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Study of the Strain Distribution and Springback for Ship Hull Curved Plate in Cold Forming

Author: YangYanQin
Tutor: ZhaoYao
School: Huazhong University of Science and Technology
Course: Design and manufacture of ships and marine structures
Keywords: Hull curved plate forming In-plane strain Bending strain Springback Numerical simulation
CLC: U671
Type: Master's thesis
Year: 2013
Downloads: 6
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Abstract

The curved plate forming is an essential work in the shipbuilding.The curved plateforming process can be viewed as a process to form a plate into the desired shape usingthe in-plane and bending deformation produced through the plastic deformation. Theproblem can be basically divided into two, such that: how much plastic deformation isrequired to obtain the desired shape; where to apply the plastic deformation. The initialplate is the plate before the molding process, may be flat, or it may be a curved platehaving a single curvature or bi-directional curvature.The method for hull curved plate forming has two categories: line heating formingand cold forming. For the hull curved plate cold forming, determining the in-planedeformation and bending deformation quantitatively for a given target surface is required,so as to obtain the proper forming trip. In-plane strain is uniformly distributed along thethickness direction, and the bending strain varies linear along the thickness.In-plane strainand bending strain, respectively, corresponding to the in-plane deformation and bendingdeformation. It is important to determine the relation between strain distribution and targetshape，so as to guide subsequent hull curved plate cold forming.The springback is the main factor to affect the forming quality for the hull curvedplate in cold forming process. It is necessary to research springback assessment methods,explore the springback law and seek the appropriate springback control technology, inorder to improve the forming quality in the hull curved plate forming process.Based the above two problems，the thesis uses the finite element method to simulatethe curved plate cold forming process, analyzes the key issues of numerical simulation,finally determine numerical simulation methods to calculate in-plane strain and bendingstrain, as well as the suitable method to simulate the springback in hull curved plate coldforming. The thesis aims to obtain the relationship between the target surface and straindistribution,and springback. First at all, the rotational shell is taken as example, including spherical shell andcylindrical shell. The forming process is analyzed by the Finite Element Method based onthe large deflection elastic-plastic plate theory. The forming process is simulated byapplying the target curved surface function shape as the load to the initial plate inincremental manner. The role of the strain components, such as elastic, plastic, in-plane,bending strain in forming the rotational shell is examined through numerical simulationdescribed above. Especially, to study the effect of the deformation of the plate and thedegree of the deformation due to forming，a series of computations in which the platethickness and the target radius of curvature are changed as parameters.Then, examples which are closed to the real ship’s hull will be examined. The actualform of curved plates resembles the portion of a curved surface of doughnut, such assaddle shape, pillow shape and twisted shape. Those factors which will affect the straindistribution s are analyzed using above methods, as saddle shape and pillow shape,including the geometry, initial curvature and target curvature, whereas, as twisted shape,including the thickness and the magnitude of twisting. The study found that, in the saddleshape and pillow shape forming, in-plane strain of total strain is proportional to theproduct of length and width, besides in inverse proportion to the product of two directionalradius of curvature, on the other hand, bending strain is mainly determined by the ratio ofthickness and target curvature. Fitting the strain curve, through interpolation to getin-plane strain and bending strain of other models, and comparing with the simulatingresult, the results show that the fitting strain curve can effectively be used to obtain thestrain of target surface.The springback is expected to be closely related to the thickness of plate and thetarget radius of curvature, therefore，the effect of these factors are examined throughnumerical analyses simulating the press forming. The elastic strain corresponds to thespringback observed in press forming. The difference of springback between sphericalshell and cylindrical shell is also analyzed. Roll bending forming is a kind of important way to form single curvature surface, in a given process conditions, the numericalsimulation of roll bending forming process can be used to determine the final curvature ofthe plate after springback, and guide the actual sheet metal forming.

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