NDE techniques assessment for Gr/Ep Composite Overwrapped Pressure Vessels

The Naval Research Laboratory (NRL) has developed a new bipropellant Upper Stage providing orbit transfer propulsion capabilities from a Delta II launch vehicle. This bipropellant stage utilizes a pair of Composite Overwrapped Pressure Vessels (COPV) to store helium for pressurization of the Upper Stage fuel and oxidizer propellant tanks. A systems engineering trade study was performed to determine the size requirements of the helium tanks followed by competitive vendor selection and subsequent tank development and qualification program. This paper discusses the helium tank sizing analysis and documents details of the COPV design and qualification testing effort. Additionally, spacecraft installation interfaces, a tank pressure hazard analysis, and on-orbit temperature and pressure performance are summarized and presented. Program description. The Naval Research Laboratory (NRL) is the corporate research laboratory for the United States Navy and Marine Corps and conducts a broad program of scientific research, technology and advanced development. The NRL, sponsored by the Defense Advanced Research Projects Agency (DARPA), developed the Upper Stage (US) for the Microsatellite Technology Experiment (MiTEx) program by drawing on its corporate experience building and operating spacecraft. The Upper Stage provided the orbit transfer propulsion for the MiTEx microsatellites and was launched on a Delta II in June of 2006. NRL personnel performed a series of mission analyses and trade studies that showed that an earth storable liquid bipropellant system would best satisfy the MiTEx orbital maneuvering requirements. A constant feed pressure is required to optimize bipropellant engine performance; therefore the propulsion subsystem design includes two Composite Overwrapped Pressure Vessels (COPV) to store helium pressurant. ARDE, Inc was selected as the COPV contractor after a competitive procurement. ARDE developed a tank design to meet the NRL requirements, and conducted extensive tests both to qualify the design and to provide thermal – helium expulsion data to support NRL mission planning needs. Mission sizing and blowdown. NRL personnel performed a series of analyses to support the COPV procurement. These included parametric studies, mission requirements flowdown, budget allocations to the various components including the COPV, and packaging studies. The key requirement addressed in these analyses was the pressure remaining in the COPV at the end of each maneuver and 1 Osborn, Clauss, Gorin, Netwall, “Micro-satellite Technology Experiment (MiTEx) Upper Stage Propulsion System Development,” AIAA 2007-5434 how well that pressure could satisfy feed system flow requirements. These analyses considered the propellant tank volume, the propellant volume required for various orbital maneuvers, the propellant volumetric flow rate, and the COPV’s operating pressure at the beginning of the maneuver to develop a trade-space that included helium end of maneuver pressure and helium flow path pressure drop. The parametric analyses showed that the temperature of the helium at the end of the maneuver is the driver for the final expected pressure. The helium tank blowdown can be modeled as a polytropic process using Equation 1 where n represents a constant for a particular process.

The increased use of composite overwrapped pressure vessels (COPVs) in space and commercial applications, and the explosive nature of pressure vessel ruptures, make it crucial to develop techniques for early condition based damage detection. The need for a robust health monitoring system for COPVs is a high priority since the mechanisms of stress rupture are not fully understood. Embedded Fiber Bragg Grating (FBG) sensors have been proposed as a potential solution that may be utilized to anticipate and potentially avoid catastrophic failures. The small size and light weight of optical fibers enable manufactures to integrate FBGs directly into composite structures for the purpose of structural health monitoring. A challenging aspect of embedding FBGs within composite structures is the risk of potentially impinging the optical fiber while the structure is under load, thus distorting the optical information to be transferred. As the COPV is pressurized, an embedded optical sensor is compressed between the expansion of the inner bottle, and the outer overwrap layer of composite. In this study, FBGs are installed on the outer surface of a COPV bottle as well as embedded underneath a composite overwrap layer for comparison of strain measurements. Experimental data is collected from optical fibers containing multiple FBGs during incremental pressurization cycles, ranging from 0 to 10,000 psi. The graphical representations of high density strain maps provide a more efficient process of monitoring structural integrity. Preliminary results capture the complex distribution of strain, while furthering the understanding of the failure mechanisms of COPVs.

This work presents a manufacturing-driven multiscale methodology for the design and structural assessment of filament-wound carbon fibre reinforced polymer (CFRP) composite overwrapped pressure vessels (COPVs) incorporating nano-modified epoxy matrices. The work integrates experimental characterization with predictive modelling in a unified framework. Neat epoxy resins were reinforced with nanosilica and graphene nanoplatelets (GNPs) using a controlled dispersion protocol. Composite plates were manufactured by filament winding and vacuum curing, then characterized by micro-computed tomography ( μ CT), tensile testing, and three-point bending to evaluate dispersion quality, void content, and mechanical behaviour. At the nanoscale, the effective elastic properties of the modified matrices were estimated using Mori–Tanaka homogenization, accounting for filler morphology and voids. These properties were introduced into a microscale finite element homogenization to determine orthotropic lamina properties, which were validated against experimental data. The resulting lamina properties were used to inform filament-winding design and stacking sequence of an 8L Type IV COPV using CADWIND . Manufacturing-consistent layups were exported to ABAQUS for global finite element analysis and burst-pressure prediction using the Hashin failure criterion. Comparative simulations performed for the neat and nano-modified composites indicate delayed damage initiation and increased predicted burst resistance for the nanosilica- and GNP-enhanced systems, enabling equivalent performance with reduced laminate thickness. The proposed framework provides a transferable, manufacturing-aware pathway from nano-modified resin development to COPV structural performance assessment. • Development of a manufacturing-driven multiscale methodology for COPVs using nanofiller-enhanced epoxy matrices. • Combined Mori–Tanaka and FE homogenization accurately predicts lamina and tow-scale elastic properties. • CADWIND-based stacking sequence optimization improves burst-pressure performance and thickness distribution. • Nanofillers significantly enhance tensile strength (up to ∼ 45% for GNPs) with minor increase in longitudinal modulus. • Full-scale 8L COPVs simulated, demonstrating improved structural performance and potential for weight reduction.

A novel damage localization technique for type III composite pressure vessels based on guided wave mode-matching method

A composite overwrapped pressure vessel (COPV) is a thin liner overwrapped by fiber reinforced polymer matrix composite material (FRPMC) through either wet filament winding process or layup with prepegs. Because of the weight benefit, space shuttle and International Space Station used COPVs for the storage of various fluids including gaseous oxygen, gaseous nitrogen, and gaseous helium. Other space systems such as launch vehicles and satellites also use COPVs in their propulsion or control subsystems. This chapter describes briefly the manufacturing process for producing a COPV and some key design and test requirements that are being imposed on space flight COPVs.

A study of residual burst strength of composite over wrapped pressure vessel due to low velocity impact

Carbon fiber (CF) is central to produce lightweight, high pressure, composite overwrapped pressure vessels (COPVs), which are used for on-board storage of hydrogen for fuel cell vehicles. In 2015, carbon fiber cost accounted for 62% of the cost of a hydrogen storage system, as COPVs were manufactured with T700S CF at $29.40/kg CF. The high cost of hydrogen storage, largely attributed to the carbon fiber cost, limits the application of fuel cells in vehicles. Therefore, our team endeavored to develop fiber processing to demonstrate carbon fiber tensile properties similar to T700S with a production cost potential of $12.60/kg or less. Here, the overarching goal of the project was the development of low cost, high strength carbon fiber for composite overwrapped pressure vessels. Within the project, our team investigated a new low-cost, high-volume, high quality polyacrylonitrile (PAN)-based precursor terpolymer for the production of carbon fiber precursors, methods for increasing water and energy use efficiency in wash-water/solvent separation, and ultimately developed a multifilament spinning and conversion process for the production of hollow carbon fibers from a segmented arc spinneret utilizing a solution spinning method.

A process and Structural Health Monitoring system was implemented on a Composite Overwrapped Pressure Vessel (COPV) for hydrogen storage at 350 bar to be used in a fuel-cell system of an Unmanned Aerial Vehicle. This work reports the embedment strategy of optical fibre Bragg grating (FBG) sensors to monitor the full life cycle of the vessel, consisting of an aluminium liner and a wound carbon fibre reinforced polymer composite overwrap. A FBG sensing array, bonded on the aluminium liner circumferential section, was covered with a localised unidirectional prepreg composite tape, enabling composite winding and curing monitoring. The sensing array strategy allowed to detect and locate Barely Visible Impact Damage resulting from drop-weight impact tests, based on the ratio of the residual strain amplitude between FBG sensor pairs. Errors as small as 17 mm and up to 56 mm were determined between the predicted and ‘real’ impact locations. To simulate the real-life operational pressure charging and discharging cycles, the COPV was subjected to cycling testing at different pressure ranges. The FBG sensors were able to monitor a total of 20 980 pressure cycles, revealing a linear response to the applied pressure, and remained operational after COPV failure. Furthermore, the FBG sensing array was able to detect the residual plastic strain caused in the aluminium liner by the autofrettage process that the COPV was subjected to prior to pressure cycling, at 600 bar for 2 min, to improve its fatigue performance. This manuscript also reports the COPV structural design by Finite Element Modelling (FEM), its manufacturing process and burst pressure testing for the FEM analysis validation. A small difference of 0.7% was found between the simulated and experimental determined burst pressure of 1061 ± 26 bar.

: NDE methods evaluated included eddy current measurements, ultrasonics, radiography, acoustic emission monitoring, acoustic-ultrasonics, and interferometric (e.g., shearography) methods. The major interest was in the detection of damage to COPVs caused by impacts at energy levels that would leave no visible scar on the COPV surface. Each of the above-mentioned NDE methods was assessed with regards to its utility for the detection of impact damage to COPVs and its applicability in the field. Three COPVs were impacted at various energies and then inspected using each of the NDE methods. The results for each method are presented, followed by a summary comparing the advantages and disadvantages. Several NDE techniques were shown to be effective for detecting impact damage sites on graphite/epoxy overwrapped pressure vessels even if the impact energy was below the threshold for creating visible surface damage. These techniques (ultrasound, radiography, thermography, shearography, and eddy current testing) are sensitive because a small dent is left in the liner following the impact. The impact damage detection thresholds for the various NDE techniques employed depend on the size and shape of the vessel. The tup characteristics and the internal pressure of the vessel during impact also affect the degree of damage and, hence, the detectability of the impact site. The size of the indication associated with a flaw is also dependent on the technique used. Visual inspection can lead to an underestimate or lack of detection of the damage. For both thermography and shearography, consistent sizing could be accomplished through the use of calibration standards and a consistent test set-up. The ultrasonic and eddy current results are most likely accurate measures of how much of the liner was debonded.

Hydrogen storage composite overwrapped pressure vessels (COPVs), as a core component of fuel cells, are widely used in the new energy vehicles and other fields. However, the service performance of COPVs, especially the burst pressure, has a great dispersion, which may come from the microscopic defects inside the vessels. In this paper, the industrial computed tomography scan was carried out on a type III hydrogen storage COPV with an aluminum alloy liner and a carbon fiber wound layer, and it was found that many delamination defects with random sizes and locations exist inside the fiber wound layer. Among these defects, there are 164 delamination defects with an area of <500 mm2 (accounting for 87.7% of the total number of defects), but the area of the defects only comprises 19.14% of the entire defects area. Delamination defects were the predominant type of manufacturing defect in the fiber wound layer of COPVs. Then the burst test of the COPV was carried out, which showed that the aggregated area of large-sized delamination defects located in the composite layer of the cylindrical part triggered the crack initiation during the burst test. Thus, in turn, directly affected both the burst pressure and the post-burst morphology. Finally, a 3D equivalent model of the laminated structure is proposed to model the expansion of the composite cylinder. By carrying out a finite element simulation of the COPV, the most dangerous area during the loading process of the COPV is confirmed. The experimentally measured burst pressure of the cylinder is 115.5 MPa, which deviates by 18.6% from the pristine model prediction and by 9.09% from the model with delamination defects.

Theoretical and experimental research on the critical buckling pressure of the thin-walled metal liner installed in the composite overwrapped pressure vessel

Composite overwrapped pressure vessels (COPVs) have become the core unit for high-pressure hydrogen storage and transportation. However, excessive autofrettage pressure could induce unilateral buckling damage of the metal liner because of large rebound compressive stress induced by large plastic deformation in the depressurization stage. When the liner contains initial defects, its critical unilateral buckling pressure would be further reduced. In this paper, a critical buckling pressure calculation formula was established by finite element analysis and theoretical derivation. Firstly, the classical theoretical calculation models and research methods were analyzed and discussed. Then, by discussing the key influencing parameters, a semi-empirical calculation formula of nonlinear confined buckling pressure of a metal liner with ovality defects was established. Finally, the proposed semi-empirical formula was used to predict the critical internal pressure of a Type-III COPV, and the predicted value was compared with the experimental result. The predicted result was higher than the experimental result and the error range was −2.8%~−23%. The proposed semi-empirical formula of nonlinear confined buckling could provide theoretical support for designing the autofrettage pressure of Type-III COPVs and help to reduce the uncertainty and repeated test cost in the design process.

Air Products and Chemicals, Inc. (Air Products) is reporting on the results of a DOE-funded program to develop and validate Advanced Hydrogen Fueling Station hydrogen tube trailers to support hydrogen and fuel cell market applications. The project involved the engineering design and development of a new composite over-wrapped pressure vessel to increase the pressure capability of tube trailers to deliver hydrogen to greater than 8500 psi (586 bar). The new composite over-wrapped pressure vessel tube trailer was expected to provide improved delivery technology and reduce the cost of transporting hydrogen from central hydrogen production sources. The project aimed to meet or exceed DOE’s 2020 delivery targets for tube trailer hydrogen distribution. A gated approach was taken for the project, with early project Go/No-Go decision points. Specific goals for this four-stage product development project included: Preliminary Study, Modeling, and Analysis: Complete a preliminary design analysis and cost viability of a >8500 psi (586 bar) hydrogen storage vessel with an appropriate frame and chassis configuration for a complete trailer to US DOT requirements; Develop New Tube Size for >8500 psi (586 bar) Vessel: Fabricate tubes for test units, perform testing, and submit test results for a DOT Special Permit; Design, fabricate, and test a delivery trailer unit; Deploy the delivery trailer to hydrogen stations and collect data. Work conducted under DE-EE0006524 established costs for US DOT Type III vessels at 8500 psi (586 bar), 12500 psi (862 bar), and 14000 psi (965 bar) with manufactured water volumes up to 375 liters. Trailer module designs were established for the maximum US DOT design limits and for Air Products’ design limit of a 40-foot trailer length that addresses California hydrogen fueling station ingress and egress operating constraints. Conceptual trailer designs were not capable of achieving the DOE 2020 capacity and cost targets, but they did exceed the DOE pressure target of 500 bar. Based on new trailer cost and simulated operating performance, the 8500 psi (586 bar) new trailer design is not feasible in comparison to Air Products’ DOT-approved 7500 psi (520 bar) trailer design in operation today. Also, the project was expected to incur significantly higher product development costs associated with new US DOT vessel development and approval. Alternative trailer design concepts were explored that included 700 bar compression-less mobile fueling that could be promising for early-market hydrogen infrastructure fueling station placement and development and niche military applications. Due to the economic and budget challenges identified in the feasibility phase of the program, Air Products, in agreement with the Department of Energy, decided to terminate this development program.

The potential of dry winding for rapid, inexpensive manufacture of composite overwrapped pressure vessels

This study reports on signal recovery of optical fiber Bragg gratings embedded in a carbon fiber composite overwrapped pressure vessel's (COPV) structure which have become chirped due to microcracks. COPVs are commonly used for the storage of high pressure liquids and gases. They utilize a thin metal liner to seal in contents, with a composite overwrap to strengthen the vessel with minimal additional mass. A COPV was instrumented with an array of surface mounted and embedded fiber Bragg gratings (FBGs) for structural health monitoring (SHM) via strain sensing of the material. FBGs have been studied as strain sensors for the last couple decades. Many of the embedded FBGs reflected a multi-peak, chirped response which was not able to be interpreted well by the current monitoring algorithm. Literature and this study found that the chirping correlated with microcracks. As loading increases, so does the number of chirped FBGs and microcracks. This study uses optical frequency domain reflectometry (OFDR) to demultiplex the array of FBGs, and then sub- divide individual FBGs. When a FBG is sub-divided using OFDR, the gratings' strain along its length is recovered. The sub-divided chirped FBGs have strain gradients along their length from microcracks. Applying this to all chirped gratings, nearly the entirety of the embedded sensors' readings can be recovered into a series of single peak responses, which show very large local strains throughout the structure. This study reports on this success in recovering embedded FBGs signal, and the strain gradient from microcracks.