Mechanical Behaviors Of Laser-welded Steel Sandwich Structure Plates And Design Of Structure

With the rapid development of national economy, industry and the urgent demand of the bigger, faster and lighter vehicle, more and more attention was attracted for the efficient utilization of raw materials and the protection of environmental. Under this background, the lightweight structure has become one of the important and effective solutions for the resource conservation and environmental protection, such as be applied in ship, automobiles, buildings, bridges, and many other fields. The sandwich structure with all steel plate is a kind of important new structure for the purpose of lightweight. In this structure, the high intensity panel and the core board were connected by the deep penetration laser welding method. The effect of "sandwich" for the designed structure make it do not only have the characteristics of light weight, large rigidity, high strength, but also have good shock absorption, noise reduction and impact resistant ability, etc. At present, this designed steel structure was successfully applied in the hull of the deck, bulkheads, aerial platforms and etc.This thesis studied the effect of the mechanical behavior, the joint geometric parameters and structure parameters on the steel sandwich structure for the three kinds of structures of the laser welded Web-core steel sandwich structure by using the overhanging three point bending experiment and the finite element calculation. Base on the results, by the consideration of the structural optimization theory, the Web-core steel sandwich structure was designed by the method of moving asymptotes (MMA), and we derived the optimal stiffness, strength and weight of the Web-core steel sandwich structure finally, so that the advantage of the specific stiffness and the specific strength of sandwich plate can be maximized. the main research content of this thesis are, as following:(1) Based on the formulas for the strength and stiffness design of steel sandwich plate, we designed the three kinds of Web-core steel sandwich structures. According to the national standard GB/T1456-2005and the American standard C393for the bending performance test method of sandwich structure, we further designed and fabricated the test device for the characterization of the performance of steel sandwich plate bending. By overhanging three points bending test method, the bending performance experiment was carried out for the three kinds of Web-core steel sandwich structures, and the finite element analysis of the stiffness, strength, bending deformation behavior and failure and failure form was also carried out. Results shows the mechanical behavior of these three kinds of Web-core steel sandwich structures is not identical, and the effect of joint geometric parameters on the mechanical behavior of Web-core steel sandwich structure significantly.(2) By the analysis of the stiffness of Web-core steel sandwich structure with two kinds of joint geometry parameters (weld width and gap), results indicates that the joint geometry discontinuity has an important influence on the stiffness and strength, the weld width is the main influence factor. When the weld width of less than60%of the core board thickness, weld width for the Web-core steel sandwich plate stiffness has obviously weakened, and more than60%of core board thickness, such extinction is not obvious. The effect of the gap on stiffness of Web-core steel sandwich plate is very small, and can be negligible. However, the used design formula for the calculation of stiffness of Web-core steel sandwich plate based on the assumption of integrity in the joint, and then the stiffness would be overestimated. On the basis of experimental and finite element analysis and together with the theory of material mechanics, the formula for the design of the stiffness of Web-core steel sandwich plate was revised which was proposed by Lok&Cheng. By using the revised formula, the stiffness calculation for Web-core sandwich plate with the incomplete joint can be better expressed.(3) By using the experiment and finite element method, we studied the effect of joint geometric parameters on strength, bending deformation behavior, failure and failure forms of Web-core steel sandwich plate systematically. Results show that when the weld width is less than60%of the core board thickness, and the weld width plays an obvious role on the weakened strength of Web-core steel sandwich structure. Further, the Web-core steel sandwich plate showed a complete failure for joint plastic hinge in appearance, and joint cracking as the destructive form. By increasing of weld width, the strength weakening caused by weld width decreased, meanwhile joint is not easy to form a complete plastic hinge, and yield and failure forms of sandwich plate also changed from joint yield and panel local yield, joint yield and core board local yield to core board yield and panel local yield. The effect of the gap on strength, bending deformation behavior, failure and failure form is small. Therefore, joint yield should be the failure form for metal sandwich plate; joint cracking should also be as one of its destructive form.(4) The strength of T-joint determined the deformation, failure and failure forms of Web-core steel sandwich plate. Through the bending experiments and finite element calculation for Web-core steel sandwich plate, the stress state of the T-joint was determined, and the experimental device for the characterization of joint mechanical properties was further designed. T-joints with different process parameters (welding speed, power, gap and migration) were tested by tensile, shear and bending, and the influence of process parameters on the joint strength was also analyzed. Results show that the process parameters, welding defects and joint organization determined the strength of the structure. In all process parameters, the welding speed has a close relationship with the joint strength, and the other parameters have a complex effect on the joint strength. Therefore, the nominal stress was used for the comprehensive expression of the relationship between the mechanical properties of T-joint and process parameters, welding defects and joint organization.(5) The effect of structural geometric parameters (the panel thickness, core board height and core plate spacing) on the stiffness, strength and bending deformation behavior, failure and failure forms of Web-core steel sandwich plate was studied. Using the multi-constrained optimization design theory,_we constructed multi-constrained optimization design mathematical model for the Web-core steel sandwich plate This mathematical model considered the influence of the joint mechanical properties, and used the method of moving asymptotes (MMA) on the solution, and the design results is verified by experiment finally. By the comparing of the mechanical behavior of the four kinds sandwich structure plate, the results show that the proposed design method is accurate.