The transmission of Lamb waves across adhesively bonded lap joints

The transmission of Lamb waves across adhesively bonded lap joints is investigated using finite element analysis. The studies consider three modes for excitation and reception, s0, a0, and a1, applied to lap joints consisting of parallel aluminum sheets bonded with an epoxy adhesive. Transmission coefficient results for a two-dimensional range of bond thicknesses and bond overlap lengths are presented for all three modes. The transmission coefficients are calculated from the spectra of the received and transmitted signals using an approach which is insensitive to the presence of multimode signals and reverberated signals, and which approximates to a power transmission coefficient. Detailed analysis is then performed for one of the modes in order to investigate the nature of the mode conversion in the overlap region of the joint. It is found that the relative amplitudes of the different modes which propagate in the overlap region can be estimated reliably and simply from the properties of the incident wave mode. As well as demonstrating the physics of the mode conversion behavior, the study provides a basis for the selection of modes for nondestructive evaluation (NDE) of the bond region and for measuring the bond dimensions.     

Ultrasonic testing of anaerobic bonded joints

The paper deals with ultrasonic testing of adhesive joints between sheets of thickness greater than 1 mm, bonded by a "zero thickness" layer of anaerobic adhesive. In the case of equally thick sheets, interference phenomena can arise that make impossible the use of the "decay method" (a method previously developed by the authors for the case of non-zero adhesive thickness). Consequently an alternative method, termed "first peak", has been proposed. In order to evaluate the applicability of the two methods, a series of tests has been carried out on specimens having geometry suitable for both types of inspection. Also two different levels of surface roughness have been considered. The results allow for evaluating the defect detection capability of each method, accounting for the roughness effect, and give information on the suitable control procedures.     

等厚铝板粘接件超声反射频谱的低频特征

利用弹簧模型研究了"铝-胶-铝"等厚粘接件的超声反射谱的低频特征(使用的超声波频率远低于胶层的共振频率),指出了利用其谐振频率的"右峰"漂移量反演弱粘接情况的胶层劲度系数的可能性。我们通过胶层固化时间的变化来模拟胶层劲度系数的变化,对理论结果进行验证,实验结果与理论吻合良好。     

单层与衬底胶接结构超声反射波谱的低频特征

理论研究了单层-衬底胶接结构超声反射波谱的低频(使用的超声波频率低于胶层的1/4波长共振频率)特征,指出了利用分层系统的谐振频率飘移量反演胶层厚度并进而评价胶层内聚强度的可能性。用Taylor展开给出了谐振频率所满足的非线性方程的显式解,其与Newton法数值解吻合得很好,适用范围远大于弹簧模型。在更低的频率范围,胶层可用熟知的线性弹簧模型描述。     

循环载荷下粘接接头真空性能的实验研究

粘接接头在经历反复冷却、复温后能否保持良好的真空密封性,是决定无磁杜瓦耐用性的一个重要因素。该文实验研究了循环载荷对粘接接头气密性的影响。用MTS 810材料试验机对玻璃钢管状粘接试样施加循环载荷,并用动态真空法测量试样真空度的变化。结果一半试样的真空度随循环次数的增加而降低,而其余试样破坏前真空度变化不大。因此,无磁杜瓦在使用过程中反复冷却及复温所产生的循环应力载荷,是导致其真空性能下降的一个重要原因。     

A novel technique for measuring 3D deformation of adhesively bonded single lap joint

An easy-to-implement yet practical single-camera microscopic stereo-digital image correlation(stereo-DIC) technique is proposed for surface three-dimensional(3D) deformation measurement of singe lap joint(SLJ) samples subjected to mechanical loads. The basic principles, optical configurations and implementation procedures of the proposed technique are described in detail. Compared with existing single-camera 2D-DIC technique, which has been regularly used for in-plane deformation measurement of a SLJ specimen, the proposed technique offers the special merit of simultaneously determining all the three displacement components by simply adding two additional optical elements to existing single-camera 2D-DIC systems. The accuracy and effectiveness of the proposed technique is demonstrated by measuring the 3D deformation of a SLJ specimen subjected to quasi-static tensile loads.     

An electro-mechanical impedance model of a cracked composite beam with adhesively bonded piezoelectric patches

A model of a laminated composite beam including multiple non-propagating part-through surface cracks as well as installed PZT transducers is presented based on the method of reverberation-ray matrix (MRRM) in this paper. Toward determining the local flexibility characteristics induced by the individual cracks, the concept of the massless rotational spring is applied. A Timoshenko beam theory is then used to simulate the behavior of the composite beam with open cracks. As a result, transverse shear and rotatory inertia effects are included in the model. Only one-dimensional axial vibration of the PZT wafer is considered and the imperfect interfacial bonding between PZT patches and the host beam is further investigated based on a Kelvin-type viscoelastic model. Then, an accurate electro-mechanical impedance (EMI) model can be established for crack detection in laminated beams. In this model, the effects of various parameters such as the ply-angle, fibre volume fraction, crack depth and position on the EMI signatures are highlighted. Furthermore, comparison with existent numerical results is presented to validate the present analysis.     

Pulse-echo measurement of longitudinal sound velocity in nanometer thin films

We propose a method to measure the longitudinal sound velocity in thin films of a few nanometer thickness using laser-based picosecond ultrasonics. In periodic multilayer structures, picosecond pulse-echo techniques were used to measure the effective sound velocity, which is related to the velocities of individual constituents through the superlattice phonon dispersion relation. The individual sound velocities can then be extracted, provided two or more effective velocities are obtained from multilayers of different thickness ratios. Longitudinal sound velocities in ion-beam sputtered Mo and amorphous Si films of 2 to 5 nm thickness have been determined to be 98 and 94% of the bulk speed, respectively. We believe this technique has general applicability to sound velocity measurement in ultra-thin films. PACS 68.60.Bs; 68.65.Ac; 78.47.+p     

The measurement of cohesive and interfacial toughness for bonded metal joints with epoxy adhesives

The types of crack growth in adhesive joints are reviewed and three are identified, namely central cohesive, asymmetric cohesive and interfacial. Test methods for measuring fracture toughness associated with these cracks are then outlined and include a Tapered Double Cantilever Beam (TDCB) test for a central cohesive crack and peel tests on flexible laminates for the other types of crack. In particular, fixed arm and mandrel peel tests are used. Two aerospace adhesives are used to prepare test specimens in order to conduct these tests. For one of these adhesives, all three types of crack growth were recorded and this provided an opportunity to make detailed comparisons of the three associated fracture toughness values. Of particular interest was the use of the mandrel peel method because it enabled a fracture transition (asymmetric cohesive to interfacial fracture) to be observed during the test. The fracture toughness value associated with a central cohesive crack was similar in magnitude to that for an asymmetric cohesive crack. However, the fracture toughness for interfacial fracture was much lower, but similar in magnitude to the expected value of half the fracture toughness from a TDCB test.     

Laser speckle decorrelation in NDT

The random noise of the laser speckle field which develops at the focusing plane of an imaging system, is, by now, efficiently used in several interferometric techniques as an information carrier of the macroscopic wavefront distortion induced by the surface displacement field of the object under investigation. The actual noise in this kind of techniques is represented by the speckle decorrelation at the image plane — i.e. the destruction of the carrier — which may be caused by the modification of the texture surface (e.g. by yielding under a severe stress state), but it is inherently produced by the same displacement field under measurement. In the paper the phenomenon of laser speckle decorrelation is numerically simulated and experimentally investigated with the aim of estimating its sensitivity to local deformation and assessing a possible field of application. Satisfactory results in the field of NDT of multilayer fiber-reinforced composites were obtained by reducing the diaphragm of the lens to increase the sensitivity of the imaging system to speckle decorrelation induced by local deformation; unfortunately this simple approach requires a considerable amount of laser power for illuminating the object. Different aperture shapes were therefore numerically simulated which provided improved efficiency and sensitivity and whereby a semi-quantitative analysis of the displacement field could be experimented.     

Application of ESPI to NDT

Interest in optical methods of NDT increased rapidly with the advent of holographic interferometry which made it possible to observe surface increments of less than 1 μm on practically any surface texture. Invariably the optical method is coupled with a mechanical loading cycle which tests the component directly in terms of its strength, a claim which cannot be made for most other NDT methods. Holographic interferometry is unfortunately slow and expensive to apply and other methods of optical examination based on coherent optical scattering have been investigated. Of particular interest in the author's laboratories is the technique of electronic speckle pattern interferometry. This technique enables a process directly comparable with holographic interferometry to be carried out with a specially adapted closed circuit television system, giving on line information. This work is discussed together with some other relevant processes and methods of optical NDT.     

A review of three magnetic NDT technologies

Magnetic techniques are most important NDT technologies to characterize the mechanical features of ferromagnetic materials based on the physical principle of magnetic-stress coupling. A review is presented in this paper about the development of the magnetic NDT technologies. After a brief outline of the theoretical studies of the magnetic-stress coupling effect, the three popular magnetic NDT technologies are reviewed, which are magnetic flux leakage (MFL), magnetic Barkhausen noise (MBN) and recently developed metal magnetic memory (MMM). The first two are ascribed to the active magnetic method, and the last one is the passive method. Based on an extensive literature survey in this field, this paper focuses on the discussion of the physical mechanism and some important experimental results relevant to the three NDT technologies. The challenges for each technique in this field are also summarized.     

Noise reduction in ultrasonic NDT using undecimated wavelet transforms

Translation-invariant wavelet processing is applied to grain noise reduction in ultrasonic non-destructive testing of materials. In particular, the undecimated wavelet transform (UWT), which is essentially a discrete wavelet transform (DWT) that avoids decimation, is used. Two different UWT processors have been specifically developed for that purpose, based on two UWT implementation schemes: the "à trous" algorithm and the cycle-spinning scheme. The performance of these two UWT processors is compared with that of a classical DWT processor, by using synthetic grain noise registers and experimental pulse-echo NDT traces. The synthetic ultrasonic traces have been generated by an own-developed frequency-domain model that includes frequency dependence in both material attenuation and scattering. The experimental ultrasonic traces have been obtained by inspecting a piece of carbon-fiber reinforced plastic composite in which we have mechanized artificial flaws. Decomposition level-dependent thresholds, which are suitable for correlated noise, are specifically determined in all cases. Soft thresholding, Daubechies db6 mother wavelet and the three well-known threshold selection rules, Universal, Minimax and SURE, are applied to the different decomposition levels. The performance of the different de-noising procedures for single echo detection has been comparatively evaluated in terms of signal-to-noise ratio enhancement.     

NDT capability of digital shearography for different materials

In this paper, the capabilities of shearography for detecting hole and crack type defects in polymeric and metallic materials using thermal loading were investigated. In order to improve the ability of the identifying defect, the fringe multiplication technique was applied to some suspectable shearography fringe patterns. Based on the test results empirical rules for inspection plates of different material with specific thicknesses were established. For aluminium plates with thicknesses of 3 mm, minimum diameter of a surface breaking hole which is detectable is approximately equal to 2.5–3.0 times the depth of the defect, and for polymer plates with a thickness of 10 mm, the minimum detectable diameter is 0.8–1.3 times of the depth. For polymeric materials results from the increasing temperature period are approximately the same as those from the decreasing temperature period. When the orientation of the image shearing is not perpendicular to the longitudinal direction of the crack type defects, the sensitivity for defect detection is relatively higher than with perpendicular image shearing.     

Disorder in DNA-linked gold nanoparticle assemblies

We report experimental observations on the effect of disorder on the phase behavior of DNA-linked nanoparticle assemblies. Variation in DNA linker lengths results in different melting temperatures of the DNA-linked nanoparticle assemblies. We observed an unusual trend of a nonmonotonic "zigzag" pattern in the melting temperature as a function of DNA linker length. Linker DNA resulting in unequal DNA duplex lengths introduces disorder and lowers the melting temperature of the nanoparticle system. Comparison with free DNA thermodynamics shows that such an anomalous zigzag pattern does not exist for free DNA duplex melting, which suggests that the disorder introduced by unequal DNA duplex lengths results in this unusual collective behavior of DNA-linked nanoparticle assemblies.     

Evolution of multilevel order in supramolecular assemblies

The process of self-assembly at multiple length scales of bis-urea substituted toluene on a Au(111) surface was studied by low temperature scanning tunneling microscopy. Pattern formation is controlled by specific hydrogen bonds between these molecules but also by significantly weaker lateral coupling between the resulting supramolecular polymers and a quasiepitaxial interlocking with the substrate. The ordered assemblies exhibit a tunnel transparency. Our experiments indicate the necessity of multiple interactions of different strengths for obtaining ordered structures with hierarchical levels of organization.     

Continuous phase transformation in nanocube assemblies

The phase behavior of 3D assemblies of nanocubes in a ligand-rich solution upon solvent evaporation was experimentally investigated using small-angle x-ray scattering and electron microscopy. We observed a continuous transformation of assemblies between simple cubic and rhombohedral phases, where a variable angle of rhombohedral structure is determined by ligand thickness. We established a quantitative relationship between the particle shape evolution from cubes to quasispheres and the lattice distortion during the transformation, with a pathway exhibiting the highest known packing.