1、Designation: D790 152D790 17Standard Test Methods forFlexural Properties of Unreinforced and Reinforced Plasticsand Electrical Insulating Materials1This standard is issued under the fixed designation D790; the number immediately following the designation indicates the year oforiginal adoption or, in
2、 the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the U.S. Department of Defense.1 NOTEEdi
3、torially corrected 4.3 in January 2016.2 NOTEEditorial corrections were made in February 2016.1. Scope*1.1 These test methods are used to determine the flexural properties of unreinforced and reinforced plastics, including highmodulus composites and electrical insulating materials utilizing a three-
4、point loading system to apply a load to a simply supportedbeam (specimen). The method is generally applicable to both rigid and semi-rigid materials, but flexural strength cannot bedetermined for those materials that do not break or yield in the outer surface of the test specimen within the 5.0 % st
5、rain limit.1.2 Test specimens of rectangular cross section are injection molded or, cut from molded or extruded sheets or plates, or cutfrom molded or extruded shapes. Specimens must be solid and uniformly rectangular. The specimen rests on two supports and isloaded by means of a loading nose midway
6、 between the supports.1.3 Measure deflection in one of two ways; using crosshead position or a deflectometer. Please note that studies have shownthat deflection data obtained with a deflectometer will differ from data obtained using crosshead position. The method of deflectionmeasurement shall be re
7、ported.NOTE 1Requirements for quality control in production environments are usually met by measuring deflection using crosshead position. However,more accurate measurement may be obtained by using an deflection indicator such as a deflectometer.NOTE 2Materials that do not rupture by the maximum str
8、ain allowed under this test method may be more suited to a 4-point bend test. The basicdifference between the two test methods is in the location of the maximum bending moment and maximum axial fiber stresses. The maximum axial fiberstresses occur on a line under the loading nose in 3-point bending
9、and over the area between the loading noses in 4-point bending. A four-point loadingsystem method can be found in Test Method D6272.1.4 The values stated in SI units are to be regarded as the standard. The values provided in parentheses are for information only.1.5 The text of this standard referenc
10、es notes and footnotes that provide explanatory material. These notes and footnotes(excluding those in tables and figures) shall not be considered as requirements of the standard.1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the resp
11、onsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.NOTE 3This standard and ISO 178 address the same subject matter, but differ in technical content.1.7 This international standard was devel
12、oped in accordance with internationally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.2. Referenced Documen
13、ts2.1 ASTM Standards:2D618 Practice for Conditioning Plastics for TestingD638 Test Method for Tensile Properties of Plastics1 These test methods are under the jurisdiction of ASTM Committee D20 on Plastics and are the direct responsibility of Subcommittee D20.10 on Mechanical Properties.Current edit
14、ion approved Dec. 1, 2015July 1, 2017. Published January 2016July 2017. Originally approved in 1970. Last previous edition approved in 20102015 asD790 10.D790 152. DOI: 10.1520/D0790-15E02.10.1520/D0790-17.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer S
15、ervice at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the prev
16、ious version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.*A Summary of Cha
17、nges section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1D883 Terminology Relating to PlasticsD4000 Classification System for Specifying Plastic MaterialsD4101 Specification for Polypropylene Inj
18、ection and Extrusion MaterialsD5947 Test Methods for Physical Dimensions of Solid Plastics SpecimensD6272 Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials byFour-Point BendingE4 Practices for Force Verification of Testing MachinesE83 Pra
19、ctice for Verification and Classification of Extensometer SystemsE691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test MethodE2309 Practices for Verification of Displacement Measuring Systems and Devices Used in Material Testing Machines2.2 ISO Standard:3ISO 178
20、PlasticsDetermination of Flexural Properties3. Terminology3.1 DefinitionsDefinitions of terms applying to these test methods appear in Terminology D883 andAnnexA2 of Test MethodD638.4. Summary of Test Method4.1 Atest specimen of rectangular cross section rests on two supports in a flat-wise position
21、 and is loaded by means of a loadingnose located midway between the supports. Unless testing certain laminated materials (see 7 for guidance), a support span-to-depth(of specimen) ratio 16:1 shall be used. The specimen is deflected until rupture occurs in the outer surface of the test specimen orunt
22、il a maximum strain (see 5.1.6) of 5.0 % is reached, whichever occurs first.4.2 Procedure A is designed principally for materials that break at comparatively small deflections and it shall be used formeasurement of flexural properties, particularly flexural modulus, unless the material specification
23、 states otherwise. Procedure Aemploys a strain rate of 0.01 mm/mm/min (0.01 in./in./min) and is the preferred procedure for this test method.4.3 Procedure B is designed principally for those materials that do not break or yield in the outer surface of the test specimenwithin the 5.0 % strain limit w
24、hen Procedure A conditions are used. Procedure B employs a strain rate of 0.10 mm/mm/min (0.10in./in./min).4.4 Type I tests utilize crosshead position for deflection measurement.4.5 Type II tests utilize an instrument (deflectometer) for deflection measurement.4.6 The procedure used and test type sh
25、all be reportedNOTE 4Comparative tests may be run in accordance with either procedure, provided that the procedure is found satisfactory for the material beingtested. Tangent modulus data obtained by ProcedureAtends to exhibit lower standard deviations than comparable results obtained by means of Pr
26、ocedureB.5. Significance and Use5.1 Flexural properties as determined by this test method are especially useful for quality control and specification purposes.They include:5.1.1 Flexural Stress (f)When a homogeneous elastic material is tested in flexure as a simple beam supported at two pointsand lo
27、aded at the midpoint, the maximum stress in the outer surface of the test specimen occurs at the midpoint. Flexural stressis calculated for any point on the load-deflection curve using equation (Eq 3) in Section 12 (see Notes 5 and 6).NOTE 5Eq 3 applies strictly to materials for which stress is line
28、arly proportional to strain up to the point of rupture and for which the strains are small.Since this is not always the case, a slight error will be introduced if Eq 3 is used to calculate stress for materials that are not true Hookean materials.The equation is valid for obtaining comparison data an
29、d for specification purposes, but only up to a maximum fiber strain of 5 % in the outer surfaceof the test specimen for specimens tested by the procedures described herein.NOTE 6When testing highly orthotropic laminates, the maximum stress may not always occur in the outer surface of the test specim
30、en.4 Laminatedbeam theory must be applied to determine the maximum tensile stress at failure. If Eq 3 is used to calculate stress, it will yield an apparent strength basedon homogeneous beam theory. This apparent strength is highly dependent on the ply-stacking sequence of highly orthotropic laminat
31、es.5.1.2 Flexural Stress for Beams Tested at Large Support Spans (f)If support span-to-depth ratios greater than 16 to 1 are usedsuch that deflections in excess of 10 % of the support span occur, the stress in the outer surface of the specimen for a simple beamis reasonably approximated using equati
32、on (Eq 4) in 12.3 (see Note 7).NOTE 7When large support span-to-depth ratios are used, significant end forces are developed at the support noses which will affect the moment ina simple supported beam. Eq 4 includes additional terms that are an approximate correction factor for the influence of these
33、 end forces in large support3 Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http:/www.ansi.org.4 For a discussion of these effects, see Zweben, C., Smith, W. S., and Wardle, M. W., “Test Methods for Fiber Tensile Strength, Composite Flexu
34、ral Modulus and Propertiesof Fabric-Reinforced Laminates,” Composite Materials: Testing and Design (Fifth Conference), ASTM STP 674, 1979, pp. 228262.D790 172span-to-depth ratio beams where relatively large deflections exist.5.1.3 Flexural Strength (fM)Maximum flexural stress sustained by the test s
35、pecimen (see Note 6) during a bending test. Itis calculated according to Eq 3 or Eq 4. Some materials that do not break at strains of up to 5 % give a load deflection curve thatshows a point at which the load does not increase with an increase in strain, that is, a yield point (Fig. 1, Curve b), Y.
36、The flexuralstrength is calculated for these materials by letting P (in Eq 3 or Eq 4) equal this point, Y.5.1.4 Flexural Offset Yield StrengthOffset yield strength is the stress at which the stress-strain curve deviates by a given strain(offset) from the tangent to the initial straight line portion
37、of the stress-strain curve. The value of the offset must be given wheneverthis property is calculated.NOTE 8Flexural Offset Yield Strength may differ from flexural strength defined in 5.1.3. Both methods of calculation are described in the annex toTest Method D638.5.1.5 Flexural Stress at Break (fB)
38、Flexural stress at break of the test specimen during a bending test. It is calculated accordingto Eq 3 or Eq 4. Some materials give a load deflection curve that shows a break point, B, without a yield point (Fig. 1, Curve a)in which case fB = fM. Other materials give a yield deflection curve with bo
39、th a yield and a break point, B (Fig. 1, Curve b).The flexural stress at break is calculated for these materials by letting P (in Eq 3 or Eq 4) equal this point, B.5.1.6 Stress at a Given StrainThe stress in the outer surface of a test specimen at a given strain is calculated in accordancewith Eq 3
40、or Eq 4 by letting P equal the load read from the load-deflection curve at the deflection corresponding to the desiredstrain (for highly orthotropic laminates, see Note 6).5.1.7 Flexural Strain, fNominal fractional change in the length of an element of the outer surface of the test specimen atmidspa
41、n, where the maximum strain occurs. Flexural strain is calculated for any deflection using Eq 5 in 12.4.5.1.8 Modulus of Elasticity:5.1.8.1 Tangent Modulus of ElasticityThe tangent modulus of elasticity, often called the “modulus of elasticity,” is the ratio,within the elastic limit, of stress to co
42、rresponding strain. It is calculated by drawing a tangent to the steepest initial straight-lineportion of the load-deflection curve and using Eq 6 in 12.5.1 (for highly anisotropic composites, see Note 159).NOTE 9Shear deflections can seriously reduce the apparent modulus of highly anisotropic compo
43、sites when they are tested at low span-to-depthratios.4 For this reason, a span-to-depth ratio of 60 to 1 is recommended for flexural modulus determinations on these composites. Flexural strength shouldbe determined on a separate set of replicate specimens at a lower span-to-depth ratio that induces
44、 tensile failure in the outer fibers of the beam along itslower face. Since the flexural modulus of highly anisotropic laminates is a critical function of ply-stacking sequence, it will not necessarily correlate withtensile modulus, which is not stacking-sequence dependent.5.1.8.2 Secant ModulusThe
45、secant modulus is the ratio of stress to corresponding strain at any selected point on thestress-strain curve, that is, the slope of the straight line that joins the origin and a selected point on the actual stress-strain curve.It shall be expressed in megapascals (pounds per square inch). The selec
46、ted point is chosen at a pre-specified stress or strain inNOTE 1Curve a: Specimen that breaks before yielding.Curve b: Specimen that yields and then breaks before the 5 % strain limit.Curve c: Specimen that neither yields nor breaks before the 5 % strain limit.FIG. 1 Typical Curves of Flexural Stres
47、s (f) Versus Flexural Strain (f)D790 173accordance with the appropriate material specification or by customer contract. It is calculated in accordance with Eq 6 by lettingm equal the slope of the secant to the load-deflection curve. The chosen stress or strain point used for the determination of the
48、secant shall be reported.5.1.8.3 Chord Modulus (Ef)The chord modulus is calculated from two discrete points on the load deflection curve. Theselected points are to be chosen at two pre-specified stress or strain points in accordance with the appropriate material specificationor by customer contract.
49、 The chosen stress or strain points used for the determination of the chord modulus shall be reported.Calculate the chord modulus, Ef using Eq 7 in 12.5.2.5.2 Experience has shown that flexural properties vary with specimen depth, temperature, atmospheric conditions, and strainrate as specified in Procedures A and B.5.3 Before proceeding with these test methods, refer to the ASTM specification of the material being tested. Any test specimenpreparation, conditioning, dimensions, or testing parameters, or combination thereof, covered in the
