Effects of Bending and Heat on the Ductility and Fracture Toughness of Flange Plate

mahyarov · 11-10-2012, 11:54 AM

Bridge fabricators for the Texas Department of Transportation (TxDOT) have occasionally experienced the formation of cracks in flange plate during bending operations, particularly when heat is applied. Bending the flange plate is necessary for certain details used in the fabrication of steel highway girders such as dapped end details. Heat is sometimes used to assist in the bending operation, particularly to help reduce the forces required to bend the plate. This report documents the findings of a TxDOT-sponsored research project that investigated the possible causes of the cracking and developed recommendations to prevent the occurrence of such cracking. The research project investigated the cracking problem using both experimental and analytical studies. The experimental study involved the use of small tensile specimens loaded to different strain levels under varying temperature conditions. Strain levels up to 15 percent were investigated. Temperature conditions included testing at room temperature, 450ºF, and 1150ºF. The results showed that strain levels above 10 percent generally reduced the ductility and fracture toughness of the plate. Additionally, it was found that the application of heat during the bending process significantly reduced ductility and was the major contributor to the formation of cracks. A finite element study of the heating process was used to extend the results for the experimental study. As a secondary study, the fatigue behavior of non-loaded bolted connection details was investigated along with the influence of plate thickness. These details occur when gusset plates are bolted to flanges of girders. Unlike flange or web splices where the load must transfer out of the main member, the load in these details passes through the main plate, resulting in higher stresses around the bolt holes. Pre-tensioning the bolts normally shields the bolt holes from fatigue damage due to the resulting compressive stress field. This study found that for plate thicknesses greater than 1.0 inch, a reduction in fatigue strength was warranted.

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