1.1 本指南讨论了当前和潜在的无损检测（NDT）程序，用于查找纤维缠绕压力容器（也称为复合缠绕压力容器）复合外包装中的不连续性和累积损伤迹象。通常，这些容器的金属衬里厚度小于2.3 mm（0.090 In.），复合材料外包装中的纤维负荷大于60%（按重量计）。在COPV中，复合外包装厚度约为2.0 mm（0.080 In.）对于较小的容器和小于等于20毫米（0.80英寸）对于较大的。 1.2 本指南重点介绍在环境温度下使用的具有非负载共享金属内衬的COPV，其最接近代表压缩气体协会（CGA）III型金属- 内衬复合罐。然而，它也与 (1) 单片金属压力容器（PV）（CGA I型）， (2) 金属内衬环包COPV（CGA II型）， (3) 塑料衬里复合材料压力容器（CPV），带有非负载共享衬里（CGA IV型），以及 (4) 一种全复合、无线性COPV（未定义类型）。本指南还与低温下使用的COPV相关。 1.3 本指南涵盖的船舶用于航空航天应用；因此，与用于非航空航天应用的容器相比，不连续性和检查点的检查要求通常不同且更严格。 1.4 本指南适用于 (1) 低压COPV用于在最大允许工作压力（MAWP）高达3.5 MPa（500 psia）和体积高达2.5 MPa的情况下存储航空航天介质 L（70英尺 3. )，和 (2) 高压COPV用于在最大允许工作压力70 MPa（10 000 psia），体积降至8 L（500英寸。 3. ). 内部真空储存或暴露不适用于任何容器尺寸。 注1: 一些容器在填充操作期间被排空，要求储罐承受外部（大气）压力，而其他容器可能包含或浸入低温流体，或两者兼有，要求储罐承受差热收缩的任何潜在有害影响。 1.5 考虑中的复合外包装包括但不限于由各种聚合物基体树脂（例如，环氧树脂、氰酸酯、聚氨酯、酚醛树脂、聚酰亚胺（包括双马来酰亚胺）和聚酰胺）制成的，具有连续纤维增强（例如，碳、芳纶、玻璃或聚（苯撑苯并双氧唑）（PBO））的外包装。正在考虑的金属内衬包括但不限于铝合金、钛合金、镍铬合金和不锈钢。 1.6 本指南描述了既定无损检测方法的应用；即声发射（AE，截面 7. )，涡流检测（ET，第 8. )，激光剪切成像（第 9 )，射线检测（RT，第 10 )，红外热成像（IRT，第 11 )，超声波检测（UT，第节 12 )，和目视测试（VT，第节 13 ). 这些方法可供认知工程组织用于检测和评估新型和在役COPV复合外包装中的缺陷、缺陷和累积损伤。 注2: 尽管目前的范围标准讨论并要求进行目视检测，但重点是补充无损检测程序，这些程序对检测COPV表面上没有可见迹象的缺陷、缺陷和损伤非常敏感。 注3: 在航空航天应用中，高度重视轻质材料，而在商业应用中，通常牺牲重量以获得更高的鲁棒性。 因此，在航空航天船舶中，需要检测低于视觉损伤阈值的损伤。 注4: 目前，无法通过任何无损检测方法确定残余强度。 1.7 本指南中讨论的所有方法（AE、ET、剪切成像、RT、IRT、UT和VT）均在覆盖和结构固化后对复合覆盖层进行。用于检测纤维缠绕压力容器中薄壁金属衬里或外包装前裸露金属衬里中不连续性的无损检测程序；即AE、ET、激光轮廓术、泄漏检测（LT）、渗透检测（PT）和RT；参考指南 E2982年 . 1.8 对于冲击损伤敏感且需要实施损伤控制计划的COPV，重点放在无损检测方法上，该方法对检测能量级冲击引起的复合材料外包装损伤敏感，并且可能或可能不会在COPV复合材料表面上留下任何可见指示。 1.9 本指南未规定接受和拒绝标准( 4.9 )用于采购或作为批准纤维缠绕压力容器使用的手段。规定的任何验收标准仅用于完善和进一步阐述本指南中所述的程序。如果可用，应使用项目或原始设备制造商（OEM）特定的验收/拒收标准，并优先于本文件中包含的任何验收标准。如果没有可用的验收/拒收标准，则本指南中讨论的任何用于识别断裂纤维的无损检测方法都需要由认可的工程组织进行处理。 1.10 本指南既参考了有经验基础并产生数值结果的既定ASTM方法，也参考了尚未验证且更好归类为定性指南和实践的较新程序。后者作为前一种方法纳入本指南，以促进研究和随后的阐述。 1.11 为了确保正确使用参考标准文件，有公认的无损检测专家，他们根据行业和公司无损检测规范进行认证。建议无损检测专家参与任何复合材料部件设计、质量保证、在役维护或损坏检查。 1.12 单位- 以国际单位制表示的数值应视为标准值。括号中的英文单位仅供参考。 1.13 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、健康和环境实践，并确定监管限制的适用性。第节给出了一些具体的危害说明 7. 关于危险。 1.14 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====== 4.1 本指南中涵盖的COPV由一个金属内衬组成，内衬有嵌入聚合物基体树脂（通常为热固性树脂）中的高强度纤维( 图1 ). 金属内衬可以由深拉/挤压整体坯料旋转形成，也可以通过焊接成型部件制造。为了减轻重量，设计师通常寻求最小化衬里厚度。使用的COPV衬里材料可以是铝合金、钛合金、镍铬合金和不锈钢、不渗透聚合物衬里（如高密度聚乙烯）或集成复合材料。纤维材料可以是碳、芳纶、玻璃、PBO、金属或混合物（两种或更多类型的纤维）。 基体树脂包括环氧树脂、氰酸酯、聚氨酯、酚醛树脂、聚酰亚胺（包括双马来酰亚胺）、聚酰胺和其他高性能聚合物。常见的粘接线粘合剂为FM-73、聚氨酯、West 105和Epon 862，厚度为0.13 mm（0.005 in.）至0.38毫米（0.015英寸）。ANSI/AIAA S-080和ANSI/AIAA S-081分别给出了金属内衬和复合外包装材料的要求。 注6: 当使用碳纤维时，应使用物理屏障（如树脂基体中的玻璃布）或类似的粘合线粘合剂为金属衬里提供电流保护。 注7: 根据认可工程组织的判断，复合材料未根据MIL中的指南开发和鉴定- HDBK-17第1卷和第3卷应具有经批准的材料使用协议。 图1 典型的碳纤维增强COPV（NASA） 4.2 然后，对伤口处的COPV进行固化，并进行自增强/验证循环，以评估性能并增加疲劳特性。 4.3 航空航天应用中减轻重量和空间需求的强大驱动力促使设计师采用环氧树脂基体中嵌入高模量碳纤维的COPV。不幸的是，高模量纤维的剪切力较弱，因此极易因机械损伤而断裂。外包装的机械损伤可能在复合材料表面上没有可见的迹象，但会产生地下损伤。 注8: 复合外包装的冲击损伤容限将取决于容器的尺寸和形状、复合厚度（层数）以及复合外包装相对于衬里的厚度。 4.4 根据MIL-HDBK-340和ANSI/AIAA S-081，本指南中讨论的COPV的主要预期功能将是在以下一种或多种适用的情况下存储加压气体和流体: 4.4.1 根据理想气体的绝热膨胀，包含19 310 J（14 240 ft-lbf）或更大的储能。 4.4.2 含有气体或液体，如果释放，会危及人员或设备或造成灾难（事故）。 4.4.3 设计极限压力大于690 kPa（100 psi）。 4.5 根据NASA-STD-（I）-5019，COPV应符合ANSI/AIAA S-081的最新版本。实施S-081时，以下要求也适用: 4.5.1 最大设计压力（MDP）应取代S-081中对最大预期工作压力（MEOP）的所有参考。 4.5.2 COPV在使用寿命期间，复合材料壳体无应力断裂失效的概率至少为0.999。 注9: 对于其他航空航天应用，认可的工程组织应根据预期故障模式、损伤容限、安全系数或故障后果或其组合，选择适当的生存概率，例如0.99、0.999、0.9999等。 例如，生存概率为0.99意味着平均每100个COPV中就有1个会失败。具有灾难性失效模式（BBL复合材料壳体应力破裂与LBB衬里泄漏）、较低损伤容限（圆柱形容器与球形容器）、较低安全系数和较高失效后果的COPV将接受更严格的无损检测。 4.6 本指南中讨论的无损检测程序的应用旨在降低复合外包装失效的可能性，通常表示为“先爆后漏”（BBL），其特征是外包装发生灾难性破裂，并释放大量能量，从而减轻或消除与受压商品和可能的地面支持人员损失相关的伴随风险，机组人员或任务。 4.6.1 对断裂关键零件（如COPV）进行无损检测，以确定硬件中存在先前存在缺陷的可能性较低。 4.6.2 根据认可的工程组织的决定，COPV断裂控制的无损检测应遵循MIL-HDBK-6870、NASA-STD-（I）-5019、MSFC-RQMT-3479或ECSS-E-30-01A或本指南未涵盖的其他一般和详细指南。 4.6.3 作为验收的一部分进行验证测试的硬件（即，不筛选特定缺陷）应在关键焊缝和其他关键位置进行验证后NDT。 4.7 不连续类型- 特定的不连续类型与COPV的特定加工、制造和使用历史有关。 金属衬里可能有裂纹、屈曲、泄漏和各种焊接不连续性（见指南中的4.6） E2982年 ). 衬里和复合材料外包装之间也可能出现非粘合缺陷（空隙）。类似地，复合材料外包装可能在制造过程中引入预先存在的制造缺陷，以及在投入使用之前由自增强或验证测试引起的损坏。一旦投入使用，低速或微流星体轨道碎片撞击、切割/划痕/磨损、火灾、暴露于航空航天介质、负载应力、热循环、物理老化、氧化降解、风化和空间环境影响（暴露于原子氧和电离辐射）可能会造成额外的损坏。 这些因素将导致外包装中出现可见或不可见、宏观或微观的复杂损伤状态。这些损伤状态的特征是存在孔隙、凹陷、气泡、起皱、侵蚀、化学改性、异物碎片（夹杂物）、丝束终止误差、丝束滑移、丝束错位、丝束扭曲、基体开裂、基体富区、基体欠固化和过固化、纤维富区、纤维基体脱粘、纤维拔出、，纤维分裂、纤维断裂、桥接、衬里/外包装脱粘和分层。这些不连续性通常可分为四大类: (1) 制造业 (2) 划痕/擦伤/磨损； (3) 机械损伤；和 (4) 变色。 4.8 缺陷的影响- 除非对具有已知缺陷类型和尺寸的试样或物品进行失败测试，否则难以确定给定复合缺陷类型或尺寸的影响（“缺陷影响”）。鉴于这种潜在的不确定性，除非已证明缺陷的影响，否则缺陷检测不一定是拒收的理由（即缺陷）。即使是给定缺陷类型和尺寸的检测也可能存在疑问，除非使用所选的无损检测方法对具有已知缺陷类型和尺寸的物理参考试样进行评估。指南中的表1给出了检测常见复合缺陷类型的各种无损检测方法的适用性 E2533 . 4.9 验收标准- 应由认可的工程组织确定COPV是否符合验收标准并适用于航空航天服务。根据本指南进行检查时，工程图纸、规范、采购订单或合同应注明验收标准。 4.9.1 验收/拒收标准应包括预期缺陷类型的清单以及每种缺陷的拒收水平。 4.9.2 根据各种验收/拒收标准，应根据合同文件确定受试物品的分区。 4.9.3 拒绝COPV- 如果发现缺陷的类型、尺寸或数量超出图纸、采购订单或合同规定的允许范围，则应将复合物品与可接受物品分开，适当确定为不一致，并由认可的工程组织提交材料审查，并进行以下处理之一: (1) 按现状可接受， (2) 进行进一步返工或修理，以使材料或部件可接受，或 (3) 合同文件要求时报废（永久无法使用）。 4.9.4 在进行检查之前，应在要求文件中定义验收标准和结果解释。买方和供应商应就检查结果的解释达成事先协议。所有信号超过工艺要求文件规定的拒收水平的不连续性都应拒收，除非零件图纸确定可拒收的不连续性不会保留在成品中。 4.10 COPV认证- ANSI/AIAA S-081定义了COPV的设计、分析和认证方法。更具体地说，COPV应表现出先泄后爆（LBB）故障模式，或应具有足够的损伤容限寿命（安全寿命），或两者兼有，这取决于临界性以及应用于危险或非危险流体。因此，无损检测方法应检测在COPV寿命期间预期操作条件下可能导致爆裂的任何不连续性。损伤容限寿命要求衬里中存在的任何不连续性在COPV的预期寿命内不会发展为故障。裂纹扩展的断裂力学评估是对可以安全存在的不连续尺寸设定限制的典型方法。 这确立了缺陷标准:所有不连续性等于或大于最小尺寸或具有 J -在预期使用寿命内导致容器失效的基于整体或其他适用断裂力学的标准被归类为缺陷，应由认可的工程组织解决。 4.10.1 设计要求- ANSI/AIAA S-081中给出了与复合外包装相关的COPV设计要求。关键要求是规定COPV应表现出LBB故障模式，或应具有足够的损伤容限寿命（安全寿命），或两者兼有，具体取决于临界性和应用。外包装设计应确保，如果衬里发生泄漏，复合材料将允许泄漏的流体（液体或气体）通过，从而不会有复合材料破裂的风险。 然而，在长时间、高应力的使用条件下，应保证COPV外包装也不会因应力（蠕变）破裂而失效，这通过实验数据的理论分析（确定风险可靠性因素）或测试（试件或飞行硬件）进行验证。 4.11 检测概率（POD）- 使用复杂结构（如COPV）的POD评估无损检测数据可靠性的详细说明超出了本指南的范围。因此，仅提供一般指导。更详细的说明，用于评估无损检测方法在POD方面的能力，作为缺陷尺寸的函数， 一 ，可在MIL中找到- HDBK-1823。估计POD的统计精度 （a） 功能( 图2 )取决于具有目标的检查点的数量、检查点目标的大小以及检查结果的基本性质（命中/未命中或信号响应的幅度）。 图2 检测概率作为缺陷尺寸的函数 注1: 吊舱( 一 )，显示最小可检测缺陷的位置，以及 一 90 （左）。吊舱( 一 )添加置信边界并显示 一 90/95 （右）。 4.11.1 鉴于此 一 90/95 已经成为事实上的设计标准，更重要的是估算POD的第90个百分位 （a） 函数比曲线的较低部分更精确。 这可以通过在目标区域放置更多目标来实现 一 90 值，但具有一系列大小，因此仍然可以估计整个曲线。 注10: 一 90/95 对于复合外包装和POD的生成( 一 )功能是基于以下假设进行预测的:缺陷的影响已经证明，并且已知特定的复合缺陷类型和尺寸，并且检测相同类型和尺寸的缺陷是拒收的理由，即该缺陷是可拒收的缺陷。 4.11.2 在POD估计中提供合理的精度( 一 )功能、经验表明，如果系统仅提供二进制命中/未命中响应，则样本测试集至少包含60个目标位点；如果系统提供定量目标响应，则至少包含40个目标位点， â . 这些数字是最小值。 4.11.3 在POD研究中，NDT方法应分为三类: 4.11.3.1 仅产生关于缺陷存在或不存在的定性信息的信息，即命中/未命中数据。 4.11.3.2 还提供了一些定量测量目标尺寸（例如，缺陷或裂纹）的方法，即， â 对 一 数据 4.11.3.3 产生目标及其周围环境的视觉图像。

1.1 This guide discusses current and potential nondestructive testing (NDT) procedures for finding indications of discontinuities and accumulated damage in the composite overwrap of filament wound pressure vessels, also known as composite overwrapped pressure vessels (COPVs). In general, these vessels have metallic liner thicknesses less than 2.3 mm (0.090 in.), and fiber loadings in the composite overwrap greater than 60 % by weight. In COPVs, the composite overwrap thickness will be of the order of 2.0 mm (0.080 in.) for smaller vessels and up to 20 mm (0.80 in.) for larger ones. 1.2 This guide focuses on COPVs with nonload-sharing metallic liners used at ambient temperature, which most closely represents a Compressed Gas Association (CGA) Type III metal-lined composite tank. However, it also has relevance to (1) monolithic metallic pressure vessels (PVs) (CGA Type I), (2) metal-lined hoop-wrapped COPVs (CGA Type II), (3) plastic-lined composite pressure vessels (CPVs) with a nonload-sharing liner (CGA Type IV), and (4) an all-composite, linerless COPV (undefined Type). This guide also has relevance to COPVs used at cryogenic temperatures. 1.3 The vessels covered by this guide are used in aerospace applications; therefore, the inspection requirements for discontinuities and inspection points will in general be different and more stringent than for vessels used in non aerospace applications. 1.4 This guide applies to (1) low pressure COPVs used for storing aerospace media at maximum allowable working pressures (MAWPs) up to 3.5 MPa (500 psia) and volumes up to 2 L (70 ft 3 ), and (2) high pressure COPVs used for storing compressed gases at MAWPs up to 70 MPa (10 000 psia) and volumes down to 8 L (500 in. 3 ). Internal vacuum storage or exposure is not considered appropriate for any vessel size. Note 1: Some vessels are evacuated during filling operations, requiring the tank to withstand external (atmospheric) pressure, while other vessels may either contain or be immersed in cryogenic fluids, or both, requiring the tanks to withstand any potentially deleterious effects of differential thermal contraction. 1.5 The composite overwraps under consideration include, but are not limited to, ones made from various polymer matrix resins (for example, epoxies, cyanate esters, polyurethanes, phenolic resins, polyimides (including bismaleimides), and polyamides) with continuous fiber reinforcement (for example, carbon, aramid, glass, or poly-(phenylenebenzobisoxazole) (PBO)). The metallic liners under consideration include, but are not limited to, aluminum alloys, titanium alloys, nickel-chromium alloys, and stainless steels. 1.6 This guide describes the application of established NDT methods; namely, Acoustic Emission (AE, Section 7 ), Eddy Current Testing (ET, Section 8 ), Laser Shearography (Section 9 ), Radiographic Testing (RT, Section 10 ), Infrared Thermography (IRT, Section 11 ), Ultrasonic Testing (UT, Section 12 ), and Visual Testing (VT, Section 13 ). These methods can be used by cognizant engineering organizations for detecting and evaluating flaws, defects, and accumulated damage in the composite overwrap of new and in-service COPVs. Note 2: Although visual testing is discussed and required by current range standards, emphasis is placed on complementary NDT procedures that are sensitive to detecting flaws, defects, and damage that leave no visible indication on the COPV surface. Note 3: In aerospace applications, a high priority is placed on light weight material, while in commercial applications, weight is typically sacrificed to obtain increased robustness. Accordingly, the need to detect damage below the visual damage threshold is more important in aerospace vessels. Note 4: Currently, no determination of residual strength can be made by any NDT method. 1.7 All methods discussed in this guide (AE, ET, shearography, RT, IRT, UT, and VT) are performed on the composite overwrap after overwrapping and structural cure. For NDT procedures for detecting discontinuities in thin-walled metallic liners in filament wound pressure vessels, or in the bare metallic liner before overwrapping; namely, AE, ET, laser profilometry, leak testing (LT), penetrant testing (PT), and RT; consult Guide E2982 . 1.8 In the case of COPVs which are impact damage sensitive and require implementation of a damage control plan, emphasis is placed on NDT methods that are sensitive to detecting damage in the composite overwrap caused by impacts at energy levels and which may or may not leave any visible indication on the COPV composite surface. 1.9 This guide does not specify accept-reject criteria ( 4.9 ) to be used in procurement or used as a means for approving filament wound pressure vessels for service. Any acceptance criteria specified are given solely for purposes of refinement and further elaboration of the procedures described in this guide. Project or original equipment manufacturer (OEM) specific accept/reject criteria should be used when available and take precedence over any acceptance criteria contained in this document. If no accept/reject criteria are available, any NDT method discussed in this guide that identifies broken fibers should require disposition by the cognizant engineering organization. 1.10 This guide references both established ASTM methods that have a foundation of experience and that yield a numerical result, and newer procedures that have yet to be validated and are better categorized as qualitative guidelines and practices. The latter are included to promote research and later elaboration in this guide as methods of the former type. 1.11 To ensure proper use of the referenced standard documents, there are recognized NDT specialists that are certified according to industry and company NDT specifications. It is recommended that an NDT specialist be a part of any composite component design, quality assurance, in-service maintenance, or damage examination. 1.12 Units— The values stated in SI units are to be regarded as standard. The English units given in parentheses are provided for information only. 1.13 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. Some specific hazards statements are given in Section 7 on Hazards. 1.14 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 ====== 4.1 The COPVs covered in this guide consist of a metallic liner overwrapped with high-strength fibers embedded in polymeric matrix resin (typically a thermoset) ( Fig. 1 ). Metallic liners may be spun-formed from a deep drawn/extruded monolithic blank or may be fabricated by welding formed components. Designers often seek to minimize the liner thickness in the interest of weight reduction. COPV liner materials used can be aluminum alloys, titanium alloys, nickel-chromium alloys, and stainless steels, impermeable polymer liner such as high density polyethylene, or integrated composite materials. Fiber materials can be carbon, aramid, glass, PBO, metals, or hybrids (two or more types of fibers). Matrix resins include epoxies, cyanate esters, polyurethanes, phenolic resins, polyimides (including bismaleimides), polyamides, and other high performance polymers. Common bond line adhesives are FM-73, urethane, West 105, and Epon 862 with thicknesses ranging from 0.13 mm (0.005 in.) to 0.38 mm (0.015 in.). Metallic liner and composite overwrap materials requirements are found in ANSI/AIAA S-080 and ANSI/AIAA S-081, respectively. Note 6: When carbon fiber is used, galvanic protection should be provided for the metallic liner using a physical barrier such as glass cloth in a resin matrix, or similarly, a bond line adhesive. Note 7: Per the discretion of the cognizant engineering organization, composite materials not developed and qualified in accordance with the guidelines in MIL-HDBK-17, Volumes 1 and 3 should have an approved material usage agreement. FIG. 1 Typical Carbon Fiber Reinforced COPVs (NASA) 4.2 The as-wound COPV is then cured and an autofrettage/proof cycle is performed to evaluate performance and increase fatigue characteristics. 4.3 The strong drive to reduce weight and spatial needs in aerospace applications has pushed designers to adopt COPVs constructed with high modulus carbon fibers embedded in an epoxy matrix. Unfortunately, high modulus fibers are weak in shear and therefore highly susceptible to fracture caused by mechanical damage. Mechanical damage to the overwrap can leave no visible indication on the composite surface, yet produce subsurface damage. Note 8: The impact damage tolerance of the composite overwrap will depend on the size and shape of the vessel, composite thickness (number of plies), and thickness of the composite overwrap relative to that of the liner. 4.4 Per MIL-HDBK-340 and ANSI/AIAA S-081, the primary intended function of COPVs as discussed in this guide will be to store pressurized gases and fluids where one or more of the following apply: 4.4.1 Contains stored energy of 19 310 J (14 240 ft-lbf) or greater based on adiabatic expansion of a perfect gas. 4.4.2 Contains a gas or liquid that would endanger personnel or equipment or create a mishap (accident) if released. 4.4.3 Experiences a design limit pressure greater than 690 kPa (100 psi). 4.5 According to NASA-STD-(I)-5019, COPVs shall comply with the latest revision of ANSI/AIAA S-081. The following requirements also apply when implementing S-081: 4.5.1 Maximum Design Pressure (MDP) shall be substituted for all references to Maximum Expected Operating Pressure (MEOP) in S-081. 4.5.2 COPVs shall have a minimum of 0.999 probability of no stress rupture failure of the composite shell during the service life. Note 9: For other aerospace applications, the cognizant engineering organization should select the appropriate probability of survival, for example, 0.99, 0.999, 0.9999, etc., depending on the anticipated failure mode, damage tolerance, safety factor, or consequence of failure, or a combination thereof. For example, a probability of survival of 0.99 means that on average, 1 in 100 COPVs will fail. COPVs exhibiting catastrophic failure modes (BBL composite shell stress rupture versus LBB liner leak), lower damage tolerance (cylindrical versus spherical vessels), lower safety factor, and high consequence of failure will be subject to more rigorous NDT. 4.6 Application of the NDT procedures discussed in this guide is intended to reduce the likelihood of composite overwrap failure, commonly denoted “burst before leak” (BBL), characterized by catastrophic rupture of the overwrap and significant energy release, thus mitigating or eliminating the attendant risks associated with loss of pressurized commodity, and possibly ground support personnel, crew, or mission. 4.6.1 NDT is done on fracture-critical parts such as COPVs to establish that a low probability of preexisting flaws is present in the hardware. 4.6.2 Following the discretion of the cognizant engineering organization, NDT for fracture control of COPVs should follow additional general and detailed guidance described in MIL-HDBK-6870, NASA-STD-(I)-5019, MSFC-RQMT-3479, or ECSS-E-30-01A, or a combination thereof, not covered in this guide. 4.6.3 Hardware that is proof tested as part of its acceptance (that is, not screening for specific flaws) should receive post-proof NDT at critical welds and other critical locations. 4.7 Discontinuity Types— Specific discontinuity types are associated with the particular processing, fabrication, and service history of the COPV. Metallic liners can have cracks, buckles, leaks, and a variety of weld discontinuities (see 4.6 in Guide E2982 ). Non-bonding flaws (voids) between the liner and composite overwrap can also occur. Similarly, the composite overwrap can have preexisting manufacturing flaws introduced during fabrication, and damage caused by autofrettage or proof testing before being placed into service. Once in service, additional damage can be incurred due to low velocity or micrometeorite orbital debris impacts, cuts/scratches/abrasion, fire, exposure to aerospace media, loading stresses, thermal cycling, physical aging, oxidative degradation, weathering, and space environment effects (exposure to atomic oxygen and ionizing radiation). These factors will lead to complex damage states in the overwrap that can be visible or invisible, macroscopic or microscopic. These damage states can be characterized by the presence of porosity, depressions, blisters, wrinkling, erosion, chemical modification, foreign object debris (inclusions), tow termination errors, tow slippage, misaligned tows, distorted tows, matrix crazing, matrix cracking, matrix-rich regions, under and over-cure of the matrix, fiber-rich regions, fiber-matrix debonding, fiber pull-out, fiber splitting, fiber breakage, bridging, liner/overwrap debonding, and delamination. Often these discontinuities can placed into four major categories: (1) manufacturing; (2) scratch/scuff/abrasion; (3) mechanical damage; and (4) discoloration. 4.8 Effect of Defect— The effect of a given composite flaw type or size (“effect of defect”) is difficult to determine unless test specimens or articles with known types and sizes of flaws are tested to failure. Given this potential uncertainty, detection of a flaw is not necessarily grounds for rejection (that is, a defect) unless the effect of defect has been demonstrated. Even the detection of a given flaw type and size can be in doubt unless physical reference specimens with known flaw types and sizes undergo evaluation using the NDT method of choice. The suitability of various NDT methods for detecting commonly occurring composite flaw types is given in Table 1 in Guide E2533 . 4.9 Acceptance Criteria— Determination about whether a COPV meets acceptance criteria and is suitable for aerospace service should be made by the cognizant engineering organization. When examinations are performed in accordance with this guide, the engineering drawing, specification, purchase order, or contract should indicate the acceptance criteria. 4.9.1 Accept/reject criteria should consist of a listing of the expected kinds of imperfections and the rejection level for each. 4.9.2 The classification of the articles under test into zones for various accept/reject criteria should be determined from contractual documents. 4.9.3 Rejection of COPVs— If the type, size, or quantities of defects are found to be outside the allowable limits specified by the drawing, purchase order, or contract, the composite article should be separated from acceptable articles, appropriately identified as discrepant, and submitted for material review by the cognizant engineering organization, and given one of the following dispositions: (1) acceptable as is, (2) subject to further rework or repair to make the materials or component acceptable, or (3) scrapped (made permanently unusable) when required by contractual documents. 4.9.4 Acceptance criteria and interpretation of results should be defined in requirements documents prior to performing the examination. Advance agreement should be reached between the purchaser and supplier regarding the interpretation of the results of the examinations. All discontinuities having signals that exceed the rejection level as defined by the process requirements documents should be rejected unless it is determined from the part drawing that the rejectable discontinuities will not remain in the finished part. 4.10 Certification of COPVs— ANSI/AIAA S-081 defines the approach for design, analysis, and certification of COPVs. More specifically, the COPV should exhibit a leak before burst (LBB) failure mode or should possess adequate damage tolerance life (safe-life), or both, depending on criticality and whether the application is for a hazardous or nonhazardous fluid. Consequently, the NDT method should detect any discontinuity that can cause burst at expected operating conditions during the life of the COPV. The Damage-Tolerance Life requires that any discontinuity present in the liner will not grow to failure during the expected life of the COPV. Fracture mechanics assessments of flaw growth are the typical method of setting limits on the sizes of discontinuities that can safely exist. This establishes the defect criteria: all discontinuities equal to or larger than the minimum size or have J -integral or other applicable fracture mechanics based criteria that will result in failure of the vessel within the expected service life are classified as defects and should be addressed by the cognizant engineering organization. 4.10.1 Design Requirements— COPV design requirements related to the composite overwrap are given in ANSI/AIAA S-081. The key requirement is the stipulation that the COPV shall exhibit a LBB failure mode or shall possess adequate damage tolerance life (safe-life), or both, depending on criticality and application. The overwrap design shall be such that, if the liner develops a leak, the composite will allow the leaking fluid (liquid or gas) to pass through it so that there will be no risk of composite rupture. However, under use conditions of prolonged, elevated stress, assurance should be made that the COPV overwrap will also not fail by stress (creep) rupture, as verified by theoretical analysis of experimental data (determination of risk reliability factors) or by test (coupons or flight hardware). 4.11 Probability of Detection (POD)— Detailed instruction for assessing the reliability of NDT data using POD of a complex structure such as a COPV is beyond the scope of this guide. Therefore, only general guidance is provided. More detailed instruction for assessing the capability of an NDT method in terms of the POD as a function of flaw size, a , can be found in MIL-HDBK-1823. The statistical precision of the estimated POD (a) function ( Fig. 2 ) depends on the number of inspection sites with targets, the size of the targets at the inspection sites, and the basic nature of the examination result (hit/miss or magnitude of signal response). FIG. 2 Probability of Detection as a Function of Flaw Size Note 1: POD( a ), showing the location of the smallest detectable flaw and a 90 (left). POD( a ) with confidence bounds added and showing the location of a 90/95 (right). 4.11.1 Given that a 90/95 has become a de facto design criterion, it is more important to estimate the 90th percentile of the POD (a) function more precisely than lower parts of the curve. This can be accomplished by placing more targets in the region of the a 90 value but with a range of sizes so the entire curve can still be estimated. Note 10: a 90/95 for a composite overwrap and generation of a POD( a ) function is predicated on the assumption that effect of defect has been demonstrated and is known for a specific composite flaw type and size, and that detection of a flaw of that same type and size is grounds for rejection, that is, the flaw is a rejectable defect. 4.11.2 To provide reasonable precision in the estimates of the POD( a ) function, experience suggests that the specimen test set contain at least 60 targeted sites if the system provides only a binary, hit/miss response and at least 40 targeted sites if the system provides a quantitative target response, â . These numbers are minimums. 4.11.3 For purposes of POD studies, the NDT method should be classified into one of three categories: 4.11.3.1 Those which produce only qualitative information as to the presence or absence of a flaw, that is, hit/miss data. 4.11.3.2 Those which also provide some quantitative measure of the size of the target (for example, flaw or crack), that is, â versus a data. 4.11.3.3 Those which produce visual images of the target and its surroundings.