Bolted joint studies in GRP
Fox, David M.
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An experimental study was carried out to determine the bolt clampup force relaxation behavior in countersunk, single-lap bolted joints between glass/vinylester resin laminates and steel panels. Additionally, the effect of bolt clampup force relaxation on the bearing strength of such joints was studied. Specially-instrumented bolts were used to measure clampup force relaxation in ten countersunk joint specimens. A 100,000 pound capacity MTS testing machine was used to evaluate the bearing strength in tension of sixteen countersunk bolted joint specimens torqued to four different levels of initial torque, as well as four similar protruding-head bolted joint specimens (at the same levels of torque). The results of the relaxation experiments indicate that clampup force varies widely in nominally identical joints at identical torque levels, by as much as a factor of two. In countersunk bolt joints with sufficiently high initial clampup force, the clampup force relaxed in accordance with the inverse power equation proposed by Shivakumar and Crews for protruding-head bolt joints in graphite/epoxy laminates. Relaxation in the present experiments proceeded faster than in the protruding-head joints studied by Shivakumar and Crews; this is believed to have been due to the relative lack of constraint provided at the GRP surface by the countersunk bolt. The results from the bearing strength experiments suggest that clampup force has only a small beneficial effect on bearing strength in both countersunk and protruding head single-lap joints, when compared to double-lap joints. The reduced effectiveness of clampup force in increasing the bearing strength is believed to be a result of the increased joint rotation and bolt bending inherent in single-lap joints. These phenomena lead to delamination and brooming failure in the GRP, which can be avoided in double lap joints with sufficient bolt clampup force. The countersunk joints failed at a load approximately 20% lower than the protruding head joints.
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