On The Stress Concentration Analysis,Measurement And Optimization Of Notched Thin-Walled Structures

Notched thin-walled structures are often encountered in the main bearing cylinder shells of launched vehicles in aerospace engineering.Since the structural]characters of stiffness discontinuity always come with concentrated stresses which may cause a considerable knock-down effect on the bearing capacity of a structure,the analysis,measurement and optimization of the stress concentration in notched thin-walled structures are of great significance to the design and optimization of aerospace structures.However,on the one hand,the stiffness discontinuity results in a complex bearing mechanism of notched thin-walled structures,which makes it difficult to analyze or optimize the notched thin-walled structures’stress concentration.On the other hand-there lack effective methods to accurately measure the concentrated stress since the region of notch is often excessively small and curved.resulting in the fact that it is hard to compare the analytical or numerical results with actual experimental data,which further restricts the development of existing theoretical and numerical analysis approaches.Therefore.under the design trend of large diameter-to-thickness ratio in aerospace structures,there is an urgent need to establish credible stress concentration prediction and design method for notched thin-uwalled structures so as to achieve the rapid design of high-performance main bearing cylinder shells.Meanwhile,a simplified measuring method for the stress concentration at notch root is also needed in order to realize the full-field real-time measurement of hot-spot stress of notched thin-walled structures.The research reported in the dissertation takes notched thin-walled structures as the research object-seeking to establish a theoretical method in analyzing the concentrate stress at the notch root in both elastic and plastic stages,to provide an engineering oriented approach for in-situ concentrated stress measurements,and to propose an innovative configuration to overcome the difficulty of the separation ring’s weight bearing and separation requirements.In order to achieve the above research objectives,the following researches are conducted.In the elastic analysis of a notched thin-walled structure,based on the theory of notch stresses,the basic elasticity equation is discussed,and the notch stress state is analyzed.Considering the asymmetric boundary conditions of a notched thin-walled structure in actual service environment,the general stress concentration factors(SCFs)for single-side notches of arbitrary depth subjected to eccentric tension are obtained.Further studies reveal the phenomenon of zero stress concentration factor of notched structures under eccentric tension.The approximate design criterion of zero stress concentration factor for notched thin-walled structures is given.In the plastic analysis of a notched thin-walled structure,facing the fact that the classic analytic methods and finite element method(FEM)are either complex or time-consuming for rapid engineering design,a rapid analytic method for determining the hot-spot plastic stress is proposed.Numerical validations of the analytical solutions have been conducted by the refined FEM.The predicted axial concentrated stress shows satisfactory agreement with the numerical results during the entire loading process until plastic flow occurs,thus confirming the feasibility and effectiveness of the present analytic method for determining the plastic concentrated stress.A new simplified indirect measuring method(SIMM)is proposed for the hot-spot stress of a stiffness-discontinued thin-walled structure by measuring the stresses away from the hot spot with the conventional strain gauges.The explicit relationships between the measurable stresses and hot-spot stress in both the elastic and plastic stages are derived.A refined finite element modeling indicates that the developed measuring method for notch stress is feasible,and the measuring accuracy is satisfactory.A series of quasi-static tensile experiments have been conducted to validate the above theoretical studies and the proposed SIMM for notch stress measurement,with both the strain gauges and advanced optical measuring method applied.Good agreement with the optical measuring results further confirms the validity and of the theoretical studies and the accuracy of the proposed SIMM for notch stress measurement.Facing the linear separation ring’s axial bearing and radial vulnerability design requirements,utilizing the structure’s eccentric load characteristic.an innovative configuration is proposed whose tensile load transmission route is optimized.The innovative linear separation ring’s axial bearing capacity is improved on the premise of enhanced radial vulnerability.Both the numerical and experimental results demonstrate the advantage of the innovative configuration of linear separation ring.The work described in this dissertation develops the current theoretical studies on stress concentration in notched thin-walled structures,yielding an analytical method for predicting the cloncentrated stress of notched thin-walled structures in actual service environment and providing a simplified indirect measuring method for the notch stress which has the advantages of low cost.easy implementation.and independence of the environmental disturbance.The prop osed innovative configuration of linear separation ring overcomes the design difficulty of axial bearing and radial vulnerability design requirements and provides new ideas for the optimum design of a notched thin-walled structure.