Investigation of the mode-I delamination behavior of Double-Double laminate carbon fiber reinforced composite

Double-Double (DD) laminate, consisting of a repeat of a 4-ply sub-laminate [±Φ/±Ψ], has attracted widespread attention. The delamination damage of composites is always a significant failure problem in the application and a challenge for DD laminate. However, the mode-I delamination propagation behavior and fracture toughness of DD laminate with different interface angles were seldom reported. In this study, the interface angles of DD laminate were variable and could be classified into two types: Ψ/Φ and –Ψ/Φ interfaces. The effect of interface angle on the delamination propagation of DD laminate was explored using the double cantilever beam test. The delamination damage mechanisms of the above interfaces were revealed using macroscopic and microscopic characterization methods. The results showed that the initial fracture toughness values of laminates at the 0°/0°, 90°/0°, Ψ/Φ, and –Ψ/Φ interfaces were similar, and the steady-state fracture toughness values of laminates at the Ψ/Φ interfaces were at least 20% higher than that of the –Ψ/Φ interfaces. Moreover, the maximum bridging stresses at Ψ/Φ and –Ψ/Φ interfaces were approximately equal, while the final failure displacements at Ψ/Φ interfaces were about 1.4–1.5 times than that at –Ψ/Φ interfaces.