Mechanical properties and cold and warm forming characteristics of sandwich sheets with a three-dimensional CFRTP core

Abstract The application of sandwich sheets containing a three-dimensional core is limited due to low manufacturing efficiency and poor formability . Metal-based sandwich sheets, where thin metallic face sheets are used to sandwich and bond with a three-dimensional core fabricated by stamping an initially flat carbon fibre reinforced thermoplastic (CFRTP) sheet, are able to provide high specific stiffness with suitability for mass production. Aiming at secondary forming such sandwich sheets into three-dimensional shapes, cold and warm forming characteristics of sandwich sheets composed of various face sheets and a three-dimensional CFRTP core with different fibre lengths and volume fractions were investigated. The numerically predicted pure bending rigidity and out-of-plane shear modulus were converted into numerical three-point bending stiffness through theoretical formulas and compared with experimental values to clarify the mechanical properties of different sandwich sheets. A three-dimensional core was successfully fabricated by warm stamping ultrathin chopped carbon fibre tape reinforced thermoplastic (UTCTT) with carbon fibre volume fraction of 40%. Draw-bending demonstrates that formability of sandwich sheets can be improved by optimising material combinations and forming temperatures. Sandwich sheets composed of metallic face sheets and a short carbon fibre reinforced thermoplastic (SCFRTP) core can be easily cold formed, while those with a UTCTT core can be easily warm formed.