Finite element study of cracked steel circular tube repaired by FRP patching

As the development and application of fiber reinforced polymer (FRP) composite materials to different engineering structures are increasing gradually, composite fiber patching techniques are being considered as alternatives to traditional methods of strengthening and fatigue crack repair in steel structures, such as the boom members of draglines. In general, the dragline boom members are in the form of circular tube type structure, therefore, application of FRP patching is usually bonded on one side (the outside surface) of the tube structure. In order to examine the efficiency of the repair by single-side patching of FRP material to tube structures, finite element study of cracked steel tube structure is conducted. The objectives of the finite element study are (1) to study the reduction of stress intensity factor (SIF) of cracked steel tube member repaired with FRP patching and (2) to predict the fatigue life of welded steel tube member repaired with FRP patching. The cracked steel circular tube was modeled by using 3-D brick elements and the adhesive and FRP patching were modeled by using shell elements. The adherend stiffness ratio (Efrptfrp/ Ests) of this study is about 0.34. Comparison of the SIF of cracked steel circular tube with and without FRP patching showed that the SIF was reduced significantly for models with FRP patching and the reduction was more than 50%. Prediction of the fatigue life of cracked circular tube member with and without FRP patching was carried out by means of the Paris equation. Due to the significant reduction of SIF for members with FRP patching, it is shown that the corresponding predicted fatigue life is increased significantly as well. In particular, the increase of fatigue life is about 22 times for model with CFRP sheet patching when the circumference half crack length grows from 25.4 mm (1 inch) to 63.5 mm (2.5 inch).