Effect of Holes on the Biaxial Strength of Woven Composites

Bi-axial strength of woven composites is particularly important due to the use of such materials in pressure vessels. Pressure vessels typically have interconnecting fixtures, which often are secured by bolted connections. In this research, two cruciform bi-axial test specimens were developed with the aide of Finite Element Analysis (FEA) and tested in the tri-axial test facility located at the Air Force Research Lab (AFRL) Space Vehicles Directorate Kirtland Air Force Base, Albuquerque, NM. Tests results were then compared to specimens with a central hole to study the decrease in strength due to stress concentration at bolted connections. Two hole diameters were drilled into each of the two cruciform types; 6.33mm (0.25") and 3.175mm (0.125"). All specimens are thick cross-ply and quasi-isotropic, [(0/90)₄]_s and [(0/90)₄(45/-45)₄]_s, IM7/UF3352 woven carbon fiber specimens. Analysis of stress concentration and test results will be provided, that are not currently available in the literature.