Design of composite structures containing bolt holes and open holes

Abstract: This thesis is concerned with stress analysis and strengthprediction in composite laminates containing bolt holes andopen holes. It covers both two- and three-dimensional analyses,and experiments have been carried out to determine criteriaparameters and to validate the analysis methods.The objective of the work on two-dimensional analysismethods has been to develop accurate and cost-efficient stressanalysis and failure prediction methods valid for complexloading conditions. Both the finite element method and a methodbased on Lekhnitskii’s complex stress functions have beenused to determine the stress distribution around the hole. Inthe case of bolted laminates, the frictionless contact betweenthe bolt and the hole has been taken into account. Differentfailure criteria, including the Point Stress Criterion (PSC)and the Damage Zone Criterion (DZC) have been used to predictthe strength of test specimens subjected to complex loadingconditions. In the case of bolted laminates, simple failurecriteria for preliminary design are proposed. The validity ofthese criteria is demonstrated on multi-fastener tension- andshear-loaded test specimens. A comprehensive test programme wascarried out to establish criteria parameters and to generatedata to validate the stress analysis and strength predictionunder complex loading conditions. A special test fixture wasdesigned for this purpose. Good agreement was found betweenpredicted and measured results.The objective of the work on three-dimensional methodshas been to develop tools for three- dimensional stress andfailure analyses and to study the effect of various factors onthe strength of single-lap joints. An experimental programmewas conducted to characterize the failure mechanisms and tomeasure deformation, strain, and bending effects. Athree-dimensional finite element model of a bolted single-lapjoint has been developed to determine the non-uniform stressdistribution through the thickness of the laminate in thevicinity of the hole. The model was validated against measuredstrains and displacements from the experimental programme. Theagreement between predicted and experimental results wasgenerally good. In the failure characterisation part of theexperimental programme, it was found that the failure of thejoints was dominated by kinking. A failure analysis procedurefor the prediction of bearing failure dominated by kinking hasbeen developed. In this procedure, failure is predicted using aquadratic failure criterion evaluating fibre stress andtransverse shear stress at a characteristic distance from theedge of the bolt hole. Experimental results were used tovalidate the analysis procedure and good agreement was foundbetween predicted and experimental failure loads. The analysisprocedure were then used study the effect of eight differentparameters which affect the strength of bolted compositelaminates. The parameters studied were: laminate thickness,lay-up, bolt diameter, bolt configuration (countersunk orprotruding head), friction, clamping force, clearance andlateral support. The parametric study was organised as areduced two-level factorial test. The results from theparametric study show that laminate thickness, friction,clamping force and bolt configuration are the parameters whichhave the strongest influence on the strength of the joints.Key words:Composite, laminate, bolted joints, openholes, static strength, failure characterization, damage,complex loading, stress analysis, three-dimensional analysis,contact analysis, failure prediction, factorial test