Investigation on the Bending Properties and Geometric Defects of Steel/Polymer/Steel Sheets—Three-Point and Hat-Shaped Bending

Steel/polymer/steel laminates, also known as laminated steels, are composite materials consisting of bonding layers of steel and polymer. The polymer layer acts as a bonding agent between the steel layers, imparting additional properties such as low density, impact resistance, and thermal insulation, while the steel layers provide strength and formability. These laminated steels have found increasing applications in automotive, aerospace, and construction industries to reduce weight and improve fuel efficiency. The bending behavior of this laminates is more complex compared to that of a single layer of metallic sheets. This complexity arises from significant differences in mechanical properties, as well as the thickness ratio between the skin and the core. The flexural properties and behavior of different St14/TPU/St14 laminate sheets that were fabricated using the direct roll bonding (DRB) process were investigated through three-point and hat-shaped bending tests. The direct roll bonding process involves the bonding of steel and semi-melt polymer sheets under the pressure of rollers, ensuring a cohesive and durable composite material. The microscopic analysis of the cross-section of the SPS laminates after the bending processes shows the absence of delamination or slippage between the layers, which indicates the correct selection of materials and the bonding method. The results showed that the springback of three-layer laminates has an inverse relationship with the work-hardening exponent, yield strength, and yield point elongation value, while possessing a direct relationship with normal anisotropy and elastic modulus. Furthermore, the flexural strength and flexural modulus decrease with the increase in the volume fraction of the polymeric core, while the flexural rigidity increases. The findings indicate the DRB technique as a promising method for manufacturing a lightweight metal–polymer laminate with a high formability performance.