Unexpected softening in helical compression springs: Experimental Characterization and numerical modeling of longitudinal wire cracks

Abstract

Cylindrical helical compression springs were experimentally found to exhibit significant softening — up to 40% loss in stiffness — despite having passed all conventional quality control procedures, including tensile tests, chemical analysis, wire and spring coiling trials, and visual inspections. Unlike documented failure modes that typically involve wire rupture under static or fatigue loading, these springs remained intact, making the anomaly difficult to identify. A multi-technique investigation combining optical and electron microscopy, dye penetrant testing, eddy current inspection, and X-ray tomography revealed a continuous longitudinal crack along the wire. This axial defect, introduced during wire drawing, had a depth close to the wire radius and a sub-micrometric width, placing it at the edge of detectability. Numerical simulations confirmed that such cracks do not affect the wire’s response in tensile or bending loading — explaining why traditional inspections failed — but significantly reduce torsional stiffness. Analytical models from the literature were compared with simulations; some were validated, and a simplified model was proposed. The role of torsional warping in setting the softening limit was also clarified. Finally, a parametric study on crack positioning along the wire’s circumference showed that while overall stiffness was only slightly affected, internal stress flow was notably disrupted when the crack lay on highly stressed fibers. This study is the first to report and explain the mechanical effects of longitudinal cracking in compression springs. The findings offer practical insights for improving defect detection and mechanical prediction in drawn wire products.