Influence of Specimen Preparation and Specimen Size on the Transverse Tensile Strength and Scatter of Glass Epoxy Laminates
The influence of specimen polishing, specimen configuration, and specimen size on the transverse tension strength of two glass epoxy materials loaded in three and four point bending was evaluated. Polishing machined edges, and/or tension side failure surfaces, was detrimental to specimen strength characterization instead of yielding a higher, more accurate, strength as a result of removing inherent manufacture and handling flaws. Transverse tension strength was sensitive to span length due to the classical weakest link effect. However, strength was less sensitive to volume changes achieved by increasing specimen width. The Weibull scaling law over-predicted changes in transverse tension strengths in three point bend tests and under-predicted changes in transverse tension strengths in four point bend tests. Furthermore, the Weibull slope varied with specimen configuration, volume, and sample size. Hence, the utility of this scaling law for predicting transverse tension strength is unclear.
Tension-Torsion Behavior of Glass Epoxy Flexbeam Laminates
Quasi-static tension-torsion and tension-torsion fatigue tests were conducted on several candidate S2/F584 glass-epoxy flexbeam layups to establish their torsional strength and fatigue behavior in the presence of an axial load, determine typical damage mechanisms, and provide a database for developing predictive analytical methods. Tension-torsion specimens contained 0° longitudinal plies and 15, 30, or 45° angle plies in seven different stacking sequences. The angle plies were ± 45 woven cloth in one layup and prepreg tape in the other layups. In the tape laminates, fatigue failures began with matrix cracks forming on the edge that initiated delaminations in the interfaces between the angle plies and 0° plies. In the cloth laminates, a series of small stair step delaminations formed between the ± 45 cloth layers and initiated delaminations in the neighboring 0/45 interfaces. Tests indicated that the three layups with only 0° and ± 45° plies had the highest torsional strength in the presence of a constant axial load. Of the three strongest layups, the one in which the ± 45° plies were made of woven cloth fabric instead of prepreg tape was the strongest and had the longest fatigue life.
Measurement of Multiaxial Ply Strength by an Off-Axis Flexure Test
An off-axis flexure (OAF) test was performed to measure ply strength under multiaxial stress states. This test involves unidirectional off-axis specimens loaded in bending, using an apparatus that allows these anisotropic specimens to twist as well as flex without the complications of a resisting torque. A 3D finite element stress analysis verified that simple beam theory could be used to compute the specimen bending stresses at failure. Unidirectional graphite/epoxy specimens with fiber angles ranging from 90 deg to 15 deg have combined normal and shear stresses on their failure planes that are typical of 45 deg plies in structural laminates. Tests for a range of stress states with AS4/3501-6 specimens showed that both normal and shear stresses on the failure plane influenced cracking resistance. This OAF test may prove to be useful for generating data needed to predict ply cracking in composite structures and may also provide an approach for studying fiber-matrix interface failures under stress states typical of structures.