共查询到20条相似文献,搜索用时 31 毫秒
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采用波动分析法研究了纵向弹性波在两半无限梁耦合点的传播。应用梁的一维波动方程,耦合点模拟为质量弹簧系统,根据耦合点的连续条件和平衡条件,计算得到了纵向波传输至耦合点的振动功率传播效率和反射效率。传播效率和反射效率确定为梁纵向波阻抗和耦合点阻抗的函数,分析了梁纵向波阻抗和耦合点阻抗对纵向波传播效率和反射效率的影响。研究表明通过合理的选择激励梁、耦合梁和耦合点阻抗之间的匹配关系,可有效控制耦合点的纵向波传播。本研究对于振动控制和结构优化设计具有重要的意义。 相似文献
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Nondestructive measurement of concrete strength gain by an ultrasonic wave reflection method 总被引:3,自引:1,他引:2
A one-sided, nondestructive, ultrasonic technique for monitoring the setting and hardening process of concrete has recently been developed. The technique is based on monitoring the reflection coefficient of ultrasonic transverse waves at the surface of a laboratory scale hardening concrete in steel molds. The technique has been shown to reliably estimate the rate of strength gain of concrete under isothermal and outdoor conditions. Results of an experimental study to investigate the influence of curing temperature and mix design on the rate of strength gain and the ultrasonic transverse wave reflection loss are presented in this paper. Simultaneous measurements of wave reflection loss and compressive strength have been performed on various concrete compositions under different curing conditions. The ultrasonic technique is shown to produce reliable estimates of the rate of strength gain at early ages. 相似文献
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J. J. Leahy J. A. G. Drake C. Birkinshaw I. L. Jamieson 《Journal of Materials Science》1990,25(8):3688-3692
Bitumen derived from peat was blended with petroleum bitumen and subjected to laboratory evaluation for use as road binder material. Standard empirical procedures used included penetration, softening point and Fraass brittle point determinations. Rheological behaviour has been assessed at low shear rate using a sliding plate microviscometer and at high shear rate using a cone and plate instrument. Thermal analysis techniques including calorimetry and dielectric thermal analysis have been used to obtain fundamental structural information. Properties have also been measured after accelerated ageing. Peat bitumen is shown to differ significantly from petroleum bitumen in having a relatively high (30%) crystal content; however, as crystal melting commences below 20 ° C, the initial effect obtained in blending with petroleum bitumen is that of a plasticizer, reducing blend viscosity and increasing penetration. Crystallinity in the blends is lower than expected and this is ascribed to diffusional control of the crystallization process. Peat bitumens show a marked propensity to harden on heating, presumably due to condensation reactions increasing molecular weight, and this more than compensates for the plasticizing effect. The results suggest that age hardening will limit technically useful blends to a maximum of 17% wt/wt peat bitumen. 相似文献
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据Hamilton线性理论解,给出沿椭球面镜轴线的电弧放电等离子声源(AD-PSS)反射波计算结果,分析由镜面引起的相位变化对中心波、边缘波及尾波传播影响。在聚焦前区中心波压力为正,边缘波、尾波压力为负;聚焦后区则相反。在线性条件下,反射波压力峰值出现于椭球面镜几何焦点,越过焦点后反射波压力幅值迅速衰减。利用有限元软件COMSOL对椭球面镜声反射过程进行数值模拟,揭示反射波传播演化过程及声场分布规律。据KZK方程及等效声源法,分析非线性效应对声传播过程影响。研究表明,非线性效应将使椭球面镜实际焦点位置偏离几何焦点,即正压实际焦点出现在几何焦点后,负压实际焦点出现在几何焦点前;随非线性系数增加正压实际焦点后移,负压实际焦点前移。 相似文献
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《International journal for numerical methods in engineering》2018,115(6):651-694
Within the framework of dynamic calculations, the hybrid multi–time‐step method proposed by Gravouil and Combescure (GC method) has proven to be an efficient algorithm that enables the use of arbitrary time steps and Newmark time schemes in each subdomain. Nonetheless, when dealing with wave propagation problems, the amount of reflections at the interfaces between subdomains strongly depends on the choice of the time integrators and the time steps used for the simulation study. In this paper, we deal with both one‐ and two‐dimensional wave propagation problems (only the anti‐plane shear wave problem is considered for the two‐dimensional case) with the aim of deriving an analytical estimation of the numerical reflection coefficient at the interface between two linear elastic subdomains having their own time integrators and time scales. The model is approximated using the lowest‐order finite elements, whereas the propagation process is described using harmonic waves. The study is carried out on the explicit/implicit and explicit/explicit integrations using arbitrary time‐step ratios. The numerical reflection coefficient is then analyzed with emphasis on the effect of the time‐step ratio and the direction of incidence. 相似文献
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P. R. Herrington 《Journal of Materials Science》1992,27(24):6615-6626
Vacuum distillation bottoms from the rerefining of waste automotive oils (WODB) can be successfully blended with standard petroleum bitumen to produce a stable binder with acceptable physical properties for roading applications. The increase in viscosity and shear susceptibility after accelerated ageing of thin films at 163°C is slightly less for bitumen extended with WODB than for bitumen itself; but at temperatures above 200°C the WODB appears to accelerate the rate of bitumen oxidation. Air blowing at 240°C can be used to produce an acceptable binder from blends of WODB and a softer grade of bitumen. The rate of hardening during air blowing depends on both the batch and concentration of WODB. 相似文献
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D. R. Cohn S. C. Han P. P. Woskov B. L. Zhou A. Ferdinand R. H. Giles J. Waldman D. W. Cooke R. E. Muenchausen 《Journal of Superconductivity》1992,5(4):389-393
Submillimeter wave laser reflection measurements of surface resistance can provide improved capability in the combination of sensitivity, spatial resolution, and frequency range. We have made reflectivity measurements on metals at 1630 GHz with an uncertainty of less than 0.3%. This sensitivity corresponds to a measurement sensitivity for surface resistance of 0.3 . Assuming anf
2 frequency scaling of high-temperature superconductor surface resistance from the microwave to the terahertz frequency range, this sensitivity corresponds to about 1 ×10–5 at 10 GHz. Capability for 10–7 sensitivity could eventually be possible. Preliminary submillimeter wave reflection measurements of a YBCO thin film have been made with a sensitivity of 1%. Submillimeter wave reflectometry can make it possible to determine the spatial dependence of surface resistance in a wide range of material sizes and shapes. The spatial resolution could be on the order of 0.3–0.5 mm. 相似文献
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Yashar Javadi Khaled Azari Seyed Mahmoud Ghalehbandi M. J. Roy 《Research in Nondestructive Evaluation》2017,28(2):101-122
Fusion welding is a joining process widely used in the industry. However, undesired residual stresses are produced once the welding process is completed. Post-weld heat-treatment (PWHT) is extensively employed in order to relieve the welding residual stresses. In this study, effect of PWHT time and temperature on the residual stresses of a ferritic stainless steel is investigated. Residual stress distributions in eight welded specimens were measured by using an ultrasonic method. Ultrasonic stress measurement is a nondestructive method based on acoustoelasticity law, which correlates mechanical stresses with velocity of an ultrasonic wave propagating within the subject material. The ultrasonic wave employed could be longitudinal or shear wave produced by the longitudinal (normal) or transverse (shear) transducers, respectively. Ultrasonic stress measurements based on longitudinal waves use longitudinal critically refracted (LCR) waves in this direction, while shear wave methods use an ultrasonic birefringence phenomenon. The results show that the effect of PWHT can be successfully inferred by both longitudinal and shear wave methods, but the former is found to be more sensitive to stress variation. Furthermore, the distribution of subsurface residual stresses is found to be more distinguishable when the LCR method is employed. 相似文献
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传统的兰姆波多采用压电陶瓷换能器激发和接收。建立了新的超声兰姆波无损检测系统,其基本思想是采用布拉格光纤传感器作为兰姆波的接收器。光纤光栅传感的基本原理是通过检测光栅反射的中心波长移动实现对外界参量如超声的测量。超声作用下光纤光栅的反射谱发生变化,对超声作用下光纤光栅的反射谱变化进行了数值分析,结果表明,超声对光栅反射谱的影响与超声波长与光栅长度的比值是高度相关的。只有当这个比值相当大时,反射谱的形状才不会变化而中心波长发生偏移,此时光纤传感器可用来探测兰姆波。这个结论为利用新的兰姆波无损检测系统在布拉格光栅长度的设计和兰姆波波长的选择方面提供了有用的工具。 相似文献
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The standard mechanical penetration approach for monitoring cement paste stiffening (Vicat needle method, ASTM C191) does not distinguish responses associated with false set of cement paste caused by secondary gypsum formation. The objective of this research is to determine whether ultrasonic wave reflection, using a testing set up with high measurement sensitivity, could be used to monitor false set of cement paste. Penetration resistance, P-wave, and S-wave reflection coefficients were measured on cement pastes with water-to-cement ratio 0.5. The S-wave reflection coefficient showed a sharp and abrupt linear drop associated with secondary gypsum formation, thereby indicating that S-wave ultrasonic wave reflection can be used to monitor false set of cement pastes. False set could not be distinguished in penetration resistance or P-wave UWR test data. 相似文献
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基于Flugge理论,建立了薄壁均质常曲率曲梁面内运动的6阶微分控制方程,得到了曲梁的频散特性曲线和6种波的轴向位移和径向位移的比值,推导了位移和内力响应的表达式以及物理域和波数域的变换矩阵。利用波的传递和反射矩阵对曲梁和半无限长直梁耦合时的能量传递系数和反射系数进行了求解分析。对于半无限长直梁中给定的拉伸波或弯曲波入射,得到了和频率,曲率半径和伸展角度相关的各种波传递和反射的能量系数表达式。数值结果表明,纵波和弯曲波在经过曲梁结构之后发生了波形转换,并研究了能量传递和反射系数随频率,伸展角度,曲梁曲率半径和截面尺寸比的变化。结果表明,无限长直梁和曲梁耦合系统中,低频时,经过曲梁反射和传递后的弯曲波和纵波会相互转化;高频时弯曲波和纵波都能够没有散射地通过曲梁而进行传播。为改善高频时曲梁中的能量衰减效果,研究了在曲梁结构中插入单个、多个中间支撑或阻振质量块时的能量传递和反射系数。结果表明,阻振质量块能够很好地阻止高频时曲梁中能量的传递,对于周期分布的多个阻振质量块,能量传递系数随频率的变化存在周期结构的阻带特征。这些研究结果为曲梁结构的设计提供定性的理论基础。 相似文献
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通过不同炸药量、不同爆炸距离、不同起爆深度的水中爆炸模型实验,研究了浅水爆炸条件下高桩钢管柱表面压力特征和空间分布规律,分析了比例爆距对冲击波峰值及空间分布影响,给出了钢管柱表面冲击波反射系数、绕射系数和抗爆设计中实际作用冲击波的工程算法。研究结果表明:水中爆炸作用下,反射和绕射冲击波近似同时作用在钢管柱表面,峰值沿柱身高度方向非均匀分;冲击波受水面影响程度相对较小,二次气泡脉动受水面影响程度较大;反射和绕射冲击波峰值均随炸药量增加、作用距离减小而增加。比例爆距相同,反射冲击波峰值相同,但炸药量小、爆炸距离近的实验工况绕射冲击波峰值相对较小;钢管柱表面冲击波反射系数和绕射系数随比例爆距增加而减小。比例爆距≥1.71时,钢管柱实际作用冲击波峰值可近似按自由场冲击波峰值的1.37倍计算。 相似文献
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运用非饱和孔隙介质理论阐述了弹性波在非饱和土中的传播特性,分析了平面S波在非饱和土层自由边界上的反射问题。根据边界条件,分别导出了在非饱和土自由边界上的四种反射波:反射P1波、反射P2波、反射P3波及反射S波的振幅反射率及能量反射率的理论表达式,并在此基础上进行了数值计算。算例中讨论了四种反射波的振幅反射率及能量反射率受平面S波入射角度及土层饱和度变化的影响情况。计算结果表明:各反射波的振幅反射率及能量反射率不仅与入射角有关,也受到饱和度变化的影响,这些结论对土动力学的理论研究以及相关工程地震勘探具有一定指导意义。 相似文献