土木结构损伤多尺度并发计算方法及其应用

大型土木结构的损伤破坏是跨尺度演化的结果, 因此单一尺度下的结构分析难以正确地反映结构的非线性损伤失效过程。该文根据结构损伤在宏观、细观尺度下的不同特征建立结构一致多尺度模型, 并通过多点约束法进行跨尺度关联, 实现了结构整体线弹性响应分析和局部细节易损部位的细观层次上弹塑性损伤分析的并发进行。计算结果表明:该文提出的结构损伤多尺度并发计算方法能够兼顾结构整体上的线弹性响应和局部易损部位在细观层次上的塑性损伤特征, 在对结构多尺度响应与损伤特征进行准确描述的基础上, 能够获得结构易损局部的细观损伤状态、演化过程及其对结构宏观响应与失效的影响。     

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。     

化学侵蚀后砂浆力学特性的劣化及其细观结构损伤定量化方法

研究了在不同水化学环境下砂浆试样力学特性劣化及其微细观结构损伤的演化,并探讨其力学特性劣化的细观机制。结果表明,化学腐蚀后试样峰值前的塑性变形有所增大。溶液对试样的化学腐蚀作用越强其塑性变形越大,并具有明显的时间阶段性。腐蚀后试样塑性变形的变化,间接地反映了化学溶液对砂浆试样的腐蚀程度。提出无损量测方法计算不同腐蚀时间段内砂浆试样的孔隙率,并基于其孔隙率的变化建立了新的损伤变量。研究发现,砂浆试样的化学损伤程度与其物理力学参数之间的一致性比较明显。这说明,用基于化学腐蚀产生的次生孔隙率建立的损伤变量定量描述试样微细观结构的化学损伤程度及其物理力学特性随化学损伤的演化过程,是合理的。     

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。     

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。     

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。     

Si3N4结合SiC耐火陶瓷裂纹萌生细观数值分析

将复合球模型导入损伤力学模型中,模拟了材料在高温环境下受拉和受压损伤时的细观演化过程;推导了材料损伤参数与界面相参数之间的函数关系。研究表明,当外加载荷小于材料损伤临界值、介于材料损伤临界值和最大承载值之间时,不论材料承受拉载还是压载,都存在无损伤演化的线弹性阶段和微裂纹扩展区逐渐增大的损伤强化阶段;若应力达到最大承载值后仍继续增大,材料损伤将过渡到损伤局部化阶段,损伤局部化的连续也就是材料宏观裂纹萌生的开始。     

冲击荷载作用下混凝土动态本构模型的研究

基于混凝土冲击荷载作用下的实验研究,以修正Ottosen四参数破坏准则为屈服法则,引入损伤,构造了一个动态本构模型用于描述混凝土材料的冲击特性。宏观上,假设混凝土材料是一个均匀连续体;而从细观角度来看,混凝土材料内部存在大量随机分布的微裂纹损伤。假设微裂纹均匀分布,且符合理想微裂纹体系统条件,定义含裂纹材料中单位体积内微裂纹所占的比例来表征微裂纹损伤所引起的混凝土材料宏观力学性能的劣化,并给出了损伤的演化方程。通过模型计算模拟结果与实验结果比较发现,模拟曲线与实验曲线拟合良好,因而可以用该模型模拟混凝土材料在冲击荷载下的动态特性。     

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。     

冲击压缩下准脆性材料含微裂纹损伤的本构模型

基于准脆性材料中翼型拉伸裂纹的成核准则,运用细观损伤理论推导了翼型裂纹损伤对材料弹性模量的弱化作用.考虑裂纹扩展对材料动态断裂的滞后效应,建立了动态裂纹扩展准则,并给出损伤演化方程,在此基础上建立了准脆性材料单轴冲击压缩下的动态损伤本构模型.结合氧化铝陶瓷材料独特的力学响应和破坏特性,讨论了模型中微裂纹成核参数、微裂纹尺寸对动态断裂强度的影响,并用该模型计算了单轴压缩下氧化铝陶瓷的应力应变曲线,数值结果与实验结果吻合良好.     

Multi‐scale experimental study on fatigue damage behaviour and its effect on structural nonlinear response

Experimental analyses on the structural response caused by local fatigue damage accumulation in welded details are accomplished to perform failure process and nonlinear effect analysis at different structural levels. The experiment is carried out by using welded compact tension (CT) specimens and a scaled truss specimen, and all of them have a notch at the weld toe to facilitate damage initiation. Cyclic loads are applied to those specimens to generate accumulative fatigue damage, respectively. The process of fatigue accumulation including initiation and propagation of fatigue cracks in the welded detail and resultant structural responses of CT specimens and the truss are measured with integration of multiple testing techniques. Multi‐scale experimental results show that microscopic‐/mesoscopic‐concentrated strain and extension of plastic zone in the vicinity of notch tip are both affected significantly by the fatigue damage accumulation and present appreciable nonlinear behaviour; however, the macroscopic response such as the frequency and stiffness parameters of the welded truss specimen are less sensitive to the low‐level fatigue damage. It is concluded that the fatigue failure of the welded truss is a multi‐scale progressive process due to fatigue damage trans‐scale evolving, in which the local meso‐damage firstly affects local strain of plastic zone in the vicinity of the notch tip, and then fatigue damage evolving from meso‐ to macro‐scale affects nonlinear responses of the damaged components; lastly, the fatigue failure could be expected as the results of the propagation of macroscopic fatigue cracks.     

A high-cycle fatigue accumulation model based on electrical resistance for structural steels

For high-cycle fatigue of metals, the DC electrical resistance is a more sensitive parameter to the initiation of micro-cracks during the irreversible fatigue damage accumulation process. This implies that the electrical resistance is a suitable parameter that can be consistent with the fatigue damage physical mechanism. The relation between the ratio of electrical resistance changes and the cyclic fraction of the fatigue specimen may reasonably represent deterioration in mechanical properties of structural steels during the high-cycle fatigue process. The high-cycle fatigue damage accumulation model based on electrical resistance for structural steels was proposed. The model was verified by some experimental data for three structural steels; normalized 45C steel, 20 Mn steel and 16 Mn steel, and good agreement was obtained. The corresponding fatigue lifetime on the basis of the electrical resistance model was also performed. The results show that the approach to fatigue lifetime prediction and failure based on the electrical resistance is a good non-destructive technique.     

An inverse analysis approach to determine fatigue performance of bituminous mixes

In pavement engineering, fatigue resistance is evaluated using different tests protocols and different specimen geometries. The dependency of the specimen shape geometry on fatigue performance does not allow the evolution of intrinsic material properties. This paper deals with the calibration of intrinsic fatigue damage parameters for bituminous materials. A fatigue damage model is implemented. The decrease of stiffness of the specimen during fatigue tests for different laboratory testing conditions is calculated from finite element computations. An inverse optimization technique is used in order to adjust the fatigue damage parameters on bending fatigue tests. A Levenberg-Marquardt algorithm is implemented to fit the finite element specimen global response on experimental results. An application on bending laboratory fatigue tests is presented to illustrate the applicability of the method for pavement engineering.     

Hygrothermal ageing effects on the static fatigue of glass/epoxy composites

A study of the effects of water ageing on the static fatigue behaviour of unidirectional glass/epoxy composites is presented. The failure mechanisms associated with fatigue damage were investigated under three-point bending loading. Depending on the ageing temperature, two failure features were identified: either fibre microbuckling on the compressive side of the specimen, or progressive cracking on the tensile side. Microbuckling has been found to be related to the reversible plasticization of the epoxy matrix, as measured by dynamic mechanical thermal analysis. On the other hand, tensile failure was associated with an irreversible weakening of the fibres and interfaces at elevated ageing temperatures. Some similarity is identified between damage processes in static and dynamic bending fatigue.     

Experimental study on effects of freeze‐thaw fatigue damage on the cracking behaviors of sandstone containing two unparallel fissures

This paper investigates the effects of freeze‐thaw (FT) fatigue damage on the cracking behaviors of sandstone specimens containing two unparallel fissures under uniaxial compression. First, the effects of FT fatigue damage and fissure angle on the mechanical properties of sandstone specimens are analyzed. Second, the real‐time cracking process of sandstone specimens is captured by a high‐speed digital video camera system. Seven crack coalescence patterns are observed in this experiment. Local strong fatigue‐damaged zones, which are visualized as white zones, are observed in the specimens subjected to FT cycles during loading. Finally, scanning electron microscopy (SEM) observations show that the local strong fatigue‐damaged zones mainly consisted of microcracks and micropores induced by the FT fatigue damage. These experimental results are helpful for improving the understanding of the cracking process in cold‐region engineering.     

Fatigue damage analysis based on energy parameters in reinforced polyamide

This article proposes an analysis method for the identification of the damage caused in a reinforced polyamide fatigue process through the evolution of the generated energy per cycle. Monotonous fatigue tests have been carried out at different stress levels using Polyamide 6 reinforced with short fibre glass tensile specimens. The loss of mechanical properties is measured using a damage parameter, defined as the variation in energy per cycle with respect to the initial conditions. Three clearly differentiated zones can be distinguished in the evolution of damage as a function of the energy per cycle. The first zone is one of deaccelerated rapid growth; in the second, the damage grows linearly, so that the damage growth speed can be determined as a function of the potential of variation of the stress applied; and finally, the third is a zone of accelerated growth of the damage until fracture.     

Micro- and Macroscale Damage Detection Using the Nonlinear Acoustic Vibro-Modulation Technique

Subjected to in-service and environmental loads, even relatively new structural components may reveal signs of microscopic deterioration. Very often, this initial damage further progresses into meso- and macroscales leading to development of one or several macrocracks that cause ultimate structural failure. Although the onset of macroscale cracking can be reliably detected by modern NDE methodologies, there is an increasing need for inspection technologies that may allow for assessing structural damage at a wide range of scales, i.e., from micro to macro. This article explores application of the nonlinear acoustic vibro-modulation technique (VMT) to incipient damage detection and monitoring. The nonlinear acoustic detection of the macroscopic damage is illustrated with examples: inspection of the cast aluminum automotive parts and testing of the aging aircraft fuselage. The microscale damage assessment is realized by real-time monitoring of the acoustic nonlinearity in the strain controlled three-point-bending fatigue test. In the experiment, a stable increase of the nonlinear response during specimen fatigue was observed indicating early damage accumulation before the macroscopic fracture.     

A fatigue damage indicator parameter for P91 chromium‐molybdenum alloy steel and fatigue pre‐damaged P54T carbon steel

Two grades of structural steel were subjected to fully reversible, constant stress amplitude cyclic loading. The local strain response of the material was measured and recorded during the test, with the applied testing technique enabling the monitoring of hysteresis loop variation for the narrowest cross‐section of the hourglass specimen. Changes in hysteresis loop width, representing the local inelastic response of the material, were recorded in order to monitor the density of structural imperfections. Material ratcheting behaviour was observed as changes in the mean strain for selected load cycles. Ratcheting was attributed to local deformation of the material in the vicinity of imperfections such as voids or inclusions, as well as deformation induced by the propagation of microcracks. Definitions of a damage indicator parameter and damage parameter were proposed. The fatigue behaviour of the two investigated grades of steel was finally illustrated in the form of damage curves for different stress amplitudes and for undamaged and fatigue pre‐damaged material.     

基于实测频响函数主成分的在役网架损伤识别方法

鉴于从实际网架动测中得到的频响函数已受到噪声的污染,会使模态分析出现较大的误差,提出了基于实测频响函数和主成分分析的网架损伤识别方法:用网架实测的频响函数数据作为损伤识别的基本变量,建立损伤识别矩阵,通过主成分分析和变量重构对频响函数进行减消噪处理,利用重构的频响函数前几阶主成分,在低维空间中对损伤信息进行分析、提取,并通过多元控制图,来识别网架的损伤。该方法不需要模态参数,避开了模态参数误差所引起的损伤识别不准问题,不需建立力学模型,对网架边界约束条件和结构型式没有特别的限制。为了验证该方法的可靠性,在试验室完成了足尺网架模型在不同损伤情况下动测试验。结果表明,所提出的损伤识别方法简便可行,结果可靠,尤其对噪声环境下和具有一定非线性网架的损伤识别有良好的适应性。     

The effects of texture and grain morphology on the fracture toughness and fatigue crack growth resistance of nanocrystalline platinum films

The fracture toughness and fatigue crack growth resistance of nanocrystalline materials are significantly affected by the thickness of the specimen. In this work we relate the mechanical properties of nanocrystalline platinum films to their texture and grain morphology. Tensile, creep and fatigue testing of annealed, ∼1 μm films resulted in mechanical properties similar to the as-received films (yield strength of ∼1.2 GPa, fracture toughness ∼17.8 MPa √m, and a fatigue crack growth power law exponent of ∼4.2). However, the breakdown of the initially columnar grain morphology had a marked effect on the transition point from an intergranular to transgranular fatigue cracking mode. Finite element modeling suggests that cyclic (fatigue) grain coarsening and the transition from inter- to transgranular cracking modes are a result of the relative importance of dislocation slip accommodation on in-plane and through-thickness oriented slip directions.