The invention relates generally to testing of flexure specimens to determine strain and elastic modulus, and in particular to a device designed to allow testing under pure bending conditions of thin composite laminates used as flexural elements.
Thin composite laminates are commonly used in deployable space structures. In application, the laminates are elastically folded to allow a structure to be compactly stowed during launch and subsequently deployed to an operational state once in orbit. This basic technology enables operational systems containing structures such as solar arrays, reflectors, antennas, and booms to be efficiently packaged and launched within the payload envelope of conventional launch vehicle fairings or within tightly allocated volume constraints common to compartmentalized payloads such as CubeSats.
While the use of composite laminates in deployable structures is widely acceptable, in operation they are subjected to strain levels and deformations outside of traditional composite structural applications. As such, their behavior is poorly understood, making deployable structure design and analysis extremely difficult. Currently, standardized ASTM test methods are used to determine composite material strengths and linear-elastic stiffnesses under traditional loading applications such as axial and transverse tension, compression, and shear. Data from these tests yield accurate, basic properties that are useful for laminate design, but they fail to characterize the nonlinear constitutive behavior over the full strain range common to deployable structures.
Studies have shown that flexural loading will result in higher compressive and tensile strengths than are determined from traditional tensile and compressive coupon tests. Single fiber tests further support this behavior, showing significantly higher flexural strengths as compared to tensile strength data. Attempts at analytically quantifying these increased strengths have largely been ineffective or are not applicable to the thin laminates used in deployable structures. For example, the commonly used Weibull statistical model under predicts flexural strength, while single fiber data does not consider structural stabilization or the role of the laminate matrix. Furthermore, the extrapolation of classical bending theory, which assumes linear-elastic behavior, is of little value due to nonlinear composite stiffening and softening with increasing and decreasing strains respectively. Finally, the matter is further complicated by laminate thickness concerns for composites in bending. Thinner specimens have been found to have higher compressive strengths due to the steep stress gradient through the specimen thickness because of the close proximity of fibers under tension and compression.
The current method for such testing is demonstrated in FIG. 1 in which two parallel platens 2, 3 are pressed together with the specimen 1 between them. The applied force F measured by the load cell 4 and the platen separation data d are used to calculate the applied moment and specimen curvature by using the rather complex Euler elastic analytical model. This test method does not generate a pure stress state in the specimen and does not allow a direct measurement of curvature, bending moment, or strain.