The Wyoming Combined Loading Compression (CLC) Test Method

Abstract

Based upon extensive experimental testing and analytical modeling of compression test methods at the University of Wyoming and elsewhere during the past several years, a specific procedure is recommended for obtaining representative axial compressive strength design values for continuous fiber composite materials. A specimen in the form of an untabbed, straight-sided strip of a [90/0]ns cross-ply composite laminate is tested in a newly developed Wyoming combined loading compression (CLC) test fixture. The standard specimen is 140-mm (5.50-in.) long, 12.7-mm (0.50-in.) wide, and of uniform but arbitrary thickness. This length produces a 12.7-mm (0.50-in.) gage section in the fixture. By increasing the torque applied to the fixture clamping bolts, the ratio of shear-loading to end-loading is readily increased as required to avoid end crushing of the untabbed cross-ply specimen. Typically, the required torque is low, in the range of 2.26 to 2.83 N-m (20 to 25 in.-lb). Classical (linear) lamination theory is adequate to calculate a “back-out factor,” which is used to determine the unidirectional ply axial strength. Using a Hercules AS4/3501-6 carbon/epoxy composite material, it is demonstrated that the axial compressive strength obtained is fully comparable to the stress at failure in the 0° plies of laminates of any general lay-up configuration, and also to that measured for unidirectional composites tested using special mini-sandwich and thickness-tapered test specimens.