The Fatigue Crack Initiation Life Assessment Of Guyed Mast's Earplate Welded Joint

A guyed mast, which is slender, tall and flexible, is widely used in communications engineering structures. The guyed mast is quite sensitive to the wind loading because of its tallness and flexibility, and the collapse of guyed masts due to wind-induced damages has been reported from time to time. The collapse of a guyed mast is often caused by the fatigue damage of the joint welds at the earplates, which connects the mast with the cables. The crack was first occur at the earplate joint which connecting the tow rope and the shaft under wind loads. The welding methods were used to connecting the ear plate and the mast shaft.The welding residual stresses are inevitable in the welds at earplate joints. Although certain measures can be taken to eliminate part of the welding residual stressses, different ways to eliminate residual stresses proportion are in different inconsistency, and hardly to eradicate,.Hence in the dual role of the welding residual stresses and disturbance wind load,are more likely to initiate crack. The study that considers the effects of the welding residual stresses in assessing the crack initiation cumulative fatigue damage assessment and fatigue life calculation at welds does not appear to be reported hitherto in the open literature.The guyed mast's overall wind-induced response are related with the distribution of the tow rope,wind load and so on. Different wind direction,different wind load strength may lead to different dynamic response. Considering the nonlinearities of the guyed mast and the mechanical characteristics of the cable, the Link10element was used to simulate the cables, while the mast was simulated using equivalent space Beam44elements.Fluctuating wind speed along the height of the guyed mast have been simulated with harmonic wave superpose method improving by introducing FFT algorithm. A nonlinear dynamic model has been established and the nonlinear dynamic response has been analyzed via Newmark-β direct integration combining to Newton-Raphson integration, so we can get the stress response time history of cable.The welding process of the welded joints is very complex, and the welding residual stress would influence the fatigue performances of the guyed mast structure. However, there is no method to determinate the welding residual stresses for practical engineering welded structures. Therefore, it is reasonable to research it with the finite element numerical simulation method. A commercial finite element package ANSYS is used to carry out a coupled thermodynamic and thermal elastic-plastic analysis to obtain the welding residual stresses. In this paper, the3D dynamic simulation for welding process is accomplished, and the finite element analysis of temperature fields and stress fields for joints of guyed mast earplate are finished. At the same time, the technology of eliminating the welding residual stress was also being used to simulate the welding residual stress results of different eliminating proportion.The paper introduces a complete dynamic stress finite element analysis for the welded joint of guyed mast earplate with multi-scale method, which included the effects of the welding residual stress for the first time. The first step of the method is to get the dynamic stress response of the large scale of the structure, and imposing fixed end constraints boundary conditions at both end of the shift two ends; the second step is to establish the refined solid finite element model of the joint, taking the welding residual stress as the initial stress, and the boundary condition are applied to the joint with small scale dynamic analysis; the next step is to analysis the strain and stress histories for each critical joint are obtained from the dynamic analysis; the von-Mises equivalent stress at each critical joint is calculated and the joint with the maximum stress amplitude is considered as the most critical one. The result for calculation of the joint of guyed mast earplate show that the fatigue critical point is near the weld toe.This paper presents a computational method together with a critical damage plane method for detecting fatigue crack initiations at the welded joints of guyed mast structures. This method consists of three steps. Firstly, the critical damage plane is determined. Secondly, the shear and normal strains are used to formulate the multiaxial fatigue damage parameter. Lastly, the fatigue damage of the joint is assessed by using the multiaxial fatigue life prediction model of Mason-coffin formula,Morrow formula and Mason-Halford formula to caclulate the multiaxial fatigue damage in the load case of0°wind direction and12m/s wind speed level. The results show that the role of mean stress on fatigue damage can not be ignored, especially for elastic stage fatigue damage. Finally, consider the probability distribution of wind speed and direction, the Miner guidelines was used to estimate the earplate sub-structure of guyed mast's fatigue crack initiation life.The high strength residual tensile stress can reduce fatigue life, whereas the residual compressive stress have a positive effect on fatigue life of the structure and/or the components when takeing the effect of residual welding stress into account.Under the double action of load and welding residual stress during the service, the structure and/or the components could lightly developed to fatigue failure.Many methods and various standpoints takes the mean stress into consideration to quantificational evaluate the influence of welding residual stress to fatigue strength,but the correlation between residual stress and material status is ignored. In this paper the study that take residual stressas as the load apply to structure to evaluate fatigue life corresponds the characteristics of fatigue failure rationally.To evaluate the influence of welding residual stress to crack initiation life of the earplate substructure of guyed mast, comparative analysis of fatigue accumulation damage is conducted in different working conditions.Take the standard height with V(10)=12m/s as a example, the mean stress under different wind angles with six cases that no residual stress eliminated,eliminated20%,40%,60%,80%and without regard to residual stress is calculated, meanwhile, the fatigue accumulation damage is calculated on the different life calculation formula. Finally, consider the probability distribution of wind speed and direction, the Miner guidelines was used to estimate the earplate sub-structure of guyed mast's fatigue crack initiation life under six different welding residual stress eliminating proportion.