Fracture of Welded Steel Structures

The objective of this program was to investigate dynamic fracture of metal weldments and develop modeling capabilities that can be used to analyze fracture strength and to design different weldment geometries. To meet this objective, we developed a local ductile fracture model and implemented it into SRI's version of the finite element code DYNA3D. The local fracture model calculates local damage in each element as a function of the stress state and plastic strain increments and allows discrete separation of the elements as the crack propagates through fully damaged regions of the mesh.

A comparison of the calculated and measured fracture behavior with an explosively loaded dynamic fracture test on a welded steel specimen is shown in Figure 1 . The local damage model successfully reproduced the experimentally observed fracture behavior. We then used the method to investigate the fracture behavior of different weld geometries and weld materials. Figure 2 shows how using a one-sided asymmetric weld results in a change of fracture location from the base plate into the stiffener. Thus, this example illustrates how the method can be used to design stronger and safer weldments for metal structures.

Development and calibration of the ductile fracture model is based on a series of laboratory material and fracture tests that include smooth round bar tensile tests, notched round bar tensile tests, cracked round bar tensile tests, compact tension fracture tests, and blunt notch bend bar fracture tests. Mechanical properties for the ductile steel and calibration of the damage model are obtained by comparison of experimental measurements and numerical simulations of the laboratory tests up to and including ultimate failure. An example of a simulation of the smooth round bar tensile test including the strain localization produced by necking is shown in Figure 3 . A comparison of the experiment and simulation that includes the necking and failure processes is used to calibrate and validate the material constitutive and damage model.

Figure 1 Dynamic Weld Fracture Experiment and Calculation of a Welded T-stiffener Specimen

Figure 2 Dynamic Weld Fracture Effect of the Weld Geometry on the Fracture Behavior

Figure 3 Material Characterization Simulation of a Tensile Test With Necking Localization

References

• J.H. Giovanola, S.W. Kirkpatrick, and J.E. Crocker, "Investigation of Scaling Effects in Elastic-Plastic Ductile Fracture Using the Local Approach," Proceedings of the First European Mechanics of Materials Conference on Local Approach to Fracture, Euromech-Mechamat '96, Fontainebleau-France, Sept. 9-11, 1996.
   
• S.W. Kirkpatrick, J.H. Giovanola, and J.W. Simons, "DYNA3D Analysis of Dynamic Fracture of Weldments," IA'96, Proceedings of International Seminar on Quasi-Impulsive Analysis, Nov. 20-22, 1996, Osaka, Japan, pp. A6.1-15, K. Wakiyama, E. Tachibana, K. Imai and T. Kitano Eds.
   
• J. H. Giovanola and S. W. Kirkpatrick, "Applying a Simple Ductile Fracture Model to Fracture of Welded T-Joints," in Advances in Local Damage/Fracture Models for the Analysis of Engineering Problems, ASME-AMD Vol. 137 (ASME Publications, 1992).

Contact Us
Don Shockey
Director, Center for Fracture Physics
Phone: 650-859-2587
Email: donald.shockey@sri.com