Reidual stress measurements in welded steel beam column connections by scanning acoustic microscopy

摘要

Residual stresses which arise from thermalexpansion and contracton due to welding may have contributed tothe brittle fracture exhibited by welded steel beam-to-columnconnections during the Northridge Earthquake. These residualstresses have a strong influence on crack initiation and crakepropagation in the vicinity of stress concentrations (I.e., unfusedbackup bar in welded steel beam-to-connections) and account forchanges in the driving force for fracture. They affect materialtoughness by changing the constraint condition under whichfracture occurs. Currently, all methods of dealing with residualstresses are hampered by the lack of a consistent means ofmeasuring the magnitudes and distribution of these stresses. Thispaper describes a new acoustic microscopy technique that allowsthe mapping of residual stresses in welded connections with highspatial resolution. The technique is based on the sensitivity ofpolarized acoustic modes to local elastic anisotropy induced bystress. The technique furthermore allows the mapping of residualstresses in a "tomographic" way by changing the frequencies of theacoustic waves. The results reveal that the magnitude of theresidual stresses is influenced by the local microstructure of thesteel and the weld metal. Kuctile microstructures within the weldand the heat affected zone release residual stressed by yielding,whereas brittle microstructures retain residual stresses.