This course has been developed based on requirements developed for space applications for COPVs. The relative requirements are documented in NASA-developed standards applicable to US and international partners for use on the International Space Station, as well as for future programs such as the NASA Commercial Requirements. These various standards reference the appropriate AIAA requirements and these will be directly addressed in this course. The course is directly relevant to individuals concerned with COPVs in automotive applications. The failure modes are common across these industries. However, there is a difference in usage and need for robustness of typical pressure vessels and a difference in materials commonly selected for these products. Consequently there are different standards and approaches to certification.
The class will explore these differences.Participants in this workshop will gain appreciation of a wide range of epoxy-matrix composites that are used in overwraps based on fibers such as: S-glass, aramids (e.g., Kevlar®49), carbon (e.g., T1000), and PBO (e.g., Zylon®), and also various current liner materials including metals such as aluminum, stainless steel, titanium and Inconel, and polymers such as high density polyethylene. Attention will be paid to the potential effects of processing variables (e.g., heat treatment, welding, annealing) on ultimate liner performance as influenced also by the fiber used in the overwrap. Various steps in the COPV design and manufacturing processes will be discussed, particularly aspects strongly influenced by end-use requirements and vessel geometry (cylindrical vs. spherical). To manufacture the overwrap, both wet filament winding and prepreg winding methods will be discussed, including their respective pros and cons and their relative importance in various designs.
Another topic discussed will be the potential for liner distortion and buckling during winding, the consequences and candidate countermeasures to protect this phenomenon from occurring. Advantages and risks in bonding the overwrap to the liner will be discussed with respect to the overall design and potential failure mechanisms. Autofrettage and proof-testing will be discussed in terms of plastic yielding of the liner that induces a significant compressive stress component beneficial to improving fatigue life. In this context, the Bauschinger effect on the final liner stress state and the potential for liner buckling will also be discussed.
The relevant analysis and test methods used to demonstrate compliance to appropriate certification standards are presented. These include factors of safety set to mitigate against stress rupture failure modes of the overwrap and Leak-Before-Burst liner/overwrap concepts and demonstration, and finally FEA/NDE approaches to establish Safe Life with respect to risk of liner fatigue failure from crack initiation and growth. Current non-destructive evaluation (NDE) techniques will be discussed as are used to detect flaws and damage in the liner and overwrap. NDE methods for detecting flaws and small cracks in liners include: visual, dye penetrant, X-ray, ultrasonic, eddy current, and borescope inspection. NDE methods for the overwrap include: Acoustic emission, Flash/Infrared thermography, laser shearography, digital image correlation of overwrap strains, and Raman spectroscopy to measure residual fiber stress.
There will be hands-on experience in computational design tools used to analyze COPV. Each participant will receive a licensed student version of the Abaqus FEA product suite (from Dassault Systèmes). Through structured learning example programs, users will gain an appreciation for stress distributions in the composite overwrap layers and the liner, and will gain an appreciation of analysis tools used to prevent metal liner failure due to fatigue in the parent material and any weld regions. These workshops will be made available to students to follow on their own.