不同频率激励下LY12铝合金悬臂梁的疲劳特性

为了研究激励频率对悬臂梁结构疲劳特性的影响,本文针对LY12铝合金悬臂梁在不同频率(139Hz、141Hz、143Hz)激励下进行了同一应力水平的振动疲劳试验。结果表明:该结构的疲劳特性在不同频率激励下存在差异。在激励频率高于固有频率振动时,结构疲劳寿命最长;激励频率低于固有频率振动时,结构疲劳寿命较短;在固有频率振动时,结构疲劳寿命最短。     

考虑温度影响的饱和软黏土应变软化研究

循环荷载作用下饱和软黏土将产生应变软化现象,通过对宁波饱和软黏土开展温控动三轴试验,研究了温度、频率、初始偏应力、动应力和含水率对土体应变软化的影响,并在试验基础上建立了一个能够考虑温度影响的应变软化模型。结果表明:随着振动次数、初始偏应力、动应力和含水率的增加,土体软化加快,软化指数逐渐减小;随着频率和温度的提高,土体软化程度降低,软化指数增大;建立的软化模型可以较为合理地描述试验温度、频率、初始偏应力、动应力和含水率对土体应变软化特性的影响。     

疲劳振动效应对瓦楞纸板承载能力与缓冲性能的影响

瓦楞纸板作为产品运输包装的承载与缓冲基础材料,其使用过程中的性能保持性至关重要。运输振动将引起纸板疲劳破损,导致其相应性能的变化。对单瓦楞纸板进行加速度为0.5－2.0g、振动频率为20Hz的疲劳振动试验;进行瓦楞纸板的准静态压缩、跌落冲击试验,测定其承载能力与冲击加速度。结果表明,随着振动次数、振动强度的增加,纸板的承载能力、剩余屈服应力显著降低,缓冲性能下降。在此基础上,基于瓦楞纸板冲击试验数据,分析不同振动次数对瓦楞纸板的冲击最大加速度和缓冲系数的影响,获得了瓦楞纸板冲击最大加速度—静应力—振动次数、缓冲系数—最大应力—振动次数三维关系。研究表明瓦楞纸板存在疲劳破坏效应,研究结果为瓦楞纸板的疲劳缓冲包装设计提供技术依据     

基于模态频率的缺口梁疲劳裂纹扩展寿命预测

从试验出发,研究了含V型缺口悬臂梁在循环载荷作用下的疲劳裂纹扩展特性及其模态频率变化规律,分析了模态频率与裂纹扩展增量间的关联性。将裂纹扩展增量作为损伤参量,建立了基于模态频率下降率与损伤参量的关系。基于损伤力学,建立了裂纹损伤与循环加载次数的演化模型。结合模态频率下降率与裂纹损伤参量的关系,提出了一种基于模态频率下降率的缺口梁疲劳裂纹扩展寿命预测方法,实现了基于当前裂纹损伤和对应循环次数的疲劳裂纹扩展剩余寿命预测。结果表明,模态频率下降率对缺口梁的疲劳裂纹扩展寿命敏感,该方法预测的疲劳裂纹扩展寿命与实测的疲劳裂纹扩展寿命基本吻合。     

铝合金薄板的铣削动力响应特性仿真与实验研究

铣削加工时铣削力周期性变化,引起的工件动态应力和应变对残余应力及振动有重要影响,明确铣削过程中的动力响应规律可以有效调控残余应力的产生与再分布。以铝合金工件为例,基于ABAQUS仿真软件建立了三维动态铣削仿真模型,获得了不同铣削时刻工件应力场的分布规律,分析了铣削不同位置时工件的动力响应特性。通过铣削实验测量了工件的动态应变和铣削力,并分析了加载频率对动力响应的影响。结果表明:铣削加工边缘时更易出现应力波峰和应力集中现象;不同加载频率下应变的波动频率恒定为267 Hz,应变波动的最大范围为40με;工件的动力响应与铣削位置紧密相关。     

建筑物对爆破振动中不同频率能量成分的响应特征

爆破地震对建筑物的影响实质上是一种能量的传递与转化过程,这种传递与转化的过程受到建筑物动力特性的影响。利用结构动力学理论,结合典型工程实例,研究了建筑物对爆破地震中不同频率能量成分的响应特征,结果表明:建(构)筑物对于爆破振动中的不同频率能量成分存在明显的选择放大效应。在爆破地震中,与结构固有频率相一致的能量成分将被最大程度的放大。在制定爆破振动安全判据时,要将爆破地震自身特征和建筑物动力特性更好的结合起来,重视爆破振动中与建筑物固有频率相对应的能量大小。     

轴流式压缩机叶片固有频率及动应力分析

以某轴流式压缩机的一级动叶片为研究对象,建立叶片的三维有限元模型,研究轴流式压缩机叶片的频率特性。从理论方面分析了影响频率的旋转软化和应力刚化效应,分析了旋转软化和应力刚化效应对叶片固有频率的影响。应用完全法瞬态分析,计算了叶片的动应力,分析结果为叶片的调频和振动特性的改善提供了依据。     

形状记忆合金梁的建模及混沌阈值计算

从形状记忆合金（SMA）的等应变拉压实验数据出发,利用van-der-pol环模型模拟了形状记忆合金在加载和卸载过程中的应力应变迟滞环特性。并根据弹性理论和Galerkin方法建立了形状记忆合金简支梁在受轴向激励时的振动模型。随后得出了自由振动系统的分岔特性。在利用待定固有频率法研究了模型的非线性参数对系统固有频率的影响后,根据待定固有频率法的计算结果和时间尺度变化提出了系统Melnikov函数的改进表达式,提高了计算形状记忆合金梁模型在参数激励下发生混沌的阈值的精度。数值模拟的结果证明了该途径的有效性。     

RV减速器振动特性分析

为了研究RV减速器的振动特性,建立了RV-20E型RV减速器的刚柔耦合动力学虚拟样机模型,利用灰色关联度分析了样机的准确性。之后通过模态分析,分析了RV减速器的整机模态频率、振型等。进一步使用有限元方法对RV减速器进行了瞬态动力学分析,得到了不同工况下的仿真信号,并与试验测得的不同转速、负载下RV减速器的加速度振动信号进行对比,仿真分析结果与试验结果吻合度较高。分析结果表明负载对RV减速器振动的影响较小,而转速对于RV减速器的振动有明显影响。     

考虑温度影响的软黏土长期动孔压模型研究

以宁波轨道交通④层淤泥质黏土为对象,开展了不同温度、动应力、初始偏应力、围压作用下的动三轴试验,获得了不同试验条件对孔压的影响规律。在此基础上,对现有的孔压-振动次数双曲线模型进行改进,建立了考虑温度影响的动孔压-振动次数模型;其次,利用孔压试验结果对模型进行验证,并给出了不同试验工况下的模型参数;最后,基于动态平衡假设,建立了长期动荷载作用下考虑温度影响的归一化孔压预测模型,模型预测值与试验结果吻合性较好,可以为轨道交通设计和施工提供理论依据。     

A small cantilever beam test for determination of creep properties of materials

The application of small specimen creep test techniques in the evaluation of creep properties of materials in‐service has been increasing. To obtain the creep data accurately and conveniently, a new creep test method with small cantilever beam specimens is proposed. Analytical equations are derived that can convert the load to equivalent uniaxial stress and the displacement rate to equivalent uniaxial strain rate. Three types of the cantilever beam specimens are designed. The optimal configuration of the cantilever beam specimens is recommended with the aid of finite element method, which is further validated by the cantilever beam and uniaxial specimen tests. The results show that parameters obtained from the cantilever beam tests correspond reasonably well with those from uniaxial tests at low stress levels. With a relatively large equivalent gauge length, the cantilever beam specimen allows the small creep strain rate data obtained with a high accuracy.     

The size-dependent natural frequency of Bernoulli-Euler micro-beams

The dynamic problems of Bernoulli-Euler beams are solved analytically on the basis of modified couple stress theory. The governing equations of equilibrium, initial conditions and boundary conditions are obtained by a combination of the basic equations of modified couple stress theory and Hamilton’s principle. Two boundary value problems (one for simply supported beam and another for cantilever beam) are solved and the size effect on the beam’s natural frequencies for two kinds of boundary conditions are assessed. It is found that the natural frequencies of the beams predicted by the new model are size-dependent. The difference between the natural frequencies predicted by the newly established model and classical beam model is very significant when the ratio of characteristic sizes to internal material length scale parameter is approximately equal to one, but is diminishing with the increase of the ratio.     

时变AR模型的一种定阶方法

自回归（AR）模型的定阶对其分析结果的准确性、效率和可靠性具有重要影响。对时变AR模型的定阶问题进行了深入研究,提出了一种基于采样频率以及分析频率之比的定阶方法。对一质量随时间变化的悬臂梁进行了有限元建模,运用计算所得到的振动响应建立了梁的时变AR模型,利用所提方法进行了该模型的定阶,采用递推最小二乘法对梁的模态频率进行了估计。对一质量时变的悬臂梁进行了试验,对采集的振动响应信号建立了时变AR模型并识别了其振动模态频率。模拟计算和试验结果都表明,提出的时变AR模型定阶方法是有效且可行的,且识别算法具有一定的抗噪性。     

The effects of film thickness variations on the residual stress distributions in coated Cr thin films

Generally, the residual stress of thin film coatings is calculated using Stoney's equation. However, variables in the manufacturing of the coated film, such as crystalline particle size and the unevenness of the thickness of the film, cause the radius of curvature of the beam to vary all over the beam. The cantilever beam curves not only in the axial direction but also in the transverse direction. Therefore, the residual stress in a film coating comprises not only axial residual stress but also transverse residual stress, and its distribution is also not uniform. Under such conditions, Stoney's equation must be modified. In this study, Si was used as a substrate in the production of cantilever beam specimens. Chromium thin films of various thicknesses were coated onto the Si substrates. The 3D digital image correlation technique was used to measure the out‐of‐plane displacement of the specimens at various positions. Then the modified Stoney's equation was used to obtain the axial and transverse residual stress at each measurement point to study the effect of variations in the thickness of the thin film on the magnitude and uniformity of the distribution of the residual stresses. Three thin film thicknesses 1, 2, and 3 μm were studied, and three specimens for each thickness were used. For each specimen, axial and transverse residual stresses were obtained at nine test points, and the equivalent residual stress was calculated. The results of this study reveal that as the difference between the thicknesses of the coating increased, average equivalent residual stress decreased and the distribution of stresses became more uniform. By comparing the corresponding results for the 1‐ and 3‐μm‐thick films revealed that the confidence levels in the average value and uniformity of the equivalent residual stress distribution, which increased with thickness, were 92.81% and 80.57%, respectively.     

含电流变体砂浆复合梁结构振动固有频率的理论研究

以修正的RL线弹簧模型为基础，通过定义等效裂纹长度，建立了含电流变体的砂浆复合悬臂梁的振动模型，并得到理论计算复合梁频率的特征方程。根据实验数据确定模型的参数，并编制MATLAB程序来模拟含电流变体的砂浆悬臂梁随电场强度变化时自振频率的变化规律。研究结果表明，该模型的模拟值与实验值吻合得较好，尤其对一阶频率的模拟效果最好。本项研究为电流变体在实际工程结构振动控制领域的应用奠定了实验和理论基础。     

On the stability of a microbeam conveying fluid considering modified couple stress theory

In this paper, the size-dependent vibrational behavior of a microbeam conveying fluid was investigated using the Modified Couple Stress Theory. For cantilever and clamped-clamped microbeams, the small amplitude vibration equation of the micro-beams was solved using a Galerkin based reduced order model and the effects of material length-scale parameter on its natural frequencies were evaluated. It was found that for the both cantilever and clamped-clamped conditions, the critical fluid velocities predicted by the modified couple stress theory are higher than those predicted by the classical beam theory. In addition, the differences between the eigen-frequencies and the critical fluid velocities predicted by the modified couple stress theory and classical beam theory depends on the ratio of the material length-scale parameter to the beam height. In addition an unexpected result in the difference between the first eigen-frequency of the cantilever micro-beam obtained by the classical and the modified couple stress theory has been achieved.     

逆虚拟激励法随机载荷识别试验研究

采用逆虚拟激励法进行载荷识别,分别在阻尼比不同的钢和有机玻璃悬臂梁上进行了多次随机激励下的载荷识别试验。此方法通过采用逆虚拟激励法和奇异值分解法的结合在一定程度上克服了频响函数在近固有频率附近的病态,并减少了计算工作量,同时以相干函数和频响函数为依据来调整试验中激振杆等的安装状态,经验证取得了较好的识别结果。研究表明,逆虚拟激励法在工程实践中有广阔的应用前景。     

A phenomenological analysis of Mode I fracture of adhesively-bonded pultruded GFRP joints

The fracture behavior of adhesively-bonded pultruded joints was experimentally investigated under Mode I loading using double cantilever beam specimens. The pultruded adherends comprised two mat layers on each side with a roving layer in the middle. An epoxy adhesive was used to form the double cantilever beam specimen. The pre-crack was introduced in three different depths in the adherend in order to induce crack initiation and propagation between different layers and thus investigate the effect of these different crack paths on the strain energy release rate. Short-fiber and roving bridging increased the fracture resistance during crack propagation. Specific levels of critical strain energy release rates could be attributed to each of the crack paths, with their levels depending on the amount of short-fiber bridging and the presence of a roving bridge. The different levels of critical strain energy release rate could be correlated to the morphology of the fracture surface and the strain energy release rate can thus be determined visually without any measurement.