Accurate three-dimensional stress-strain constitutive properties are essential for understanding of complex deformation and failure mechanisms for glass-fiber and carbon-fiber reinforced polymer-matrix composites. A large number of different methods and specimen types, which are currently required to generate three-dimensional allowables for structural design, slow down material characterization. Also, some of the material constitutive properties are never measured due to prohibitive cost of the specimens needed. This work shows that simple short-beam shear specimens are well-suited for measurement of 3D constitutive properties for composite systems. In particular, a methodology to measure tensile and compressive material properties, generate shear stress-strain curves and measure the shear strength in a simple short beam shear test will be presented. The methodology is based on the Digital Image Correlation (DIC) full-field deformation measurement. Short-beam and curved-beam tests are accomplished to generate 3D stress-strain response for glass/epoxy and carbon/epoxy tape composite material systems. Accuracy of constitutive properties is also verified using standard methods and data available in the public domain.
The short beam shear (SBS) test method combined with digital image correlation (DIC) deformation measurement is used in this work to study constitutive properties of the IM7/8552 carbon/epoxy composite material system. Dependence of the shear stress approximation on the SBS specimen span-to-thickness (s/t) ratio has been investigated. A stress model developed in a previous study is expanded to account for this dependency. In addition, the effect of nonzero transverse normal stresses on a simple closed-form approximation for tensile and compressive axial moduli has been assessed. Results from the expanded closed-form solution verify that transverse normal stress has little influence on the measurement of the axial moduli. Also, the SBS test setup has been refined as a result of the sensitivity of the measured axial strain distribution to the specimen alignment in the test fixture. The test results obtained in this work are compared with previously published results to demonstrate the ability of a single unidirectional SBS test to capture multiple constitutive properties including axial tensile and compressive moduli, the nonlinear shear stress–strain response in the plane of loading, and to show that measured material properties do not depend on the test specimen size.
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