Multiaxial Stress Interaction and Failure Initiation in Internally Pressurized Thick-Walled Carbon/Epoxy Composite Cylinders

The strength assessment of thick-walled composite cylinders under internal pressure requires consideration of the nonuniform stress distribution and the interaction of stress components arising from material orthotropy and axial restraint.Conventional assessments based primarily on circumferential stress may not adequately represent the multiaxial stress state governing failure initiation.This study investigates the elastic stress response and initial failure of an internally pressurized thick-walled cylinder made of Hexcel AS4/3501-6 carbon fiber/epoxy composite, with particular attention to the effects of wall thickness and three-dimensional stress interaction.An analytical formulation was developed using orthotropic elasticity under plane-strain conditions, and the radial, circumferential, and axial normal stresses were determined across the cylinder wall.The three-dimensional Tsai-Wu criterion was then applied to evaluate the combined influence of these stresses on failure initiation and to determine the allowable internal pressure.The calculations showed a nonuniform stress distribution, with the circumferential stress reaching its maximum at the inner surface and decreasing toward the outer wall.For the representative configuration, the maximum circumferential stress was approximately 188 MPa, while the axial stress remained nonzero because of the imposed axial constraint.Variations in wall thickness produced substantial changes in the stress distribution and the predicted failure response.The analysis also established a procedure for identifying the pressure at which the Tsai-Wu failure threshold is first reached.These findings clarify the role of orthotropic elasticity and multiaxial stress interaction in the strength assessment of pressurized composite cylinders.The analytical framework provides a basis for evaluating initial failure margins and examining the influence of wall thickness on structural pressure capacity.