Effects of layups with negative and positive Poisson’s ratios on the stress–strain behaviors of plain and holed hybrid composite laminates

In this paper, classical laminate theory is employed to calculate Poisson’s ratios for glass/epoxy, carbon/epoxy, and hybrid composite laminates with various layup configurations. A total number of 67,368,400 different multilayer combinations with varying layers, compositions, and materials are theoretically analyzed, exhibiting negative Poisson’s ratios. Given the significance of materials showing negative Poisson’s ratio, auxetic composite laminates are selected from the obtained layups, and some of them are further analyzed using experimental and finite element methods. The results from the classical laminate theory are confirmed with the static tensile tests conducted on selected layups with both negative and positive Poisson’s ratios. The theoretical and experimental results are then compared to validate the obtained mechanical properties. Hybrid plain and perforated laminates with both negative and positive Poisson’s ratios, but similar longitudinal Young’s modulus, are manufactured and subjected to tensile loading. Strain distributions are obtained using finite element and DIC methods for all tested specimens. The obtained results shown that the use of carbon/epoxy layers and hybrid laminates exhibits the potential to achieve negative Poisson’s ratios. Moreover, the findings demonstrated that layer angles close to 10° and 70° for carbon/epoxy layers, as well as angles close to −20° and −80° for glass/epoxy layers, result in lower Poisson’s ratio values. Furthermore, it was shown that the auxetic perforated hybrid laminates exhibits higher ultimate tensile strength and longitudinal elastic modulus compared to the non-auxetic specimens. Experimental results also shown that in the auxetic perforated hybrid laminates, failure occurs in the fibers, whereas in the non-auxetic specimen, failure initiates in the matrix, with crack propagation from the hole towards the entire specimen. Additionally, auxetic samples experience gradual failure at higher strains, while non-auxetic samples fail suddenly at lower strains.