Fatigue Performance Prognosis Analysis And Structural Integrity Assessment On Exiting Steel Industrial Structures

The fatigue performance degradation of steel structures has been a hot topic for both engineers and scholars and the research data of the American Society of Civil Engineers showed that 80%-90% of steel structure failure is related to fatigue damage and fracture.Because of the heavy production task,the steel industrial structures are frequently and chronically subjected to cyclic load caused by the crane running.As a result,they more likely failed compared with the common steel structures.The majority of steel industrial structures are planned to serve under high temperature in which the mechanical properties of metallic material could be weakened and the time-dependent strain(i.e.creep)of the metal could be generated,which accelerates the fatigue degradation of the structures.Therefore,the fatigue prognosis analysis method and structural integrity assessment method for existing steel industrial structures have been developed.The developed method is not only applicable to the fatigue prognostic analysis of steel industrial structures under high temperature environment,but also is suited to the fatigue prognostic analysis on steel structures under room temperature(only by making the creep of metallic material to be zero).The main research works and conclusions are summarized as follows:The current operating state of existing steel industrial structures has been systematically studied and a staged prognostic analysis system has been developed.The service loading of the structures exhibits obvious dynamic effect so that the dynamic explicit method is needed to simulate the loading caused by the crane's reciprocating operation.In the service life of the existing steel industrial structures,the damage of the structure is gradually transformed from the damage dominated by distributed microcracks to the damage dominated by macroscopic cracks.The macroscopic fatigue crack of the structure can be simplified to elliptic surface crack with type I.According to the fatigue degradation mechanism,the whole service process of the structure is divided into the early service stage,the middle service stage and the end service stage.The fatigue damage prognostic models for steel industrial structures at the early service stage and the middle service stage have been established,respectively.At these two stages,the fatigue damage of the structure is caused by the collective behavior of distributed microcracks,namely,the initiation,propagation and the interaction among microcracks.The interaction among the microcracks is the result of the counterbalance between the stress shielding and stress amplification,which is one of the important factors affecting the structural fatigue degradation.The developed fatigue damage prognosis model for steel industrial structures at the early service stage has the following two advantages: One is that the relationship between the collective behavior of microcracks and the macro fatigue damage variable has been established in which the interaction among the microcracks has been considered.The other is that the nonlinear coupling between creep damage and fatigue damage has been considered.The following originality is included in the developed fatigue damage prognosis model for steel industrial structures at the middle service stage: the Weibull distribution is used to express the scatter of microcrack length;both the relationship between the collective behavior of microcracks(including the interaction among the microcracks)and the macro fatigue damage variable and the nonlinear coupling between creep damage and fatigue damage have been considered;the model can be used to determine the critical state between the early service stage and the middle service stage.The multi-scale fatigue damage finite element model of the steel industrial structure has been established,and the fatigue prognosis analysis on the structure at different stages has been performed based on the developed fatigue damage prognosis models.The stress concentration obviously occurs at these areas of the steel industrial structure where fatigue crack likely initiates: the joint weld between the upper flange and the web,the joint weld between the lower flange and the web,the high-temperature operating platform near the furnace,the supporting components of the steel crane runway girders.At the early service stage of the structure,the fatigue life of the steel crane runway girder is about 15 years when the service environment temperature is 500? and the stress amplitude(35)? =115.5MPa.It demonstrates that the early service stage of the steel industrial structure is a long process which accounts for the majority of the whole life of the structures.At the middle service stage of the structure,the fatigue life of the structure is about 40 days when the service environment temperature is 450? and the stress amplitude(35)? =202.5MPa.It demonstrates the rationality and necessity of replacing vulnerable key components regularly in engineering practice.At the end of the service stage of the steel industrial structure,a method for predicting the fatigue life of steel structures has been developed.The fatigue crack propagation process of the structure subjected to both cyclic loading and creep has been analyzed based on numerical simulation results.The runway eccentricity exhibited little effect on the fatigue crack propagation at the weld joint between the lower flange and the web.With the increase of crane wheel pressure value,the fatigue life of the structure decreases significantly.Under the crane's full loading operation condition,the crane only runs about 400 times(about 4 days)could lead to the fatigue crack growing by 0.9mm.Special attention should be paid on the response of the structure caused by the crane with full loading.At the end of the service stage of the steel industrial structure,the constraint-based R6 criterion has been firstly introduced to assess the integrity of the structure with a macro crack,which provides a theoretical method to solve the safe operation problem of the cracked steel structures.The high level of constraint effect could occurs at the crack front at the weld joint of the upper flange and the web of the steel crane runway girder under the service loading,which indicates that the fracture toughness of the structural materials will be decreased.In this study,structural integrity assessment for the three groups of cracked steel crane runway girders under different runway eccentricity conditions have been performed and all the assessment results are safe,indicating that the cracked structures can still run safely.