Machining and fatigue induced damage in bolted composite laminates

Abstract: The effects of hole machining defects on strength andfatigue life of carbon/epoxy laminates with open holes andpin-loaded holes subjected to static and fatigue loading havebeen investigated. Traditional drilling techniques causingvarying extent of damage and a novel method causing nodetectable damage were employed to machine the holes. Intrinsicdamage and propagation of damage during fatigue loading wasdetected using non destructive examination techniques. Theresults showed that intrinsic damage affect the strength andfatigue life considerably. They also show that during a largeportion of the fatigue life the growth of damage (mainlydelaminations ) do not affect the overall structural responseof the specimens.Several factors affecting static strength and fatigue lifeof multiple-row bolted joints in carbon/epoxy laminates wereinvestigated experimentally and ranked in order of importance.the experiments were set up according to a design ofexperiments plan. Design factors such as the ratio between holediameter and spacing between fasteners, and type of fastenerhead proved to have significant influence on the strength andfatigue life.The strain energy density used as a failure criterion hasbeen employed to predict delamination initiation in apin-loaded carbon/epoxy laminate. The finite element method wasused to determine the stress and strain fields in the vicinityof the pin-loaded hole. Experiments of pin-loaded specimens andsubsequent non destructive examination were used to validatethe analysis. The analytical results agree well with theexperiments.Design variables such as hole to pin shape configuration andhole to edge distance were investigated experimentally andanalytically. Pin-loaded specimens instrumented with straingages determined the strains far and near the hole. Twodimensional analysis using the modified mapping collocationmethod was employed to determine the stresses and strains farand near the hole. The analytical predictions and experimentalmeasurements indicate that the load carrying capacity of apin-loaded hole can be altered by changing the pin-shape.Key words:Composite, laminate, bolted-joints,pin-loaded, hole machining, damage, computerized tomography,fatigue, delamination, strength, strain energy density,boundary collocation method.