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（也称为复合材料外包装压力容器（COPV））中薄壁金属内衬的不连续迹象。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，最大直径为20毫米（0。 80英寸。）对于较大的。 1.2 本指南重点介绍了在环境温度下使用非负载共享金属内衬的COPV，它最接近于代表压缩气体协会（CGA）III型金属内衬COPV。然而，它也与 (1) 单片金属压力容器（PV）（CGA I型），以及 (2) 金属内衬环包COPV（CGA II型）。 1.3 本指南涵盖的船舶用于航空航天应用；因此，不连续性和检查点的检查要求通常不同于非连续性容器，且更为严格- 航空航天应用。 1.4 本指南适用于 (1) 低压COPV和PV用于在最大允许工作压力（MAWP）高达3.5 MPa（500 psia）和容量高达2000 L（70 ft）的情况下存储航空航天介质 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 in。 3. ). 内部真空储存或暴露不适用于任何容器尺寸。 注1: 一些容器在加注操作期间被排空，要求储罐承受外部（大气）压力。 1.5 正在考虑的金属衬里包括但不限于由铝合金、钛合金、镍基合金和不锈钢制成的衬里。在COPV的情况下，包覆后应通过其进行无损检测的复合材料包括但不限于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）、聚酰胺），具有连续纤维增强（例如，碳、芳纶、玻璃或聚甲醛）- （苯撑苯并双恶唑）（PBO））。 1.6 本指南描述了既定无损检测程序的应用；即声发射（AE，截面 7. )，涡流检测（ET，第 8. )，激光轮廓术（LP，第 9 )，泄漏测试（LT，第节 10 )渗透检测（PT，第 11 )和射线检测（RT，第 12 ). 这些程序可供认可的工程组织用于检测和评估金属PV、覆绕前COPV的裸露金属内衬以及新的和新的金属内衬中的缺陷、缺陷和累积损伤- 服务COPV。 1.7 本指南中讨论的所有方法（AE、ET、LP、LT、PT和RT）均在覆层和结构固化前后在COPV的金属内衬上进行。同样的方法也可以在金属PV上执行。用于检测纤维缠绕压力容器中复合外包装中不连续性的无损检测程序；即AE、ET、剪切成像检测（ST）、RT、超声波检测（UT）和视觉检测（VT）；参考指南 E2981年 . 1.8 由于与检查薄板相关的困难- 通过复合外包装的有壁金属COPV衬里，以及中列出的无损检测方法的可用性 1.6 为了在外包装和金属PVs之前检查COPV衬里，本标准未涉及超声波检测（UT）。UT仍可以按照供应商和客户之间的约定进行。超声波要求可利用实践 E2375 根据具体的衬里应用和金属厚度，视情况而定。替代超声波检测方法，如兰姆波、表面波、剪切波、反射板等。 可根据商定的合同要求建立和记录。测试要求应结合工程分析确定的具体标准制定。 1.9 一般来说，声发射和渗透检测在覆波前在光伏或COPV的裸金属内衬上进行（对于COPV，声发射在覆波前进行，以最大限度地减少复合覆波的干扰）。ET、LT和RT在PV上进行，PV是COPV的裸金属内衬，在覆层之前，或在as上进行- 制造的COPV。LP在PV的内外表面上进行，或在覆层前后在COPV衬里的内表面上进行。此外，AE和RT非常适合评估焊接PVs和COPV衬里的焊接完整性。 1.10 只要可能，所述无损检测程序应足够灵敏，以检测1.3 mm（0.050 in.）量级的关键缺陷尺寸长宽比为2:1的长度。 注2: 衬垫经常因焊接不当而失效，导致多个0级小不连续的萌生和生长。 050毫米（0.002英寸）长这些缺陷将形成1毫米（0.040英寸）的宏观缺陷仅在较高应力水平下的长度。 1.11 对于检测纤维缠绕压力容器复合外包装中不连续性的无损检测程序（即AE、ET、剪切成像、热成像、UT和目视检查），请参阅指南 E2981年 . 1.12 对于冲击损伤敏感且需要实施损伤控制计划的COPV，重点放在无损检测程序上，该程序对检测能量水平下冲击引起的金属内衬损伤敏感，该能量水平可能会或可能不会在COPV复合材料表面上留下任何可见指示。 1.13 本指南未规定接受/拒绝标准( 4.10 )用于采购或用作批准PV或COPV服务的手段。本文提供的任何验收标准主要用于完善和进一步阐述指南中所述的程序。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。 1.14 本指南参考了已有的ASTM试验方法，这些方法具有经验基础并产生数值结果，以及尚未验证的更新程序，这些程序更好地归类为定性指南和实践。 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。 1.15 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薄壁金属部件设计、质量保证、在役维护或损坏检查。 1.16 单位- 以公制单位表示的值应视为标准。 括号中的英文单位仅供参考。 1.17 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。 1.18 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 无损检测的目标是检测与COPV金属衬里故障有关的缺陷，或导致泄漏、内容物丢失、受伤、死亡或任务或其组合的缺陷。通过无损检测检测发现的需要认可工程组织特别注意的衬里缺陷包括贯穿裂纹、部分贯穿裂纹、衬里屈曲、点蚀、变薄和在循环载荷、持续载荷、温度循环、机械冲击和其他预期或非预期使用条件影响下的腐蚀。 注3: 不锈钢和镍基合金制成的衬里比铝制成的衬里具有更高的抗冲击损伤能力。 注4: 安全寿命是任何COPV的目标，以便在使用寿命期间不会在衬里中形成贯穿裂纹。 注5: 使用具有良好疲劳和缓慢裂纹扩展特性的材料非常重要。例如，镍基合金优于沉淀硬化不锈钢。铝还具有良好的延展性和抗裂性。 4.2 本指南中涵盖的COPV由一个金属内衬组成，内衬有高- 嵌入聚合物基体树脂（通常为热固性树脂）中的高强度纤维。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。常见的粘合剂通常是环氧树脂（FM-73、West 105和Epon 862）或厚度在0.13 mm（0.005 in.）之间的聚氨酯至0.38毫米（0.015英寸）。金属内衬和复合外包装材料要求分别见ANSI/AIAA S-080和ANSI/AIAA S-081。指南中显示了代表性COPV的图片 E2981年 . 4.3 COPV金属内衬和金属PV的操作失效模式，按可能性的近似顺序为: （a） 疲劳开裂， （b） 屈曲， （c） 腐蚀 （d） 环境开裂，以及 （e） 超载。 注6: 对于运载火箭和卫星，为了减轻重量，设计者们不得不采用带有更薄金属内衬的COPV。不幸的是，这种配置更容易受到衬里屈曲的影响。因此，作为缸套疲劳的前兆，应注意缸套屈曲。 4.4 每密耳- HDBK-340，本指南中讨论的COPV的主要预期功能是在以下一个或多个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中所有对最大预期工作压力（MEOP）的引用。 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7: 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 本标准中讨论的无损检测程序的应用旨在降低衬里失效的可能性，通常表示为爆裂前泄漏（LBB），其特征是压力商品的泄漏和损失，从而减轻或消除与压力商品损失和可能的任务相关的伴随风险。 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-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）应收到post- 在关键焊缝和其他关键位置进行无损检测。 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。COPV复合覆盖层可能有无数种可能的不连续类型，在性能影响方面具有不同程度的重要性（见指南中的4.7） E2981年 ). 对于金属衬里中的不连续性，从无损检测角度来看，主要关注的是检测可能在寿命周期内产生裂纹或将衬里残余强度降低到所需水平以下的不连续性。 因此，不连续性应分类如下: 4.7.1 固有材料不连续性:在试验过程中检测到的夹杂物、晶界等 （a） 和 （b） 属于 5.5 . 注8: 固有材料不连续性通常比损伤容限尺寸小得多。任何不符合本声明的设计都应进行修改。制造过程中制定的质量控制程序应消除任何不符合规范的原材料。 4.7.2 制造引起的不连续性: 由焊接、机加工、热处理等引起，在 （b） 和 （c） 属于 5.5 . 注9: 制造引起的不连续性取决于制造过程，可能包括加工痕迹、不当热处理和与焊接相关的不连续性，如未熔合、气孔、夹杂物、局部材料脆化区、收缩和开裂。 4.7.3 服务引起的不连续性:在运行期间检测到的疲劳、腐蚀、应力腐蚀开裂、磨损、意外损坏等 （d） 和 （e） 属于 5.5 （安装COPV后）。在这些情况下，无损检测应在“移除并检查”或 现场 “依据取决于使用的程序和设备。 4.8 保守损伤容限寿命评估是通过假设存在类似裂纹的不连续或不连续系统，并确定该不连续的最大尺寸或其他特征来进行的，该不连续在容器投入使用时可能存在，但在预期使用条件下不会发展为失效。 然后定义了应通过无损检测检测的裂纹或类裂纹不连续或类裂纹不连续系统的尺寸或其他特征。 注10: 焊接或机加工可能会导致非裂纹状缺陷/缺陷/条件，这可能很重要，这些缺陷/缺陷/条件的无损检测选择可能与裂纹状缺陷不同。 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，适当地确定为不一致，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 PV认证- ANSI/AIAA S-080定义了金属PV的设计、分析和认证方法。 4.11 COPV认证- ANSI/AIAA S-081定义了COPV的设计、分析和认证方法，而ANSI/AIAA S-080定义了PV的设计、分析和认证方法。更具体地说，PV或COPV薄壁金属衬里应表现出先泄后爆（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，这取决于临界程度以及应用于危险或非危险流体。 因此，无损检测程序应检测在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设定安全存在的不连续尺寸限制的典型方法。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。 4.11.1 设计要求- ANSI/AIAA S-080中给出了与金属衬里相关的COPV设计要求。关键要求是规定PV或COPV薄壁金属内衬应表现出LBB故障模式或应具有足够的损伤容限寿命（安全- 生活），或两者兼而有之。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。 4.12 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。根据POD评估无损检测程序能力的更详细说明，作为缺陷尺寸的函数， 一 ，可在MIL-HDBK-1823中找到。估计POD的统计精度( 一 )功能( 图1 )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1 检测概率作为缺陷尺寸的函数，POD( 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） 4.12.1 鉴于此 一 90/95 已成为事实上的设计标准，重要的是估计90 第 POD百分比( 一 )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11: 一 90/95 用于金属衬里和POD的生成( 一 )函数基于以下假设进行预测:给定厚度的衬里的临界初始缺陷尺寸（CIFS）可以检测到90/95的能力（95%置信水平下90%的检测概率）。 这对于具有非常薄金属内衬的COPV来说是有问题的，其中CIFS将小于NASA-STD-5009表1中给出的最小可检测缺陷尺寸。在此检测极限（CIFS）下< 一 90/95 ), 一 90/95 对于薄壁COPV无效。 4.12.2 NASA-STD-5009定义了各种无损检测程序和应用的典型无损检测能力极限。鉴于损伤容限寿命要求和待检测的潜在不连续性建立的缺陷标准，NASA- STD-5009可用于选择可能达到所需检测能力的无损检测程序。 注12: 断裂关键硬件的无损检测应检测损伤容限断裂分析中使用的初始裂纹尺寸，其能力为90/95。NASA-STD-5009表1所示标准无损检测程序的最小可检测裂纹尺寸满足90/95能力要求。NASA-STD-5009表1中的裂纹尺寸数据主要基于在平坦疲劳条件下进行的无损检测能力研究- 航天飞机项目早期，2219-T87铝板开裂。尽管此后进行了许多其他类似的能力研究和测试，但无论是单独还是组合，都没有普遍应用。在测试所有变量的情况下，进行理想的无损检测能力演示显然是不可管理和不切实际的。 4.12.3 纵横比和等效面积考虑- 管理航空航天金属压力容器（ANSI/AIAA S-080）和COPV衬里（ANSI/AIAA S）的现行标准- 081）要求进行断裂分析，以确定纵横比在0.1到0.5之间的裂纹的CIFS。然而，没有足够的数据支持仅在一个纵横比下进行测试，然后使用等效面积方法将结果扩展到所需的纵横比范围的方法 ( 1- 9 ) . 20 因此，应在所需裂纹纵横比范围内对金属COPV衬里进行POD测试。 注13: 注意事项: 为了最大限度地减少质量，航空航天系统的设计师正在减少金属压力容器和COPV衬里的壁厚。 对于给定的内部压力，壁厚的减少会产生更高的净截面应力，从而导致较小的CIFS。这些较小的裂纹尺寸接近当前无损检测的局限性。如果未能充分证明给定无损检测程序在所需裂纹纵横比范围内的能力，可能导致无法检测到导致储罐灾难性故障的关键缺陷。 4.12.4 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。 4.12.5 为了进行POD研究，NDT程序应分为三类: 4.12.5.1 仅产生缺陷存在或不存在的定性信息，即命中/未命中数据， 4.12.5.2 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，以及 4.12.5.3 产生目标及其周围环境的视觉图像。 4.12.6 详细的吊舱引导- 有关如何进行POD研究的详细指导，包括系统定义和控制、校准、噪声、演示设计、演示测试、数据分析、结果展示、重新测试和过程控制计划，请参阅MIL-HDBK-1823。 4.12.6.1 有关如何对ET、PT和UT进行POD研究的详细指南，请分别参阅MIL-HDBK-1823附录a至D。 4.12.6.2 详细的测试程序指导；样品设计、制造、记录和维护；无损检测数据的统计分析； POD的模型辅助测定；专题；和相关文件，分别查阅MIL-HDBK-1823附录E至J。 4.13 无损检测数据可靠性- MIL-HDBK-1823提供了无约束力的指南，用于评估无损检测程序的检测能力，以检查需要测量无损检测可靠性的新硬件或在用硬件。MIL-HDBK-1823中给出了ET、PT和UT的具体指南。MIL-HDBK-1823可用于其他无损检测程序，如RT或轮廓术，前提是它们提供定量信号， â ，或二进制响应， 命中/未命中 . 由于其目的是将POD与目标尺寸（或任何其他有意义的特征，如化学成分）联系起来，“尺寸”（或特征特征）应明确定义，并可明确测量，即具有类似尺寸的其他目标将从NDT设备产生类似输出。这对于非晶态目标尤其重要，如腐蚀损伤或具有重要化学反应区的埋藏夹杂物。关于无损检测数据可靠性的其他文献在别处给出 ( 2- 7. ) . 注14: 一般情况下，AE不会产生COPV金属内衬中的缺陷大小；但是，可用于接受和拒绝COPV（参见第节 7. 在本指南和指南中 E2981年 ). 4.14 进一步指导- 其他政府文件（NASA-STD-5003、SSP 30558、SSP 52005、NSTS 1700.7B）和非政府文件（NTIAC-DB-97-02、NTIAC-TA-00-01）中提供了裂缝控制的额外指南。

1.1 This guide discusses current and potential nondestructive testing (NDT) procedures for finding indications of discontinuities in thin-walled metallic liners in 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 percent 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 COPV. However, it also has relevance to (1) monolithic metallic pressure vessels (PVs) (CGA Type I), and (2) metal-lined hoop-wrapped COPVs (CGA Type II). 1.3 The vessels covered by this guide are used in aerospace applications; therefore, examination 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 and PVs used for storing aerospace media at maximum allowable working pressures (MAWPs) up to 3.5 MPa (500 psia) and volumes up to 2000 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. 1.5 The metallic liners under consideration include, but are not limited to, ones made from aluminum alloys, titanium alloys, nickel-based alloys, and stainless steels. In the case of COPVs, the composites through which the NDT interrogation should be made after overwrapping include, but are not limited to, various polymer matrix resins (for example, epoxies, cyanate esters, polyurethanes, phenolic resins, polyimides (including bismaleimides), polyamides) with continuous fiber reinforcement (for example, carbon, aramid, glass, or poly-(phenylenebenzobisoxazole) (PBO)). 1.6 This guide describes the application of established NDT procedures; namely, Acoustic Emission (AE, Section 7 ), Eddy Current Testing (ET, Section 8 ), Laser Profilometry (LP, Section 9 ), Leak Testing (LT, Section 10 ), Penetrant Testing (PT, Section 11 ), and Radiographic Testing (RT, Section 12 ). These procedures can be used by cognizant engineering organizations for detecting and evaluating flaws, defects, and accumulated damage in metallic PVs, the bare metallic liner of COPVs before overwrapping, and the metallic liner of new and in-service COPVs. 1.7 All methods discussed in this guide (AE, ET, LP, LT, PT, and RT) are performed on the metallic liner of COPVs before or after overwrapping and structural cure. The same methods may also be performed on metal PVs. For NDT procedures for detecting discontinuities in the composite overwrap in filament wound pressure vessels; namely, AE, ET, Shearography Testing (ST), RT, Ultrasonic Testing (UT) and Visual Testing (VT); consult Guide E2981 . 1.8 Due to difficulties associated with inspecting thin-walled metallic COPV liners through composite overwraps, and the availability of the NDE methods listed in 1.6 to inspect COPV liners before overwrapping and metal PVs, ultrasonic testing (UT) is not addressed in this standard. UT may still be performed as agreed upon between the supplier and customer. Ultrasonic requirements may utilize Practice E2375 as applicable based upon the specific liner application and metal thickness. Alternate ultrasonic inspection methods such as Lamb wave, surface wave, shear wave, reflector plate, etc. may be established and documented per agreed upon contractual requirements. The test requirements should be developed in conjunction with the specific criteria defined by engineering analysis. 1.9 In general, AE and PT are performed on the PV or the bare metallic liner of a COPV before overwrapping (in the case of COPVs, AE is done before overwrapping to minimize interference from the composite overwrap). ET, LT, and RT are performed on the PV, bare metallic liner of a COPV before overwrapping, or on the as-manufactured COPV. LP is performed on the inner and outer surfaces of the PV, or on the inner surface of the COPV liner both before and after overwrapping. Furthermore, AE and RT are well suited for evaluating the weld integrity of welded PVs and COPV liners. 1.10 Wherever possible, the NDT procedures described should be sensitive enough to detect critical flaw sizes of the order of 1.3 mm (0.050 in.) length with a 2:1 aspect ratio. Note 2: Liners often fail due to improper welding resulting in initiation and growth of multiple small discontinuities of the order of 0.050 mm (0.002 in.) length. These will form a macro-flaw of 1-mm (0.040-in.) length only at higher stress levels. 1.11 For NDT procedures that detect discontinuities in the composite overwrap of filament-wound pressure vessels (namely, AE, ET, shearography, thermography, UT and visual examination), consult Guide E2981 . 1.12 In the case of COPVs which are impact damage sensitive and require implementation of a damage control plan, emphasis is placed on NDT procedures that are sensitive to detecting damage in the metallic liner caused by impacts at energy levels which may or may not leave any visible indication on the COPV composite surface. 1.13 This guide does not specify accept/reject criteria ( 4.10 ) used in procurement or used as a means for approving PVs or COPVs for service. Any acceptance criteria provided herein are given mainly for purposes of refinement and further elaboration of the procedures described in the 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. 1.14 This guide references established ASTM test methods that have a foundation of experience and that yield a numerical result, and newer procedures that have yet to be validated which 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. 1.15 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 thin-walled metallic component design, quality assurance, in-service maintenance, or damage examination. 1.16 Units— The values stated in metric units are to be regarded as the standard. The English units given in parentheses are provided for information only. 1.17 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. 1.18 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 goal of the NDT is to detect defects that have been implicated in the failure of the COPV metal liner, or have led to leakage, loss of contents, injury, death, or mission, or a combination thereof. Liner defects detected by NDT that require special attention by the cognizant engineering organization include through cracks, part-through cracks, liner buckling, pitting, thinning, and corrosion under the influence of cyclic loading, sustained loading, temperature cycling, mechanical impact and other intended or unintended service conditions. Note 3: Liners made from stainless steel and nickel-based alloys exhibit a higher damage resistance to impact than those made from aluminum. Note 4: Safe life is the goal for any COPV so that a through crack in the liner will not develop during the service life. Note 5: The use a material with good fatigue and slow crack growth characteristics is important. For example, nickel-based alloys are better than precipitation-hardened stainless steel. Aluminum also has good ductility and crack resistance. 4.2 The COPVs covered in this guide consist of a metallic liner overwrapped with high-strength fibers embedded in polymeric matrix resin (typically a thermoset). 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 fiber). Matrix resins include epoxies, cyanate esters, polyurethanes, phenolic resins, polyimides (including bismaleimides), polyamides and other high performance polymers. Common bond line adhesives are generally epoxies (FM-73, West 105, and Epon 862) or urethanes with thicknesses ranging from 0.13 mm (0.005 in.) to 0.38 mm (0.015 in.). Metal liner and composite overwrap materials requirements are found in ANSI/AIAA S-080 and ANSI/AIAA S-081, respectively. Pictures of representative COPVs are shown in Guide E2981 . 4.3 The operative failure modes COPV metal liners and metal PVs, in approximate order of likelihood, are: (a) fatigue cracking, (b) buckling, (c) corrosion, (d) environmental cracking, and (e) overload. Note 6: For launch vehicles and satellites, the strong drive to reduce weight has pushed designers to adopt COPVs with thinner metal liners. Unfortunately, this configuration is more susceptible to liner buckling. Therefore, as a precursor to liner fatigue, attention should be paid to liner buckling. 4.4 Per MIL-HDBK-340, 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 Per NASA-STD-(I)-5019, COPVs should 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) should 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 7: 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 standard is intended to reduce the likelihood of liner failure, commonly denoted leak before burst (LBB), characterized by leakage and loss of the pressurized commodity, thus mitigating or eliminating the attendant risks associated with loss of the pressurized commodity, and possibly 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 Per 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-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. COPV composite overwraps can have a myriad of possible discontinuity types, with varying degrees of importance in terms of effect on performance (see 4.7 in Guide E2981 ). As for discontinuities in the metallic liner, the primary concern from an NDT perspective is to detect discontinuities that can develop cracks or reduce residual strength of the liner below the levels required, within the context of the life cycle. Therefore, discontinuities should be categorized as follows: 4.7.1 Inherent material discontinuities: inclusions, grain boundaries, etc., detected during (a) and (b) of 5.5 . Note 8: Inherent material discontinuities are generally much smaller than the damage-tolerance limit size. Any design that does not satisfy this statement should be revised. Quality control procedures in place in the manufacturing process should eliminate any source materials that do not satisfy specifications. 4.7.2 Manufacturing-induced discontinuities: caused by welding, machining, heat treatment, etc., detected during (b) and (c) of 5.5 . Note 9: Manufacturing-induced discontinuities depend on the manufacturing process, and can include machining marks, improper heat treatment, and weld-related discontinuities such as lack of fusion, porosity, inclusions, zones of local material embrittlement, shrinkage, and cracking. 4.7.3 Service-induced discontinuities: fatigue, corrosion, stress corrosion cracking, wear, accidental damage, etc. detected during (d) and (e) of 5.5 (after the COPV has been installed). In these cases, NDT should either be made on a “remove and inspect” or “ in-situ ” basis depending on the procedure and equipment used. 4.8 A conservative damage-tolerance life assessment is made by assuming the existence of a crack-like discontinuity or system of discontinuities, and determining the maximum size or other characteristic of this discontinuity(s) that can exist at the time the vessel is placed into service but not progress to failure under the expected service conditions. This then defines the dimensions or other characteristics of the crack or crack-like discontinuity or system of crack-like discontinuities that should be detected by NDT. Note 10: Welding or machining may result in non-crack like flaws/imperfections/conditions that may be important, and NDT choices for these flaws/imperfections/conditions may be different than for crack-like ones. 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 result 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 PVs— ANSI/AIAA S-080 defines the approach for design, analysis, and certification of metallic PVs. 4.11 Certification of COPVs— ANSI/AIAA S-081 defines the approach for design, analysis, and certification of COPVs, while ANSI/AIAA S-080 defines the approach for design, analysis, and certification of PVs. More specifically, the PV or COPV thin-walled metal liner should exhibit a leak before burst (LBB) failure mode or shall 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 procedure 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 assessment of crack growth is the typical approach used for 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. 4.11.1 Design Requirements— COPV design requirements related to the metallic liner are given in ANSI/AIAA S-080. The key requirement is the stipulation that the PV or COPV thin-walled metal liner should exhibit an LBB failure mode or should possess adequate damage tolerance life (safe-life), or both. The overwrap design should 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. 4.12 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 procedure 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. 1 ) depends on the number of examination sites with targets, the size of the targets at the examination sites, and the basic nature of the examination result (hit/miss or magnitude of signal response). FIG. 1 Probability of Detection as a Function of Flaw Size, 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) 4.12.1 Given that a 90/95 has become a de facto design criterion, it is important to estimate the 90 th 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 11: a 90/95 for a metallic liner and generation of a POD( a ) function is predicated on the assumption that critical initial flaw size (CIFS) for a liner of a given thickness can be detected with a capability of 90/95 (90 percent probability of detection at a 95 percent confidence level). This is problematic for COPVs with very thin metallic liners where the CIFS will be smaller than the minimum detectable flaw sizes given in Table 1 in NASA-STD-5009. At this limit of detection (CIFS < a 90/95 ), a 90/95 will have no validity for a thin-walled COPV. 4.12.2 NASA-STD-5009 defines typical limits of NDT capability for a wide range of NDT procedures and applications. Given the defect criteria established by the Damage-Tolerance Life requirements and the potential discontinuities to be detected, NASA-STD-5009 can be used to select NDT procedures that are likely to achieve the required examination capability. Note 12: NDT of fracture critical hardware should detect the initial crack sizes used in the damage tolerance fracture analyses with a capability of 90/95. The minimum detectable crack sizes for the standard NDT procedures shown in Table 1 of NASA-STD-5009 meet the 90/95 capability requirement. The crack size data in Table 1 of NASA-STD-5009 are based principally on an NDT capability study that was conducted on flat, fatigue-cracked 2219-T87 aluminum panels early in the Space Shuttle program. Although many other similar capability studies and tests have been conducted since, none have universal application, neither individually or in combination. Conducting an ideal NDT capability demonstration where all of the variables are tested is obviously unmanageable and impractical. 4.12.3 Aspect Ratio and Equivalent Area Considerations— Current standards governing aerospace metallic pressure vessels (ANSI/AIAA S-080) and COPV liners (ANSI/AIAA S-081) require that fracture analysis be performed to determine the CIFS for cracks having an aspect ratio ranging from 0.1 to 0.5. However, there is insufficient data to support the approach of testing at only one aspect ratio and then using an equivalent area approach to extend the results to the required range of aspect ratios ( 1- 9 ) . 20 Accordingly, POD testing on metallic COPV liners should be performed at the bounds of the required range of crack aspect ratios. Note 13: Caution: To minimize mass, designers of aerospace systems are reducing the wall thickness for metallic pressure vessels and COPV liners. This reduction in wall thickness produces higher net section stresses, for a given internal pressure, resulting in smaller CIFS. These smaller crack sizes approach the limitations of current NDT. Failure to adequately demonstrate the capabilities of a given NDT procedure over the required range of crack aspect ratios may lead to the failure to detect a critical flaw resulting in a catastrophic tank failure. 4.12.4 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. 4.12.5 For purposes of POD studies, the NDT procedure should be classified into one of three categories: 4.12.5.1 Those which produce only qualitative information as to the presence or absence of a flaw, that is, hit/miss data, 4.12.5.2 Those which also provide some quantitative measure of the size of the target (for example, flaw or crack), that is, â versus a data, and 4.12.5.3 Those which produce visual images of the target and its surroundings. 4.12.6 Detailed POD Guidance— For detailed guidance on how to conduct a POD study, including system definition and control, calibration, noise, demonstration design, demonstration tests, data analysis, presentation of results, retesting, and process control plan, consult MIL-HDBK-1823. 4.12.6.1 For detailed guidance on how to conduct a POD study for ET, PT, and UT, consult MIL-HDBK-1823, Appendices A through D, respectively. 4.12.6.2 For detailed test program guidance; specimen design, fabrication, documentation, and maintenance; statistical analysis of NDT data; model-assisted determination of POD; special topics; and related documents, consult MIL-HDBK-1823, Appendices E through J, respectively. 4.13 NDT Data Reliability— MIL-HDBK-1823 provides nonbinding guidance for estimating the detection capability of NDT procedures for examining either new or in-service hardware for which a measure of NDT reliability is needed. Specific guidance is given in MIL-HDBK-1823 for ET, PT, and UT. MIL-HDBK-1823 may be used for other NDT procedures, such as RT or Profilometry, provided they provide either a quantitative signal, â , or a binary response, hit/miss . Because the purpose is to relate POD with target size (or any other meaningful feature like chemical composition), “size” (or feature characteristic) should be explicitly defined and be unambiguously measurable, that is, other targets having similar sizes will produce similar output from the NDT equipment. This is especially important for amorphous targets like corrosion damage or buried inclusions with a significant chemical reaction zone. Other literature on NDT data reliability is given elsewhere ( 2- 7 ) . Note 14: AE as generally practiced does not yield the size of a flaw in a metallic liner of a COPV; however, can be used for accept-reject of COPVs (see Section 7 in both this guide and Guide E2981 ). 4.14 Further Guidance— Additional guidance for fracture control is provided in other governmental documents (NASA-STD-5003, SSP 30558, SSP 52005, NSTS 1700.7B), and non-government documents (NTIAC-DB-97-02, NTIAC-TA-00-01).