Analysis of Bending‐Twisting Coupling on the Flexural Response of Quasi‐Isotropic Laminates

ABSTRACT

Quasi‐isotropic (QI) laminates can exhibit similar in‐plane properties while developing different flexural responses because the bending stiffness matrix depends on both ply orientation and ply position through the laminate thickness. In balanced and symmetric QI laminates, non‐zero and terms can produce bending‐twisting (B–T) coupling under out‐of‐plane loading. This study investigates the influence of B–T coupling on the flexural and damage behaviour of 16‐ply QI carbon‐fibre laminates with 0°, 90° and ±45° ply orientations. Classical laminate theory was used to derive the stacking configurations and determine their stiffness and coupling terms. Three laminates with identical extensional stiffness matrices and comparable primary bending stiffness terms, but different and values, were evaluated using short‐beam and three‐point bending tests, followed by post‐failure damage examination. The stacking‐sequence analysis identified 32 extensional isotropic laminate (EIL) configurations with = = 0. The laminate without B–T coupling recorded the highest short‐beam strength, failure load and flexural strength, while the laminate with the highest B–T coupling recorded values approximately 7%–8% lower. Post‐failure observations also showed more extensive internal damage and ply separation in the laminate with higher B–T coupling. These results show that stacking‐sequence B–T coupling is associated with measurable differences in the flexural and damage responses of balanced and symmetric DI laminates.