Many automotive body structure components are made of press-hardened steel (PHS) to ensure occupant safety, and they are often subjected to bending-dominated deformation such as axial folding and side intrusion and fracture usually occurs under plane strain bending. Therefore, fracture strain under plane strain condition is an important material property for designing mass-efficient components while maintaining vehicle crashworthiness. The VDA 238-100 standard uses bending angle to qualitatively assess fracture resistance of PHS. However, bending angle is thickness dependent and it cannot be utilized in finite element models to characterize materials' fracture limit for predicting structure failure. In this study, a methodology to determine fracture strain of PHS is developed by applying interrupted bending tests and extrapolation, without using any direct strain measurement system. Then this method is applied to determine fracture strains of bare and AlSi-coated PHS with various thicknesses and prior austenite grain sizes, and the results are compared to that obtained from direct strain measurements. An error analysis is also performed to assess factors/assumptions affecting the accuracy of the proposed methodology. The new method is easy to implement and can be broadly applied to determine fracture strain for other sheet metals whose bendability was previously evaluated by VDA bending angle.
A novel alloy design, designated as 1·2C–1·5Cr–5Al, has been proposed with high aluminium(∼5 wt-) and more carbon(∼1·2 wt-) addition into the classical 1C–1·5Cr bearing steel for lowering density and improving performance simultaneously, which is approximate 8 wt- lighter than convention. In order to understand preliminarily the suitability of the novel alloy for bearing application, the martensite starting temperature and hardness, related to microstructure evolution and mechanical properties, respectively, after partial austenitisation treatment with undissolved carbides have been investigated carefully. The martensite starting temperature is comparable with conventional 1C–1·5Cr alloy. The hardness of 860±3 HV20 achieved is much higher than convention.
The 7A62 as one kind of Al-Zn-Mg alloys is the highest strength weldable aluminum alloy currently. Plates and forgings of 7A62 alloy have been widely used in the defense system and main bearing parts of special vehicles. The microstructure, dynamic mechanical response and weldability of 7A62 aluminum alloy were investigated by OM, TEM, SHTB, DSC, microhardness and other tests. The morphology and mechanical test results showed that the 7A62 aluminum alloy strengthened by trans-scale precipitates had high strength, high hardness and good plastic toughness. The mechanical responses of the 7A62 aluminum alloy were found to be strain-rate sensitive and the dynamic response behavior was significantly higher than that of the 7A52 aluminum alloy through the SHTB test. In addition, for the as-welded 7A62 the tensile strength reached a scope of 260 MPa-305 MPa, which is considerably higher than the tensile properties reported when using ER5356. Because the weld of 7A62 alloy was free of macroscopic imperfections. The grains were highly equiaxed and homogeneous throughout the melting zone, showing smooth grain boundaries. Compared with 7A52, the lower melting point, low thermal enthalpy and low specific heat capacity of 7A62 alloy are more conducive to weld performed.
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