A test method for evaluating the material properties of closed-cross-section composite laminates has been developed. A finite-element investigation of the first five eigenvalues for the specimen in buckling shows that the critical load is at least a factor of three higher than the failure load observed during testing. This same analysis shows that the highest stresses and strains occur in the gage section of the specimen.
The test specimens are 228.6 cm (9 in.) in length, 50.8 cm (2 in.) in diameter, and have fiberglass end tabs co-cured onto them. Several specimens were manufactured in a quasi-isotropic configuration ([±45/0/90]s and [±45/90/0]s) in order to experimentally evaluate the robustness of the test method, observe typical failure modes, and to catalog the accumulation of damage in the specimens up to failure. Several of these tubes were outfitted with strain gages to detect the presence of buckling and bending in the specimens. Damage was observed by loading specimens to 45%, 60%, 75%, 90%, and 95% of the mean ultimate load. At each of these load levels specimens were cut along their length, a dye penetrate was applied, and then the tubes were X-rayed. The experimental investigation shows that the specimens did not buckle and that the mode of failure is a compressive material failure in the gage section of the tube.