An experimental investigation of the biaxial strength of IM6/3501-6 carbon/epoxy cross-ply laminates using cruciform specimens

2-D biaxial testing and failure predictions of IM7/977-2 carbon/epoxy quasi-isotropic laminates

“…1, it is therefore reasonable to expect unacceptable failures to occur in the arms (which are loaded uniaxially) rather than in the biaxially loaded, hence strengthened, gage section. In previous research [3,4] cross-ply laminates have been successfully tested, in part, due to their low in-plane Poisson's response (off-axis terms in the stiffness matrix, which account for internally generated multiaxial stress states, are a function of Poisson's ratios). Furthermore, the experience gathered by the current authors led them to believe that a fiber reinforced quasi-isotropic laminate (which exhibit a larger in-plane Poisson's response) could be successfully tested provided appropriate specimen geometry and sufficient reinforcement was placed in the loading arms.…”

“…Although various cruciform configurations have been studied [4], the focus of this program was to fabricate and test a quasi-isotropic laminate. Existing failure theories indicate that a quasi-isotropic laminate would be significantly (2·) stronger when loaded biaxially than when loaded uniaxially.…”

“…Tsai-Wu, that the 2-D failure envelope will be an ellipse (an ellipsoid failure surface in 3-D) with the major axis +45°to the abscissa and a zero ordinate intercept, i.e., occupying quadrants I and III in r 1 -r 2 stress space when plotted in rectangular coordinates. The composites community has clearly and repeatedly demonstrated the need for reliable experimental biaxial data [1][2][3][4]. Although the Air Force Research Laboratory (AFRL) has an operational triaxial test facility dedicated to the evaluation of composite materials, a quasi-isotropic laminate configuration had never been successfully tested.…”

Biaxial Tensile Strength Characterization of Textile Composite Materials

“…Testing biaxially-loaded cruciform specimens represent a more direct approach for obtaining true biaxial stress states, and consequently this method has gained wide acceptance [7], [8], [10], [11], [15]. As suggested by many researchers in the field [7], [10], [13], an ideal cruciform specimen should accomplish the following features: i) It should be capable of generating a sufficiently wide and homogenous biaxial stress/strain field in the gauge area, ii) failure must occur in the predefined gauge zone, iii) the cruciform should accept arbitrary biaxial load ratios for generating a complete failure envelope (within a desired range), iv) both the tested and the reinforcement layers should be of the same material, v) the transition between the gauge zone and the reinforced regions should be gradual enough as to avoid undesirable high stress concentrations, vi) the cruciform fillet radius should be as small as possible in order to reduce stress coupling effects, and vii) stress measurements in the test area should be comparable to nominal values obtained by dividing each applied load by its corresponding cross-sectional area.…”

“…[5] expressed some concerns about the generality of the experimental methodology for the case of non quasi-isotropic lay-up configurations, such as the one studied in their work. In view of the lack of consensus for accurate TC strength prediction [7], [8], and as stated by researchers who participated in the World Wide Failure Exercises (WWFE) [2] more experimental data, better testing methods and properly designed specimens are needed to generate reliable biaxial strength models.…”