Load Spectrum Acquisition Methods And Fatigue Residual Life Estimate Studies Of Overhead Travelling Crane

The lifting equipment is indispensable to the national economic construction as special equipment, and its operation safety and economy are concerned by the public and government. The differences between design and actual usage result in the variation of service life and design life for bridge crane. Therefore, assessing reliably and predicting the remaining life of the bridge crane, doing the maintenance and reinforcement in a safe period can play the economic benefits and reduce the updated replacement cost to ensure safety and reliable operation, which are major propositions for attention and resolved.The fatigue analysis and life assessment are based on the acquisition and preparation of the stress spectrum in the fatigue accounting points, and the stress spectrum of high confidence can be obtained by the prototype. It costs much more time and is difficult to test the bridge crane in different usage occasions and different types by trials. The simulation is fast, low cost, but it has differences with the actual working conditions, so the confidence level is not high. In order to solve this critical problem, this paper will take a strategy of "take chance and overcome own weakness" for the bridge crane stress spectrum.First of all, this paper aims to determine the accounting point of the fatigue analysis and life assessment of the metal structure of the bridge crane by finite element analysis and the trial results of model beam produced based on a true main beam cross-section ratio of1:2. Select a75t/28.5m bridge crane using assembly workshop as the prototype, paste strain gauges on the dangerous sections, and perform the actual time-stress history test for a work cycle. Simultaneously, some key parameters can be recorded by manual or instrument, including each actual lifting loop lifting weight, lifting and unloading position, and whether the crane car goes across the beam midpoint, which would affect the stress spectrum of the fatigue accounting point.Then, conduct the statistical analysis of the goodness of fit test for these parameters, and get the statistical distribution model which best depicts the key parameters. At the same time, analyze the lifting times for a period of time to determine the average daily lifting number. Then, based on the probabilistic analysis model of the key parameters, use the Latin Hypercube Sampling (LHS) of Monte Carlo method, to simulate the pseudo random numbers. The four critical parameters include a lifting load size, the liter and unloading position, and whether the lifting car goes through the main beam mid-span in the inspection cycle.Second, create the bridge crane simulation model by means of Matlab Simulink simulation software. In order to verify the accuracy of the simulation model, firstly compare6typical fatigue accounting point tests and simulation stress data with the lifting load in the mid-span, and then the tests and simulation stress-time history with the same lifting load, and, the lifting and unloading position. By comparing, the error is in the credible range, and it indicates that the simulation model can be used to simulate the stress-time history in the work process.Then, with the simulation model through lifting loop simulation, the analog random numbers of key parameters generated in the inspection cycle can be used to obtain the simulation stress-time course of the fatigue accounting point. The paper can get the two-dimensional simulation stress spectrum in the test cycle, using self rainflow counting procedure to do the equivalent number compression, peak-valley value detection, invalid amplitude removing and dual-parameter rainflow counting data compression processing. At the same time, with the same processing of stress-time history data in a work cycle, a two-dimensional test stress spectrum can be obtained to provide basic data for the remaining the fatigue life assessment and reliability analysis.Finally, put forward the remaining fatigue life assessment methods and steps of the crane in service and new crane, divide the crane into a finite-life and infinite-life crane according to the NDT results and loading of the key components (the main beam) before the pre-assessment, and raise the assessment criteria for crane safety in different situations. As crane metal structure is welded structure, even if the welding quality meets the standard requirements, the weld surface and interior have a certain number of initial defects, which the naked eye can not see but can not be ignored. As these defects can be technically measured, the paper puts forward the remaining fatigue life assessment methods for the cracked metal structure. Using fracture mechanics theory and damage tolerance concept, establish a mathematical model of a cracked mental structure fatigue remaining life estimation and security assessment in the detection cycle, and discuss the key parameter value of the crane structure remaining life assessment.As crane metal structure suffers a random stress, the stress spectrum, even if got by rainflow counting, can not be used to establish mathematical model for life estimation and reliability analysis.Therefore, this paper proposes equivalent stress method, Miner theory method, and the cycle continued circulation calculation three approaches to handle the random stress or stress spectrum.According to the characteristics of Cranes, based on the fracture mechanics and reliability theory, establish the remaining life reliability safety margin equation of stochastic processes for the mental structure, and determine the key parameters distribution and distribution parameters values of safety margin equation in joint points each detecting time. As safety margin equation is highly nonlinear and failure probability is very small, put forward the improved digital simulation based on the Monte Carlo method-the importance sampling method to analyze the reliability estimation, In order to construct the importance sampling, take Multi-constrained nonlinear optimization methods-sequential quadratic programming (SQP) method to find the design point, using test stress spectrum and simulation stress spectrum, for the design life of both U6and U7, and, do the reliability analysis of fatigue remaining life for25detection time joint points of the prototype. To learn about which distribution parameters of key parameters are more influential, more sensitive to the reliability, do the distribution parameters sensitivity analysis for the residual fatigue life reliability, and, finally, do the remaining fatigue life reliability sensitivity analysis for the prototype metal structure working for6years.