Excellent torsional stiffness generated by angle-ply laminates at ±45° configurations favor their wide applicability in the industry, while in laboratories it is commonly used for the purposes of determining material in-plane shear properties due to the relative simplicity of the tensile test method applied. When it comes to uniaxial testing, the ±45° angle-ply laminates at times develop pseudo-ductile effects. Pseudo-ductility could be sought in composite structures to avoid their brittle behavior and, consequently, to withstand higher levels of external loading due to an extended non-linear response. Presently, there exists a plethora of literature regarding the angle-ply laminates submitted to uniaxial tests. Nevertheless, the pseudo-ductile effects appear also under flexural loading because tension and compression are applied in different regions of the cross-sections simultaneously. In this sense, bending testing presents a higher degree of complexity introduced by the variation of the strain through the cross-section thickness. Therefore, there is need, urge and drive to comprehend pseudo-ductile flexural response of symmetric ±45° angle-ply laminates as flexion is a very frequent loading case in structures under working conditions.
Recently, a team of researchers from the University of Castilla-La Mancha in Spain investigated the three-point bending response of symmetric ±45° angle-ply laminates built with unidirectional and continuous carbon fiber-reinforced epoxy laminae. They purposed to cross-examine the pseudo-ductile effects considering that pseudo-ductility refers to the non-linearity driven by the accumulation of damage and yielding of the matrix that favors the fiber reorientation. Their work is currently published in the research journal, Composites Science and Technology.
For the purposes of their work, the research technique utilized entailed evaluation of the classical laminated plate theory in order to quantify the coupling of forces based on the theoretical knowledge of the neutral axis deviation and the different material behavior under tension and compression. Next, Serna Moreno and colleagues tested the proposed laminates in order to verify the correctness of the analytical predictions acquiring the displacement and strain fields by means of Digital Image Correlation techniques. In addition, optical visualization and Scanning Electron Microscopy were employed in order to assess experimentally the first failure mode induced.
By comparing the effects of the pseudo-ductility in flexural testing and the experimental response of both laminates, the researchers observed that similar in-plane linear response were recorded. Additionally, it was also seen that the main differences in the mechanical response could be attributed to the stacking sequence, as the effective ply-thickness in the first sample was half that of the alternate laminate. Finally, the researchers were able to develop analytical predictions of different parameters of the linear response by considering the different tensile and compressive mechanical performance of the material, as well as the numerical optimal design of the stacking sequences that reduce the bending-twisting coupling and increase the pseudo-ductile effect.
In conclusion, University of Castilla-La Mancha study presented an excellent investigation of the flexural response of [+45 -45]6S and [+452 -452]3S laminates. In so doing, they discovered that similar linear behavior was eminent for the two samples, but different non-linear phases. Altogether, it has been shown in this work that it is possible to remove the inherent limitation and enlarge the design space by using thin ply angle-ply laminates that exhibit a non-linear ‘pseudo-ductile’ stress-strain response.
*This work was financially supported by the Ministerio de Economía y Competitividad of Spain under the grant DPI2016-77715-R.
M.C. Serna Moreno, S. Horta Munoz, A. Romero Gutiérrez, C. Rappold, J.L. Martínez Vicente, P.A. Morales-Rodríguez, J.J. Lopez Cela. Pseudo-ductility in flexural testing of symmetric ±45° angle-ply CFRP laminates. Composites Science and Technology, volume 156 (2018) page 8-18Go To Composites Science and Technology