Experimental Investigation of Puncture Behavior and Fractured Morphology of Steel/Graphene-Reinforced Polymer/Steel Sandwich Composite Structure

Graphene has emerged as a revolutionary two-dimensional (2D) nanomaterial, profoundly altering the fields of materials science and mechanical engineering. Driven by its superior mechanical, electrical, and thermal properties, including a Young's modulus of approximately 1 TPa, high electrical conductivity, and high thermal conductivity, researchers have extensively explored its potential as a reinforcing filler in various composites. The thin composite steel/graphene-reinforced polymer/steel sandwich structure proposed in this study can be applied in the development of precision electromechanical devices due to its advantages of high dimensional stability, high rigidity in a confined space, light weight, and ability to reduce micro-vibrations. This study evaluates the effect of adding graphene to the alkyd-based polymer core layer on the mechanical properties and fracture mechanisms of a sandwich system with a total measured thickness of 0.35 ± 0.01 mm. Through small punch tests (SPTs) with controlled die and punch geometry coefficients, the local load-bearing behavior of the material was investigated in detail. Experimental results show that the dispersion of graphene enhances the flexural stiffness of the core layer, thereby improving the flexural stiffness of the entire structure and increasing the maximum load by 12.6% (from 0.79 ± 0.03 kN to 0.89 ± 0.04 kN). However, this structure exhibits a clear mechanical trade-off as the fracture strain decreases from 1.80 ± 0.08 mm to 1.50 ± 0.09 mm, resulting in a slight 6.1% decrease in approximated energy absorption capacity. Morphological observations at the fracture groove suggest a shift in the fracture mechanism from macroscopic ductile tearing with large plastic deformation to localized abrupt brittle shear plug, accompanied by instantaneous elastic energy release and delamination. These findings indicate that the graphene-reinforced sandwich structure is suitable for thin-film applications requiring high static rigidity, but careful consideration is needed in environments subject to dynamic impact.