Improving bearing performance of composite bolted joints using z-pins

The effect of z-pins on the bearing properties and damage tolerance of composite bolted joints is experimentally studied in this paper. The region around bolt-holes in carbon/epoxy laminates was reinforced in the through-thickness direction with different volume contents and sizes of fibrous z-pins. Bearing test results show that the z-pins improved the bearing stiffness (by 7.5-9.6%), ultimate load (7.7-12.8%), failure strength (7.4-9.8%), and elastic strain energy absorption to bearing failure (8.5-16.3%) of the composite joints. The bearing properties increased at a quasi-linear rate with the z-pin content, but were not dependent on pin diameter. Stiffness is improved by z-pins increasing the through-thickness tensile modulus around the bolt-hole of the joint. Post-mortem microstructural examination of the failed joint specimens revealed that z-pins improve the bearing strength by reducing cracking near the bolt-hole via an interlaminar bridging toughening mechanism that involves debonding and frictional sliding of pins within the damaged region. The elastic strain energy to failure is increased by the through-thickness stiffening and toughening provided by the z-pins. This study proves that the reinforcement of bolt-holes with z-pins increases the bearing properties without the weight penalty incurred with the traditional strengthening method of thickening the laminate around holes. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.