结构疲劳损伤演化过程的多尺度同步观测与分析

大型土木结构的失效劣化过程是一个跨尺度损伤演化过程,在相同的时间节点不同的空间尺度下,通常表现出独特的损伤演化响应特征和力学特性。该文以某大型桥梁纵向加劲钢桁架的缩尺模型为研究对象开展疲劳损伤演化过程多尺度同步观测试验工作,综合运用电测技术、光测技术和动态测试技术实时监测桁架疲劳损伤演化过程,据此分析对比桁架在细观和宏观尺度下的损伤演化响应特征和力学特性。在电测技术观测到桁架局部疲劳裂纹萌生之后,光测和动态测试技术得以继续追踪疲劳裂纹的扩展过程并评价局部损伤演化对构件以及结构整体力学特性的影响,结果表明:结构局部细观尺度上的损伤演化不至于影响结构宏观尺度的整体力学特性,但是局部损伤的累积必将导致结构失效劣化。     

三维离散元与壳体有限元耦合的时空多尺度方法

壳体结构的局部失效及其对整体结构稳定性影响涉及到跨宏细观多尺度力学问题。该文推导出元/网格动量传递的多尺度算法,建立了三维离散元与壳体有限元耦合的时空多尺度数值计算方法。通过激光辐照下充内压圆柱壳局部失效算例的数值模拟,验证该多尺度方法能够完善地并行实现时间多尺度与空间多尺度计算,不仅能够准确模拟壳体结构局部细观非均匀演化及其对整体结构的影响,而且计算时间很少,有效地发挥了时空多尺度模拟高效率优势。     

点蚀损伤下海洋平台结构剩余强度的多尺度分析方法

均匀腐蚀损伤影响结构承载力的研究已趋于成熟,但点蚀损伤影响整体结构承载力的规律及其局部损伤的演化行为仍不明确。该文采用均匀分布的圆柱体点蚀损伤模型,探讨在宏观结构尺度中点蚀损伤的描述、多尺度区域的划分和跨尺度界面的连接以及结构非线性的求解方法,提出了研究考虑细观尺度点蚀损伤的海洋平台结构多尺度模型的构建和计算方法。计算结果表明:该文提出的多尺度模型构建策略有效地解决了细观尺度点蚀损伤在宏观结构尺度中的描述问题,且所建多尺度模型能够体现点蚀损伤削弱结构极限承载力的规律,为评估在役海洋平台结构的剩余强度提供了可行的方法。     

复合材料多尺度分析方法与典型元件拉伸损伤模拟

复合材料具有多尺度特性,多尺度模拟方法能够考虑细观损伤、演变对宏观材料性能和力学行为的影响,是复合材料响应分析的一种重要方法和手段。基于多尺度渐进展开理论,对复合材料弹性问题控制方程进行尺度分解,推导了细观尺度与宏观尺度的控制方程,建立了复合材料宏观和细观尺度响应之间的关联。基于协同多尺度计算策略,利用通用有限元软件的用户子程序与脚本二次开发,在宏观模型计算中实时调用细观模型进行多尺度渐进损伤模拟,实现了宏观和细观尺度的信息传递与反馈。建立的复合材料多尺度数值模拟方法可以快速集成细观损伤模型以及宏观唯象强度理论,具有良好的通用性。碳/碳复合材料拉伸模拟算例结果与试验结果吻合较好。     

强载荷下结构动力响应与损伤的跨尺度分析

结构多尺度有限元模拟是考虑结构易损部位损伤演化进行大型结构失效过程分析的有效方法。如何方便、准确地建立有限元大尺度与小尺度模型的跨尺度界面连接是多尺度有限元模拟中的关键。本文根据损伤多尺度分析的需求,研究了两种跨尺度界面连接方法及其实施过程,并对计算方法的精度进行了比较和验证;将塑性损伤演化本构关系通过ABAQUS提供的UMAT子程序写入,考察狗骨式刚节点在强动载荷作用下其薄弱区域的损伤演化特性,结合跨尺度界面连接方法初步实现了强载荷下结构损伤的跨尺度分析;针对某大跨悬索桥钢箱梁纵向加劲钢桁架结构,采用本文建立的跨尺度分析方法研究了强载荷作用下结构中的损伤演化过程。研究结果为揭示结构在强载荷作用下的损伤破坏机理奠定了基础。     

基于双重网格的混凝土自适应宏细观协同有限元分析方法

混凝土损伤开裂表现出明显的跨尺度特征,其演化过程与细观材料结构直接相关。如何在兼顾效率和精度前提下分析混凝土损伤开裂的跨尺度演化过程一直是比较棘手的问题。在假定处于弹性阶段的混凝土为宏观均匀材料和处于损伤开裂阶段的混凝土为细观非均匀材料的基础上,提出了一种基于双重网格的混凝土自适应宏细观协同有限元分析方法。该方法通过在分析域内布置宏细观双重网格分别建立宏观尺度和细观尺度有限元计算模型,通过宏观尺度至细观尺度的自适应转换在分析过程中动态确定宏细观协同分析的宏观区域和细观区域,通过基于形函数插值的多点位移约束实现宏细观协同有限元模型中的非协调重叠网格连接。算例分析表明,采用该文方法不仅可通过考虑损伤开裂区的细观材料结构保证模拟精度,亦可通过在分析中自适应确定细观尺度分析区域提高模拟效率。     

复合材料跨尺度失效准则及其损伤演化

提出了一种新的基于物理失效模式的复合材料跨尺度失效准则,从细观层面分别对纤维和基体的失效模式进行了表征,将纤维失效分为拉伸失效和压缩失效,将基体失效分为膨胀失效和扭曲失效.建立了相应的失效准则及损伤演化方法.通过正方形和六边形的代表体积单元(RVE)模型,计算了宏观应力到细观应力的机械应力放大系数和热应力放大系数.以IM7/5250-4复合材料拉伸试验作为算例对失效模型进行了验证.计算结果与试验结果吻合较好,表明跨尺度失效准则能够准确预测复合材料层合板的破坏.     

复合材料跨尺度失效准则及其损伤演化

提出了一种新的基于物理失效模式的复合材料跨尺度失效准则, 从细观层面分别对纤维和基体的失效模式进行了表征, 将纤维失效分为拉伸失效和压缩失效, 将基体失效分为膨胀失效和扭曲失效。建立了相应的失效准则及损伤演化方法。通过正方形和六边形的代表体积单元(RVE)模型, 计算了宏观应力到细观应力的机械应力放大系数和热应力放大系数。以IM7/5250-4复合材料拉伸试验作为算例对失效模型进行了验证。计算结果与试验结果吻合较好, 表明跨尺度失效准则能够准确预测复合材料层合板的破坏。     

考虑微观界面的2D编织SiC/SiC复合材料宏-细-微多尺度渐进损伤失效分析

该文将微观界面组分纳入宏-细-微三个尺度的多尺度渐进损伤失效分析中.对细、微观单胞模型施加周期性边界条件获取放大因子并采用k-means聚类方法进行缩聚;通过缩聚的放大因子求解宏观模型对应的细、微观组分应力并进行损伤失效判定;计算宏观模型退化后的弹性参数用于后续计算;针对2D编织SiC/SiC复合材料开展了渐近损伤失效...     

焊接结构内部孔洞演化特征及其多尺度损伤表征

为了研究结构失效过程中焊接细节处的宏细观损伤演化过程,以含细观孔洞的焊接构件为对象进行拉伸试验,综合采用X-CT(X射线计算机断层扫描)和电测技术分别同步记录拉伸过程中构件内部细观结构和宏观力学性能的演化过程。试验结果发现随着构件变形的增加,其宏观性能变化表现为弹性模量的线性减小,同时其内部结构演化表现为新孔洞的萌生和原有孔洞的增长,孔洞总体积随塑性变形呈线性增加趋势,构件在不同的变形阶段,孔洞的体积与数量之间的关系都呈现出分形特征,分形维数随塑性变形的增加呈平缓增加趋势。根据构件宏观力学性能与其内部孔洞总体积之间的关系,并结合分形特征的意义,提出一种多尺度损伤表征方法,并用基于Lemaitre理论的损伤量化方法对其进行验证。结果表明新的损伤表征方法不但具有更加明确的物理意义,同时还可以很好地描述细观孔洞萌生、增长直到构件断裂的损伤跨尺度演化导致失效的过程。     

Multiscale modeling of progressive failure in polymer nanocomposites using nanoscale informed damage mechanics

In this article, a novel technique to model damage and damage evolution in polymer nanocomposites (PNCs) using the internal state variable approach is proposed. The multiscale aspects of the nanocomposite are captured by embedding local inhomogeneities and the localized nanoparticle (nanographene) and polymer atom interactions at the interface into a continuum scale damage model. This approach assumes that the damage evolution is primarily due to changes in nonbonded interactions at the nanoscale, hence the moniker nanoscale-informed damage mechanics (NIDM) model. The unknown coefficients in the NIDM model are obtained by a linear regression fit of the polymer and the PNC stress-strain behavior using molecular dynamics simulation. It is envisioned that the resulting nonlinear constitutive model can be readily incorporated into a hierarchical multiscale finite element model and used by the aerospace industry for structural scale applications to model progressive failure.     

Local/global model order reduction strategy for the simulation of quasi‐brittle fracture

This paper proposes a novel technique to reduce the computational burden associated with the simulation of localized failure. The proposed methodology affords the simulation of damage initiation and propagation while concentrating the computational effort where it is most needed, that is, in the localization zones. To do so, a local/global technique is devised where the global (slave) problem (far from the zones undergoing severe damage and cracking) is solved for in a reduced space computed by the classical proper orthogonal decomposition while the local (master) degrees of freedom (associated with the part of the structure where most of the damage is taking place) are fully resolved. Both domains are coupled through a local/global technique. This method circumvents the difficulties associated with model order reduction for the simulation of highly nonlinear mechanical failure and offers an alternative or complementary approach to the development of multiscale fracture simulators. Copyright © 2011 John Wiley & Sons, Ltd.     

A local thermomechanical model of friction for computing structures under extreme loading conditions

A multiscale thermomechanical model of friction is proposed for metallic interfaces submitted to extreme loading conditions with large sliding velocities ( [v] > 100 ms^?1) and high contact pressures (P > 1 GPa). This model decomposes the friction problem into a global multidimensional structural problem and a local interface subproblem. At global scale, the structure behavior is governed by an elastoplastic model in large strains, and thermal conduction is neglected. The local model describes the micrometric scale and the underlying thermomechanical mechanisms: mechanical hardening, frictional heating, and plastic work. Using dimensional and asymptotic analysis, the corresponding set of equations at local scale is simplified and reduces to a one‐dimensional quasistatic thermoelastoplastic friction model. It is solved locally at each global point by a finite differences subgrid model using a nonlinear time‐implicit solver. This solver is coupled to a time‐explicit Lagrangian hydrocode used at the global structural scale. The coupling strategy between the solvers is force‐based: the frictional stress is evaluated at the local scale by our multiscale friction model and transferred to the global model that can then predict local velocity corrections. This coupling strategy has been successfully tested on real‐life cases. Copyright © 2011 John Wiley & Sons, Ltd.     

超高层钢-混凝土混合结构地震损伤模型研究

以型钢混凝土框架-混凝土核心筒结构为研究对象,考虑不同类型构件在某典型超高层钢-混凝土混合结构中的地震损伤分布特点及损伤演化规律,建立了考虑不同类型构件重要性的钢-混凝土混合结构楼层地震损伤模型,并基于提出的楼层地震损伤模型,选取了位置权重系数和损伤值权重系数二者组合的形式,建立了能够反映钢-混凝土混合结构楼层损伤分布规律的整体结构损伤模型。并通过PERFORM-3D软件对一钢-混凝土混合结构进行了非线性地震反应计算,分析了该结构在地震作用下的损伤分布规律和损伤发展情况,利用本文提出的地震损伤模型对结构各层次的地震损伤指标进行了计算,验证了该损伤模型的合理性。     

Energy-based collapse assessment of concrete structures subjected to random damage evolutions

The assessment of structural capacity against collapse is conducive to the optimal design of new structures as well as checking the safety of existing structures. However, the evaluation cannot be typically carried out by means of destructive tests on prototype or reduced scale structures. In this regard, the numerical models that adequately represent the prototype structures can be alternatively used. Specifically, both the nonlinearities and randomness as well as their coupling effect of materials need to be represented in a unified manner in structural analysis. The present paper aims at providing an effective approach to incorporate the stochastic nature of damage constitutive relationships in collapse analysis and assessment of concrete structures subjected to earthquake ground motions. Within the framework of stochastic damage mechanics, the spatial variability of concrete is represented by a two-scale stationary random fields. The concept of covariance constraint is introduced to bridge the two-scale random fields such that the scale-of-fluctuation of the random material property is satisfied at both scales. Random damage evolution induced structural collapse analysis is achieved via the nonlinear stochastic finite element method. To address the randomness propagation across scales, the probability density evolution method is employed. By exerting the absorbing boundary condition associated with an energy-based collapse criterion on the generalized probability density evolution equation, the anti-collapse reliability of concrete structures can be evaluated with fair accuracy and efficiency. Numerical investigation regarding an actual high-rise reinforced concrete frame-shear wall structure indicates that the random damage evolution of concrete dramatically affects the structural nonlinear behaviors and even leads to entirely different collapse modes. The proposed method provides a systematic treatment of both uncertainties and nonlinearities in collapse assessment of complex concrete structures.     

基于子结构的Woodbury非线性分析方法

非线性分析是研究结构性能的重要手段,准确并高效的模拟非线性行为,对评估结构安全性具有重要意义。工程结构材料非线性行为一般仅发生在局部区域,考虑结构的局部非线性特征,往往能有效提高计算效率。Woodbury公式被应用于多种数值算法中来高效地求解局部材料非线性问题,该公式仅需对小规模的Schur补矩阵进行分解,避免了对整体刚度矩阵的分解运算。然而,Schur补矩阵通常不具备稀疏特征,且其阶数与非线性规模相关,因此,当非线性区域较大时,Woodbury公式的高效性受到限制。为此,该文提出了基于子结构的Woodbury非线性分析方法,该方法将Schur补矩阵分解为若干个子矩阵,大幅降低了非线性分析过程中Schur补矩阵的规模。最后将该方法应用于某钢框架结构的动力非线性分析,并从精度和效率两方面与传统Woodbury法做了对比;结果表明:该文方法在保证计算精度的前提下改善了Woodbury公式的计算性能,进一步拓宽了其适用范围。     

未知地震激励下结构恢复力及质量非参数化识别

地震作用过程中结构恢复力是描述结构损伤发生发展过程定量描述的基础,更有助于描述结构在地震中破坏模式的演化。该文研究地震激励下结构部分质量及动力响应信息未知时,结构非线性恢复力、质量以及未知加速度时程的非参数化识别方法。首先,根据部分已知动力响应测量和质量信息,识别地震加速度时程。随后,利用记忆衰退全局加权迭代扩展卡尔曼滤波算法,引入幂级数多项式表征结构恢复力,实现了结构质量与非线性恢复力的非参数化识别。将具有非线性恢复力的磁流变阻尼器中引入到一个剪切型多自由度结构构成非线性系统,考虑测量噪声的影响,通过数值模拟验证了在噪声及较大质量初始误差情况下该方法识别结构质量、非线性恢复力及地震动加速度时程的识别效果。     

Structural damage detection using improved particle swarm optimization

An approach based on an improved particle swarm optimization (PSO) algorithm is proposed for structural damage detection in this study. A disturbance is introduced in the evolution process to avoid the occurrence of premature. The present algorithm focuses on the mutation of global or individual best known positions to guide the swarm to escape from the local minimum. The feasibility and robustness of the modified PSO are verified by three different structures, including a beam, a truss and a plate. The results show that the method is efficient and effective for structural damage identification when measurement noise is considered.     

On a multiscale strategy and its optimization for the simulation of combined delamination and buckling

This paper investigates a computational strategy for studying the interactions between multiple through‐the‐width delaminations and global or local buckling in composite laminates taking into account possible contact between the delaminated surfaces. To achieve an accurate prediction of the quasi‐static response, a very refined discretization of the structure is required, leading to the resolution of very large and highly nonlinear numerical problems. In this paper, a nonlinear finite element formulation along with a parallel iterative scheme based on a multiscale domain decomposition is used for the computation of three‐dimensional mesoscale models. Previous works by the authors already dealt with the simulation of multiscale delamination assuming small perturbations. This paper presents the formulation used to include geometric nonlinearities into this existing multiscale framework and discusses the adaptations that need to be made to the iterative process to ensure the rapid convergence and the scalability of the method in the presence of buckling and delamination. These various adaptations are illustrated by simulations involving large numbers of DOFs. Copyright © 2012 John Wiley & Sons, Ltd.