Defect: Wall Thinning at Critical Section
缺陷：壁厚不均与危险断面变薄
标签：变薄, 危险断面, 圆角半径, 拉裂前兆
拉深中材料在凹模圆角处受弯曲+拉伸，凸模圆角处受双向拉应力，此处即"危险断面"。若凹模圆角过小（<3t）或压边力偏大，危险断面减薄超 25% 即临近拉裂。对策：增大圆角半径、降低单道拉深率、优化润滑，并用超声波测厚监控最薄点。
Material at the die radius undergoes bending plus tension, and at the punch radius it sees biaxial tension—this is the "critical section." If the die radius is too small (<3t) or blank-holder force too high, thinning beyond 25% there precedes fracture. Remedies: enlarge radii, lower per-pass draw ratio, improve lubrication, and monitor the thinnest point with ultrasonic gauging. ---