1、Designation: D 6048 07Standard Practice forStress Relaxation Testing of Raw Rubber, UnvulcanizedRubber Compounds, and Thermoplastic Elastomers1This standard is issued under the fixed designation D 6048; the number immediately following the designation indicates the year oforiginal adoption or, in th
2、e case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.INTRODUCTIONThere are a number of techniques used to identify the processability of raw rubber, u
3、nvulcanizedrubber compounds, and thermoplastic elastomers (rubber and rubberlike materials). Most measure asingle averaged value of non-Newtonian viscosity with some doing so after a period of steady shear.These techniques may not provide sufficient information with regards to the processability of
4、aselected material, as they: (1) fail to provide a measure of the viscoelastic behavior, and (2) destroystructure of the selected material through the steady shearing.Stress relaxation testing provides a measure of the viscoelastic response of a material over a periodof time without destroying the s
5、tructure of the sample. In such testing both the instantaneous andtime-dependent response to an applied deformation are measured. The information from this singleexperiment can then be used to examine a materials reaction to various different process conditions.There are several different techniques
6、 for measuring stress relaxation properties of rubber andrubberlike materials. This practice serves to provide the reader with some background informationabout those techniques in terms of the theory of testing and the interpretation of results. Manyconcepts are put forward that are not discussed in
7、 depth, for to do so would require a textbook, nota practice. The reader is therefore encouraged to consult the identified references.1. Scope1.1 This practice covers several different techniques fordetermining the stress relaxation characteristics of rubber andrubberlike materials and for the possi
8、ble interconversion of thisstress relaxation information into dynamic mechanical proper-ties.1.2 The techniques are intended for materials having stressrelaxation moduli in the range of 103to 108Pa (0.1 to1.5 3 104psi) and for test temperatures from 23 to 225C (73to 437F). Not all measuring apparatu
9、s may be able toaccommodate the entire ranges. These techniques are alsointended for measurement of materials in their rubbery ormolten states, or both.1.3 Differences in results will be found among the tech-niques. Because of these differences, the test report needs toinclude the technique and the
10、conditions of the test. Thisinformation will allow for resolving any issues pertaining tothe test measurements.1.4 The generalized descriptions of apparatus are based onthe measurement of force as a function of time. Mathematicaltreatment of that relationship produces information that can berepresen
11、tative of material properties. Mathematical transfor-mation of the force measurements will first yield stressrelaxation moduli with subsequent transformation producingdynamic mechanical properties.1.5 The values stated in SI units are to be regarded as thestandard. The values given in parentheses ar
12、e provided forinformation only.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitatio
13、ns prior to use.2. Referenced Documents2.1 ASTM Standards:2D 1566 Terminology Relating to RubberD 1646 Test Methods for RubberViscosity, Stress Relax-ation, and Pre-Vulcanization Characteristics (Mooney Vis-cometer)1This practice is under the jurisdiction of Committee D11 on Rubber and is thedirect
14、responsibility of Subcommittee D11.12 on Processability Tests.Current edition approved May 1, 2007. Published June 2007. Originallyapproved in 1996. Last previous edition approved in 2002 as D 6048 02.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Servi
15、ce at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.D 5992 Guide for Dynamic Testing of Vulcanize
16、d Rubberand Rubber-Like Materials Using Vibratory MethodsE 328 Test Methods for Stress Relaxation for Materials andStructures3. Terminology3.1 DefinitionsEach of the following terms applies tolinear viscoelastic behavior. The terms have been arrangedlogically so that simple basic terms are defined f
17、irst and arethen used to define more complex terms that either contain thesimple terms or depend upon the simple terms for comprehen-sion. Some terms in this section have been excerpted fromTerminology D 1566, Guide D 5992, or Test Methods E 328.3.1.1 stress, nthe force per unit area that acts on th
18、e faceof a cubical element that is perpendicular to the force.3.1.2 strain, nthe change in the size or shape of a bodydue to force when referred to its original size or shape.3.1.3 initial stress, nthe stress occurring in a specimenimmediately upon achieving the given input strain.3.1.4 damping, na
19、material property that results in theconversion of mechanical energy to heat when the material issubjected to deformation.3.1.5 modulus, na material property that is the ratio ofstress to strain.3.1.6 stress relaxation, nthe time-dependent decrease instress under constant strain at constant temperat
20、ure.3.1.7 relaxation curve, na plot of the force, stress, orrelaxation modulus as a function of time.3.1.8 relaxation time, nthe time required for a body torelax after deformation (synonym: time constant).3.1.9 relaxation spectrum, nthe response of a group ofbodies to deformation, each having a uniq
21、ue relaxation time;normally defined as a mathematical function, an integral overwhich describes linear viscoelastic behavior.3.1.10 elastic, adjthe tendency for a material to return tooriginal shape after release of stress; specifically, descriptive ofthat component of the complex force in phase wit
22、h dynamicdeflection that does not convert mechanical energy to heat andthat can return energy to an oscillating mass-spring system(synonym: storage).3.1.11 loss, adjdescriptive of that fraction of energyabsorbed by a strained object that is converted to heat, that is,that which is hysteretic.3.1.12
23、viscous, adjdescriptive of that type of energy lossin which the damping component of stress is proportional tothe rate of deformation.3.1.13 compression, nthe type of strain parallel to thedirection of displacement that results in a decrease in the heightof the strained body.3.1.14 shear, nthe type
24、of strain that is perpendicular tothe direction of displacement.3.1.15 shape factor, nfor disks or cylinders to be tested incompression, the ratio of the diameter of the specimen to itsheight.3.1.16 compression modulus, na material property ofresistance to change in height when subjected to a compre
25、ssiveforce; a ratio of compressive stress to compressive strain.3.1.17 viscoelasticity, na unique response to deformationcharacterized by both the storage and loss of energy; theresponse is dependent on time and temperature.3.1.18 linear viscoelasticity, na unique response to defor-mation characteri
26、zed by both the storage and loss of energywhere modulus is independent of strain.3.1.19 reptation, nthe mechanism by which the motion ofa polymer molecule is restricted by the proximity of segmentsof other polymer molecules.3.2 Symbols:Gshear modulus.G(t)shear relaxation modulus as a function of tim
27、e.P(t)pressure (in compression) as a function of time.tcharacteristic relaxation time.t0zero time.sstress.eextensional strain.gshear strain.grate of shear strain.Dcompressive strain (in bulk).Drate of compressive strain (in bulk).trunning time.Kespring constant (elastic).Kvdashpot constant (viscous)
28、.3.3 Symbols:Symbols for Dynamic Properties: G*complex shear modulus.E*complex extensional modulus in either tension or com-pression.G8 = G*cosdshear storage modulus; the in-phase componentof G*.G9 = G*sindshear loss modulus; the component of G* out ofphase by 90.E8 = E*cosdextensional storage modul
29、us, the in-phase com-ponent of E*.E9 = E*sindshear loss modulus; the component of E* out ofphase by 90.hviscosity.h8dynamic viscosity.vangular frequency.4. Summary of Practice4.1 The methods covered in this practice are divided intofour general categories.4.1.1 Shear stress relaxation after sudden s
30、tep strain,4.1.2 Compressive stress relaxation after sudden step strain,4.1.3 Shear stress relaxation after cessation of steady shearflow, and4.1.4 Shear stress relaxation after sudden stress application.4.2 Descriptions of these methods are given in Section 8.Sufficient mathematical formulae are al
31、so provided to indicatehow results are calculated.D60480725. Significance and Use5.1 The processing behavior (processability) of rubber orrubberlike materials is closely related to their viscoelasticproperties. The viscoelastic properties as well as the mechani-cal properties are related to the poly
32、meric, including macro-molecular and micromolecular structure. Therefore, a determi-nation of the viscoelasticity of a material will provideinformation to predict processing and service performance.5.2 Stress relaxation testing provides a methodology forinvestigating the viscoelasticity of rubber or
33、 rubberlike mate-rials. Certain structural characteristics that have been demon-strated to be evaluated by this test method are: (1) averagemolecular weight, (2) molecular weight distribution, (3) lin-earity or chain branching, (4) gel content, and (5) monomerratio.5.3 This practice is intended to d
34、escribe various methods ofmeasuring the stress relaxation properties of raw rubber,unvulcanized rubber compounds, or thermoplastic elastomersfor determining the processability of these materials throughviscoelastic measurements. Factory performance characteris-tics that this methodology may correlat
35、e with include die swellor shrinkage, extrusion rate, mill banding, carbon black incor-poration time, and mold flow.6. Hazards6.1 There are no hazards inherent to the techniques to bedescribed. There is no use of reagents or hazardous materials.The design of the various different test apparatus may
36、havecreated possible pinch points; caution shall be exercised and aguard shall be provided for these sites.6.2 Normal safety precautions and good laboratory practiceshould be observed when using any equipment. This isespecially true when performing tests at elevated temperatureswhere electrical heat
37、ers are used.7. Theory of Stress Relaxation7.1 Mechanical Models of Viscoelasticity:7.1.1 Polymeric materials exhibit both elasticity and viscousresistance to deformation. The materials can retain the recov-erable (elastic) strain energy partially, but they also dissipateenergy if the deformation is
38、 maintained. Mechanical analoguesof a viscoelastic solid (Fig. 1) and a viscoelastic liquid (Fig. 2)help identify this behavior.7.1.2 In Fig. 1, a dashpot is connected in parallel with aspring. This is known as a Voigt element. If deformed, the forcein the spring is assumed to be proportional to the
39、 elongation ofthe assembly, and the force in the dashpot is assumed to beproportional to the rate of elongation of the assembly. With noforce acting upon it, the assembly will return to its referencestate that is dictated by the rest length of the spring.7.1.3 In this example, if a sudden tensile fo
40、rce is applied,some of the work performed on the assembly is dissipated inthe dashpot while the remainder is stored in the spring. Theapplied force is analogous to the deforming stress and theelongation is analogous to the resulting strain. The viscousresistance to deformation represented by the das
41、hpot intro-duces time dependency to the response of the assembly wherethis dependency is dictated by the spring and dashpot con-stants. The assembly cannot respond instantaneously tochanges in stress; this indicates that the viscoelastic solid has atime dependency.7.1.4 In Fig. 2, a dashpot is conne
42、cted in series with aspring. This is called a Maxwell element. Unlike the Voigtelement, there is no dictated reference state so that theassembly will deform indefinitely under the influence of anapplied force, assuming that the dashpot has infinite length, acharacteristic of a viscoelastic fluid.7.1
43、.5 In this example, if a sudden tensile force is applied, itis the same in both the spring and the dashpot. Some of thework performed on the assembly is dissipated in the dashpotwhile the remainder is stored in the spring. The total displace-ment experienced by the element is the sum of the displace
44、-ments of the spring and the dashpot. As with the Voigt elementrepresenting a viscoelastic solid, the Maxwell element repre-sents a combination of viscous and elastic properties. Thisindicates that the viscoelastic liquid is also time-dependent andhas a characteristic time constant. However, as the
45、timeconstant becomes smaller and smaller, the elastic quality of theliquid becomes less and less and appears to behave more likea purely viscous material.7.1.6 The response of polymers to changes in stress or strainis actually a combination of multiple elements of both me-chanical models; one such e
46、xample is illustrated in Fig. 3. Theresponse is always time-dependent and involves both theelastic storage of energy and viscous loss.7.2 Molecular Behavior:NOTEF = force,Ke= spring constant (“e” denotes “elastic”), andKv= dashpot constant (“v” denotes “viscous”).FIG. 1 Voigt Element Representing th
47、e Response of aViscoelastic SolidD60480737.2.1 In the rubbery or molten state, the polymer moleculesare flexible and can be considered like entangled coils. Undersmall deformation, the coils change shape. The coils will returnto their original shape when the deformation is removed. Thatreturn is ret
48、arded by molecular friction as the coils overlap; thedensity of the molecules in the space that they occupy at anygiven instant is much less than the observed density. Thisoverlapping or entanglement strongly affects the motion ofneighboring molecules.7.2.2 The mechanics of these coil models have be
49、en pro-posed (1)3and modified (2) but only explain polymer behaviorin dilute solution where there is little or no interaction betweenindividual polymer coils. A theory (3) of reptation betterconsiders the interaction between polymer molecules. Thistheory places the molecule within a tube of known diameterand length. Under deformation, and at very short times, theonly reaction that occurs within an entangled molecule is theredistribution of segments between the constraining points ofentanglement. Once the maximum strain energy that ca
