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This study examines the effect of sliding speed and surface temperature on the wear behavior of an unlubricated mild steel–tool steel contact pair using the pin-on-disc test. The operating conditions and contact pair are of interest to the automotive sheet metal stamping industry and the broader metal forming community, where high contact pressures and moderate forming speeds can result in significant frictional heating and thus affect tool life. It will be shown that, while adhesive wear is dominant at the tool steel surface for all sliding speeds examined, the adhesive wear rate is very sensitive to sliding speed during slow speed conditions but relatively insensitive to sliding speed during higher speed conditions. These higher sliding speeds result in high frictional heating, however, the effect of increasing bulk temperature results in a transition from adhesive wear to material removal-dominated mechanisms. It is concluded that there is a distinct difference in the wear response for comparable surface temperature and bulk temperature conditions, at the low to moderate sliding speeds and temperatures examined in this study. The SEM and profilometry analysis show that the technique of increasing sliding speed to replicate bulk temperature conditions (or vice versa), may not result in equivalent wear rates and mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: This paper investigates the influence of the die corner radius shape on wear behavior during sheet metal stamping. In particular, small and localized changes from an ideal circular die profile shape were examined and the effect on tool life determined. A novel technique to test and characterize the die profile shape and wear behavior, based on a semi-industrial channel forming process, is presented. It will be shown experimentally that small changes in die radius profile shape (of less than 15μm) can result in significantly reduced tool life. In all cases examined, the small variations in shape were the result of inherent variations associated with the machining, grinding and polishing processes typically used to manufacture stamping dies and die corner inserts. Numerical simulation shows that these small shape ‘defects’ at the die corner radius can result in a significant increase in local contact pressures, explaining the large adverse changes in tool life. These results are of direct importance to stamping engineers and tool designers and indicate that careful control over die manufacture and maintenance procedures may result in significant improvements in tool life. [Copyright &y& Elsevier]

A new approach was used to characterise the hardening, instability and fracture behaviour of ultra-thin (0.1 mm) stainless steel sheets under stress triaxiality ranging from 0.37 to 0.66. The Swift and the linear hardening laws were used to capture the stress–strain relationship for the complete level of plastic deformation that exceeds that achievable by a uniaxial tensile test. For this, the Virtual Field Method (VFM) was applied, which uses the strain field measured on the surface of the notched samples, to output the material hardening parameters. To determine one set of hardening parameters that fits all conditions of stress triaxiality, an upper bound of major strain was selected for the VFM fitting that optimises a single set of hardening parameters across all of the stress triaxiality conditions. The void coalescence parameter in a Gurson-Tvergaard-Needleman (GTN) fracture model was calibrated and the model results for fracture initiation were validated experimentally with a quasi-biaxial stretching test. [ABSTRACT FROM AUTHOR]

Abstract: The contact conditions at the die radius are of primary importance to the wear response for many sheet metal forming processes. In particular, a detailed understanding of the contact pressure at the wearing interface is essential for the application of representative wear tests, the use of wear resistant materials and coatings, the development of suitable wear models, and for the ultimate goal of predicting tool life. However, there is a lack of information concerning the time-dependant nature of the contact pressure response in sheet metal stamping. This work provides a qualitative description of the evolution and distribution of contact pressure at the die radius for a typical channel forming process. Through an analysis of the deformation conditions, contact phenomena and underlying mechanics, it was identified that three distinct phases exist. Significantly, the initial and intermediate stages resulted in severe and localised contact conditions, with contact pressures significantly greater than the blank material yield strength. The final phase corresponds to a larger contact area, with steady and smaller contact pressures. The proposed contact pressure behaviour was compared to other results available in the literature and also discussed with respect to tool wear. [Copyright &y& Elsevier]

• Performed in-planar shear test on ultra-thin stainless steel sheet. • Buckling is suppressed by adding supporting on two-side surfaces. • Evaluated the strain path at the notch region experimentally and numerically. • Optimised the notch geometries to extend the shear strain path. Bipolar plates are essential components in proton exchange membrane (PEM) fuel cells for electric vehicles. Micro-formed thin (thickness <0.15 mm) stainless steel bipolar plates have advantages over conventional CNC-machined graphite plates in terms of weight and cost, but the geometry of the final part shape is limited due to the fracture limits of the material. The further optimisation of micro-forming processes, therefore, requires the development of reliable fracture models, with fracture locus covering a wide range of stress states, including shear fracture at zero (or close to zero) stress triaxiality conditions. Few studies have investigated shear tests on thin specimens, as it is challenging to maintain a stable shear condition during the test. There exists a twofold problem that: (a) the sheet buckles prematurely before the failure strain is reached; (b) plane strain fracture initiates prematurely from the edge of the gauge area. In this study, in-planar shear tests are performed on 0.1 mm-thick stainless steel sheet. To achieve fracture initiation in shear, an anti-buckling device was developed and the shear test sample shape was optimised. The shape optimization process is based on finite element analysis (FEA) and verification with experimental shear test trials. The optimised shape has a minimum value of a goal function that encompasses the Lode parameter, stress triaxiality and the ratio of the centre to edge equivalent plastic strain. The experimental trials indicate that the optimised sample shape, in combination with the clamping device, enables testing thin stainless steel for fracture limits in shear. The test routine can be used to calibrate ductile fracture models. [ABSTRACT FROM AUTHOR]