Improvement of bearing strength of laminated composites

Carbon fibre reinforced composite structures generally out perform metallic ones, but this is not the case when the structure is loaded in bearing. This work examines several different strategies for improving bearing performance. The first strategy was fibre steering (directed fibre placement). Three different methods were used to determine the trajectories for the steered fibres, referred to as the principal stress method, the load path method and the genetic algorithm method. In the principal stress method the fibre trajectories were determined from the major (tensile), s11, and minor (compressive), s22, principal stresses. An improvement of 36% in bearing strength was obtained using a modified principal stress pattern in which the s11 trajectories were displaced 5 mm away from the bearing-loaded hole to reduce fibre waviness which resulted from local overcrowding of the s11 and s 22 tows directly beneath the bearing surface. In the load path method, the steered fibre trajectories followed the dominant load path. A similar improvement in bearing strength of 33% was obtained using this method. Moreover, it was found that the load path method provided surplus reinforcement against net section failure and it was possible to more than double the efficiency of carbon laminate joints by reducing the ratio of joint width to bolthole diameter w/d from the standard value of 5 to a more compact 2.5, while simultaneously maintaining the bearing strength of the laminate. The third method employed an optimisation technique in which a genetic algorithm was used to determine the trajectories for the steered fibres. The trajectories obtained were intermediate between those from the principal stress method and those from the load path method. Matrix stiffening using nanoreinforcement was also examined. In this part of the work montmorillonite clay at loadings of 7.5 and 12.5 parts per hundred resin was incorporated into the matrix resin. While the addition of the clay nanoparticles substantially stiffened the neat resin, its incorporation into the composite laminates did not increase the bearing strength due to the introduction of an alternative, premature failure mode. The incorporation of the nanoparticles did however stiffen the bearing response, indicating that the method could produce improved bearing strength if premature failure could be avoided. Through thickness reinforcement using z-pins was also examined by inserting carbon fibre z-pins, at an aerial density of 4%, locally in the vicinity of the bearing-loaded hole. This technique increased the ultimate bearing load by 7%. However this did not translate into improved bearing strength because of local thickening of the laminate in the z-pinned region due to the presence of surplus resin displaced by the z-pins. It is considered that local thickening could be readily avoided by using a caul plate in the vicinity of the z-pinned region, thereby allowing the potential of this technique to be realised.