Abstract

Heat-straightening has been employed for repairing damaged steel for many years. Only recently has heat-straightening made the transition from early experience-based handicraft to a scientifically-parameterized modern repair technique. Still, some aspects about this technique are not fully understood.

The degree of damage and the jacking force, which are represented by the strain ratio (the ratio of the largest strain at the damaged cross-section to the yield strain) and the jacking ratio (the ratio of the applied jacking force to the force that causes plastic moment at the damaged cross-section at ambient temperature) respectively, are regarded as key parameters of heat-straightening. Current research data have shown that damaged steel with strain ratios up to 100 can be repaired successfully if the jacking ratio is limited to 50%. However, cracks occasionally occur during the heat-straightening process and are the primary impetus for this research. Additionally, some researchers believe that damaged steel with strain ratios larger than 100 may still be heat-straightened and it may be possible to expedite the repair by applying larger jacking ratios, but there is no supporting research for these opinions.

Motivated by these existing research gaps, this dissertation examines the fracture-related properties of heat-straightened steel. In addition to evaluating fracture properties, this work examines strain ratios and jacking ratios beyond previously studied values. Series of steel plates were bent along the weak and strong axis to various strain ratios and then heat-straightened under various jacking ratios. Charpy V-Notch toughness tests, tensile tests and J-integral tests were performed on coupons extracted from heat-straightened plates, damaged but unstraightened plates, and original plates.

The conclusions of this work are: for both weak-axis and strong-axis damaged steel plates, if the strain ratio is bigger than 100, the plates may be too brittle to be heat-straightened; if the strain ratio is larger than 40, the J-characterized fracture toughness is significantly reduced (for the specimen thickness used in this research) and thus the pre-existing flaw at the damaged zone needs to be fixed; if the strain ratio is less than 65, the jacking ratio could be increased to 90% to expedite repair.