Risk,Life Assessment And Repairable Decision-making Of Mobile Crane Jib Structure In Service

As the modern materials handling equipment,mobile cranes are widely used in construction,metallurgy,shipbuilding,power,petrochemical,energy and other fields.Jib structures,the main loaded members of mobile cranes,are prone to damage and failure,whose structural forms are complex and service conditions are rigorous.With high added value and large quantity,jib structures are a kind of common key component in construction machinery.In order to enter the best repair period of jib structures,prolong service life,maximize the material utilization,and realize energy saving and emission reduction,how to evaluate current security statuses and residual life of mobile crane jib structures in service effectively and reliably is a mutual concerned problem for enterprises,customers and special equipment supervision departments.Up to now,there are no relevant methods and standards in risk assessment,life assessment and repairable decision-making of mobile crane jib structures.How to determine the use,repair,replacement or special treatment is lack of systematic theoretical support.Accounting for these problems,the following researches have been carried out in this paper:1)To forecast potential failure modes of mobile crane jib structures more effectively,the predicting model of potential failure modes of jib structures based on case-based reasoning technology is built.Through the investigation and collection of historical information of finite failure modes of mobile crane jib structures,the failure mode fuzzy database of jib structures is established.For the same type moble cranes,the case database of failure modes of jib structures under different typical working conditions is also set up.The case-based reasoning technology is applied to predict potential failure modes of jib structures of mobile cranes in service.The potential failure modes are modified by the parameterized finite element model and simulation platform of mobile crane jib structures,which provide the basic information and data for subsequent risk assessment and fatigue residual life estimation.2)For security issues of mobile crane jib structures with multiple coupling failure modes,the risk assessment method of mobile crane jib structures based on potential failure modes is developed.According to potential failure modes of mobile crane jib structure,the improved DEMATEL method is adopted to establish the direct and indirect influence fuzzy relation matrixes between dangerous points or evaluation indexes at all levels.Influence matrixes and affected matrixes of every evaluation index are deduced,and then obtained the centrality and reason degree.The weighting factor of each index is determined by making an analysis of the correlation and influence of indexes.The fuzzy comprehensive evaluation theory is used to quantify the failure risk of jib structure,thus realized the risk assessment of mobile crane jib structures with multiple coupling failure modes.3)The randomness,intermittence and circulation of working loads of mobile crane in time and space result in the randomness of measured load spectrum and the limitation of test conditions makes it difficult to carry out large-scale load spectrum test.To solve these problem,the acquisition method of random load spectrum based on mathematical statistics and prediction methods of equivalent load spectrum based on artificial intelligence technology are proposed.The small-sample measured load spectrum is formed by collecting working cycles of mobile crane under different characteristic parameters within a specific time period.By mathematical statistics technology combined with Latin Hypercube Sampling or by a combination of new intelligent optimization algorithms and artificial intelligence prediction technology,the random weight sample or equivalent weight sample is determined within the period of inspection.In accordance with the rated lifting table,lifting height curve and rated lifting moment table,the working length and working range of jib structure corresponding to the lifting weight are obtained,thus described the stochastic parameters and corresponding work cycles in line with engineering practice.Therefore,the random load spectrum or equivalent load spectrum is established,which provides basic data for the evaluation of fatigue residual life.4)Mobile cranes are confronted with the multiple failure modes coupling effect in operation process,which leads to the safety of jib structures being of great uncertainty.Considering the effect of dynamic load and coupling effect of multiple failure modes,the residual fatigue life assessment method of mobile crane jib structure is proposed.With the random load spectrum or equivalent load spectrum being as the basis,the first principal stress time-history theoretical simulation model of dangerous points of jib structures is establised in accordance with the work cycle processes of mobile crane.The variation of first principal stress with time of dangerous points during detection period is obtained and the double stress spectrum of corresponding dangerous points is described by the rain flow counting method.On this basis and combining with the damage tolerance design and fracture mechanics theory,the fatigue life evaluation model based on equivalent load spectrum or random load spectrum is introduced.For single failure mode or coupling failure modes,fatigue residual life values of corresponding dangerous points of jib structures are calculated,which provide the threshold for the repairable evaluation and making-decision of damaged jib structures.Combining with the purchase cost of new jib structures,the comprehensive decision-making result for the repairable of damaged jib structure is given.5)For the problem of use,repair,replacement or scrap of damaged jib structures of mobile crane in service,the comprehensive evaluation and decision-making model of the repairable of damaged jib structure is established,from the perspective of current characteristics of structures,repair technologies,economic and environmental resources.For dangerous points in crosssection tensile stress region of damaged jib sections with fatigue residual life(for dangerous points in compressive stress region of damaged jib sections that enter the repair period),the reliability of fatigue residual life of dangerous points(the strength redundancy of dangerous points)is analyzed at the detection period nodes.In light of the reliability threshold(the minimum intensity redundancy factor),the availability of current characteristics of structures at dangerous point is determined.According to the repair process,the feasibility of repair technologies are confirmed by quantitative analysis of technical feasibility evaluation indexs.Based on the actual repair expenses of damaged jib structures of mobile crane in enterprises within a specific time period,Activity-based Costing is applied to trace the repair cost of damaged jib sections corresponding to diverse failure characteristics,thus formed the repair cost database.The repair cost of damaged jib sections of mobile crane that need to be evaluated is forecasted by the improved relevance vector machine theory.With the purchase cost of new jib structure,the economic evaluation of damage jib structure is given and the environmental resources benefit is determined.Finally,the comprehensive decision-making result for the repairable of damaged jib structure is provided,which provides an important theoretical basis for the formulation of the relevant standards.6)For convenience in engineering application,based on the VC++6.0/C# platform and aboved theories,the system of risk,life and repairable decision-making of jib structures of mobile cranes in service is developed,which provides the technical support for the prediction of potential failure modes,risk assessment,load spectrum prediction,fatigue life estimation,and comprehensive evaluation and decision-making of the repairable of jib structures.