Study On Failure Path-independent Method For Identification Of Structural Failure Mode And System Reliability Analysis

Identification of structural failure mode and system reliability analysis are the important foundation of structural optimization design,maintenance and safety evaluation.Traditional methods for identification of structural failure mode require updating the material constitutive law step by step,which rely on the failure path analysis.Meanwhile,traditional methods of structural system reliability analysis are difficult to identify the dominate failure modes and calculate the joint failure probability of multi-failure modes.Therefore,there is important theoretical significance and engineering application value to study on the efficient methodology for identification of structural failure mode and system reliability analysis.The elastic modulus reduction method(EMRM)developed in recent years simulates the damage evolution of the structure by reducing the elastic moduli of the highly stressed elements in the process of iterative analysis based on the linear elastic finite element computational model.The EMRM is simple in calculation principle,high efficiency and independent on the failure path(i.e.,path-independent).In view of this,this thesis foucs on the study on identification of structural failure mode and system reliability analysis based on the EMRM:(1)Based on the traditional elastoplastic incremental method(EPIA),the relative height of the elastic core in cross section of members was presented to determine the degree of plastic yield in the cross section.Meanwhile,the method for identification of failure mode of truss structure was proposed,which provides a new way for failure mode identification of truss structures based on the EPIA.Analysis results show that the failure cross sections could attain to plastic yielding state of full section in the plastic limit state when appropriate finite element parameters were used,and there is no elastic core in the failure cross sections.However,due to the subjectivity of parameter selection,there probably exist elastic cores of a certain height in the failure cross sections of the structure when the limit load results have higher accuracy.According to the analysis results of examples in this paper,it can be considered that the cross section has entered a plastic yielding state and formed a plastic hinge when the relative height of elastic core of the cross section is less than or equal to 1/8.(2)An efficient failure path-independent method for structural damage evolution simulation and failure mode identification was presented by combining the elastic modulus reduction strategy with the homogeneous generalized yield function.First,the characteristic parameters of the element bearing ratio(EBR)distribution were defined in terms of the homogeneous generalized yield function,by which the effect of combined internal forces could be considered,while a dynamic criterion was presented to identify the highly stressed elements.Subsequently,structural damage evolution and internal force redistribution was simulated according to reduce the elastic moduli of the highly stressed elements,and a quantitative recognition criteria of the failed sections among the failure elements was established according to the reduction rate of elastic moduli of the highly stressed elements in the last iteration.Finally,each failed section was replaced with a plastic hinge,based on which the potential failure mode was identified.The failure path-independent methodology of structural damage evolution and failure mode identification overcomes the disadvantages of traditionalmethods,i.e.,require updating continuously the material constitutive law or the finite element governing equation of damaged elements due to the failure path-dependent property.(3)A failure path-independent method for failure mode identification of structural system with permanent load effect was presented by combining the strength incorporation technique with the elastic modulus reduction strategy.First,a modified computational model was established using the strength incorporation technique,so that the permanent load effect could be reasonably considered.Then,the damage evolution process of structural system with permanent load effect was analyzed by using the elastic modulus reduction strategy,and the failure mode of structural system with permanent load effect was identified according to the reduction rate of elastic moduli of the highly stressed elements in the plastic limit state.Analysis results show that the proposed method can reasonably consider the effects of the permanent load and combined internal forces,and it is failure path-independ because it does not need to update the material constitutive law or the finite element governing equation in the iterative analysis process.Meanwhile,the proposed method is simple to implement and of high accuracy.(4)A path-independent method for structural system reliability analysis was developed by combining the stochastic response surface method(SRSM)with the EMRM.First,the explicit expression of stochastic ultimate bearing capacity of structural system was presented based on the SRSM and the EMRM.Then the performance function of global structural bearing limit state was established and the path-independent method for structural system reliability analysis was developed based on the stochastic ultimate bearing capacity.Based on the proposed method,the structural system reliability analysis problem can be solved by analyzing the limit bearing capacity of the structure on the.limited sample points in random space,which greatly reduce the dimension of the problem.The proposed method is of high accuracy and efficiency as well as wide applications,since it avoids two difficulties of traditional methods of system reliability analysis including the identification of doninant failure modes and the calculation of joint failure probability.