Crane girder fatigue life determination using SN and LEFM methods

- Obtain a crane girder fatigue life using SN and LEFM methods
- Performance a technique to determine crane girder real enforces under operational conditions
- Establish the best maintenance techniques to detect fatigue cracks in the structure
- Evaluate crack growth in the structure

This paper examines the fatigue life of a Melt Shop crane girder with over 30 years of operation. Research indicates that the most frequent failures in these structures is related to fatigue in welded joints, thus an initial approach was performed according to AASHTO standard for calculating the remaining fatigue life of the structure. The alternating stress at the most severe loading conditions was obtained by numerical simulation in ANSYS software. Based on history and production forecasts, each critical welded joint fatigue life was determined. To calibrate the model, a field test was performed with strain gages to determine the real stress variation during regular plant operation. At this stage, crane load was parameterized in the computational model and varied to simulate the real behavior of the structure under the measured stresses. The analysis pointed out the most critical points which guided a visual inspection that identified cracks that could lead to a structural collapse. Identified crack growth was simulated through FRANC2D software that applies the LEFM method to determine the stress intensity factor. The calculated values were used to reach fatigue life for crack growth from an initial to a critical value which is expected to lead to a brittle fracture. It was concluded that the S-N method associated with finite element analysis has proven to be an efficient technique for determining the most critical structure joints damage reducing inspection work significantly. The application of linear elastic fracture mechanics techniques reached more conservative results. This technique can assist the maintenance crew in determining inspection intervals and the best moment for intervention to repair existent cracks based on the calculated remaining life.