Stiffness Degradation Modeling for Composite Wind Turbine Blades Based on Full-Scale Fatigue Testing

In order to provide more insights into the damage propagation composite wind turbine blades(blade)under cyclic fatigue loading,a stiffness degradation model for blade is proposed based on the full-scale fatigue testing of a blade.A novel non-linear fatigue damage accumulation model is proposed using the damage assessment theories of composite laminates for the first time.Then,a stiffness degradation model is established based on the correlation of fatigue damage and residual stiffness of the composite laminates.Finally,a stiffness degradation model for the blade is presented based on the full-scale fatigue testing.The scientific rationale of the proposed stiffness model of blade is verified by using full-scale fatigue test data of blade with a total length of 52.5 m.The results indicate that the proposed stiffness degradation model of the blade agrees well with the fatigue testing results of this blade.This work provides a basis for evaluating the fatigue damage and lifetime of blade under cyclic fatigue loading.

Stiffness Degradation of Undisturbed Saturated Soft Clay in the Yangtze Estuary Under Complex Stress Conditions

Stiffness degradation will occur due to the generation of accumulated pore pressure in saturated soft clays under cyclic loading. The soil static-dynamic multi-purpose triaxial and torsional shear apparatus in Dalian University of Technology was employed to perform different types of test on the saturated soft marine clay in the Yangtze Estuary. Undisturbed samples of the clay were subjected to undrained cyclic vertical and torsional coupling shear and cyclic torsional shear after three-directional anisotropic consolidation with different initial consolidation parameters. Investigated were the effects of the initial orientation angle of the major principal stress, initial ratio of deviatoric stress, initial coefficient of intermediate principal stress and continuous rotation of principal stress axes on the stiffness degradation. It is found that the degradation index decreases （or degradation degree increases） significantly with increasing initial orientation angle of the major principal stress and initial ratio of deviatoric stress. Compared with the effects of the initial orientation angle of the major principal stress and initial ratio of deviatoric stress, the effect of initial coefficient of intermediate principal stress is less evident and this trend is more clearly reflected by the results of the cyclic torsional shear tests than those of the cyclic coupling shear tests. At the same cycle number, the degradation index obtained from the cyclic torsional shear test is higher than that from the cyclic coupling shear test. The main reason is that the continuous rotation in principal stress directions during cyclic coupling shear damages the original structure of the soil more than the cyclic torsional shear does.Based on a series of experiments, a mathematical model for stiffness degradation is proposed and the relevant parameters are determined.

Stiffness degradation-based damage model for RC members and structures using fiber-beam elements

To meet the demand for an accurate and highly efficient damage model with a distinct physical meaning for performance-based earthquake engineering applications, a stiffness degradation-based damage model for reinforced concrete （RC） members and structures was developed using fiber beam-column elements. In this model, damage indices for concrete and steel fibers were defined by the degradation of the initial reloading modulus and the low-cycle fatigue law. Then, section, member, story and structure damage was evaluated by the degradation of the sectional bending stiffness, rod-end bending stiffness, story lateral stiffness and structure lateral stiffness, respectively. The damage model was realized in Matlab by reading in the outputs of OpenSees. The application of the damage model to RC columns and a RC frame indicates that the damage model is capable of accurately predicting the magnitude, position, and evolutionary process of damage, and estimating stow damage more precisely than inter-story drift. Additionally, the damage model establishes a close connection between damage indices at various levels without introducing weighting coefficients or force-displacement relationships. The development of the model has perfected the damage assessment function of OpenSees, laying a solid foundation for damage estimation at various levels of a large-scale structure subjected to seismic loading.

CONSTITUTIVE RELATION AND STIFFNESS DEGRADATION OF CRACKED ANISOTROPIC COMPOSITE LAMINATES(II)─DETERMINATION OF RESOLVED STIFFNESS AND INVESTIGATION OF STIFFNESS DEGRADATION FOR CRACKED LAMINATES

The study on properly degradation of damaged,.onlj,osile laminates is extendedto anisotropic laminates with matrix cracking. In (II) of the paper, a solution forpartitioned stiffness is given to complete the constitutive relations developed in (I) Thestiffness degradation in cracked laminates is calculated and the results arediscussed.

Simulation of Fatigue Stiffness Degradation in Prestressed Concrete Beams under Cyclic Loading

In order to investigate and research the fatigue cracking of prestressed concrete fatigue properties and loading and stiffness degeneration process,cyclic loading tests were carried out on six prestressed concrete beams and the stiffness degradation under fatigue was investigated. A simulation model of stiffness degradation is proposed based on the stiffness analysis of the fatigue-damaged section. The elastic modulus of damaged concrete and the effective residual area of steel were introduced as well as an adjusted three-stage concrete fatigue damage evolution model. The strip method was used to analyze concrete damage due to changing stress along the depth of the beam section. The simulation and test results were compared and a method of predicting fatigue deflection was presented based on the simulation model. The predicted results were compared with that of the neural network method. It is in good agreement for the simulation results with the test results. It is only less than5% error for the simulation model which can reveal the two-stage degradation of prestressed concrete beams under cyclic loading. It is more precise for the simulation prediction method under proper conditions.

Simplified stiffness-based approach for seismic performance evaluation of moment-resisting frame

Based on the concept of stiffness degradation, a damage index of the whole frame and the storey is proposed for the frame seismic performance evaluation. The index is compatible with the non-linear static analysis （e. g. the pushover analysis）, and the structural damage is considered via plastic hinges. Simultaneously, a practical approach is developed to obtain the relationships between the proposed index and earthquake intensities based on the capacity spectrum method. The proposed index is then illustrated through two low-rise reinforced concrete frames, and it is also compared with some other indices. The results indicate that the proposed index is on the safe side and not sensitive to the lateral load pattern. The storey index is helpful to reflect the storey damage and to uncover the position of the weak storey. Finally, the relationship between performance levels and damage index values is also proposed through statistical analysis for the performance-based seismic evaluation.

Development and verification of large deformation model considering stiffness deterioration and shear dilation effect in FLAC^(3D)

The existing constitutive models of rock with strain softening cannot successfully reflect the damageinduced anisotropy and nonlinearity of the post-peak failure behavior under progressive loading. In order to better reflect the nonlinear stress-strain behavior of rock, especially for the post-peak failure behavior,with expected stiffness degradation and large deformation, a modified constitutive model of rock considering stiffness degradation and dilatation behavior at large deformation was proposed in this study. This study analyzes and discusses various attenuation parameters in the proposed nonlinear plastic constitutive model using FLAC^(3D) software. The excavation-induced stress in rocks was calculated by FLAC^(3D) using the Mohr-Coulomb model, conventional strain model, and the proposed modified model. The obtained results of these models were analyzed and compared with field data. This study shows that the simulated results using the proposed new constitutive model matched much more closely with the measured field data, with an average error less than 5%. This new model can successfully reflect the damage-induced stiffness degradation at large deformation, and can provide a theoretical basis for stability design and evaluation of underground excavation.

含水率对红层泥岩填料刚度劣化特性及微观结构影响研究

为研究含水率对红层泥岩填料刚度劣化规律和微观结构的影响,开展了不同含水率下三轴不排水剪切试验、压汞试验和扫描电镜试验,基于能量守恒定律推导了填料刚度劣化与弹塑性应变能增量间的关系。结果表明:填料黏聚力随含水率增加而降低,内摩擦角则随含水率增加先增加后降低。填料结构损伤由应变能转化引起,从宏观上表现为刚度劣化。归一化刚度比值随塑性应变能增量占比增加而线性降低,随应力发展呈非线性衰减。以最优含水率为界,当试样在干侧时,填料有较好的荷载抵抗能力;湿侧填料荷载抵抗能力远低于干侧。损伤应力等值线与等含水率线构成上下界为对数曲线的状态面。填料孔隙分布在干侧为单峰形态,粒间接触较为紧密,结构稳定性好。在湿侧呈双峰结构,团聚体排列定向性程度较低。填料应在干侧压实填筑,推荐采用5%~7%作为填筑目标含水率。

基于双信号融合的主轴/刀柄结合面刚度退化程度预测

为了预测主轴/刀柄结合面刚度退化程度,提出了一种基于激励和响应信号融合的主轴/刀柄结合面刚度退化程度预测方法。首先进行钛合金矩形工件侧铣实验,采集瞬时铣削力信号和主轴/刀柄结合面附近的响应振动信号,构建反映主轴/刀柄结合面刚度退化的数据库。然后根据数据库中瞬时铣削力和振动信号各方向的时域、频域和时频域特征,基于相关性分析优选出瞬时铣削力信号和振动信号的时域均值、频域中心频率、时频域一阶小波包能量3个特征,分别使用低频滤波卷积核和高频滤波卷积核对优选后的特征矩阵进行双通道卷积池化处理,获取深度融合的主轴/刀柄结合面刚度退化程度特征向量。最后以支持向量机模型(SVM)的概率模式转化为朴素贝叶斯分类器(NBC)的条件概率,构建混合分类器模型(NBC-SVM),提高了分类器的分类性能。在主轴/刀柄结合面刚度退化数据库的基础上,基于双通道卷积池化的特征融合方法(CP-FF)和NBC-SVM模型实现了主轴/刀柄结合面刚度退化程度的预测,预测精度达96%。

双工字钢板组合梁桥的时变体系冗余性评估

为评估双工字钢板组合梁桥应对非对称极端作用的时变体系冗余性,采用数值增量算法,开展了双工字钢板组合梁桥非对称荷载下的弹塑性全过程分析.提出了基于弹塑性累积应变能的构件承载贡献率指标,研究了双工字钢板组合梁桥时变体系冗余性的影响因素及变化规律,揭示了双工字钢板组合梁桥冗余性不足的内在原因.结果表明:横向刚度和主梁延性是影响钢板组合梁桥体系冗余性的主要因素,横向传力刚度越大,主梁延性越高,则结构体系冗余性越强;与典型多主梁体系相比,双工字钢板组合梁桥的横向传力刚度降低约24%,导致结构冗余度降低约55%;材料劣化引起的主梁延性退化会导致冗余度在寿命周期最大下降约25%.

多元荷载作用下软基深水高桩码头结构承载特性研究

软基深水高桩码头结构受到恶劣外海环境荷载及复杂工作荷载等多元荷载的共同作用。研究表明,船舶撞击荷载是该结构水平向控制荷载,但波浪长期循环荷载作用会引起桩周土体软化,进而导致码头结构在其它荷载作用下承载特性的劣化。鉴于此,首先基于ABAQUS有限元软件建立了深水高桩码头结构-地基土体相互作用的三维弹塑性有限元模型;然后,借助USDFLD子程序实现了同时考虑土体强度弱化和刚度衰减的模拟,进而开展了未考虑土体软化、仅考虑土体强度弱化、仅考虑土体刚度衰减以及同时考虑土体强度弱化和刚度衰减对码头结构承载特性影响的对比分析。研究结果表明,与未考虑土体软化时撞击荷载作用下码头结构安全系数相比,仅考虑土体强度弱化时其值降低14.72%,仅考虑土体刚度衰减时其值降低15.28%,同时考虑土体强度弱化和刚度衰减时其值降低19.44%,且考虑土体软化后桩周土体的塑性应变范围明显增大,极限状态时桩身应力值减小,结构稳定性显著降低。

标准化Z字形钢结构梁柱连接节点转动刚度研究

为研究标准化Z字形钢结构梁柱连接节点的转动刚度,对翼缘焊接和搭接两种形式的Z字形钢结构梁柱连接节点进行了单调和往复循环加载试验及有限元分析。研究了不同连接节点设计强度对Z字形钢结构梁柱连接节点转动刚度和循环刚度退化等力学性能的影响。结果表明,翼缘焊接和搭接两种形式的Z字形钢结构梁柱连接节点均具有良好的延性和滞回性能。翼缘焊接Z形节点转动刚度表现为刚性连接,翼缘搭接Z字形钢结构节点转动刚度表现为铰接。对于翼缘焊接Z字形钢结构梁柱连接节点,其转动刚度和循环刚度退化程度受焊缝连接强度影响显著。较小的强度设计倍数(0.2~0.4)时节点破坏模式为焊缝率先发生破坏,其节点转动性能表现为半刚性。较大节点强度设计倍数(0.7~1.1)时节点破坏模式转变为构件率先发生破坏,节点表现为刚性连接且其循环刚度退化程度(转角小于0.02 rad条件下)明显小于强度设计倍数较小的Z字形钢结构节点循环刚度退化程度。总体上看,可通过改变节点强度设计倍数来调节其转动刚度。

往复压载下纤维增强混凝土力学行为退化及本构关系

为研究钢纤维混凝土在往复轴压荷载下的力学性能及其本构关系,开展单轴往复压缩试验,以不同纤维掺量和长径比为变量,探究钢纤维混凝土的力学行为退化及损伤演化规律。结果表明,与单调加载相比,往复加载作用下钢纤维混凝土试件强度有轻微退化;混凝土塑性应变累积随钢纤维体积掺量的增加而大幅降低,弹性刚度退化率随之增大,但对纤维长径比的影响不显著;应力退化率随卸载应变呈先减小后增大的变化趋势;基于力学行为退化现象和纤维增强效应,建立了钢纤维混凝土往复轴压下混凝土损伤演化模型,与试验结果进行比较发现该模型与试验数据吻合较好。

二级减速器故障系统建模及SVD-MMSE劣化评估

为检测故障齿轮劣化程度并进行有效的程度评估,通过有限元法建立含有正常、裂纹和断齿等3种齿轮状态的二级直齿轮减速器系统模型。首先,分别计算3种状态的齿轮时变啮合刚度,并综合考虑轴承支撑刚度,得到了3种不同状态下的轴承振动响应;其次,引入多元多尺度样本熵(multivariate multiscale sample entropy,简称MMSE)对故障齿轮的劣化程度进行分析;最后,引进奇异值分解(singular value decomposition,简称SVD)算法进行预处理,以达到更好的诊断效果来综合评定故障齿轮生命周期的劣化程度。结果表明:齿轮发生故障时,主要导致时频域信号发生转频调制,时域存在有规律的冲击,频域出现边频带,且分布在输入轴的转频及其倍频和啮频及其倍频处;随着故障程度的增加,劣化越发明显,频率成分也发生改变,致使MMSE值也随之变化,且整体呈单调递减趋势;SVD-MMSE算法能有效地对齿轮故障程度进行判别,降低了噪声对于劣化程度检测准确性的影响。

拉伸及弯曲载荷作用下风电叶片主梁面外褶皱缺陷的失效预测研究

叶片主梁褶皱严重危及风力机运行安全,该文提出一种基于连续损伤力学(CDM)模型来预测含不同高宽比的面外褶皱单向板的失效方法。首先,基于ABAQUS建立5种含不同高宽比的面外褶皱的5层单向板数值分析模型,通过编写USDFLD子程序并采用连续壳单元(SC8R)进行拉伸失效载荷预测和渐进损伤失效历程研究,并与试验对比,验证该文分析方法的精准性。进一步,基于AeroDyn 2 MW-45.3 m叶片主梁厚度分布,建立3种含不同高宽比的面外褶皱的15层大厚度单向板的数值分析模型,为了探究剪切效应对层合板性能的影响,编写VUMAT子程序并采用三维Hashin准则分别预测拉伸载荷和弯曲载荷作用下,褶皱对主梁单向板抗拉与抗弯性能的影响规律。研究结果表明:褶皱的高宽比对单向板承载能力影响较大,随着褶皱高宽比增大,叶片主梁的抗拉和抗弯性能逐渐降低,并受单向板厚度方向应变ε_(33)和面外应变ε_(13)的耦合作用影响,在高应变区域出现分层,显著降低单向板的刚度和强度。

T形PEC柱抗震性能的数值模拟分析

利用ABAQUS软件对14个T形PEC柱进行低周反复加载模拟,分析不同参数对T形PEC柱抗震性能的影响规律。结果表明:T形PEC柱的滞回曲线呈饱满的平行四边形,且刚度退化稳定,耗能能力和延性良好;提高混凝土强度等级虽然可增大柱的初始刚度,但混凝土强度等级过高则会导致柱的延性降低,刚度退化加剧,抗震性能降低;在T形PEC柱内设置横向拉杆,且将x方向和z方向的横向拉杆相互焊接,有利于对混凝土形成套箍作用从而提高柱的抗震性能,但拉杆间距不宜大于150 mm;在T形PEC柱的两侧阴角处分别设置1根纵筋并将其与横向拉杆绑定后形成钢筋笼,能够明显的改善柱的抗震性能。

基于裂缝形态的混凝土桥梁承载力退化

在役混凝土桥梁经过长时间的服役工作,其结构的完整性产生改变以及材料性能发生实质性退化,需要对桥梁结构进行评估分析。选取一座在役混凝土桥梁,通过有限元技术建立桥梁模型,采用离散模型对裂缝进行建模,做受弯模拟进行计算分析;根据受力裂缝类型以及分布位置,分别研究存在不同形态下的弯曲裂缝与剪切裂缝对混凝土桥梁结构的破坏行为以及承载力退化的影响。研究发现存在弯曲裂缝对桥梁结构的破坏行为影响较小,在不同裂缝高度与宽度下荷载-位移曲线变化较小,结构破坏前底部纵筋与开裂处箍筋受力达到屈服状态,裂缝分布均匀且从加载点向两端高度递减;剪切裂缝高度对桥梁结构的破坏行为有着显著的影响,在剪切裂缝高度较高时,荷载-位移曲线发生明显的下降,剪切裂缝宽度对桥梁结构承载力影响较小,不同宽度下荷载-位移曲线基本保持一致,结构破坏前底部纵筋并没有达到屈服状态,在剪切裂缝处开裂贯穿梁体导致结构破坏而梁体其余部分开裂高度相对较低。考虑到开裂桥梁结构产生的变形由两部分组成,由此引入结构刚度折减计算公式,依据相关标准推导出剩余承载力计算公式,并对选取桥梁进行承载力验算,偏差在5%之内。

疲劳荷载作用下钢-混组合梁界面滑移对其性能影响研究

为分析界面滑移对钢-混组合梁疲劳性能的影响,在考虑界面滑移影响的栓钉疲劳刚度退化模型基础上,提出考虑界面滑移损伤的钢-混组合梁疲劳性能分析方法,以某工字钢-混组合连续梁为背景开展缩尺模型试验,建立考虑界面滑移损伤的钢-混组合梁疲劳性能分析模型梁,对比分析疲劳加载过程中试验梁与模型梁的疲劳性能,并进行界面滑移系数影响因素的参数分析。结果表明:疲劳荷载作用下,钢-混组合梁中钢梁下翼缘发生开裂,并最终导致组合梁失效,加载过程中钢-混组合梁跨中挠度及界面残余滑移均呈现明显的三阶段特征;模型梁跨中挠度及界面残余滑移随加载次数的变化趋势分析结果与试验梁吻合良好,验证了考虑界面滑移损伤的钢-混组合梁疲劳性能分析方法的可行性;疲劳荷载作用下钢-混组合梁界面滑移将引起钢梁中应力增加,从而降低组合梁疲劳寿命,建议在疲劳寿命预测中引入界面滑移疲劳折减系数,建议其取值为1.2。

多自由度结构-串并联调谐质量阻尼器减震性能

针对提出的串并联调谐质量阻尼器(TTMD)减震系统,采用粒子群算法,在频域内对多自由度结构⁃TTMD系统进行优化分析。建立多自由度结构⁃TTMD系统减震控制仿真分析模型,分别考虑不同类型实际地震记录,在时域内研究了TTMD对结构地震响应的控制效率,并与相同总质量比的调谐质量阻尼器(TMD)进行比较。进一步考虑了结构刚度发生10%和30%退化的情况,分析了TTMD系统对刚度退化结构的减震效果。数值结果表明,TTMD系统减震性能和鲁棒性能优于TMD系统,且具有阻尼需求小、安装简单、易于实现等优势,是一种增强型减震系统。

拉挤工艺单向玻纤复合材料的疲劳衰减特性

拉挤工艺制备的玻璃纤维增强复合材料(GFRP)由于其良好的电气、力学性能被广泛运用于特高压输电工程,其中动态载荷作用下的性质研究是其应用的基础。文中通过开展沿纤维方向的静载实验和疲劳实验,建立了基于S-N曲线和分段线性等寿命图的疲劳寿命预测方法;根据不同应力比下刚度退化规律,提出了基于改进三角函数形式的损伤累积模型,准确描述了拉-拉和拉-压疲劳载荷下的材料非线性刚度退化规律;建立了剩余刚度-剩余强度关联模型,实现了疲劳载荷下强度性能衰减的准确预测。结果表明,GFRP在拉-拉和拉-压疲劳下具有显著不同的刚度退化规律,文中所提出的模型可准确预测两种条件下的剩余强度;而压-压疲劳载荷下,刚度退化曲线存在两种显著不同的模式;同时,剩余强度结果表明,前70%疲劳寿命内材料强度可能并不会发生明显退化。文中建立的针对拉挤GFRP在不同疲劳加载下剩余强度与剩余刚度的预测模型,为GFRP耐久性设计提供了指导。