Adhesive joint evaluation by ultrasonic interface and lamb waves

Some results on the application of interface and Lamb waves for the study of curing of thin adhesive layers were summarized. In the case of thick substrates (thickness much more than the wave length) the interface waves can be used. In this case the experimental data can be inverted and the shear modulus of the adhesive film may be explicitly found based on the measured interface wave velocity. It is shown that interface waves can be used for the study of curing of structural adhesives as a function of different temperatures and other experimental conditions. The kinetics of curing was studied. In the case of thin substrates the wave phenomena are much more complicated. It is shown that for successful measurements proper selection of experimental conditions is very important. This can be done based on theoretical estimations. For correctly selected experimental conditions the Lamb waves may be a sensitive probe of adhesive bond quality and may be used or cure monitoring

Determination of Residual Stress in Composite Materials Using Ultrasonic Waves

The performance of high temperature composites can be significantly affected by the presence of residual stresses. These stresses arise during cooling processes from fabrication to room temperature due to mismatch of thermal expansion coefficients between matrix and fiber materials. This effect is especially pronounced in metal matrix and intermetallic composites. It can lead to plastic deformations, matrix cracking and fiber/matrix interface debonding. In this work the feasibility of ultrasonic techniques for residual stress assessment in composites is addressed. A novel technique for absolute stress determination in orthotropic materials from angular dependencies of ultrasonic velocities is described. The technique is applicable for determination of both applied and residual stresses and does not require calibration measurements on a reference sample. The important advantage of this method is that stress is determined simultaneously with stress-dependent elastic constants and is thus decoupled from the material texture. It is demonstrated that when the principal plane stress directions coincide with acoustical axes, the angular velocity data in the plane perpendicular to the stress plane may be used to determine both stress components. When the stress is off the acoustical axes, the shear and the difference of the normal stress components may be determined from the angular dependence of group velocities in the plane of stresses. Synthetic sets of experimental data corresponding to materials with different anisotropy and stress levels are used to check the applicability of the technique. The method is also verified experimentally. A high precision ultrasonic wave transmission technique is developed to measure angular dependence of ultrasonic velocities. Examples of stress determination from experimental velocity data are given. A method is presented for determination of velocities of ultrasonic waves propagating through the composite material with residual stresses. It is based on the generalized self-consistent multiple scattering model. Calculation results for longitudinal and shear ultrasonic wave velocities propagating perpendicular to the fibers direction in SCS-6/Ti composite with and without residual stresses are presented. They show that velocity changes due to presence of stresses are of order 1%

Effect of Fiber-Matrix Interpahse on Low Frequency Ultrasonic Wave Scattering: Spring B.C. Approach

In modern metal matrix and ceramic matrix composites fiber-matrix interphases are specially designed to improve the fracture toughness of a composite, and to prevent fiber-matrix chemical reaction. As the interphase transfers load from the fiber to the matrix, the state of the bond between the interphase and the surrounding materials (fiber or matrix) determines the overall mechanical performance of the composite. Ultrasonic scattering from such interphases carries important information on the fiber itself and its bonding with the surrounding matrix material, thus having potential for fiber-matrix interphase characterization. Much work has been done on the study of wave scattering from cylindrical objects embedded in elastic media since 1950, owing to the importance of the subject [1–6]. Recent studies address scattering from coated fibers in composites [7–12]

Quantification of Damage Evolution in Multilayered High Temperature Composites from Ultrasonic Measurements

New high temperature composite materials have potential to yield significant performance improvement and weight reduction in different components for aerospace applications (i.e. aircraft engines, airframe parts). For successful application of these materials in various environments their mechanical properties, life service capability and reliability must be known. The high cost of structures engineered from these materials makes experimental life determination very difficult. Thus the development of methodology for NDE of structures, combined with life prediction models using measured damage as input data, becomes very important

In-Process Ultrasonic Evaluation of Spot Weld Quality

While spot welding has wide use in the automotive and aerospace industries there is no acceptable nondestructive testing technique for evaluation of its quality. One of the major metallurgical defects of spot welds, the stick weld, cannot be evaluated at all. There is growing interest in the industry to develop simple production-oriented nondestructive techniques for evaluation of spot weld quality

Effect of Shear Modes Interference in Anisotropic Materials

Ultrasonic signals transmitted through a plate by shear waves in weakly anisotropic materials are usually not separable in the time domain and thus interfere at the receiver. The interference signal bears information on material anisotropy due to texture or residual stresses. This dependence of the interference signals on material properties can be utilized practically. Blinka and Sachse [1] used interference of two shear waves to measure stress applied normally to the direction of wave propagation in an aluminum specimen. The phenomenon is also described in [2]. The change of acoustic microscopy contrast is attributed by Dreschler-Krasicka to stress induced interference [3]. Various interference phenomena in crystalline solids are described by Wolfe in [4]

Ultrasonic Characterization of Interfacial Fatigue Damage in Metal Matrix

In recent years there has been significant interest in titanium-based metal-matrix composites for structural applications in airframe and engine components. In many applications, these composites are integrated into structural components by diffusion bonding. Thus the composites are subjected to temperature/time cycles during material processing and to temperature/stress cycles in service. One major effect of these thermomechanical cycles is that they significantly alter the microstructure of the composite constituents and the residual stresses. Even though the changes in microstructure and residual stresses may not affect the composite properties greatly, they often lead to completely different failure mechanisms. Therefore, it is important to understand the role of heat treatment on composite damage development, which can only be done by nondestructively monitoring damage initiation and development during the failure process

Modeling for Ultrasonic Health Monitoring of Foams with Embedded Sensors

In this report analytical and numerical methods are proposed to estimate the effective elastic properties of regular and random open-cell foams. The methods are based on the principle of minimum energy and on structural beam models. The analytical solutions are obtained using symbolic processing software. The microstructure of the random foam is simulated using Voronoi tessellation together with a rate-dependent random close-packing algorithm. The statistics of the geometrical properties of random foams corresponding to different packing fractions have been studied. The effects of the packing fraction on elastic properties of the foams have been investigated by decomposing the compliance into bending and axial compliance components. It is shown that the bending compliance increases and the axial compliance decreases when the packing fraction increases. Keywords: Foam; Elastic properties; Finite element; Randomnes

Wave Propagation in Stressed Composites

High temperature composites have potential for significant performance improvement and weight reduction in aircraft engines and other structures. For successful application of these materials in different environments their mechanical properties, life capability and reliability must be well known. Thus NDE of composite structures becomes very important