Thickness‐Dependent Toughening Mechanisms in Mode I Delamination of Z‐Pin‐Reinforced CFRP Composites

ABSTRACT This study investigates the thickness‐dependent Mode I interlaminar fracture behavior of z‐pin‐reinforced carbon fiber‐reinforced polymer (CFRP) composites with varying thickness. Double cantilever beam (DCB) tests were conducted to evaluate the Mode I delamination resistance of specimens reinforced with through‐thickness z‐pins. The load–displacement responses and R ‐curves were analyzed to elucidate the thickness‐dependent delamination toughening mechanisms. The experimental results demonstrate a marked increase in the Mode I fracture toughness G _{I c} with specimen thickness. Compared with unpinned specimens, the steady‐state fracture toughness G _ss of z‐pinned specimens increased to a maximum of 6934 J/m ² . Analysis of the load–displacement responses and R ‐curves reveals that the enhanced delamination resistance is governed by the evolution of the z‐pin bridging zone, where the z‐pins predominantly fail by pull‐out rather than fracture. With increasing laminate thickness, more rows of z‐pins are activated within the bridging zone, thereby increasing the cumulative pull‐out resistance and the associated energy dissipation during crack propagation. These findings provide new insight into the thickness‐dependent toughening mechanisms of z‐pin‐reinforced CFRP laminates and offer guidance for the design of through‐thickness‐reinforced composite structures with improved delamination resistance.