The typical structures that locally enter the plastic zone are studied, and the principal stress changes of the spherical head-nozzle structure at the local discontinuities, as well as the bending deformation or culvert tendency in the uniformly loaded simply supported beams and thick-walled cylindrical structures are investigated. In order to meet the requirements of process, installation and maintenance, it is often necessary to make holes in the shell of the leather container. In the conventional design, the method of equal area reinforcement is used to ensure that the hole has the same strength as the continuous part of the cylinder. In the analysis and design using the stress classification method, the primary stress in the structure is limited to less than 1.5 times the design stress intensity to ensure that the opening will not fail. There is a structural discontinuity at the intersection of the opening nozzle and the simplified form, so the secondary stress component must exist. Due to the opening of the hole, the load that the originally excavated shell needs to bear needs to be borne by the reinforcing material in the intersecting area, so the primary stress component is also essential. It is irresponsible to simply classify the stress in the intersecting zone as primary or secondary stress. It will be too conservative to classify the stress as the primary stress, and the secondary stress will be biased towards the advancing, so the stress in the complex stress region The analysis should be more detailed. Investigating the principal stress on the path is one of the methods of detailed stress analysis. When some structures enter plastic deformation, the phenomenon of Zhao Qu phenomenon will gradually appear, such as simply supported beams bearing uniformly distributed loads; some structures are restricted by themselves due to the shape characteristics of live bending, but the influence of inverse strong bending on the stress and strain state of the structure Clever is positively related to the degree of plastic deformation, such as a round and simple structure with a clever wall under internal pressure.

　　Through in-depth study of the overall plastic deformation design check and its applicability in the direct method, corresponding solutions are proposed for the problems found. Through limit analysis and elastoplastic analysis of typical structures, the advantages and disadvantages of the six main analysis methods are compared. The main conclusions are summarized as follows:

　　(1) The main structural strain is required to check the overall plastic deformation of the structure. EN13445-3 only specifies the formula for determining the structural strain of the hot spot based on the surface fulcrum strain by the secondary extrapolation method. Two structural strain components are extrapolated to the hot spot, and then the main structural strain at the hot spot is obtained; or the main structural strain at the surface fulcrum is obtained first, and then it is extrapolated to the hot spot. Strain is a tensor, and the direction of principal strain at different surface fulcrums may be inconsistent, but the direction of strain component is determined. Therefore, the second extrapolation of the strain component can ensure that the direction of the strain component of each surface fulcrum is consistent, which is a reasonable extrapolation method. If the principal strains with different directions at the three surface fulcrums are extrapolated twice numerically, there will be errors in the results. Therefore, when calculating according to the EN13445 extrapolation method, the strain component of the surface fulcrum should be extrapolated to the hot spot secondly, and then the main structural strain at the hot spot should be obtained.

　　(2) Propose a strain calculation method based on strain linearization for the main structure. Regardless of whether the structure is in an elastic state or a plastic state, the deformation along the thickness direction according to the classic plate and shell theory always satisfies the linear deformation assumption. Comparing the calculation results, it is found that the deviation between the strain linearization method and the quadratic extrapolation method is the smallest; the strain linearization method The structural strain at any position in the pressure vessel component can be calculated, including the case where the secondary extrapolation method cannot be used.

　　(3) Based on the theory of maximum elongation linear strain, the direct method selects the strain of the main structure as the characteristic parameter for judging structural failure. However, the maximum elongation linear strain theory is mainly applicable to brittle materials. For the primary materials widely used in the pressure vessel industry, the clear variable energy density theory can better describe the failure state. Therefore, corresponding to the maximum primary structural strain criterion, a maximum equivalent structural strain criterion that can be used for the overall plastic deformation check is proposed. This criterion is more suitable for the ingenious materials commonly used in pressure vessels.

　　(4) The two-fold slope criterion, the double-tangent criterion and the zero-curvature criterion are programmed, and the criteria for the tangent point and zero-curvature point of the plastic section on the load-strain or load-deformation (P-W) curve are given. , Can accurately and quickly determine the ultimate load that meets the three criteria according to the P-W curve. Comparing the limit loads determined by the three criteria in the limit analysis and elastoplastic analysis, it is found that the stability and accuracy of the double tangent criterion and the zero curvature criterion are significantly better than the double elastic slope criterion.

　　(5) Through the limit analysis of the typical structure, it is found that the limit load obtained by the double tangent criterion, the zero curvature criterion, the direct method to prevent overall plastic deformation and the limit load method is very small, which is within the allowable range of engineering error. Moreover, the zero curvature criterion and the direct method to prevent the overall plastic deformation check can limit the excessive plastic deformation of the structure, and at the same time can avoid the problem of numerical instability when approaching the limit load.

　　Through the elastoplastic analysis of typical structures, it is found that when the structure has obvious geometrical strengthening, the allowable load determined by the collapse load will be greater than the allowable load determined by the ultimate load; the zero curvature criterion satisfies the use of load-resistance coefficient method for design calibration The condition of the core can effectively limit the excessive plastic deformation of the structure. Therefore, the smaller allowable load determined by the collapse load and the ultimate load can be selected as the final allowable load of the structure.