Uncertainty analysis of strain modal parameters by Bayesian method using frequency response function

Structural strain modes are able to detect changes in local structural performance, but errors are inevitably intermixed in the measured data. In this paper, strain modal parameters are considered as random variables, and their uncertainty is analyzed by a Bayesian method based on the structural frequency response function （FRF）. The estimates of strain modal parameters with maximal posterior probability are determined. Several independent measurements of the FRF of a four-story reinforced concrete flame structural model were performed in the laboratory. The ability to identify the stiffness change in a concrete column using the strain mode was verified. It is shown that the uncertainty of the natural frequency is very small. Compared with the displacement mode shape, the variations of strain mode shapes at each point are quite different. The damping ratios are more affected by the types of test systems. Except for the case where a high order strain mode does not identify local damage, the first order strain mode can provide an exact indication of the damage location.

PROBABILISTIC METHODOLOGY OF LOW CYCLE FATIGUE ANALYSIS

The cyclic stress-strain responses (CSSR), Neuber's rule (NR) and cyclic strain-life relation (CSLR) are treated as probabilistic curves in local stress and strain method of low cycle fatigue analysis. The randomness of loading and the theory of fatigue damage accumulation (TOFDA) are considered. The probabilistic analysis of local stress, local strain and fatigue life are constructed based on the first-order Taylor's series expansions. Through this method proposed fatigue reliability analysis can be accomplished.

A COMBINED PARAMETRIC QUADRATIC PROGRAMMING AND PRECISE INTEGRATION METHOD BASED DYNAMIC ANALYSIS OF ELASTIC-PLASTIC HARDENING/SOFTENING PROBLEMS

The objective of the paper is to develop a new algorithm for numerical solution of dynamic elastic-plastic strain hardening/softening problems. The gradient dependent model is adopted in the numerical model to overcome the result mesh-sensitivity problem in the dynamic strain softening or strain localization analysis. The equations for the dynamic elastic-plastic problems are derived in terms of the parametric variational principle, which is valid for associated, non-associated and strain softening plastic constitutive models in the finite element analysis. The precise integration method, which has been widely used for discretization in time domain of the linear problems, is introduced for the solution of dynamic nonlinear equations. The new algorithm proposed is based on the combination of the parametric quadratic programming method and the precise integration method and has all the advantages in both of the algorithms. Results of numerical examples demonstrate not only the validity, but also the advantages of the algorithm proposed for the numerical solution of nonlinear dynamic problems.

PREDICTION FOR FORMING LIMIT OF AL2024T3 SHEET BASED ON DAMAGE THEORY USING FINITE ELEMENT METHOD

This paper presents the application of anisotropic damage theory to the study of forming limit diagram of A12024T3 aluminum alloy sheet. In the prediction of limiting strains of the aluminum sheet structure, a finite element cell model has been constructed. The cell model consists of two phases, the aluminum alloy matrix and the intermetallic cluster. The material behavior of the aluminum alloy matrix is described with a fully coupled elasto-plastic damage constitutive equation. The intermetallic cluster is assumed to be elastic and brittle. By varying the stretching ratio, the limiting strains of the sheet under biaxial stretching have been predicted by using the necking criterion proposed. The prediction is in good agreement with the experimental findings. Moreover, the finite element cell model can provide information for understanding the microscopic damage mechanism of the aluminum alloy. Over-estimation of the limit strains may result if the effect of material damage is ignored in the sheet metal forming study.

Steel Design by Advanced Analysis: Material Modeling and Strain Limits

Structural analysis of steel frames is typically performed using beam elements. Since these elements are unable to explicitly capture the local buckling behavior of steel cross-sections, traditional steel design specifications use the concept of cross-section classification to determine the extent to which the strength and deformation capacity of a cross-section are affected by local buckling. The use of plastic design methods are restricted to Class 1 cross-sections, which possess sufficient rotation capacity for plastic hinges to develop and a collapse mechanism to form. Local buckling prevents the development of plastic hinges with such rotation capacity for cross-sections of higher classes and, unless computationally demanding shell elements are used, elastic analysis is required. However, this article demonstrates that local buckling can be mimicked effectively in beam elements by incorporating the continuous strength method (CSM) strain limits into the analysis. Furthermore, by performing an advanced analysis that accounts for both geometric and material nonlinearities, no additional design checks are required. The positive influence of the strain hardening observed in stocky cross-sections can also be harnessed, provided a suitably accurate stress–strain relationship is adopted;a quad-linear material model for hot-rolled steels is described for this purpose. The CSM strain limits allow cross-sections of all slenderness to be analyzed in a consistent advanced analysis framework and to benefit from the appropriate level of load redistribution. The proposed approach is applied herein to individual members, continuous beams, and frames, and is shown to bring significant benefits in terms of accuracy and consistency over current steel design specifications.

钢板感应加热工艺中加热顺序对于变形的影响

复杂曲面板的水火弯板成形需要多条加热线,加热顺序是成形特定形状曲面板的重要影响因素。采用弹塑性固有应变有限元模型研究钢板在不同加热顺序下的整体变形,同时以加热顺序为唯一变量,对2张辊弯板进行多加热线感应加热实验,2张板的计算值与测量值之间的高度差符合工程应用要求。弹塑性固有应变有限元模型可用于计算钢板在不同加热顺序下的变形。对2张钢板进行三维曲面定位和偏差数据的假设检验,分析表明多条加热线的加热顺序对钢板变形有显著影响。

基于离散元的颗粒材料中应变局部化形成与演化研究

岩土颗粒材料的应变局部化失效问题广泛存在于工程设计应用中,主要表现为介观尺度上的应变局部化现象和宏观尺度上的剪切带产生,目前在微观尺度上的形成机理尚不明确。为了系统研究颗粒集合体的应变局部化的形成与演化过程,通过使用离散单元法(Discrete Element Method,DEM)模拟了指定平面应变加载路径的真三轴试验,获取了宏观和微观尺度上的颗粒材料几何、运动以及力学信息。为了找出表征应变局部化特性的最佳特征量,比较了颗粒温度、波动位移和局部剪胀角等微观指标,发现波动位移在表征应变局部化方面与其他参数相比相关性更好,并选定其作为应变局部化表征变量。为了量化颗粒集合体从应变局部化开始产生到发育完成的具体应变区间,采用莫兰指数对波动位移的空间分布特征进行统计和分析,确定了颗粒集合体在弹塑性转换阶段的发育区间。进一步对颗粒集合体应变局部化发育区间内的波动位移空间分布进行探究,并统计不同空间区域内波动位移的概率密度函数,发现研究区域外部的局部塑性在过渡阶段停止演化,而内部塑性以逾渗模式发展。最终,通过波动位移的空间分布进行聚类分析,获得了描述介观尺度上应变局部化的团簇体模型。该模型可以将颗粒集合体宏观剪切带的形成与发育和单个颗粒的微观塑性发展相联系,刻画颗粒集合体从介观尺度上的应变局部化出现到宏观尺度上剪切带完全形成的演化过程。

基于名义应力法与局部应力-应变法的绳轮寿命评估

死绳固定器绳轮在钻井生产实际使用中承受着高强度交变载荷,极易发生疲劳破坏。为准确评估绳轮的疲劳寿命,以JZG18C型死绳固定器为研究对象,结合Goodman等寿命曲线修正的名义应力法和Morrow弹性应力修正的局部应力-应变法,估算死绳固定器绳轮在最大死绳拉力下的疲劳寿命。结果表明:JZG18C型死绳固定器绳轮寿命薄弱位置为传感器支臂过渡圆角处(疲劳寿命安全系数n 1=0.4351),在名义应力法及局部应力-应变法下绳轮薄弱位置处的最小疲劳寿命分别为38194次(3.5年)、37120次(3.4年),2种疲劳寿命评估的理论计算结果与有限元分析得到的疲劳寿命结果分别相差0.38%、2.51%,验证了以名义应力法与局部应力-应变法评估死绳固定器疲劳寿命的可靠性与准确性。

弹簧操动机构分闸扣板冲击疲劳寿命估算

针对弹簧操动机构分闸扣板在工作过程中失效的问题,提出了一种适用于受强冲击载荷的疲劳寿命计算方法。运用RecurDyn对整体弹簧操动机构进行运动学分析,得到合闸速度、行程与弹簧力。基于运动学模型利用Lsdyna进行冲击动力学仿真,得到合闸过程中分闸扣板应力集中处的时间应力曲线。针对其失效特点,采用局部应力应变法加冲击损伤修正方法进行寿命估算,得到的结果为9216次,与实际测试次数10000次相比,误差为7.8%,符合工程要求。

集成排气歧管低周疲劳寿命研究

集成排气歧管的设计使排气部分收到高低温载荷的反复冲击,因此在局部区域会产生较大的应力集中,超过材料的屈服强度,产生塑性变形,会有低周疲劳破坏的风险。本文基于有限元的方法,计算某冲击循环的瞬态温度场结果,并基于此结果进行交变工况下的应力应变分析,研究排气部位危险位置的温度、应力和应变的变化规律,并基于局部应力应变法得到该位置的疲劳寿命,为缸盖设计提供一种分析方法。

考虑横隔板局部刚域的宽幅钢-混组合梁桥荷载试验研究

在主梁高度既定的情况下,长宽比大的桥梁横向抗弯刚度相对较大,桥面荷载横向分布对主梁应变及挠度在横桥向的响应程度影响较小;但对于长宽比小的宽幅桥梁,桥梁整体的横向抗弯刚度对应变及挠度的分布影响会急剧增大,对于分离式主梁桥梁,传统的梁格法荷载试验模型在模拟横向抗弯刚度时会产生误差,影响最终的检验结果评判精度。本文以宽幅钢-混组合梁桥荷载试验为例,采用梁格法模型计算了横隔板与主梁交接处局部刚域对理论应变和挠度横向分布的影响,结合板单元+实体单元模型和现场实测数据进行对比分析,研究结果可为宽幅分离式主梁桥梁的荷载试验及有限元模型修正提供参考。

饱和多孔弹性深埋复合式衬砌抗震性能分析

目前国内外学者针对地震波扰动下隧道的衬砌动力响应解析解的研究多为单层衬砌,而实际工况中隧道一般包括初衬和二次衬砌。因此,研究复合式衬砌隧道的抗震性至关重要。以深埋复合式衬砌隧道为研究对象,建立了考虑尺寸效应和非均匀性结构效应的高阶应变梯度非局部孔隙弹性计算模型,用以描述多孔介质材料和结构的具体物理特性。通过引入2个长度参数、非局部参数及尺度因子来改进Biot理论。采用波函数展开法得到了复合式衬砌的解析解。考虑深埋复合式衬砌隧道的边界条件,确定复合式衬砌隧道周围的动力集中因子大小,分析了不同入射P波频率下,尺寸因子和非局部参数对衬砌动应力集中因子大小的影响,以及内外衬砌刚度和厚度对衬砌内外边界动应力集中因子(DSCF)值的影响。结果表明:高频下,非局部参数和尺寸因子对衬砌内DSCF值的影响较大,并且非局部参数对衬砌内DSCF值有削弱的影响,尺寸因子对衬砌内DSCF值有增强的影响;低频下外衬砌刚度越大,外衬砌内DSCF值越大,内衬砌内DSCF值越小,这说明在外层衬砌不被破坏的情况下,内层衬砌适当选择刚度小一点的材料有利于隧道衬砌的安全;低频下,衬砌厚度在合理范围内越大,衬砌内DSCF值越小。解析结果可为隧道复合衬砌设计提供参考。

基于单轴损伤的货车钩尾框多轴疲劳寿命分析

基于单轴应变疲劳分析 ,提出了比例加载多轴应力状态下的疲劳寿命计算方法。采用钩尾框 AAR载荷谱和我国实测载荷谱 ,对货车 13号车钩钩尾框进行了细致的有限元计算和多轴疲劳寿命分析 ,结果与实际相吻合。

相关参数对汽轮机低压叶片疲劳寿命的影响

根据汽轮机低压叶片所受载荷的特点,建立了一种基于三维有限元法和改进局部应力应变法的叶片疲劳寿命预测模型。该模型定量考虑了频率的分散性、激振力因子、阻尼、疲劳缺口系数、表面粗造度、腐蚀、表面硬化状态、以及微动磨损等参数对叶片疲劳寿命的影响。给出了计算模型中这些相关参数的选取范围和选取原则。并用实例说明了该模型的可用性,计算也表明叶片疲劳寿命对这些参数的变化很敏感。该研究为叶片的事故分析和设计提供了一个有效手段和工具。

应用局部应力-应变法计算联轴器膜片疲劳寿命

由于轴线间的角向不对中，联轴器旋转时膜片中产生交变应力，引起疲劳问题。使用中需要考虑其疲劳寿命。本文对六孔圆环形和束腰形膜片，利用有限元方法和薄板弯曲理论建立膜片应力计算模型。引入改进的局部应力－应变法，建立计算膜片联轴器膜片疲劳裂纹形成寿命的模型和方法。定量分析两种形式膜片的应力、附加载荷和疲劳寿命，最后，得出对膜片组设计有参考价值的结论。结论表明，对于设计合理的膜片，其疲劳寿命能满足机组工作要求。

膜片联轴器膜片应力计算及疲劳寿命分析

对六孔圆环形和束腰形膜片 ,利用有限元方法和薄板弯曲理论建立膜片应力计算模型。引入改进的局部应力 -应变法 ,建立计算膜片联轴器膜片疲劳裂纹形成寿命的模型和方法。定量分析两种形式膜片的应力、附加载荷的疲劳寿命 ,最后 。

应用局部应力-应变法估算机械疲劳寿命

对局部应力 -应变方法估算机械疲劳寿命的计算思路和具体步骤进行了阐述 ,并通过试验和计算实例进一步加以说明 .该方法在解决低周随机载荷下的疲劳寿命预计问题方面具有相当的优势 。

用局部应力应变法进行高周疲劳寿命预测的研究

概述局部应力应变法的基本原理和目前的使用方法 ,指出在计算中现行方法对低周疲劳有较好的寿命预测精度 ,但对高周疲劳寿命预测精度不高。这主要是因为没有考虑应力集中、表面加工状况、尺寸和环境介质的影响。在充分考虑四者之后 ,对应变寿命曲线的弹性分量进行修正 ,并推导出高周疲劳下的局部应力应变法修正公式。本方法计算简单、精度高。通过实例对传统局部应力应变法和文中提出的方法进行比较 ,得出本方法不仅适用于低周疲劳寿命分析 ,也可用于高周疲劳寿命计算。

扭转条件下过渡圆角半径及径比对圆轴疲劳寿命的影响

提出应用局部应力应变法计算疲劳寿命的原理.建立不同圆角半径和径比圆轴的有限元计算模型,利用ANSYS软件计算纯扭转条件下过渡圆角处的局部应力、应变;应用局部应力应变法计算对称循环载荷谱下的疲劳裂纹萌生寿命;绘制寿命随圆角半径和径比变化的曲线图.计算分析结果表明,疲劳裂纹的形成寿命随着过渡圆角半径的变大而增长,随着径比的增大而呈下降趋势,径比越大,过渡圆角半径越小,疲劳裂纹的形成寿命越短.

基于局部应力应变法估算高周疲劳寿命

以往用局部应力应变法计算结构高周疲劳寿命不好的主要原因是在损伤与寿命计算中没有考虑缺口应力梯度等因素的影响。文中利用疲劳缺口系数对应变 -寿命曲线的弹性分量进行修正 ,从而可以较好地反映缺口根部应力梯度等因素对疲劳寿命的影响 ,使得局部应力应变法不仅可适用于低周疲劳寿命分析 ,也可以用于高周疲劳寿命分析。文中给出的方法简单 ,精度较好 。