Singular stress fields near contact boundaries in a composite bolted joint

Singular stresses arising in the neighborhood of contact surfaces introduced in laminated orthotropic plates by mechanical joining with clamp-up were investigated by using local asymptotic solutions and full-field numerical analysis. Three-dimensional B-spline approximation of the displacements and a penalty function-based contact solution was used in the numerical analysis. Recent work has shown that fracture in bolted composite joints may initiate near the outer edge of the bolt head or washer away from the hole edge, particularly if the joint is preloaded. Material and geometric discontinuities exist in these regions, resulting in singular stress behavior. Asymptotic stress analysis was performed to obtain the power of singularity in these regions as a function of the bolt-head (washer) stiffness. Frictionless contact conditions were assumed. It was found that the characteristics of the stress singularity for such practically important combinations as titanium bolt-head and carbon fiber composite plate are similar to a crack in terms of the power of singularity and uniqueness of the singular term. Coefficients of the singular terms of the asymptotic expansion were determined by comparison with the numerical solution in the close vicinity of the singular contour. Good agreement between the asymptotic and numerical solution in the transition regions was observed.