SPECIMEN DESIGN, MANUFACTURING AND TESTING PROCEDURES FOR FLAT CARBON-FIBER-REINFORCED PLASTIC LAMINATES UNDER BIAXIAL LOADING

The optimization of a specimen design allowing the investigation of th e biaxial strength of composite laminates over the full range of failu re strain was the primary objective of this work. Multiaxial strength criteria are often found unreliable mainly as a result of the inherent complexity of biaxial tests and, in many cases, as a result of ineffi cient specimen designs. As a result of a development program combining numerical simulations and experimental measurements, a flat cruciform -shaped specimen has been developed for carbon fiber reinforced plasti c (CFRP) laminates. The design fulfills the basic criteria for such a specimen, namely allowing for a uniform biaxial stress/strain state to exist in the gauge area and for testing the virgin material up to fai lure in both the tension-tension and tension-compression quadrants of the strain/stress space. The fabrication of the specimen is described and a three-step testing procedure for generating biaxial strength dat a is proposed. Typical results obtained from specimens of the proposed configuration tested in accordance with this procedure are presented. Results compare well with those obtained from tubular specimens, thus confirming the effectiveness of the proposed design. Experimental dat a obtained for the AS4/3501-6 carbon/epoxy composite system are finall y compared against strength predictions of recognized failure theories .