A model for the response of unreinforced and z-pin reinforced composite pi joints

Experimental data obtained from Narrow Element and Wide Element pi joints was used to motivate the development of a mechanics-based model to predict the quasi-static response of non-reinforced and z-pin reinforced composite pi joints. The progressive failure modeling approach used a novel cohesive formulation to include intra- and inter-laminar damage modes and their interactions. A smeared cohesive zone modeling approach was implemented in which effective fracture toughness and cohesive strength values were defined in the areas corresponding to the z-pin fields in the reinforced joints. Properties defining the cohesive responses were calibrated at the narrow element level and subsequently used in the wide element model to establish blind predictions prior to experimental testing. Unreinforced (unpinned) and z-pin reinforced pi joint specimens were tested in pull-off loading. Experimental and numerical comparisons were made in terms of load-displacement response, critical loads, damage modes, and failure progression. Comparisons between the experimental results and blind predictions for the unpinned joints were found to be in good agreement; however, discrepancies for the z-pin reinforced joint emphasize the challenges associated with physics-based simplified modeling techniques intended to capture complex z-pin behavior.