Abstract

The use of an anti-buckling device is proposed to avoid the appearance of undesired instabilities during biaxial testing with cruciform specimens in which the compressive loads are applied directly on the arms of the sample by means of compression plates. The device consists on a cross-shaped accessory that restricts out-of-plane displacements and a L-shaped support that forces the alignment of the fixture, the specimen and the testing machine. In addition, recommendations about the test methodology and the conditioning of the cruciform sample are given. For this purpose, analytical and numerical studies are developed to design the experiments and to describe both the global buckling of the cruciform specimen and the local instability of the central region subjected to biaxial loading. The risk of instability in the arms of the sample is reduced under tension-compression and compression-compression loading cases whenever the device is used, ensuring that the results are not affected by premature buckling of the specimen and favoring the appearance of appropriate failure modes.

Abstract

Characterization of shear properties in unidirectional fibre composite materials can be a complex process due to the difficulty in obtaining uniform and pure shear stress-strain states at the moment of failure. This limits the range of application of the most commonly used standardized methods, as they focus on small deformations. Therefore, this work proposes an alternative solution: the tension-compression test applied to cruciform specimens with ±45° symmetric laminates for the characterization of lamina in-plane shear properties. By using a test method that provides uniform and pure shear stress-strain states, the accuracy of the results can be improved, and the range of application of the method can be expanded. The analysis is based on experimental tests, supported by simulations performed using the finite element method. Special attention is paid to verifying the achieved shear state, and the obtained response is compared with standardized tests that generate non-pure shear stress-strain states on the principal material directions. Additionally, Digital Image Correlation is used during the test to verify the appearance of nonlinear response in different regions of the cruciform specimen, which allows ensuring the validity of the test.