Large-Scale Flexure Fracture Experiments on High-Toughness Steel

Abstract

Modern advances in steel production have resulted in materials with increased fracture resistance. Material characterization studies have quantified fracture behavior, while large-scale experimental tests have demonstrated large critical crack lengths. Early testing focused on demonstrating the extreme potential of high-toughness materials. Recently, a test program utilizing large-scale specimens aimed to identify the toughness level appropriate for steel bridge design of structures traditionally classified as fracture critical. The work resulted in the concept of an integrated fracture control plan (FCP), combining the intent of the original AASHTO FCP from 1978 with modern advances in steel production, analysis, and understanding of fracture mechanics. An integrated FCP prevents fracture through a series of interrelated components, which influence each other in a rational and quantifiable way. The project was comprised of material characterization, full-scale fracture testing of steel bridge components, three-dimensional finite-element analysis (FEA), and an analytical parametric study. Large-scale flexure test results, which included both traditional and high-toughness materials, are presented. Results suggest historical large-scale fracture testing practices may result in critical crack lengths larger than would be expected in service due to the generation of high compressive residual stresses at the crack tip after unsuccessful fracture attempts on a given specimen. Further, fracture toughness demands calculated using FEA compared well with material characterization testing. Results indicated that fracture toughness values indirectly obtained from large-scale experiments should be calculated using FEA. Ultimately, the high-toughness steels tested demonstrated greatly improved fracture performance. High-toughness material, examined at a Charpy V-notch (CVN) impact energy of 170 J (125 ft-lbf), exhibited a 285% increase in critical fracture toughness as compared with a material with over twice the impact resistance of the current specification.