A method for measurement of three-dimensional constitutive properties for composite materials

Accurate three-dimensional stress-strain constitutive properties are essential for understanding of complex deformation and failure mechanisms for materials with highly anisotropic mechanical properties. Among such materials, glass-fiber and carbon-fiber reinforced polymer-matrix composites play a critical role in advanced structural designs. A large number of different methods and specimen types currently required to generate threedimensional allowables for structural design slow down the material characterization. Also, some of the material constitutive properties are never measured due to prohibitive cost of the specimens used for the material characterization. This work shows that simple short-beam shear specimens are well-suited for measurement of 3D constitutive properties for composite materials, and that can enable a major shift toward accurate 3D material characterization. The material characterization is based on the Digital Image Correlation full-field deformation measurement and simple stress analysis. This work introduces three fundamental contributions to the characterization of mechanical properties for composite materials. First, tensile, compressive, and shear stress-strain relations are measured in a single experiment. Second, a counterintuitive feasibility of closed-form stress and modulus models, normally applicable to long beams, is demonstrated for short-beam coupons. The modulus and stress-strain data are presented for glass/epoxy and carbon/epoxy material systems. And third, the test method is viable for measurement of stress-strain relations at various load rates including static, fatigue, and impact load conditions. Introduction Analysis of mechanical behavior of materials and structures requires knowledge of material stressstrain constitutive properties. Growing acceptance and rapid development of materials with highlyanisotropic mechanical properties, including glassreinforced and carbon-reinforced polymer-matrix composites, left accurate characterization of their constitutive properties behind. Structural analysis of composites oftentimes requires accurate assessment of their three-dimensional stress and deformation states to understand complex failure mechanisms [1,2]. Therefore, accurate three-dimensional stressstrain constitutive relations are needed. Standard techniques for assessment of stress-strain constitutive relations for materials are based on resistance strain gage measurements. As a strain gage measures a “point” strain averaged through the gage area, such measurement imposes constraints on the test specimen designs. For example, a standard practice for assessment of transverse shear constitutive properties for composites is the Vnotched beam method [3]. Strain gage measurements in the V-notched beam method drive high specimen cost: at least 0.75-inch (19 mm) thick laminate is required to machine specimens for strain gage placement; and tight geometry tolerances are imposed to minimize variations of strain at the gage location. Also, a large number of different test specimen types used for assessment of constitutive properties, including tensile, compressive, and shear stress-strain curves in the principal material planes [3], slow down 3D material characterization process. A METHOD FOR MEASUREMENT OF THREE-DIMENSIONAL CONSTITUTIVE PROPERTIES FOR COMPOSITE MATERIALS A. Makeev, Y. He, B. Shonkwiler, E. Lee, H. Schreier, Y. Nikishkov 1 University of Texas at Arlington, Arlington, TX, USA, 2 Georgia Institute of Technology, Atlanta, GA, USA, 3 Clark Atlanta University, Atlanta, GA, USA, 4 Bell Helicopter Textron, Hurst, TX, USA, 5 Correlated Solutions, Columbia, SC, USA

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