EXPERIMENTAL EVALUATION OF COMPRESSIVE STRENGTH OF PAN-BASED CARBON MONOFILAMENT BY POISSON’S DEFORMATION IN CRUCIFORM SPECIMEN

Carbon fiber has relatively high anisotropy in tensile and compressive strength. Since, the diameter of carbon fiber is 5~10[μm], it is difficult to directly evaluate the compressive strength of monofilament. In this study, the compressive strength of carbon single fiber was evaluated using Poisson's deformation of Cruciform specimen. Using the Cruciform test method, the compressive strengths of monofilaments of PAN-based carbon fibers, T300B and T700SC, were experimentally and analytically evaluated. As a result, the compressive strengths of carbon single fibers was 5.12 [GPa] for T300/PA6 and 5.54 [GPa] for T700/PA6 in this study.

Comparison of the shear behavior in graphite-epoxy composites evaluated by means of biaxial test and off-axis tension test

Characterization of shear behavior in composite materials remains a not fully solved problem. In the last fifty years, many different approaches have been proposed to solve this problem (rail shear, thin-walled tube torsion, off-axis tensile, ±45° tensile, Arcan, Iosipescu, asymmetric four-point bend, plate twist, v-notched rail shear, off-axis flexural, and shear frame), although none of these approaches have achieved an unquestionable solution. For this reason, proposals of alternative methods and comparison between different experimental techniques are of interest. In the present work, the use of cruciform samples with the fiber oriented at 45° with respect to the load directions, and subjected to tension-compression (creating a pure shear stress state at the central part of the samples), is studied. The experimental results of the cruciform samples have been compared with the off-axis tests (with the fiber at 10°) for the same material (AS4/8552), finding a good agreement between the shear experimental curves, especially at the initial part of the curve, where the shear modulus is calculated. Nevertheless, the shear strength value obtained by means of the cruciform specimen has shown to be significantly lower than that obtained using the off-axis test. A Finite Element numerical model of the cruciform specimen has been developed to analyze the stress field of the samples. Numerical results have shown that there is a central area of the cruciform specimens where a pure and uniform shear stress state is developed, which is suitable for the evaluation of the shear constitutive law of the material. It has been observed that there is a (σ 22) stress concentration in the transition between the straight and curved parts of the boundary geometry of the samples, which explain some premature failures of the samples. This premature failure could be avoided with tabs extended up to the beginning of the central part of the sample.

Optimization Design of a Small-Sized Cruciform Specimen for Biaxial Fatigue Testing

The in-plane biaxial specimen can well reflect the complex stress state of sheet metal. However, there is no standard for small-sized specimens used in the low-power biaxial fatigue testing machine. The main goal of this paper is to apply the finite element method and orthogonal experiment method to design the cruciform specimen, considering the influence of three main parameters including the diameter of the central semispherical thinning area, the minimum thickness of the center and the arm thickness of the specimen. According to the central strain dispersion coefficient and the strain concentration coefficient proposed in this paper, we ensured that the distribution of strain in the gauge area is uniform and the strain it at its maximum value at the same time. The optimized specimen is verified by a biaxial fatigue test with the digital image correlation (DIC) technique. It is found that the fatigue crack appears in the center region, which proves that the optimized specimen can be effectively used for biaxial fatigue test.

Strength characterization of glass/epoxy plain weave composite under different biaxial loading ratios

Over the past years, various studies have been investigated in order to characterize the behavior of composite materials under different multi-axial loading conditions. One of the most used biaxial techniques is the in-plane biaxial test on cruciform specimens. To achieve reliable biaxial failure results, the design of the cruciform specimen presents a crucial part. Previous studies show that there is no well-adapted cruciform geometry for the composite biaxial tests. In this paper, an optimal cruciform specimen has been defined numerically for the composite characterization test. The specimen is composed of two aluminum tabs glued on top and bottom side of the plain-weave glass/epoxy composite. Finite element simulations have been carried out in order to study the influence of the aluminum grade and thickness on the stress distribution in the composite. An experimental validation confirms the failure of the specimen in the central zone under three different biaxial tensile ratios. The experimental strains were evaluated using the digital image correlation method. The traction/traction quadrant of the failure envelop was obtained and compared with different failure criteria. The maximum strain criterion shows a good agreement with the experimental results.