Prediction Of Germination And Propagation Life Of Wind Induced Fatigue Crack Of Earplate Connecting Guyed-mast To Cables

Guyed mast is a kind of highrising structure which mainly subjects to wind load, and consists of a slender bar body and several layer cables of 3 or 4 orientation stretching obliquely along height which has been widely used on radio communication,broadcast television,ocean and air navigation,environment protection,energy explore and other important fields of national economy.The ratio of guyed mast failure accidents is the second to none in civil engineers on account of the complicate working mechanism of guyed mast has yet not been understood and mastered completely.Reports of many guyed mast failure accidents show that most collapse and failure accidents are result of structural instability or the stress of structural member has exceeded the strength limit due to fatigue damage.So it is necessary to study fatigue damage and predict the fatigue life of guyed mast in order to provide important basis for guyed mast on evaluation of fatigue residual life and proposition of new method of fatigue damage detection.Taking crack germination life as fatigue criterion is the result of having no enough knowledge of fatigue fracture.Fatigue total life includes crack germination and crack propagation,and complete fatigue analysis should not only study crack germination but also crack propagation.This dissertation studied crack germination lift with multi-axial analytic theory.15 series of wind load along the height of the guyed mast have been simulated with harmonic wave superpose method improving by introducing FFT algorithm.Also the cross-correlation function and power spectrum have been checked and simulated wind load with the same characteristic of natural wind has been obtained by transformation wind velocity to wind pressure.A nonlinear dynamic model has been established and the nonlinear dynamic response has been analyzed via Newmark-βdirect integration combining to Newton-Raphson integration,so we can get the stress response time history of cable. On account of the symmetry of guyed mast only 0°,30°,60°orientation of wind responses have been calculated and wind response of other orientation can be obtained according to symmetry.The stress or strain state of dangerous point of fatigue on guyed mast's ear plate has been calculated via finite element software. Rotate the coordination of stress or strain with angle changes per certain interval to search the critical plane relation to maximum damage.The shear and normal strain cycle of critical plane are extracted with double rain-flow counting method and be composed a unified multi-axial fatigue damage parameter basing on Von-Mises rule. Then The multi-axis fatigue accumulative damages of different wind orientation and wind average velocity of guyed mast's ear plate connecting with sub-layer cable have been evaluated on basis of Mason-coffin formula and Miner fatigue accumulative damage rule.And germination life of crack has been calculated according to total damage.The emphasis of this dissertation is studying the propagation of initial fatigue crack of complicate structure guyed mast's ear plate with fracture mechanics.The research of crack propagation of ear-plate connecting guyed mast to cable should solve two problems:the calculating of stress intensity factors along crack front and the analysis of crack propagation.Stress intensity factors along 3-D surface crack have been calculated with finite element method.In view the difficulty on modeling 3-D crack,a solid modeling method has been proposed.This method decreases modeling word greatly by adopting Boolean operation and some super mesh skills which can be used to model all kinds of cracked finite element model with complex configuration.Stress intensity factors of central surface crack and crack at hole have been calculated with finite element method basing on solid modeling method.They are compared with the results deriving from Newman & Raju stress intensity factor empirical formula.By that the procedures of solid modeling and solving of stress intensity factor have been validated.With previous described finite element method we calculatedⅠ,Ⅱ,Ⅲtype of stress intensity factor along front of given shape and size crack at hole of guyed mast's ear-plate and transform them all to the form of no-dimension stress intensity factors which are convenient for calculation.Simulate no dimension stress intensity factor with polynomial by least square method.In order to maintain the changing trend of no-dimension stress intensity factor we replace the dispersed no-dimension stress intensity factor with data of polynomial function to calculate no-dimension Stress intensity factors of any shape crack at hole by multi-parameter Lagrange interpolation method.And then calculate stress intensity factor of any shape crack at hole.There is a comparison between interpolation value and the finite element analytical results of a certain shape crack which shows that they are close to each other.So we can replace whole 3-D finite element analysis with numerical interpolation calculation approximately.Take guyed mast' ear plate connecting with sub-layer cable as objects,we studied the characteristic of crack propagation shape by adopting Paris propagation velocity formula with consideration of mix crack propagation pattern and crack close phenomenon on the basis of above solving of stress intensity factor.Propagation life of crack at hole of different initial situation of ear-plates connecting with guyed mast's upper and lower layer cables have been predicted with one by one cycle direct integration method with the consideration of the characteristic of crack propagation shape.