Specimen size effects in the off-axis compression test of unidirectional carbon fiber tow composites

Specimen size effects in the off-axis compression test for unidirectional carbon fiber tow composites were studied by subjecting coupon specimens of different lengths and widths to off-axis compression under static and low velocity impact loading. Specimens were cut such that the fibers are at angles of 15, 30, 45 and 60° to the direction of loading. A modified compression fixture with anti-buckling side supports was used to carry out the tests. Static tests were carried out on a hydraulically activated MTS loading frame, where specimens were subjected to displacement controlled loading. Low velocity impact tests were conducted on a drop tower facility. A three strain gage rosette was used to measure global strains. Load was measured using a load cell. Due to the unique microstructure of the specimens, a modified three parameter orthotropic plasticity characterization of the plastic response was used and the constants associated with this characterization were determined uniquely. It is shown that the orthotropic plastic response of the material is affected by specimen size primarily due to the mechanism of load introduction in the off-axis test and load transfer through the fiber tow/matrix interface that is prevalent in the material.