【作者】 王文利；

【导师】 瞿伟廉；

【作者基本信息】 武汉理工大学 ， 结构工程， 2012， 博士

【摘要】 桅杆结构是一种广泛应用于通讯工程的高耸结构。由于它具有高和柔的特点,因此对风荷载的作用比较敏感,历史上曾多次发生桅杆结构风致倒塌破坏的案例。而在桅杆结构风致倒塌破坏的事故中,疲劳损伤主要发生在主桅杆与纤绳联结的拉耳节点处,桅杆结构在风荷载的作用下,其纤绳与桅杆杆身连接的拉耳处最先发生初始裂纹,拉耳耳板与桅杆杆身通常采用焊接的连接方法,而焊接不可避免的会产生残余应力,虽然可以采取一定的措施可以消除一部分焊接残余应力,但是采用不同的方法消残的程度不一致,而且很难彻底清除,于是在焊接残余应力和扰动风荷载的双重作用下,更有可能萌生裂纹。因此,如何在考虑焊接残余应力影响的基础上来进行焊缝裂纹萌生累积疲劳损伤的评定并进行疲劳寿命的计算还是一个新的课题。桅杆结构整体的风致动力响应与纤绳分布、外荷载等多种因素有关,桅杆结构在风荷载作用下,因风向不同、风荷载强度等级不同,致使其动力响应不同。于是本文首先考虑采用LINK10三维空间杆单元模拟纤绳和BEAM44三维空间梁单元模拟杆身,建立了桅杆结构整体尺度的非线性有限元模型。然后采用基于FFT算法的改进谐波叠加法进行了标准高度处各种风速等级对应的脉动风速的模拟。再采用Newmark-β直接积分并结合修正的Newton-Raphson迭代法求解桅杆结构在模拟平均风和脉动风荷载共同作用下时域内的动力响应。桅杆结构拉耳焊接节点的焊接过程复杂,焊接残余应力对结构的疲劳性能有着重要影响。然而目前无法对实际工程中的焊件测定其焊接残余应力,因此采用有限元数值模拟分析焊接节点的焊接残余应力场分布情况是进行疲劳分析的基础。本文采用ANSYS有限元分析软件,基于热弹塑性力学理论,开发了一套完整的焊接有限元程序,并对桅杆结构拉耳焊接节点的焊接过程成功实现了三维动态模拟,分析了各个时刻的温度场和最终的焊接残余应力场,并模拟消除焊接残余应力技术,进行了各个消除残余应力比例后的应力场结果。本文将焊接节点的焊接残余应力考虑在内,提出了一套完整的桅杆拉耳子结构焊接节点的动应力场多尺度有限元分析方法。该方法首先采用ANSYS有限元软件模拟出桅杆结构整体尺度的纤绳动应力响应时程,然后对需要进行疲劳分析的焊接节点施加两端固结的边界条件；接着建立焊接节点的精细实体有限元模型,将焊接残余应力当作初始应力,施加上步得到的力学和位移边界条件进行小尺度的动应力分析,得到焊接节点局部细节处的应力应变状态,最后由Von-mise等效应力准则确定应力最大的节点为疲劳危险点,结果表明拉耳子结构焊接节点的疲劳危险点出现在焊缝的焊趾处。本文采用基于应变的多轴疲劳的临界面方法,计算了桅杆拉耳子结构焊接节点的裂纹萌生寿命。首先分析疲劳危险点的应变时程确定临界面的位置,然后采用循环计数方法提取临界面上的正、剪应变循环,并将其合成统一的多轴疲劳损伤参量,再根据是否考虑平均应力,分别采用Mason-coffin公式Morrow公式和Mason-Halford公式计算0°风向、12m/s风速等级的工况下500s内的多轴疲劳累积损伤,发现平均应力对疲劳损伤的影响不可忽略,特别是对弹性阶段的影响比较大。最后考虑风速风向概率分布,利用Miner准则估算了消除40%残余应力的桅杆拉耳子结构的裂纹萌生寿命。考虑到焊接残余应力对焊接结构的疲劳强度有着很大的影响,高强度的残余拉应力对疲劳寿命有不利的影响,而残余压应力则对延长结构或构件的疲劳寿命有很好的效果。在焊接残余应力和外荷载的双重作用下,更容易产生疲劳失效。为了能定量的评定焊接残余应力对疲劳强度的影响,前人虽然提出力用平均应力的观点来考虑其对疲劳损伤的影响,但忽略了残余应力其实与材料的状态有关这一事实,于是本文提出将残余应力作为外荷载符合材料的疲劳破坏特性。为了能定量的评定焊接残余应力对桅杆拉耳子结构疲劳裂纹萌生寿命的影响程度,以标准高度处V(10)=12m/s风速等级为例,将不同风向下未消除、消除20%、消除40%、消除60%、消除80%和不考虑残余应力的六种情况下500s内的平均应力结果,以及用不同寿命公式计算出来的不同消残比例下的500s内的疲劳累积损伤进行对比分析,最后考虑风速风向概率分布,对比计算了六种消残比例的疲劳裂纹萌生寿命。更多还原

【Abstract】 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.更多还原