FEA helps explain crack propagation in structural steel
Washington County LUT, Building Services
Femap’s ability to accurately mesh narrow gap areas such as crack tip openings allowed FEA analysis of a crack in a hollow structural steel column
Using FEA to understand a crack
While performing his duties as a county building engineer, Kofi Nelson-Owusu was made aware of a problem at a job site that he later decided would make a good research project. The job site was a sixstory office building that was under construction. The problem was a fairly long (5-foot) vertical crack in one of the hollow structural steel columns located on the ground floor of the structure.
Nelson-Owusu, a member of the ASCE who has years of experience using finite element analysis (FEA), became curious about the crack. “As soon as it was discovered, they added temporary support and eventually repaired the column, but I was curious about what had caused the crack,” Nelson-Owusu explains. “I also wondered why it stopped where it did instead of propagating through the rest of the column.”
A meshing challenge
Several features of this problem made it intriguing to Nelson-Owusu. One was the location of the crack. “The crack ran along the corner of the column and I knew that in the process of forming this type of column, there is a tendency to see residual stresses at the corners where geometric thinning is common,” he says. “We performed chemical and mechanical analyses and determined that the column met the necessary requirements (for ASTMA500 Grade B carbon steel).” Another consideration was that the crack was found in January, making ice formation within the hollow column a possible cause for the cracking. This meant that it would be necessary to consider thermo-mechanical loading in the analysis.
Another interesting feature was the modeling challenge this situation presented. The crack went through the entire thickness of the metal and it had a jagged appearance with internal cleavage on the fracture face. “The challenge in modeling was to capture the crack,” explains Nelson-Owusu. “It’s very difficult for most FEA preprocessors to mesh a narrow crack tip opening, particularly when the situation is geometric corners such as this work focused on. And if they succeed in meshing it, there are no guarantees that you won’t run into analysis instability because of singularities.”
Nelson-Owusu’s FEA pre- and postprocessor of choice is the Femap™ solution from Siemens PLM Software. “In my professional opinion, Femap is the most comprehensive pre and post-processor available,” he says. He created the geometry of the column and crack using Femap’s geometric modeling tools, which he says “let you do same as CAD without the additional cost of CAD.” Femap was able to accurately mesh the narrow crack, which impressed Professor Lemmy Meekisho of Portland State University, who was co-author with Nelson-Owusu for this research project. “Professor Meekisho has a lot of experience with FEA and he was impressed by Femap’s ability to create mesh of that size in such a difficult area,” Nelson-Owusu adds.
Ice was the culprit
Analysis consisted of two phases using first a global then a local FEA model. The global model simulated the entire column under the conditions it experienced at the building site. The global finite element model consisted of 240 eight-node solid hexahedral elements including the hollow section (which was assumed to be filled with ice). The mechanical properties of both the steel and ice were specified, including the construction live loads and the structure dead loads supported by the column. Results of this analysis were transferred in the form of boundary conditions to the smaller, local model. This model focused on the area around the crack tip for a more detailed stress analysis.
Femap’s extensive post-processing capabilities were important to understanding the analysis results. “The Femap post-processor has many options for viewing an analysis like this in different ways,” Nelson-Owusu explains. “Without that flexibility, I might have missed something in the results because the area we were interested in was so small.” He also appreciated the speed at which Femap generated new variations of the output.
The results of these analyses (along with damage mechanics and elasto-plastic fracture mechanics principles) led Nelson-Owusu and his co-author to conclude that the crack probably started at a pre-existing condition such as a pore in the metal that raised the stress level at that spot. The thermal mechanical load from the freezing process of water into ice, combined with the static building loads, generated a state of stress that reached the critical intensity value. The localized plastic yielding may have occurred at the crack front in response to the thermal and mechanical loads advanced the crack propagation. The crack stopped because of the mixed mode cracking conditions (tearing and twisting), which helped blunt the crack tip.
Nelson-Owusu presented this research at the 17th ASCE Engineering Mechanics conference in 2004. He credits Femap with the ability to use FEA, which gave critical insight into the loading conditions. “Because of the narrowness of the crack, it could have been impossible to use FEA in this work without Femap,” he notes. “Femap is unique in terms of its speed, flexibility and also the level of accuracy it provides.”