validate the test specimen slab against the real component to assure they have similar cure history. Cutting small uniaxial specimens from real components can be used for this validation.Test at the oper

30、ating temperatures expected in the application.It is best to obtain data from more that one type of test. Experience shows that data from more than one mode of deformation (strain state) should be used to achieve the most accurate material model.Test Data Guidelines (4/6)Test redundancies: solid rub

31、berFor fully pressible materials the superposition of a hydrostatic pressure does not alter the deformation mode. As a result, some apparently different types of tests are equivalent and provide redundant information, such as:Uniaxial tension Equibiaxial compressionUniaxial compression Equibiaxial t

32、ensionPlanar Tension Planar CompressionUniaxial tension and uniaxial compression provide independent data.Test Data Guidelines (5/6)Test redundancies: solid rubberTest Data Guidelines (6/6)Other guidelinesNominal (engineering) stress and strain data are required for the deviatoric (shear) test data

33、input in Abaqus. Both tension and compression data are allowed. Compressive stresses and strains are given as negative values.Always use more experimental data points than unknown coefficientsVolumetric curve fit requires pressure-volume ratio test data.Testing for Time-Dependent Properties (1/4)Ela

34、stomers have time-dependent material behavior. Hyperelasticity only approximates their mechanical response.Additional tests are required to define the time-dependent, or viscoelastic, part of the material behavior.Creep test: constant stress with time-varying strainStress relaxation test: constant s

35、train with time-varying stressDynamic testing: time-varying loading (usually sinusoidal)We will discuss using test data to define viscoelasticity in Lectures 6, 7, and 8.Testing for Time-Dependent Properties (2/4)Creep TestingIn a simple creep test, the specimen is loaded “instantaneously” with an a

36、pplied stress.The strain is recorded over some time period.The creep test is often performed on the uniaxial tension specimen. True stress must be constant which means load may have to be adjusted in the case of finite creep deformations.Normalized uniaxial tensile creep test data is equivalent to n

37、ormalized shear creep test data for nearly pressible materials only.Testing for Time-Dependent Properties (3/4)Stress-Relaxation TestingIn a simple stress relaxation test, the specimen is strained “instantaneously” to some level and held fixed.The stress is recorded over some time period.The stress

38、relaxation test is often performed on the uniaxial tension specimen.In theory, normalized uniaxial stress-relaxation test data is equivalent to normalized shear stress-relaxation test data for all materials.Testing for Time-Dependent Properties (4/4)Dynamic TestingIn a dynamic test, the specimen is

39、subjected to a time-varying load.Specimens can be loaded uniaxially or in shear.DMA (Dynamic Mechanical Analysis) testing is common. An oscillatory torsion is applied to the specimen.Sinusoidal loading of a uniaxial specimen is also common.Specimen must be pre-loaded to prevent buckling.Abaqus allow

40、s viscoelastic definitions which depend on the pre-load.High-frequency material properties can be measured with wave propagation methods.ObjectivesWhen you complete this exercise you will be able to extract all the files necessary to complete the demonstrations and workshops associated with this cou

41、rseWorkshop file setup (option 1: installation via plug-in)From the main menu bar, select Plug-insTools Install Courses.In the Install Courses dialog box:Specify the directory to which the files will be written.Chooses the course(s) for which the files will be extracted.Click OK.Workshop Preliminari

42、es (1/2)5 minutesWorkshop file setup (option 2: manual installation)Find out where the Abaqus release is installed by typingabqxxx whereamiwhere abqxxx is the name of the Abaqus execution procedure on your system. It can be defined to have a different name. For example, the command for the 6.131 release might be aliased to abq6131.This command will give the full path to the direct