Design of a Fixture System and a Composite Tapered Laminate Specimen under Combined Tension–Torsion Loading

Ply drop-offs represent the regions of structural weakness in variable-thickness composite laminates, such as those found in pump-jet rotor blades. However, the structural experiment of the rotor blade entails considerable expense at the preliminary design stage. This paper proposed a combined tension–torsion loading method, which used tapered laminates to simulate the stress state of a pump-jet rotor blade under critical failure conditions. By matching the geometric and stress-state similarity, this approach offers a cost-effective means of verifying preliminary designs. The fixture system was developed to impose the pre-torsion load, whereas the tensile load was applied using a uniaxial testing machine. The design of tapered laminate specimens was further investigated to more accurately approximate the stress conditions experienced by pump-jet rotor blades. The results show that the specimen’s width has a significant effect on the stress field. The stress increases with decreasing width. A too short interval between the clamping area and the tapered part of the specimen leads to a severe stress concentration effect on Interface 1. A converse trend is observed for Interface 2. With the increase in the interval, the influence of the clamping end is weakened. Therefore, it is necessary to determine the width and interval appropriately to achieve the stress-state similarity between the tapered laminate and the rotor blade.