| With the aggravation of energy crisis,there is an urgent demand for weight reduction in the automobile industry,which promotes the application of aluminum alloy sheets to replace traditional steel sheets.However,the formability and ductility of aluminum alloy sheets are poor at room temperature,resulting in fracture failure,especially shear fracture failure,occurring easily in the forming processes of aluminum complex components subjected to multi-axial loading and multi-stage loading,thus limiting the application of aluminum alloy sheets in complex components of automobiles.In order to solve this kind of problem and make full use of the deformation potential of aluminum alloy sheets,it is necessary to have a full insight into the mechanical behavior and fracture limits for aluminum alloy sheets under multi-axial loading and multi-stage loading with the strain states between simple shear and equi-biaxial tension.Presently,the commonly used experimental method is the biaxial tensile test with a cruciform specimen.However,only the loading paths between uniaxial tension and equi-biaxial tension can be realized by using the experimental method,which cannot be used to study the problem of shear fracture failure.In view of this,the mechanical behavior and fracture limits for 6K21-T4 aluminum sheets under multi-axial loading and multi-stage loading are systematically studied through the experimental method for tension and shear biaxial loading in this paper.The research of this paper not only can provide a theoretical guidance and technical support for the research on and the development of aluminum alloy sheet forming processes,but also can promote the progresses of the experimental technique for multi-axial loading and multi-stage loading of sheet metals.Firstly,a precisely controlled tension and shear biaxial loading method is realized through a developed tension and shear biaxial loading system driven by servomotor,based on which an experimental technique for tension and shear biaxial testing more suitable for lightweight materials such as aluminum alloys,etc.is proposed.The effectiveness of the experimental technique for researching mechanical behavior of plastic deformation under biaxial loading are validated via the finite element simulation of tension and shear biaxial loading experiments.The yield loci of the 6K21-T4 aluminum sheet under different tension and shear biaxial proportional loading cases are obtained and are compared with those constructed by existing yield models.It is found that the Hill48 yield model can preferably predict the yield behavior of the aluminum alloy sheets under different tension and shear biaxial proportional loading.Secondly,to realize testing of mechanical behavior under two-stage loading with various pre-strain paths,a two-stage loading experimental method employing tension and shear biaxial loading as a pre-loading mode is proposed.Specimens suitable for the two-stage loading experimental method are determined through the finite element analyses of the strain field uniformity,calculation accuracy of the plastic work per unit volume and strain paths of specimens under different loading cases.It is proved that testing of mechanical behavior under two-stage loading with arbitrary pre-strain paths between simple shear and plane stain can be realized only by changing loading cases acting on the boundaries of the newly designed specimen,which extends the range of pre-strain paths of the existing experimental methods for mechanical behavior testing under two-stage loading.The evolution laws of the hardening behavior and mechanical properties for the 6K21-T4 aluminum sheet after being pre-deformed respectively through three different loading modes,plane strain tension,shear loading,and combined tension and shear biaxial loading with load ratio of one are obtained through the proposed experimental method.Besides,the corresponding micro mechanism is revealed by the observation of the dislocation patterns of the pre-deformed specimens.To research the fracture limits for the 6K21-T4 aluminum sheet under different tension and shear biaxial proportional loading,optimized butterfly specimens with oval shoulder boundaries are proposed.Experimental results show that the fracture positions of the optimized butterfly specimen almost locate at the center of the gage section under different tension and shear biaxial proportional loading cases,which enables to determine the fracture limit strains in different strain paths truly using the optimized butterfly specimens.The forming limit curve at fracture for the aluminum sheet between simple shear and plane strain is obtained,which extends the traditional forming limit curve at fracture to the range of strain paths between simple shear and uniaxial tension.Meanwhile,the capability of three ductile fracture criteria(Lou-Huh 2012,MM3,and Hu-Chen)to predict the fracture limits for the sheet in various quasi-linear strain paths between simple shear and plane strain is investigated.Results indicate that the Hu-Chen model can provide the highest prediction accuracy for the fracture limits.Finally,to investigate the fracture limits over a wide range of strain states in non-linear strain paths with pre-strain paths between simple shear and uniaxial tension,a two-stage loading experimental method only employing an optimized butterfly specimen is proposed.Two-stage tension and shear biaxial loading experiments on aluminum alloy 6K21-T4 sheets are conducted,which verifies that the fracture limit strains in the above various non-linear strain paths can be effectively determined by using the proposed experimental method,and the influence of the loading path changes on the fracture limits for the aluminum alloy sheets is investigated.Together with a non-linear damage accumulation rule,the capability of four ductile fracture criteria(a new model proposed in this paper,Lou-Huh 2012,MMC3,Hu-Chen)to forecast the fracture limits in the non-linear strain paths is also investigated.The research results show that the proposed new model along with the rule has a high prediction accuracy. |
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