2023 Research Reports No R972-R973

Abstract
As one of the main failure modes of steel structures, fracture in welded connections has widely been discussed based on experimental investigations and numerical simulations. However, the mechanical properties of the weld and Heat Affected Zone (HAZ), such as stress-strain relationships and fracture strains under various stress states, have rarely been considered in these analyses. Therefore, in this paper, the fracture process of welded connections is discussed to investigate the effects of the inhomogeneity of mechanical properties in the weld zone. Tensile tests are conducted on welded cruciform specimens fabricated using 8 mm or 12 mm fillet welds and finite element models are developed by considering or ignoring the material inhomogeneity in the weld zone. The simulation results are compared with the experimental and it is concluded that the assumption of homogenous properties within the weld zone using the properties of the base metal will underestimate the strength of the welded cruciform specimens and using the mechanical properties of the three material areas in the weld zone will increase the accuracy of the simulation results. Using the free parameters calibrated by the fracture strains of the three material areas, the fracture process of the welded cruciform specimens is simulated using the fracture model LMVGM, and the comparison shows that the mechanical properties of the weld and HAZ should be included in the investigation of fracture in welded connections to obtain reliable simulation results.

Keywords
Welded connections, fracture, mechanical properties, material inhomogeneity

Abstract
This paper investigates the strengths and fracture strains of weld and heat affected zone (HAZ) in welded connections for both the longitudinal and transverse directions and compares them to those of the base metal. A series of miniature coupons, including miniature flat plates, notched round bars and grooved plates, were extracted from the three zones of a butt weld and tested using a custom-built jig. The true stress-strain relationships and fracture strains of the base metal, weld and HAZ materials were obtained for both directions from the miniature coupon tests and corresponding numerical simulations. The fracture strain data were used to calibrate the Lode angle modified void growth model (LMVGM) for predicting the fracture strain of the three material zones at any given stress state. The following major conclusions were drawn: (1) The weld was generally isotropic in terms of both strength and fracture strain. The weld also had the highest values of yield and tensile strengths among the three materials in both directions, but the lowest fracture strain in both directions except for the longitudinal direction with stress triaxiality above 0.21 to 0.30, for which the base metal had the lowest fracture strain. (2) The HAZ had higher yield and tensile strengths but smaller fracture strain in the longitudinal direction than in the transverse direction. The same anisotropic characteristic applied to the base metal. Meanwhile, the HAZ had higher yield and tensile strengths than the base metal as well as similar but slightly larger fracture strains in both directions. (3) The yield and tensile strengths of the weld and HAZ can be approximated using the empirical hardness-strength correlation functions, except that the functions tend to overestimate the strengths of the weld by about 10%. (4) For the weld, HAZ and base metal, the fracture surfaces tilted towards stress states with high stress triaxiality and low Lode angle parameter, indicating that fracture can initiate more easily at these stress states. Note that the above conclusions are limited to the tested AS350 grade steel and the selected welding parameters.

Keywords
Welded connections, heat affected zone, miniature coupons, stress-strain curves, fracture