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 a90 (left). POD(a) with confidence bounds added and showing the location of a90/95 (right).
4.11.1 Given that a90/95 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 a90 value but with a range of sizes so the entire curve can still be estimated.
Note 10: a90/95 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.
Scope
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 ft3), 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.
