Size effect and damage mechanism of Double–Double open-hole composite laminates

Abstract

Double–Double (DD) laminates are emerging as a promising lay-up design, which employs $[\pm \Phi ,\pm \Psi ]$ as a basic unit to replace the traditional method of using discrete $0^o$, $\pm 45^o$, and $90^o$ layers (called QUAD laminates). This novel design offers several advantages, including simplified manufacturing processes, greater design flexibility, and significant potential for weight reduction. While DD laminates have gained increasing attention in recent years, their mechanical behavior in configurations with open holes, a common feature in practical applications, remains insufficiently studied. This paper aims to investigate the effect of open holes on the failure behavior and strength of DD laminates. To this end, we first present an experimental campaign on four groups of open-hole tensile specimens, including QUAD laminates with quasi-isotropic (QI) and soft (with more than 50% $\pm 45^o$ layers) lay-ups, as well as their DD laminate counterparts designed to maintain equivalent in-plane stiffness. Subsequently, progressive damage analysis and strength predictions were performed based on smeared crack modeling and finite fracture mechanics, respectively. Results show that QUAD_QI, serving as a baseline, presents a conventional size effect: the notched strength decreases as the hole diameter increases when the diameter-to-width is kept constant. In contrast, DD laminates demonstrate an inverse size effect, where their strength ultimately exceeds that of the corresponding QUAD laminates when the hole diameter is the largest. This phenomenon is closely tied to the evolution of damage mechanism as the geometric size changes, in which different roles of sub-critical damage events in the final failure of QUAD and DD laminates were discovered.