1.1 这些试验方法用于确定未增强和增强塑料的弯曲性能，包括高模量复合材料和电绝缘材料，使用三点加载系统向简支梁（试样）施加荷载。该方法通常适用于刚性和半刚性材料，但无法确定在5.0%应变极限内未在试样外表面断裂或屈服的材料的弯曲强度。 1.2 矩形横截面的试样是注塑的，或从模制或挤压板材或板材上切下，或从模制或挤压形状上切下。试样必须为实心且均匀的矩形。试样位于两个支架上，并通过支架中间的加载鼻加载。 1.3 用两种方法之一测量挠度； 使用十字头位置或挠度计。请注意，研究表明，使用挠度计获得的挠度数据将不同于使用十字头位置获得的数据。应报告挠度测量方法。 注1: 通常通过使用十字头位置测量挠度来满足生产环境中的质量控制要求。然而，可以通过使用挠度指示器（例如挠度计）获得更精确的测量。 注2: 在本试验方法下允许的最大应变下不会破裂的材料可能更适合进行四点弯曲试验。两种试验方法的基本区别在于最大弯矩和最大轴向纤维应力的位置。最大轴向纤维应力发生在3点弯曲加载端下方的一条线上，以及4点弯曲加载端之间的区域- 点弯曲。可以在测试方法中找到四点加载系统方法 D6272 . 1.4 以国际单位制表示的数值应视为标准。括号中提供的值仅供参考。 1.5 本标准的文本引用了提供解释材料的注释和脚注。这些注释和脚注（不包括表和图中的注释和脚注）不应视为本标准的要求。 1.6 本标准并非旨在解决与其使用相关的所有安全问题（如有）。本标准的用户有责任在使用前制定适当的安全和健康实践，并确定监管限制的适用性。 注3: 本标准和ISO 178涉及相同的主题，但技术内容不同。 1.7 本国际标准是根据世界贸易组织技术性贸易壁垒（TBT）委员会发布的《关于制定国际标准、指南和建议的原则的决定》中确立的国际公认标准化原则制定的。 ====意义和用途====== 5.1 本试验方法测定的弯曲性能对于质量控制和规范目的特别有用。它们包括: 5.1.1 弯曲应力（σ f )— 当均匀弹性材料作为两点支撑的简支梁进行弯曲试验并在中点加载时，试样外表面的最大应力出现在中点。使用公式计算荷载-挠度曲线上任意点的弯曲应力( 等式3 )在节中 12 （参见 注释5和 6. ). 注5: 等式3 严格适用于应力与应变成线性比例直至断裂点且应变较小的材料。由于情况并非总是如此，如果出现以下情况，则会出现轻微错误: 等式3 用于计算非真实胡克材料的应力。 该方程适用于获得比较数据和规范目的，但对于按本文所述程序测试的试样，其外表面的最大纤维应变仅为5%。 注6: 在测试高度正交各向异性层压板时，最大应力可能并不总是出现在试样的外表面。 4. 必须应用层合梁理论来确定破坏时的最大拉伸应力。如果 等式3 用于计算应力，它将根据均匀梁理论产生视强度。这种表观强度在很大程度上取决于高度正交各向异性层压板的铺层顺序。 5.1.2 在大支撑跨度下测试的梁的弯曲应力（σ f )— 如果使用大于16:1的支架跨距与深度比，使得挠度超过支架跨距的10%，则简单梁试样外表面的应力可使用公式合理近似( 等式4 )在 12.3 （参见 附注7 ). 注7: 当使用较大的支承跨高比时，支承端部会产生显著的端部力，这将影响简支梁中的力矩。 等式4 包括附加项，这些附加项是存在较大挠度的大支撑跨高比梁中这些端部力影响的近似修正系数。 5.1.3 弯曲强度（σ 调频 )— 试样承受的最大弯曲应力（见 附注6 )在弯曲试验期间。根据以下公式计算 等式3 或 等式4 . 一些在应变高达5%时不会断裂的材料给出了载荷-挠度曲线，该曲线显示了载荷不随应变增加而增加的点，即屈服点( 图1 ，曲线b）， Y . 通过以下公式计算这些材料的弯曲强度: P （英寸 等式3 或 等式4 )等于此点， Y . 图1 弯曲应力的典型曲线（σ f )相对于弯曲应变（ε f ) 注1: 曲线a:屈服前断裂的试样。 曲线b:屈服后在5 % 应变极限。 曲线c:在5°c之前既不屈服也不断裂的试样 % 应变极限。 5.1.4 弯曲偏移屈服强度- 偏移屈服强度是指应力-应变曲线偏离应力-应变曲线初始直线部分切线的给定应变（偏移）的应力。每当计算此特性时，必须给出偏移值。 注8: 弯曲偏移屈服强度可能不同于中定义的弯曲强度 5.1.3 . 试验方法附录中描述了两种计算方法 D638 . 5.1.5 断裂时的弯曲应力（σ fB公司 )— 弯曲试验期间试样断裂时的弯曲应力。 根据以下公式计算 等式3 或 等式4 . 一些材料给出了显示断点的荷载-挠度曲线， B ，没有屈服点( 图1 ，曲线a），在这种情况下σ fB公司 = σ 调频 . 其他材料给出了具有屈服点和断点的屈服-挠度曲线， B ( 图1 ，曲线b）。通过以下公式计算这些材料的断裂弯曲应力: P （英寸 等式3 或 等式4 )等于此点， B . 5.1.6 给定应变下的应力- 给定应变下试样外表面的应力根据以下公式计算: 等式3 或 等式4 通过让 P 等于在对应于所需应变的挠度处从荷载-挠度曲线读取的荷载（对于高度正交各向异性层压板，请参见 附注6 ). 5.1.7 弯曲应变，ɛ f — 出现最大应变的跨中试样外表面构件长度的标称分数变化。 使用以下公式计算任何挠度的弯曲应变: 等式5 在里面 12.4 . 5.1.8 弹性模量: 5.1.8.1 切线弹性模量- 切线弹性模量，通常称为“弹性模量”，是应力与相应应变在弹性极限内的比值。通过绘制与荷载-挠度曲线最陡初始直线部分的切线并使用 等式6 在里面 12.5.1 （对于高度各向异性的复合材料，请参见 附注9 ). 注9: 在低跨高比下测试时，剪切挠度会严重降低高度各向异性复合材料的表观模量。 4. 因此，建议在测定这些复合材料的弯曲模量时，跨深比为60:1。弯曲强度应在一组单独的复制试样上以较低的跨深比确定，该跨深比会导致梁的外纤维沿其下表面发生拉伸破坏。 由于高度各向异性层压板的弯曲模量是铺层顺序的关键函数，因此它不一定与拉伸模量相关，拉伸模量与铺层顺序无关。 5.1.8.2 割线模量- 割线模量是应力-应变曲线上任何选定点处的应力与相应应变之比，即连接原点和实际应力-应变曲线上选定点的直线的斜率。应以兆帕（磅/平方英寸）表示。根据适当的材料规范或客户合同，在预先指定的应力或应变下选择所选点。根据以下公式计算: 等式6 通过使m等于荷载-挠度曲线的割线斜率。应报告用于确定割线的选定应力或应变点。 5.1.8.3 弦模量（E f )— 弦模量从荷载-挠度曲线上的两个离散点计算。根据适当的材料规范或客户合同，在两个预先指定的应力或应变点处选择所选点。应报告用于测定弦模量的选定应力或应变点。计算弦模量， E f 使用 公式7 在里面 12.5.2 . 5.2 经验表明，弯曲性能随试样深度、温度、大气条件和程序A和B中规定的应变率而变化。 5.3 在继续使用这些测试方法之前，请参阅被测试材料的ASTM规范。ASTM材料规范中涵盖的任何试样制备、调节、尺寸或测试参数，或其组合，应优先于这些测试方法中提及的。 分类系统中的表1 D4000 列出了目前存在的塑料ASTM材料规范。

1.1 These test methods are used to determine the flexural properties of unreinforced and reinforced plastics, including high modulus composites and electrical insulating materials utilizing a three-point loading system to apply a load to a simply supported beam (specimen). The method is generally applicable to both rigid and semi-rigid materials, but flexural strength cannot be determined for those materials that do not break or yield in the outer surface of the test specimen within the 5.0 % strain limit. 1.2 Test specimens of rectangular cross section are injection molded or, cut from molded or extruded sheets or plates, or cut from molded or extruded shapes. Specimens must be solid and uniformly rectangular. The specimen rests on two supports and is loaded by means of a loading nose midway between the supports. 1.3 Measure deflection in one of two ways; using crosshead position or a deflectometer. Please note that studies have shown that deflection data obtained with a deflectometer will differ from data obtained using crosshead position. The method of deflection measurement shall be reported. Note 1: Requirements 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 2: Materials that do not rupture by the maximum strain allowed under this test method may be more suited to a 4-point bend test. The basic difference between the two test methods is in the location of the maximum bending moment and maximum axial fiber stresses. The maximum axial fiber stresses occur on a line under the loading nose in 3-point bending and over the area between the loading noses in 4-point bending. A four-point loading system 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 references 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 responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Note 3: This standard and ISO 178 address the same subject matter, but differ in technical content. 1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee. ====== Significance And Use ====== 5.1 Flexural properties as determined by this test method are especially useful for quality control and specification purposes. They include: 5.1.1 Flexural Stress (σ f )— When a homogeneous elastic material is tested in flexure as a simple beam supported at two points and loaded at the midpoint, the maximum stress in the outer surface of the test specimen occurs at the midpoint. Flexural stress is calculated for any point on the load-deflection curve using equation ( Eq 3 ) in Section 12 (see Notes 5 and 6 ). Note 5: Eq 3 applies strictly to materials for which stress is linearly 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 and for specification purposes, but only up to a maximum fiber strain of 5 % in the outer surface of the test specimen for specimens tested by the procedures described herein. Note 6: When testing highly orthotropic laminates, the maximum stress may not always occur in the outer surface of the test specimen. 4 Laminated beam 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 based on homogeneous beam theory. This apparent strength is highly dependent on the ply-stacking sequence of highly orthotropic laminates. 5.1.2 Flexural Stress for Beams Tested at Large Support Spans (σ f )— If support span-to-depth ratios greater than 16 to 1 are used such that deflections in excess of 10 % of the support span occur, the stress in the outer surface of the specimen for a simple beam is reasonably approximated using equation ( Eq 4 ) in 12.3 (see Note 7 ). Note 7: When large support span-to-depth ratios are used, significant end forces are developed at the support noses which will affect the moment in a simple supported beam. Eq 4 includes additional terms that are an approximate correction factor for the influence of these end forces in large support span-to-depth ratio beams where relatively large deflections exist. 5.1.3 Flexural Strength (σ fM )— Maximum flexural stress sustained by the test specimen (see Note 6 ) during a bending test. It is 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 that shows a point at which the load does not increase with an increase in strain, that is, a yield point ( Fig. 1 , Curve b), Y . The flexural strength is calculated for these materials by letting P (in Eq 3 or Eq 4 ) equal this point, Y . FIG. 1 Typical Curves of Flexural Stress (σ f ) Versus Flexural Strain (ε f ) Note 1: Curve 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. 5.1.4 Flexural Offset Yield Strength— Offset 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 of the stress-strain curve. The value of the offset must be given whenever this property is calculated. Note 8: Flexural Offset Yield Strength may differ from flexural strength defined in 5.1.3 . Both methods of calculation are described in the annex to Test Method D638 . 5.1.5 Flexural Stress at Break (σ fB )— Flexural stress at break of the test specimen during a bending test. It is calculated according to 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 σ fB = σ fM . Other materials give a yield deflection curve with both 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 Strain— The stress in the outer surface of a test specimen at a given strain is calculated in accordance with Eq 3 or Eq 4 by letting P equal the load read from the load-deflection curve at the deflection corresponding to the desired strain (for highly orthotropic laminates, see Note 6 ). 5.1.7 Flexural Strain, ɛ f — Nominal fractional change in the length of an element of the outer surface of the test specimen at midspan, 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 Elasticity— The tangent modulus of elasticity, often called the “modulus of elasticity,” is the ratio, within the elastic limit, of stress to corresponding strain. It is calculated by drawing a tangent to the steepest initial straight-line portion of the load-deflection curve and using Eq 6 in 12.5.1 (for highly anisotropic composites, see Note 9 ). Note 9: Shear deflections can seriously reduce the apparent modulus of highly anisotropic composites when they are tested at low span-to-depth ratios. 4 For this reason, a span-to-depth ratio of 60 to 1 is recommended for flexural modulus determinations on these composites. Flexural strength should be determined on a separate set of replicate specimens at a lower span-to-depth ratio that induces tensile failure in the outer fibers of the beam along its lower face. Since the flexural modulus of highly anisotropic laminates is a critical function of ply-stacking sequence, it will not necessarily correlate with tensile modulus, which is not stacking-sequence dependent. 5.1.8.2 Secant Modulus— The secant modulus is the ratio of stress to corresponding strain at any selected point on the stress-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 selected point is chosen at a pre-specified stress or strain in accordance with the appropriate material specification or by customer contract. It is calculated in accordance with Eq 6 by letting m equal the slope of the secant to the load-deflection curve. The chosen stress or strain point used for the determination of the secant shall be reported. 5.1.8.3 Chord Modulus (E f )— The chord modulus is calculated from two discrete points on the load deflection curve. The selected points are to be chosen at two pre-specified stress or strain points in accordance with the appropriate material specification or by customer contract. The chosen stress or strain points used for the determination of the chord modulus shall be reported. Calculate the chord modulus, E f using Eq 7 in 12.5.2 . 5.2 Experience has shown that flexural properties vary with specimen depth, temperature, atmospheric conditions, and strain rate 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 specimen preparation, conditioning, dimensions, or testing parameters, or combination thereof, covered in the ASTM material specification shall take precedence over those mentioned in these test methods. Table 1 in Classification System D4000 lists the ASTM material specifications that currently exist for plastics.