In order to determine the fracture mechanism and cause of the trunk lid torsion bar spring, the fracture morphology, composition, size, material strength, microstructure and residual stress were tested, and a comprehensive analysis was conducted in combination with background information. The results indicate that the torsion bar spring of the trunk lid experienced high-strength steel delayed cracking at the concentrated bending stress caused by high material strength, insufficient tempering, and high residual stress. Based on the cause of cracking, improvements were made from two aspects: controlling the strength of raw materials and optimizing the stress relief tempering process to reduce residual stress, and ultimately the delayed cracking failure of the luggage torsion bar spring was effectively solved.

Torsion bar spring  /  Delayed fracture  /  Residual stress  /  Stress relief tempering