An Improved Short-Beam Method for Measuring Multiple Constitutive Properties for Composites

Abstract

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.

Mechanical properties characterization of fiber reinforced composites by nonlinear constitutive parameter optimization in short beam shear specimens

“…The proposed NCPO method compares to the FEMU-based iterative methods that aim to improve on the closed-form solution for shear stress given in the ASTM standard D2344. 12,33,41 As shown in Figure 6, the optimized shear stress-strain curve is significantly more accurate than the closed-form solution at high values of shear stress. The analysis of the test method is flexible, which allows it to be used to explore the appropriate material model and its parameters.…”

“…The accuracy of the closed-form solution diminishes as the aspect ratio L/h of the test drops below 10. 41 Geometric and material nonlinearity effects cause shear stress to be overestimated and the neutral axis to shift. As this effect also overstates the difference between the tensile and compressive modulus, the current work seeks to find the average axial modulus.…”

“…As the transverse stress component r yy has a negligible effect on the calculations of the axial modulus, it is not considered in these calculations. 41 The axial force is equal to zero and not considered for the SBS method. The validity is limited to the cross-sections away from the contact surfaces.…”