Research On Damage Mechanism And Its Identification Method Of Crane Girder Based On Peridynamics

As one of the important equipments of modern industry,cranes have improved the production capacity and efficiency of enterprises,with its rated operating loads having been continuously enhanced.While,cranes are often operated under heavy loads with high frequency of utilization.If accidents occur,it may lead to injurious effects upon our country,so its safety has received more and more attention.Crane girder is one of the most critical components of the mechanical structure.Having complicated structures and cumbersome manufacturing processes,the crane girder bears heavy loads and cyclic impact loads in the long-trem operation,which is easy to cause damage and even lead to safety accidents.However,the existed detection methods of the crane girder are mainly local non-destructive testing methods,such as ultrasonic,eddy current flaw detection and so on,which can not fully reflect the health conditions of the mechanical structure of the crane girder,and they are not preliminary.Therefore,it is an urgent task to study the mechanism of the crane girder,and establish effective methods to identify the damage of the crane girder.Aiming at researching the complex damage mechanism of the crane girder under the operation conditions,and improving the effectiveness of damage identification method of the crane girder,the peridynamics is used to model the crane girder,and research the damage mechanism of the crane girder in the form of elastic-plastic deformation,crack initiation and propagation,as well as the generation mechanism and propagation characteristics of engineering measurable signals,such as strain and stress wave generated in the process of damage evolution.On this basis,combining with the signal analysis and processing theory,the damage of the crane girder is identified to provide a basis for the structural safety assessment of the crane girder.The main work of this paper is as follows.（1）The peridynamic theory is reviewed,and three kinds of peridynamic models are compared and analyzed,and the research state of peridynamics is reviewed from the aspects of constitutive model,numerical calculation method and coupling method,etc.The application researches of peridynamic theory in damage,failure simulation and stress wave propagation are discussed in detail.By making a systematic review of damage identification method,and summarizing the theory and application of the peridynamics,it enhances that peridynamics has advantages in damage identification.（2）By establishing the elast-plastic constitutive relationship,an improved peridynamic micropolar model is proposed,which is suitable for studying the mechanical behaviors of metal materials.And the elast-plastic deformation and damage evolution of metal materials are analyzed;then,the peridynamic micropolar model of interface of dissimilar materials is proposed to model the welded structure,which is used to analyze the elast-plastic deformation and damage evolution of the welded structures.Considered the variable Poisson’s ratio of the peidynamic micropolar model,and combined with the theory of elast-plastic mechanics,the strain values are calculated by the displacements of the material points in the peidynamic micropolar model,and the Mises yield theory is used to judge the elast-plastic deformation state.According to the strain values of material points,the different corresponding constitutive equations of the peridynamic micropolar model are used to simulate the elast-plastic deformation and damage evolution of metal materials.At the same time,the peridynamic micropolar model of interface of dissimilar materials based on the uniform cross-section integrated beam model is modeled,which is used to analyze the elast-plastic deformation and damage evolution of the welded structure that contains welding defects.（3）According to fatigue theory and virtual crack closure technique,a kind of peridynamic fatigue model is proposed on the basis of the ordinary state-based peridynamic model.In the stage of fatigue crack initiation,according to the local stress-strain method of the fatigue theory,the Manson-Coffin formula and the fatigue element model,the fatigue remaining life equation of the initial core bond is proposed by calculating the elongation of material points under cyclic load.The lifes of the fatigue crack initiation stage and the fatigue damage initial location are analyzed.In the stage of fatigue crack initiation,a dummy model is proposed to accurately describe the fatigue crack growth path by calculating the average maximum elongation of bonds.For a single crack or symmetric fatigue cracks,a peridynamic global virtual crack closure method is proposed to analyze the Strain Energy Release（SER）rate and Stress Intensity Factor（SIF）during the fatigue crack propagating.Combined with the maximum hoop tension stress theory,a crack mode decomposition method is put forward.For the complex or multiple fatigue cracks,a peridynamic local virtual crack closure method is proposed to analyze the closure work of the bonds located in the virtual crack closure region of the crack tip,and the Strain Energy Release（SER）rate during the fatigue crack propagating is got,so the Stress Intensity Factor（SIF）is obtained.Finally,the effectiveness of the proposed peridynamic fatigue models is verified by fatigue tests of multi-cracked metal plates and CT specimens.（4）The damage mechanism of the crane girder is researched by the proposed peridynamic micropolar methods.Aiming at the elastic-plastic deformation and damage evolution of the crane girder,the improved peridynamic micropolar model is used to analyze the strain distribution,crack length and reaction force,meanwhile,the damage evolution of the crane girder with stop hole is contrasted and analyzed.It is found that the existence of cracks significantly reduces the strength of the crane girder.Considering the particularity of the welded structure of the crane girder,the peridynamic micropolar model of interface of dissimilar materials is used to simulate the damage evolution of the crane girder.The law of crack propagation and the influences of different defects are obtained.Meanwhile,the crane girder under cyclic fatigue load is studied in the stages of fatigue crack initiation and fatigue crack propagation under different maximums and different stress ratios of cyclic loads,the fatigue crack length,remain life and the damage mechanism is obtained.（5）Taking the strain signal as the research object,which is generated by the crane girder under impact load during work,a method of crane girder strain modal damage identification based on the ordinary state-based peridynamic model is proposed.According to the peridynamic ordinary state-based model,a three-dimensional model of the crane girder is established to simulate the strain signals of the crane girder under working impact load.Combining with the mechanical vibration theory,the strain mode of the crane girder model is obtained,then the difference curve of strain modal is got by calculating the strain modal values of uniformly distributed check points in the crane girder.And the damage sensitive index(I_DSI)is established to identify damage location quantificationally.On the basis of damage location identification,the Auto-Regressive Moving-Average Model（ARMA）,model is established by analyzing the local strain modal difference data of the damage location,and the prediction function of the model is used to obtain the undamaged node of the crane girder.The strain difference data are used to quantitatively identify the damage degree of the local structure of the crane girder by establishing the damage degree sensitive index.Finally,the application research of the proposed damage identification method is carried out,and the proposed main beam damage identification method is verified through the strain modal test experiment of the crane girder model.