Residual Stress Distribution in Stiffened Steel Tubes for Long-Span Transmission Towers

Literature Overview

The study by Liu Haifeng, Bian Qi, Fu Dahong, Zhu Binrong, and Yang Junfen, published in Building Structure (2020, Vol. 50, No. 23, pp. 58-63), investigates the residual stress distribution in stiffened steel tubes designed for use as main materials in long-span transmission towers. Stiffened steel tubes are innovative structural members created by welding longitudinal or transverse stiffening ribs onto the inner wall of conventional steel tubes. The research is funded by the National Natural Science Foundation of China (Grant No. 51408569) and addresses a critical issue in the design and fabrication of transmission tower components, where residual stresses from welding significantly influence structural performance.

The authors designed twelve specimens comprising three different stiffening rib configurations and one unstiffened control tube, with three replicates for each configuration. Residual stresses were measured using the blind hole method, a widely accepted technique for residual stress determination in metallic components.

Core Technical Findings

The study reveals several important characteristics of the residual stress field in stiffened steel tubes. First, the residual stress field in stiffened tubes can be considered as a superposition of the residual stress fields from the butt weld of the steel tube and the residual stress fields from the stiffening rib welds. This superposition principle is a significant finding because it simplifies the prediction of residual stresses in complex welded assemblies.

The residual tensile stress peak in stiffened tubes is essentially the same as in unstiffened tubes, indicating that the butt weld remains the dominant source of high tensile residual stress. This is expected because the butt weld involves full-penetration welding of the tube wall thickness, creating a significant thermal gradient and plastic deformation zone. The stiffening rib welds, being fillet or partial-penetration welds on the inner surface, produce lower thermal input and consequently lower residual stresses.

However, the residual compressive stress peak in stiffened tubes is notably higher than in unstiffened tubes, and its location undergoes significant variation. This is attributed to the additional thermal cycles introduced by the stiffening rib welds, which redistribute the residual stress field. The compressive stress redistribution is particularly important because high compressive residual stresses can promote buckling under external loading, which is a critical failure mode for transmission tower main members.

Methodology and Measurement Techniques

The blind hole method is a strain gauge-based technique for measuring residual stresses. A small hole is drilled at the measurement point, and the strain release around the hole is measured using a rosette strain gauge. The residual stress is then calculated from the measured strain release using calibration factors that depend on the material properties and hole geometry.

The selection of this method is appropriate for the study because it provides point measurements with good accuracy and is suitable for thick-walled components like steel tubes. However, the blind hole method is destructive, requiring post-test repair of the specimen. For production components, non-destructive alternatives such as X-ray diffraction or neutron diffraction should be considered.

The test program design, with three configurations and three replicates each, provides statistical robustness to the findings. The inclusion of an unstiffened control tube allows direct comparison and isolation of the effect of stiffening ribs on the residual stress field.

Engineering Practice Implications

For the fabrication of stiffened steel tubes for transmission towers, the findings have several practical implications. First, the butt weld remains the critical zone for residual stress management, and appropriate welding procedures, such as preheating, post-weld heat treatment, or stress-relief annealing, should be applied to reduce the residual tensile stress at the butt weld. Second, the increased compressive residual stress in stiffened tubes may require careful consideration in buckling analysis, as the effective buckling load is reduced by compressive residual stresses.

From a welding process perspective, the sequence of welding operations is critical. If stiffening ribs are welded before the butt weld, the residual stress field from the rib welds may be partially relieved by the subsequent butt weld thermal cycle. Conversely, if the butt weld is performed first, the stiffening rib welds will superimpose their stress fields on the existing residual stress pattern. Process planning should account for this interaction to minimize adverse residual stress effects.

Study Insights and Reflections

This study provides valuable experimental data on a novel structural member that is being considered for use in large-scale transmission tower construction. The superposition principle finding is particularly useful because it allows engineers to predict residual stresses in complex configurations by combining simpler stress fields. This reduces the need for extensive testing of every possible configuration.

The finding that stiffening rib welds produce lower tensile residual stresses than butt welds is reassuring, as it means that the addition of stiffening ribs does not significantly increase the risk of weld cracking at the rib weld locations. However, the increased compressive residual stress is a concern that warrants further investigation, particularly through buckling tests of stiffened tube columns.

Summary

The research by Liu et al. provides essential experimental data on the residual stress distribution in stiffened steel tubes for transmission tower applications. The superposition principle, the dominance of butt weld residual stresses, and the increased compressive residual stress in stiffened tubes are key findings that have direct implications for fabrication process planning and structural design. Engineers involved in the design and fabrication of stiffened steel tube components should incorporate these findings into their residual stress management strategies, including appropriate welding sequences, stress-relief treatments, and buckling analysis considerations. The work contributes to the growing body of knowledge on innovative structural members for power transmission infrastructure.