Effect of Mechanical Cut-Edges on the Fatigue and Formability Performance of Advanced High-Strength Steels
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TECHNICAL ARTICLE—PEER-REVIEWED
Effect of Mechanical Cut-Edges on the Fatigue and Formability Performance of Advanced High-Strength Steels Daniel J. Thomas
Submitted: 1 June 2012 / Published online: 27 June 2012 Ó ASM International 2012
Abstract Mechanical cut-edge properties influence the fatigue lives and formability capacity of advanced highstrength steels. This factor is critical as S355MC and DP600 exhibited an increased sensitivity to fatigue cracks initiating from defects on the cut-edge fracture zone. Mechanical cut-edges as a result displayed a decreased level of formability that was highly dependent on the cutedge surface and internal microstructure of the cut-edge produced. It was determined that, by controlling the mechanical clearance, optimized mechanical cut-edges were produced. This was achieved through minimizing surface damage and by controlling the internal and topographical properties of the cut-edge zones. Keywords Cut-edge fatigue Automotive manufacturing Forming Hole expansion capacity Nomenclature A Elongation to failure AHSS Advanced high-strength steel HEC Hole expansion capacity HSLA High-strength low alloy HV Vickers hardness MPa Mega Pascal R Stress ratio (min stress/max stress) Ra Arithmetic mean of departures from the mean line Rp Maximum height of profile above the mean line Rv Maximum depth of profile below the mean line
D. J. Thomas (&) Materials Research Centre, College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP, UK e-mail: [email protected]
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S-N Wt
Stress-life Weight
Introduction The principal means by which automotive components are manufactured is the mechanical blanking process. This mass production manufacturing process induces rough cutedge surfaces, with a high degree of damage and resulting notch features. Previous studies have emphasized that the cutting process has a major detrimental effect on the integrity of steel structures [1–5]. It has been established that the larger number of accumulated defects on the surface of the mechanical cut-edge fracture zone that results in lower fatigue strength of components generated using mechanical cutting methods. The commercial mechanical blanking process as shown in Fig. 1 induces internal residual stresses within the material [3]. The punch– and die clearance is the chief factor in determining the formation of surface roughness features, residual stresses, and surface work hardening of mechanical cut-edges. This can result in a reduced fatigue and fracture resistance because the small defects produced during the cutting process act as stress inducers. There are comparable results of the effects of mechanical cutting process parameters on the depth of residual stresses. In terms of relating residual stress to mechanical cut-edges; fatigue cracks always appear to initiate in the mechanical fracture zone, where the greatest magnitudes of tensile residual stresses are formed. The influence of hardness on the residual stress profile can therefore be credited to the formation of plastic d
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