1.1 此测试方法 2. 涵盖了从接近阈值开始的疲劳裂纹扩展速率的确定（参见中的区域I 图1 到 K 最大值 受控不稳定性（见中的区域III 图1 .）结果用裂纹尖端应力强度因子范围（Δ K )，由线弹性理论定义。 1.9 各种试样配置的特殊要求按以下顺序出现: 紧凑型试样 附件A1 中间张力试样 附件A2 偏心加载的单边裂纹拉伸试样 附件A3 1.10 本标准并不旨在解决与其使用相关的所有安全问题（如有）。 本标准的使用者有责任在使用前制定适当的安全、健康和环境实践，并确定监管限制的适用性。 1.11 本国际标准是根据世界贸易组织技术性贸易壁垒委员会发布的《关于制定国际标准、指南和建议的原则的决定》中确立的国际公认的标准化原则制定的。 ====意义和用途====== 5.1 疲劳裂纹扩展速率表示为裂纹尖端应力强度因子范围的函数，d 一 d N 与Δ K ，表征了材料在循环载荷下对稳定裂纹扩展的抵抗力。参考文献中给出了使用线弹性断裂力学分析疲劳裂纹扩展速率数据的定量的背景信息 ( 3. ) 和 ( 4. ) . 5.1.1 在无害（惰性）环境中，疲劳裂纹扩展速率主要是Δ K 和力比， R 或 K 最大值 和 R ( 注1 ). 温度和侵蚀性环境会显著影响d 一 d N 与Δ K ，在许多情况下强调 R -并引入其他负载变量（如循环频率和波形）的影响。在研究和设计数据的生成中，需要注意这些变量的正确选择和控制。 注1: Δ K , K 最大值 和 R 不是彼此独立的。这些变量中任意两个的规格足以定义负载条件。通常指定一个应力强度参数（Δ K 或 K 最大值 )与力比一起， R . 5.1.2 表达d 一 d N 作为Δ的函数 K 提供独立于平面几何形状的结果，从而能够交换和比较从各种样品配置和加载条件获得的数据。此外，此功能使d 一 d N 与Δ K 用于工程结构设计和评估的数据。假设相似概念，这意味着不同长度的裂纹受到相同标称Δ K 将在每个循环中以相等的裂纹扩展增量前进。 5.1.3 疲劳裂纹扩展速率数据在严格意义上并不总是几何独立的，因为有时会发生厚度效应。然而，关于厚度对疲劳裂纹扩展速率的影响的数据喜忧参半。Δ范围内的疲劳裂纹扩展速率 K 据报道，随着试样厚度的增加，增加、减少或不受影响。厚度效应也可以与其他变量相互作用，如环境和热处理。例如，材料可能在d的终端范围内表现出厚度效应 一 d N 与Δ K ，与名义收益率相关( 注2 )或作为 K 最大值 接近材料的断裂韧性。在为研究或设计生成数据时，应考虑试样厚度的潜在影响。 注2: 在符合适当试样附件中所列试样尺寸要求的试验中，应避免出现这种情况。 5.1.4 残余应力可以影响疲劳裂纹扩展速率、这种扩展速率的测量以及疲劳裂纹扩展性能的可预测性。当试样从含有残余应力场的材料中移除时，这种影响可能是显著的； 例如焊接件或复杂形状的锻造、挤压、铸造或机加工厚截面，其中不可能完全消除应力，或具有复杂形状的锻制、挤压、铸件或机加工粗截面的加工零件，其中不可以完全消除应力或具有有意引起的残余应力。从含有残余应力的此类产品中提取的试样本身也同样含有残余应力图。虽然试样的提取和裂纹起始槽的引入本身部分地缓解和重新分布了残余应力的模式，但剩余的大小仍可能在随后的测试结果中造成重大误差。 残余应力叠加在施加的循环应力上，导致实际裂纹尖端的最大和最小应力强度不同于仅基于外部施加的循环力或位移的应力强度。例如，由远场3D残余应力引起的裂纹夹持可能导致部分压缩应力循环，并加剧裂纹闭合效应，即使当试样标称施加应力范围完全是拉伸时也是如此。试样制备过程中的加工变形、试样位置和配置依赖性、疲劳预裂纹过程中的不规则裂纹扩展（例如，出乎意料的缓慢或快速裂纹扩展速率、过度裂纹- 前曲率或裂纹路径偏差），以及裂纹闭合力的显著松弛（与裂纹扩展时的试样应力释放有关），通常表明残余应力对测量的da/dN与Δ K 后果 ( 5. , 6. ) 零作用力下显著的裂纹张开位移表明残余应力会影响随后的疲劳裂纹扩展特性测量。 5.1.5 在给定Δ K 价值观使用长裂纹数据来分析小裂纹扩展通常会导致非保守的寿命估计。 环境因素可能会加剧小裂缝效应。当1）与相关微观结构尺寸相比，裂纹的长度较小（连续体力学限制），2）与局部塑性尺度相比，裂纹长度较小（线弹性断裂力学限制）和3）裂纹仅在物理上较小（<1 mm）时，裂纹被定义为小裂纹。根据该方法建立的近阈值数据应被视为代表材料的稳态疲劳裂纹扩展速率响应，该响应源于长裂纹，该裂纹具有足够的长度，从而完成从疲劳起始阶段到扩展阶段的过渡。 当应用于服务载荷历史时，稳态接近阈值的数据可能导致非保守的寿命估计，特别是对于小裂纹 ( 7. 9 ) . 5.1.6 裂纹闭合对疲劳裂纹扩展速率行为具有主要影响，特别是在低应力比的近阈值状态下。这意味着裂纹尾流的条件和先前的载荷历史可能对电流传播速率有影响。对闭合过程的作用的理解对于诸如小裂纹的行为和可变振幅加载期间的瞬态裂纹扩展速率行为等现象至关重要。 闭合提供了一种机制，通过该机制，裂纹尖端附近的循环应力强度Δ K eff ，与标称应用值Δ不同 K 这一概念对疲劳裂纹扩展速率数据的断裂力学解释具有重要意义，因为它暗示了Δ K 和 R ( 1. ) . 5. 注3: 通过在高应力比下进行测试，可以在接近阈值的情况下更接近小裂纹行为的特征，其中裂纹闭合引起的异常被最小化。 5.1.7 与施加的Δ相比，除了裂纹闭合之外，其他形式的裂纹尖端屏蔽，如分支、楔入、桥接和滑动（以及其他外部效应）也可以降低裂纹尖端驱动力 K ，其中一些对相对于材料晶粒结构的裂纹取向敏感( E1823 ，附件A2）。屏蔽概念对疲劳裂纹扩展速率数据的断裂力学解释具有重要意义，因为它也意味着应用Δ K 和 R 并且可能使关于LEFM相似性的典型假设无效，因为屏蔽耗散了标准应力强度因子计算中未考虑的能量。材料的晶粒结构可能对速率行为有很大影响，特别是对于在轧制或其他成型过程中具有显著变形的材料，例如在制造铝合金板、板、锻造和挤压产品形式时发生的材料。 对于某些材料，常见的L-T和T-L取向会导致裂纹尖端应力-应变场与周围晶粒结构之间的相互作用，从而导致分层增韧等效应。一些铝厚板和锻件产品在单元化结构中的应用引入了在不太常见的方向（如L-S和T-S）上生长的可能性，导致平面外裂纹分支和在贯穿厚度的裂纹生长过程中意外的裂纹转向最弱的微观结构面。这种复杂的屏蔽机制可能会阻止数据从试样成功转移到结构应用中，其中晶粒结构和裂纹尖端应力状态可能与试样的不相似 ( 2. ) . 5.1.8 应注意:在表征过程中识别和理解意外的屏蔽机制；评估FCGR数据用于其他用途（如材料排名或结构分析）的相似性和可转移性；并防止不保守的数据和应用程序。 5.2 此测试方法可用于以下目的: 5.2.1 为了确定疲劳裂纹扩展对承受循环载荷的部件寿命的影响，提供的数据是在代表性条件下生成的，并与适当的断裂韧性数据相结合（例如，见试验方法 E399 )，缺陷表征数据和应力分析信息 ( 10 , 11 ) . 注4: 疲劳裂纹的扩展会受到载荷历史的显著影响。在可变振幅加载过程中，裂纹扩展速率可以增强或延迟（相对于给定Δ下的稳态恒定振幅扩展速率 K )取决于具体的装载顺序。在使用恒定振幅增长率数据分析可变振幅疲劳问题时，需要考虑这一复杂因素 ( 12 ) . 5.2.2 为耐损伤应用制定材料选择标准和检查要求。 5.2.3 定量地确定冶金、制造、环境和载荷变量对疲劳裂纹扩展的单独和综合影响。

1.1 This test method 2 covers the determination of fatigue crack growth rates from near-threshold (see region I in Fig. 1 ) to K max controlled instability (see region III in Fig. 1 .) Results are expressed in terms of the crack-tip stress-intensity factor range (Δ K ), defined by the theory of linear elasticity. 1.9 Special requirements for the various specimen configurations appear in the following order: The Compact Specimen Annex A1 The Middle Tension Specimen Annex A2 The Eccentrically-Loaded Single Edge Crack Tension Specimen Annex A3 1.10 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use. 1.11 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 Fatigue crack growth rate expressed as a function of crack-tip stress-intensity factor range, d a /d N versus Δ K , characterizes a material's resistance to stable crack extension under cyclic loading. Background information on the ration-ale for employing linear elastic fracture mechanics to analyze fatigue crack growth rate data is given in Refs ( 3 ) and ( 4 ) . 5.1.1 In innocuous (inert) environments fatigue crack growth rates are primarily a function of Δ K and force ratio, R , or K max and R ( Note 1 ). Temperature and aggressive environments can significantly affect d a/ d N versus Δ K , and in many cases accentuate R -effects and introduce effects of other loading variables such as cycle frequency and waveform. Attention needs to be given to the proper selection and control of these variables in research studies and in the generation of design data. Note 1: Δ K , K max , and R are not independent of each other. Specification of any two of these variables is sufficient to define the loading condition. It is customary to specify one of the stress-intensity parameters (Δ K or K max ) along with the force ratio, R . 5.1.2 Expressing d a /d N as a function of Δ K provides results that are independent of planar geometry, thus enabling exchange and comparison of data obtained from a variety of specimen configurations and loading conditions. Moreover, this feature enables d a /d N versus Δ K data to be utilized in the design and evaluation of engineering structures. The concept of similitude is assumed, which implies that cracks of differing lengths subjected to the same nominal Δ K will advance by equal increments of crack extension per cycle. 5.1.3 Fatigue crack growth rate data are not always geometry-independent in the strict sense since thickness effects sometimes occur. However, data on the influence of thickness on fatigue crack growth rate are mixed. Fatigue crack growth rates over a wide range of Δ K have been reported to either increase, decrease, or remain unaffected as specimen thickness is increased. Thickness effects can also interact with other variables such as environment and heat treatment. For example, materials may exhibit thickness effects over the terminal range of d a/ d N versus Δ K , which are associated with either nominal yielding ( Note 2 ) or as K max approaches the material fracture toughness. The potential influence of specimen thickness should be considered when generating data for research or design. Note 2: This condition should be avoided in tests that conform to the specimen size requirements listed in the appropriate specimen annex. 5.1.4 Residual stresses can influence fatigue crack growth rates, the measurement of such growth rates and the predictability of fatigue crack growth performance. The effect can be significant when test specimens are removed from materials that embody residual stress fields; for example weldments or complex shape forged, extruded, cast or machined thick sections, where full stress relief is not possible, or worked parts having complex shape forged, extruded, cast or machined thick sections where full stress relief is not possible or worked parts having intentionally-induced residual stresses. Specimens taken from such products that contain residual stresses will likewise themselves contain residual stress. While extraction of the specimen and introduction of the crack starting slot in itself partially relieves and redistributes the pattern of residual stress, the remaining magnitude can still cause significant error in the ensuing test result. Residual stress is superimposed on the applied cyclic stress and results in actual crack-tip maximum and minimum stress-intensities that are different from those based solely on externally applied cyclic forces or displacements. For example, crack-clamping resulting from far-field 3D residual stresses may lead to partly compressive stress cycles, and exacerbate the crack closure effect, even when the specimen nominal applied stress range is wholly tensile. Machining distortion during specimen preparation, specimen location and configuration dependence, irregular crack growth during fatigue precracking (for example, unexpected slow or fast crack growth rate, excessive crack-front curvature or crack path deviation), and dramatic relaxation in crack closing forces (associated with specimen stress relief as the crack extends) will often indicate influential residual stress impact on the measured da/dN versus Δ K result. ( 5 , 6 ) Noticeable crack-mouth-opening displacement at zero applied force is indicative of residual stresses that can affect the subsequent fatigue crack growth property measurement. 5.1.5 The growth rate of small fatigue cracks can differ noticeably from that of long cracks at given Δ K values. Use of long crack data to analyze small crack growth often results in non-conservative life estimates. The small crack effect may be accentuated by environmental factors. Cracks are defined as being small when 1) their length is small compared to relevant microstructural dimension (a continuum mechanics limitation), 2) their length is small compared to the scale of local plasticity (a linear elastic fracture mechanics limitation), and 3) they are merely physically small (<1 mm). Near-threshold data established according to this method should be considered as representing the materials' steady-state fatigue crack growth rate response emanating from a long crack, one that is of sufficient length such that transition from the initiation to propagation stage of fatigue is complete. Steady-state near-threshold data, when applied to service loading histories, may result in non-conservative lifetime estimates, particularly for small cracks ( 7- 9 ) . 5.1.6 Crack closure can have a dominant influence on fatigue crack growth rate behavior, particularly in the near-threshold regime at low stress ratios. This implies that the conditions in the wake of the crack and prior loading history can have a bearing on the current propagation rates. The understanding of the role of the closure process is essential to such phenomena as the behavior of small cracks and the transient crack growth rate behavior during variable amplitude loading. Closure provides a mechanism whereby the cyclic stress intensity near the crack tip, Δ K eff , differs from the nominally applied values, Δ K . This concept is of importance to the fracture mechanics interpretation of fatigue crack growth rate data since it implies a non-unique growth rate dependence in terms of Δ K , and R ( 1 ) . 5 Note 3: The characterization of small crack behavior may be more closely approximated in the near-threshold regime by testing at a high stress ratio where the anomalies due to crack closure are minimized. 5.1.7 Along with crack closure, other forms of crack tip shielding such as branching, wedging, bridging and sliding (among other extrinsic effects) can also reduce the crack tip driving force in comparison to the applied Δ K , with some of these sensitive to crack orientation relative to the material grain structure ( E1823 , Annex A2). The shielding concept is of importance to the fracture mechanics interpretation of fatigue crack growth rate data since it also implies a non-unique growth-rate dependence in terms of applied Δ K and R and may invalidate typical assumptions about LEFM similitude, because the shielding dissipates energy not accounted for in the standard stress-intensity factor calculation. Material grain structure can have a substantial influence on rate behavior, especially for materials with significant deformation during rolling or other forming processes such as those that occur in the manufacture of aluminum alloy sheet, plate, forged, and extruded product forms. For some materials, the common L-T and T-L orientations can lead to interactions between crack-tip stress-strain fields and the surrounding grain structure, leading to such effects as delamination toughening. Applications of some aluminum thick plate and forging products to unitized structure introduce possibilities of growth in less common orientations such as L-S and T-S, leading to out-of-plane crack branching and unexpected crack turning to the weakest microstructural plane during through-thickness crack growth. Such complex shielding mechanisms may prevent successful transfer of data from coupons to structural application, where grain structure and crack tip stress state may not be similar to those of the test coupon ( 2 ) . 5.1.8 Care should be taken to: identify and understand unexpected shielding mechanisms during characterization; assess similitude and transferability of the FCGR data for other uses such as material ranking or structural analysis; and prevent unconservative data and applications. 5.2 This test method can serve the following purposes: 5.2.1 To establish the influence of fatigue crack growth on the life of components subjected to cyclic loading, provided data are generated under representative conditions and combined with appropriate fracture toughness data (for example, see Test Method E399 ), defect characterization data, and stress analysis information ( 10 , 11 ) . Note 4: Fatigue crack growth can be significantly influenced by load history. During variable amplitude loading, crack growth rates can be either enhanced or retarded (relative to steady-state, constant-amplitude growth rates at a given Δ K ) depending on the specific loading sequence. This complicating factor needs to be considered in using constant-amplitude growth rate data to analyze variable amplitude fatigue problems ( 12 ) . 5.2.2 To establish material selection criteria and inspection requirements for damage tolerant applications. 5.2.3 To establish, in quantitative terms, the individual and combined effects of metallurgical, fabrication, environmental, and loading variables on fatigue crack growth.