What is it about?

Fatigue predictions and tool design are important when operating under high frequency pulse currents with large amplitudes during electromagnetic forming/welding processes. This can be attributed to the fact that the fieldshapers used in electromagnetic forming and welding processes are prone to have fatigue damage. During each shot, impulse current passes through the coil that induces an eddy current cycle and a cyclic Lorentz force on the fieldshaper. Thus, the failure of the fieldshapers during service is associated with the cyclic loading conditions during a decaying high frequency electric current pulse. Repetition of the current pulse that causes the damage and ultimately results in a failure which is analogous to that occurs during a fatigue problem.

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Why is it important?

The failure of the fieldshapers during service is associated with the cyclic loading conditions during a decaying high frequency electric current pulse. Repetition of the current pulse that causes the damage and ultimately results in a failure which is analogous to that occurs during a fatigue problem. Various studies have endeavored to correlate the electric fatigue behavior with generalized Coffin-Manson law. Unfortunately, up to date, damage originating from the electrical loading has not yet been considered. Recent technological advances have led to the development of a novel material of promising qualities such as Cu-Ni-Si alloys (e.g. Siclanic), however, the fatigue phenomenon of fieldshapers fabricated from such material have not yet been systematically investigated either. Therefore we did a fatigue case study for Siclanic fieldshaper using a strain-based fatigue analysis so as to better comprehend the scenario of the damage caused in electromagnetic pulse forming and welding (EMPF/EMPW) tools. Multiphysics simulation showed that the work-pieces deformation had a negligible influence on the stresses development on the tools. Additionally, we noticed that the radial component of the Lorentz force contributed to the large stress development and the effective stress was significantly high at the location opposite to the slot. Generally, the circumferential Lorentz force component was seen to be negligible in the fieldshaper. The study demonstrated a thorough investigation of the recently developed and promising Siclanic material, for use in EMPF/EMPW applications without compromising the electrical conductivity for its mechanical properties and vice versa. Overall, this work showed that the predicted damage locations using the fatigue analysis concurred with the experimental observations of the failed fieldshapers used in service and with the first crack formation that corresponded to the element location of the worst life repeat predicted from fe-safe calculation. Altogether, the study provides a clear understanding of the fatigue damage due to electromagnetic loading and establishes a linear correlation to predict the life cycle for an input electric current.

Perspectives

Fatigue crack evaluation was not modelled in this study, and the crack growth could be influenced by the localized electric current (around the crack tip after the initial crack formation) and the subsequent thermal effects. To model the fatigue failure evaluation, one should consider the thermal effect and material softening, and combination with various temperature dependent fatigue properties obtained for the material to predict the fatigue failure evaluation.

Dr Thaneshan SAPANATHAN
Curtin University, Curtin Corrosion Centre, Faculty of Science and Engineering

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This page is a summary of: Fatigue damage in fieldshapers used during electromagnetic forming and welding processes at high frequency impulse current, International Journal of Fatigue, April 2018, Elsevier, DOI: 10.1016/j.ijfatigue.2017.12.019.
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