Analysis Methodology for Assessing Delaminations in Composite Overwrapped Pressure Vessels

Abstract

An analysis methodology for filament wound composite overwrapped pressure vessels (COPVs) is summa- rized. The COPV finite element model is generated from an in-house pre-processing code, which is able to model the variation of the fiber orientation from boss to boss, include the thickness of each ply throughout the vessel, and represent the orthotropic properties of the ply on an individual basis. The residual stresses from manufacturing are accounted for by modeling the autofrettage process using the Abaqus finite element software. Finally, a user subroutine was implemented to simulate failure progression from the presence of an incipient defect. As an illustrative example, finite element analysis is used to determine the structural integrity of a COPV due to a delamination initiatiating in ply drop-off regions. is assumed to be taken by the fibers while ignoring the presence of the resin. The method is restricted to idealized geometries and winding patterns, and its usefulness in evaluating manufacturing issues is very limited. Finite element analyses presented in the literature, 4, 5 utilize oversimplifications for spherical tanks. Recently, a composite filament winding pre-processing tool (Wound Composite Modeler - WCM) was implemented as a plug-in to the Abaqus com- puter program, reducing the time required to build a COPV finite element model. 6 This paper describes the theoretical foundation of an in-house development computer program. The program fa- cilitates the construction of high-fidelity finite element models of COPVs and includes specialized tools required to evaluate delaminations and debonds. The in-house computer program, COPV Stress Analysis Program (COSAP), 13 is flexible enough to deal with various vessel configurations, winding patterns, irregularities resulting from manu- facturing anomalies, and peculiar situations resulting from manufacturing considerations. The program incorporates many features required to perform a comprehensive analysis of COPVs, but it may also be used for analysis of other filament-wound vessels such as solid rocket motor cases. COSAP consists of pre- and post-processing codes designed to work with the Abaqus finite element commercial code. COSAP has been used to analyze a number of spherical and cylindrical COPVs and it has been shown to correlate well with test data. 7 Since delaminations occassionally occur in COPVs, and in special cases could lead to catastrophic failure, an illustrative example of delamination initiation and growth is presented with the objective of describing the analysis methodology. The paper is structured as follows: Sec- tion II - Overview of COPV analysis procedure, Section III - COPV Model Generation, Section IV - Failure Analysis Methodology, and Section V - Analysis Demonstration.