Influence of drawing parameters on the formation of Chevron cracks in drawn surgical rods

Abstract

In this research, the prediction of Chevron cracks (central bursts) during the surgical rod drawing process is examined using a stress-based methodology. The factors influencing the formation of Chevron cracks along the rod’s length are (a) the die angle, (b) the friction coefficient between the die and the rod, (c) the reduction ratio in the rod’s cross-sectional area, and (d) material flow stress. An analytical axisymmetric equilibrium model is employed to forecast the stress state under various drawing conditions by evaluating axial stress, interface pressure, and surface drag stress as functions of die angle (1°–20°), reduction ratio (5%–30%), friction coefficient (0.01–0.10), and applied back tension (0–0.20). The analysis focuses on stress evolution along the rod axis and at the die–rod interface, where damage initiation is most likely to occur. This methodology is used to assess proximity to yielding at the die–rod interface based on a von Mises yielding criterion and to identify locations where internal defects may initiate during the drawing process. The findings indicate that the die angle, reduction ratio, friction coefficient, and back tension significantly influence the stress distribution and the likelihood of damage. Central bursts are expected to occur when critical combinations of these parameters drive the normalized stress state toward the yielding boundary, with an optimal die angle range of approximately 2°–5° identified for typical reductions of 10%–20%. Although the formulation is purely analytical, the predicted stress trends, and critical processing ranges are systematically compared with experimental data reported in the literature, demonstrating consistent agreement without introducing new experimental testing.