Change of strain field due to damage development in adhesive joints

Abstract

In this thesis six metal I-beams with a crack repaired by a composite patch been tested in four point bending. The patches used to repair the beams had three different patch thicknesses. The different thicknesses were manufactured to see how the thickness affects the stress field inside the laminates. To attach the patch to the steel beam, adhesive joints were used. However, the long-term performances of such joints are still not well understood. Monitoring the damage development in joints like this can provide a better understanding of how the damage propagates to investigate the long-term performance of the joint. A new method using optical fibers to measure strains inside the laminates was used. It is possible to measure the strains in different layers of the laminate by using optical fibers. Compared to traditional strain gauges which measures strain in a small area, the optical fibers can measure a length up to 70 meters. All of the beams have been monitored with seven traditional electrical strain gauges and up to eight optical fibers. The measurements done with the optical fibers made it possible to plot the changes of the strain field due to damage development inside the laminate through the thickness. It is concluded that the strain field over the notch in a given layer inside laminate, has the same shape independent of thickness. This thesis also confirms that the angle of the tapering at the end of the laminate has an impact on where the laminate starts to delaminate. Typically, the delamination starts at the highest strain concentration, either at the notch or one side of the laminate. It has also been possible to use the shape of the strain field to predict approximately how far the delamination has propagated.