Quasi-static and fatigue behaviour of composite bolted joints

by Starikov, Roman

Since modern aircraft composite structures often are fastened by means of bolted joints, the structural integrity of such connections has to be validated according to the certification requirements. The behaviour of full-scale composite structures can be represented on sub-structural elements including mechanically fastened joints. The objective of the dissertation is to improve the understanding of the quasi-static and fatigue behaviour of bolted joints with different configurations. In the experimental procedure composite joints with single and multi-row, single and double overlap configurations were tested. The influences of protruding and countersunk-bolt heads, fastener system, quasi-isotropic and highly orthotropic lay-ups on the joint strength and fatigue performance were studied. Strain gauge measurements were carried out in order to measure strain distribution and to calculate load transfer between bolt rows on specimens tested in quasi-static and fatigue. An experimental method of bolt-movement measurement was used to analyse the local behaviour of the bolt and the surrounding composite during loading. Fractographic SEM and optical microscopy were employed to investigate fracture and damage developed in the joint system during static and fatigue testing. As the obtained test results show, increasing the number of bolt rows to the joint configuration, improves its ability to support static and fatigue loading. A linear dependence was found between the number of bolt rows and the fatigue life of those joints, which failure mode was due to fatigue fracture of bolts. The fatigue resistance of single and double lap joints was similar. Among different fastener systems, specimens joined by protruding-head bolts displayed better static and fatigue performance than those bolted by countersunk fasteners. When comparing applied strain, joints with a quasi-isotropic lay-up yielded higher static and fatigue strength than bolted laminates with a 0?-dominated lay-up. Load-transfer measurements of bolt rows showed that the distribution of applied load on the bolt rows was not the same. The outer bolt rows appeared to be transferring a slightly larger amount of the applied load than the inner bolt row. Strain distribution between bolt rows was found to be greatly affected by the bolt presence and its behaviour during static and fatigue testing. The bolt behaviour was affected by reduction in pre-stress in the bolt, friction forces between the composite parts, and the level of damage sustained by the fastener and composite. Fractographic observation showed that bolted joints subjected to fatigue loading experienced severe damage around the bolt holes. However, the degree of damage developed at different bolt holes varied since the individual bolts were exposed to different load levels during testing. The joint configuration, the level of pre-stress in the fasteners, load transfer of the applied load between the joint parts, and the extent of damage developed during loading are very important issues which should be considered during the design procedure of composite mechanically fastened joints. Quasi-static and Fatigue Behaviour of Composite Bolted Joints v Dissertation The present thesis includes an introduction to the area of research and five appended research papers: Paper A Starikov R., and Schön J. “Quasi-static behaviour of composite joints with protruding-head bolts” Composite Structures 2001;51(4):411–425. Paper B Starikov R., and Schön J. “Quasi-static behaviour of composite joints with countersunk composite and metal fasteners” Accepted for publication in Composites Part B: Engineering. Paper C Starikov R., and Schön J. “Fatigue resistance of composite joints with countersunk composite and metal fasteners” Submitted to International Journal of Fatigue. Paper D Starikov R., and Schön J. “Experimental study on fatigue resistance of composite joints with protruding-head bolts” Submitted to Composite Structures. Paper E Starikov R., and Schön J. “Local fatigue behaviour of CFRP bolted joints” Submitted to Composite Science and Technology. Quasi-static and Fatigue Behaviour of Composite Bolted Joints vii Division of Work Between Authors Paper A Starikov performed the quasi-static tests, experimental measurements, and fractographic observations. Starikov and Schön wrote the paper. Paper B Starikov carried out the quasi-static tests, experimental measurements, and fractographic observations. Starikov and Schön wrote the paper. Paper C The fatigue tests and experimental measurements were done by Starikov. Starikov and Schön wrote the paper. Paper D Starikov performed the fatigue tests and experimental measurements. Starikov and Schön wrote the paper. Paper E The measurements and fractographic work were carried out by Starikov. Starikov and Schön wrote the paper. Quasi-static and Fatigue Behaviour of Composite Bolted Joints ix