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Steel cords are widely used to improve the mechanical properties of pneumatic tires. The steel filaments are twisted together in a different manner to build up various constructions. The important mechanical properties required for steel filaments are outstanding fatigue resistance. Since steel filaments are fabricated through repeated drawing and heat treatment, the drawn filaments possess various residual stress, which is the one of the most important factor influencing the fatigue resistance. To measure the residual stress, x-ray diffraction method and stress relaxation method has been generally used. However, these methods are not suitable in measuring the residual stress of the filaments due to their cylindrical shape and small dimension of ~100µm.
In this study, a different method to exactly measure the residual stress of the steel filaments is described. Since the steel wires were fabricated with different annealing condition, various values of the residual stress were measured along the axial direction of the filament. This method is based on the combination of the focused ion beam (FIB) milling and high resolution strain mapping program (VIC-2D). That is, the residual stress was calculated from the measured displacement field before and after the introduction of a slot along the steel wires. The displacement was obtained by the digital correlation analysis of high-resolution SEM micrographs, while the slot was introduced by FIB milling with low energy beam. The fitting of the experimental results to an analytical model with the independent Young¡¯s modulus determined allows us to find the residual stress.
The present measurement revealed that the residual stress within 8 % of the magnitude was persistent in the steel filaments fabricated. As annealing temperature increased, the residual stress decreased and the fatigue resistance increased. Since the decrease of residual stress can reduce stress concentration on the grooves formed on the surface of the wire during drawing, the formation of cracks can be suppressed. It is concluded that the decreased residual stress is the major cause of the improvement of fatigue resistance in pearlitic steel wires.
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