Characterization of compressively loaded filament wound composite cylinders using digital image correlation

In filament wound composites, the combination of undulated fiber architecture and a compliant matrix introduces unique challenges in the prediction of stiffness and strength using traditional laminated composite theories. Available micromechanical models incorporating the effects of undulated fibers on the stiffness and strength of filament wound composites have not been evaluated with high fidelity experimental data. Therefore, the objective of the current investigation is to measure full-field strains and out-of-plane displacements in compressively loaded filament wound cylinders made with rigid and flexible matrix materials. A series of [±θ/89/ ±θ] cylinders with multiple helical fiber angles, winding patterns, and matrix materials were manufactured and tested. Digital image correlation was used to measure outside surface displacements and strains. Axial and hoop direction strain fluctuations within with the filament winding pattern were found to be of the order of 20-30% of the mean values throughout the cylinders. Qualitatively, these fluctuations can be related to non-classical elastic couplings in the anti-symmetric regions of the pattern. Winding pattern affected compressive strength only in the flexible matrix composite, while it did not affect the axial modulus of elasticity appreciably in either material system. Failure of the cylinder occurred by fiber microbuckling, which initiated near the crossing of circumferential and helical cross-over bands. Based on a statistical analysis of surface strains in the local fiber coordinate system, it was determined that fiber- and shear-direction strains at incipient microbuckling were two to four times greater than their respective global counterparts. These results indicate the magnitude of strain concentration existing in the cylinders immediately before failure and highlight the importance of fiber- and shear-direction strains in the failure process.