共查询到17条相似文献,搜索用时 187 毫秒
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根据垂直入射超声波在3层介质中传播的相关理论,给出了基于超声波反射系数油膜厚度测量的谐振模型和弹簧模型,在此基础上进行实验测试。设计并制作了基于STC单片机的信号发生控制器电路,编写了脉冲串波形、频率和个数控制程序,对给定油膜厚度的试件进行了超声测量。结果表明:通过测量超声波反射系数可间接获得机械结构中。油膜厚度,并且测量误差基本保持在5%的范围内。 相似文献
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为了研究超声波在流体-孔隙介质周期性粗糙界面的传播特性,文章基于周期性锯齿粗糙界面的衍射模型分析孔隙介质开孔与闭孔状态下孔隙度对反射与透射的影响。通过孔隙介质比奥特(Biot)理论与光栅方程理论,得到包含各阶反射系数、透射系数的线性方程组,再利用傅里叶变换进行数值计算,得到孔隙度与流体-孔隙介质周期性粗糙界面反射系数、透射系数之间的关系。结果表明,由于界面的周期性,频率对反射与透射系数的影响没有呈现一定的规律。但孔隙度对反射与透射系数有显著影响,且由于孔隙介质状态的差异性,导致反射与透射系数在开孔与闭孔时变化趋势不同。 相似文献
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《振动与冲击》2016,(12)
对层状碳纤维复合材料(CFRP)超声反射系数进行建模并研究超声波在CFRP内的共振特性。首先建立了声波在多层介质中传播时的反射系数频响模型;在此基础上对多层CFRP反射系数频域响应进行数值计算并对反射系数频响随CFRP层数的变化特征进行分析;结果表明使用中心频率接近层状CFRP固有共振频率的超声探头进行检测时,超声波在层状CFRP中将产生共振现象;在CFRP层数较少时,共振结构噪声中会夹杂其他频率的信号成分。随着CFRP层数增加,共振结构噪声在信号共振区域的占比将会提高;当CFRP层数增加到一定数量后,共振结构噪声将只在离超声波入射表面较近的区域出现;随着超声波远离入射表面,共振结构噪声将逐渐消失。最后对薄板型CFRP和厚截面CFRP的实验信号进行分析,得到的实验结果和数值计算结果相一致。 相似文献
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针对薄层材料超声测厚过程中回波信号混叠、超声纵波声速未知导致薄层厚度无法测量的问题,提出一种基于声压反射系数幅度谱(Ultrasonic Reflection Coefficient Amplitude Spectrum,URCAS)匹配分析技术同时测量薄层厚度和超声纵波声速的方法。采用相关系数法对薄层试样实测声压反射系数幅度谱和理论声压反射系数幅度谱在超声检测有效频带范围内逐一进行匹配分析,通过反演计算得到相关系数最大值点对应的超声检测参数,最终实现薄层厚度和超声纵波声速的同时表征。利用该方法对铝合金基体上的雷达吸波涂层(Radar Absorbing Coatings,RAC)进行实验测试及信号分析。结果表明:该方法可以有效实现混叠信号中超声特征参量的提取,反演得到吸波涂层厚度与千分尺测量厚度间相对误差为2.53%-3.72%、纵波声速测量相对误差为2.51%-3.75%。 相似文献
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超声纵波与横波分别垂直入射到固体间的Stokes流体滑移界面层时,其反射系数与界面层的特性(包括粘滞系数、体积弹性模量、厚度)或入射波的频率有关.通过对纵横波两种波型各自在粘滞流体界面层的反射特性的讨论,结合两种粘滞性不同的流体界面层试验测量结果,说明了使用纵横波测量滑移界面层粘滞性的不同特点. 相似文献
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针对涂层结合界面刚度在非破坏条件下难以精确测量的问题,提出了一种超声检测特征参量表征刚度系数的方法。利用声波在n层各向同性介质中的反射、透射原理,结合界面的弹簧模型,建立了多层介质界面刚度系数的超声检测数学模型。基体选用钢、铸铁、铝合金,表面选用不同喷涂工艺得到的Al2O3陶瓷涂层,获得了不同界面刚度系数的超声反射频谱。仿真结果表明:分离界面和理想界面时,谐振频率都具有周期性,但周期大小不同;弱结合界面时,随着界面的刚度系数逐渐增加,谐振频率逐渐增多,这些谐振频率均向高频方向移动。与频率较高处相比,频率较低处的谐振频率随着刚度系数的增加向高频移动的速度更快。因此建立了第一个谐振频率与刚度系数之间的关系。在同一刚度系数下,由谐振频率与材料的特性阻抗关系获得如下规律:当涂层材料不变时,谐振频率随着基体特性阻抗的增大而增大;当基体材料不变时,谐振频率随着涂层特性阻抗的增大而减小。给出了以指数函数形式拟合的刚度系数与谐振频率的变化曲线。通过对该指数函数参数与材料特性阻抗之间关系的分析,获得了弱界面时谐振频率与刚度系数和材料特性阻抗三者之间的函数表达式。该方法为涂层复合材料弱界面的超声检测提供了理论支持。 相似文献
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Hadi Rahimi 《Journal of Superconductivity and Novel Magnetism》2016,29(7):1767-1772
In the ultraviolet region and by using transfer matrix method, the transmission spectra of electromagnetic waves through one-dimensional quasiperiodic photonic structures consisting of high-temperature yttrium barium copper oxide (Y123) superconductor and strontium titanate (STO) dielectric layers are studied theoretically. It is shown that for TE polarization at oblique incidence two band gaps are created, while for TM polarization three band gaps are produced. The edges of both polarizations shift to higher wavelengths by increasing incident angles. Also, for angles of incidence greater than 78°, the second gap for TM-polarized light is eliminated. It is also found that in the supposed structure the number of PBGs can be modulated by the thickness of dielectric layer, while it is nearly insensitive to the thickness of superconductor layer. Interestingly, for the incidence angles other than normal incidence the structure can exhibit some narrow resonant peaks near wavelengths where the electric permittivity of the superconductor layer changes sign. These resonant peaks are only for TM polarization and not present for TE polarization. This structure can act as a very compact polarization sensitive splitters and defect-free multichannel narrowband tunable filters. 相似文献
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This paper presents a near-field microwave nondestructive testing technique for disbond/crack detection and evaluation in a concrete structure backed by an infinite half space of any material. A model describing the interaction of waves radiated out from an open-ended rectangular waveguide, in the near-field, with any layered medium will be utilized. The theoretical model calculates the effective reflection coefficient of the structure, at the aperture of the waveguide, as a function of the frequency of operation, the thickness and dielectric properties of the layers of the structures, including the standoff distance. The frequency of operation and standoff distance (the measurement parameters) can be optimized to achieve maximum sensitivity to the presence of the disbond. The presence of a disbond in a structure is viewed as an additional layer and will change the properties of the effective reflection coefficient (phase and magnitude). This change will depend on the thickness and location of the disbond. This fact will be used to investigate the potential of utilizing multiple frequency measurements to obtain disbond location and thickness information. A fuzzy logic model relating the phase of reflection coefficient, frequency of operation, and standoff distance to the disbond thickness and depth was generated and utilized. 相似文献
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An analysis is made of the propagation of an electromagnetic wave through an infinite periodic superconductor-dielectric structure
consisting of alternating layers of dielectric and thin layers of type II superconductor. The presence of thin layers of superconductor
is taken into account by introducing a suitable boundary condition. It is observed that the reflection coefficient depends
abruptly on the angle of incidence of the wave, the thickness of the superconducting film, and the external magnetic field.
Pis’ma Zh. Tekh. Fiz. 24, 9–12 (January 12, 1998) 相似文献
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利用光学显微镜实时观测磷酸二氢钾(KDP)晶体柱面及锥面薄表面层的生长过程,测得不同过饱和度下薄表面层前端不同倾角的非正常棱边推移速度。结果表明:倾角越小,非正常棱边推移速度越慢,薄表面生长终止于其前端的正常棱边处;随着过饱和度的增大,非正常棱边推移速度线性增加。计算得到柱面及锥面薄表面层前端非正常棱边推移动力学系数,由于Eslice(010) Eslice(101),因而柱面薄表面层的生长动力学系数大于锥面。建立了基于非奇异面上台阶生长机制下的薄表面层生长体扩散模型。应用该模型解释了薄表面层生长速度随其厚度及前端非正常棱边倾角的变化关系,并讨论了溶液流动对薄表面层生长的影响。结果发现薄表面层生长存在一个使其以恒定厚度向前推移的临界厚度。 相似文献
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13. Measurement of the lattice constant of SiGe heteroepitaxial layers grown on a silicon substrate
Herzl Aharoni 《Vacuum》1978,28(12):571-578
The lattice constant of thin heteroepitaxial layers composed of single crystal SiGe solid solutions various compositions grown on silicon substracts were measured. The measurements were performed by X-ray diffraction method. The fact that the layers are thin and, therefore, there is also reflection from the silicon substrate (in addition to the reflection from the layer) is utilized for this measurement method. A review of the measurement method is given and the experimental results obtained by irradiating the samples with CuKα are presented. Specific problems related to the measurement methods and to the layer lattice structure are also presented. There are: resolution problems between the reflections from the layer and the substrate for low Ge concentrations, deformation problems, the effects of the defects and non-uniform changes in the lattice constant at different crystallographic orientations. 相似文献
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A method involving only normal reflectance measurements was developed to determine the optical properties of thin transparent layers formed on opaque substrates over a wavelength range including the visible spectrum. The spectral refractive index of Nb2O5 anodically grown on niobium samples up to 215 nm thick was obtained by this method and also the spectral phase change on reflection at the oxide-metal interface. Index determinations, however, require separate measurements of the layer thickness using, for instance, grazing incidence X-rays. 相似文献
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建立了垂直入射时复合船体板结构中前行波和回行波声压幅值在层间的传递矩阵,计算了该类复合结构的透声系数,并对典型夹芯板结构进行了分析,指出对于阻抗匹配层要降低材料的损耗因子,而对于阻抗失配层要降低厚度,才能显著提高复合板结构的透声性能,有助于水声复合材料的设计以及声学试验的数值验证。 相似文献