Properties and failure mechanisms of pinned composite lap joints in monotonic and cyclic tension

The effect of through-thickness reinforcement by fibrous pins on the static tensile strength, fatigue life and failure mechanisms of single lap joints made of carbon/epoxy composite is investigated. Pinning is highly effective in increasing the ultimate strength, elongation limit and fatigue life. Improvements to the monotonic and fatigue properties are attributed to transitions in the failure mechanisms, from unstable joint debonding in the absence of pins to stable debonding in the presence of pins followed by ultimate failure by pin pull-out or shear fracture or tensile laminate rupture. Which mechanism induces ultimate failure in the presence of pins depends on their volume content and diameter. The trends with volume content and diameter can be predicted qualitatively using analytical results from a previously published model of the deformation mechanics of pins loaded in mixed mode.