Resonant-based identification of the elastic properties of layered materials: Application to air-plasma sprayed thermal barrier coatings

This work introduces a resonant-based, mixed numerical–experimental method for the determination of the in-plane elastic properties of the constituent materials of laminates. This non-destructive method identifies elastic properties from the resonant frequencies of beam-shaped layered specimens, using a set of finite element models. The method is demonstrated on a thermal barrier coating system made of NiCoCrAlY bondcoat and yttria-stabilised zirconia topcoat deposited by air-plasma spraying on stainless steel. The stainless steel was found to be elastically anisotropic, while both bondcoat and topcoat exhibited in-plane isotropy. Moreover, the topcoat Poisson's ratio approached zero, and the bondcoat properties varied with the coating thickness. Scanning electron microscopy was used to correlate the identified elastic properties with the coating microstructure.     

A non-destructive technique for the elastic characterization of thin isotropic plates

The paper presents a non-destructive technique for determining the dynamic elastic properties of isotropic thin square plates. It is based on the measurement of at least two of the first four resonant frequencies of the samples and allows to determine the elastic constants directly using polynomial interpolating functions. These functions are derived from suitable data obtained by means of finite element analyses. The procedure is tested on metallic and ceramic specimens. Samples are excited to vibrate by drivers with continuously variable frequency output or by impact. The effect of the different sources of error on the calculation of elastic properties is also discussed.     

Inversion of ultrasonic, plane-wave transmission data in composite plates to infer viscoelastic material properties

Stiffness and damping properties of viscoelastic materials are given by the real and imaginary components, respectively, of the material constants. A new technique is proposed to experimentally measure the real and imaginary components of anisotropic (and isotropic) viscoelastic plates. Main advantage of this technique is that material properties of thin plates can be measured where many other techniques fail. Material properties are obtained by numerically inverting the transmitted ultrasonic fields, obtained for different incident angles. Simplex inversion algorithm is applied to initial estimates of plate thickness and plate properties. By this iterative technique the values of the unknown parameters (material properties and plate thickness) are continuously modified to give better agreement between the experimental and theoretical transmitted fields. After a certain number of iterations the speed of convergence of the Simplex scheme is significantly reduced. To improve the accuracy of convergence the Newton–Raphson inversion technique is adopted at that point. By this technique material properties of different types of plates are measured. These is a glass plate (isotropic plate with no damping), a polymer plate (isotropic plate with damping), and glass fiber reinforced epoxy plates with different fiber orientations (anisotropic plates with damping). Both real and imaginary components are successfully measured for all these plates. In a relative scale the measurement error for the imaginary components is higher. Reliability of the measured material constants of fiber reinforced epoxy plates is verified by the method of invariance. All experiments are carried out in the frequency range that is appropriate for satisfying two conditions—the specimen homogeneity and the plane wave conditions.     

In situ measurements of hot-mix asphalt dielectric properties

Twelve different flexible pavement sections, which comprised different layers/materials, are incorporated in the Virginia Smart Road test facility. These sections provide a good opportunity to explore the feasibility of using ground penetrating radar (GPR) to assess pavements and to verify its practicality. Thirty-one copper plates, serving as a reflecting material, were placed during construction at different layer interfaces throughout the pavement sections. Results show that enough radar energy is reaching the subgrade, but due to low dielectric contrast between some pavement materials, energy is not reflected back. In these cases, the copper plates indicate where the interface between each two layers occurs. Reflections from the copper plates are also used to determine the dielectric constant of pavement materials over the GPR frequency range. This paper presents an overview of the Virginia Smart Road test facility, data obtained from different sections using two GPR systems, and a method to calculate the complex dielectric constant of hot-mix asphalt over the frequency range of 750–1750 MHz using an air-coupled GPR system.     

Characterization of progressive microcracking in Portland cement mortar using nonlinear ultrasonics

This paper presents the successful application of a nonlinear ultrasonic technique, nonlinear wave modulation spectroscopy (NWMS) to quantitatively track the evolution of microcracks in Portland cement mortar samples. The damage type considered in this study is microcracking due to alkali–silica reaction (ASR), a deleterious reaction occurring in concrete structures around the world. Nonlinear ultrasonic measurements are conducted on six different mortar specimens that are monitored from their initial, intact state up to their fully damaged state. The objective of this research is to determine the sensitivity and suitability of NWMS to quantitatively track this damage state throughout an entire life-cycle and to nondestructively identify the initiation time and the extent of microcracking in these mortar specimens. The nonlinear ultrasonic measurements are made with standard laboratory equipment, and the inherent high attenuation of cement-based materials is overcome with a procedure that uses the sideband energy instead of measuring peak amplitudes. The results show that the NWMS method can track the progressive damage in mortar, demonstrating the feasibility of using this nonlinear ultrasonic technique to quantitatively assess the deterioration of cement-based materials.     

Measurement of the elastic modulus of dental pieces

Understanding the mechanical properties of human teeth is important to clinical tooth preparation and to the development of restorative materials. Particularly, the elastic modulus of materials used in restoration of dental parts subjected to endodontic treatment must be compatible with the original tooth. The major difficulty to measure mechanical properties of biological materials is, in general, the geometry of the specimen. Actually, even when there are different methodologies to determine the elastic modulus, samples with flat and parallel faces of uniform section are usually required. Dental parts not only do not fulfil this condition, but also constitute a composite material formed essentially by enamel and dentin with quite different elastic moduli. In this work the elastic moduli of canine and pre-molar teeth, measured by using piezoelectric excitation at high frequency, are presented. The measuring procedure considers the tooth as a composite, that is, gives a mean value according to the complex distribution of enamel and dentin in the different cross-sections of the specimen. Results are compared with Young’s modulus of enamel and dentin obtained by indentation techniques.     

Evaluation of simulated transfer functions of concrete plate derived by impact-echo method

In this study, the simulated transfer function of a concrete plate was obtained solely from displacement waveform in an impact-echo test. In the simulated transfer function, the amplitude corresponding to the modal vibration at thickness direction, called the thickness-amplitude, can be consistent during various tests on the same location. An empirical formula was obtained by numerical simulation to predict the thickness-amplitude for given thickness, impact–receiver distance, and P-wave speed. The formula was confirmed experimentally using plates made from various mixed materials with various thicknesses. The differences between the experimental and predicted thickness-amplitude are mostly within 10%. The formula has the potential for quantitative evaluation of the bond between concrete and the substrate layer.     

Application of ultrasonic Rayleigh waves to thickness measurement of metallic coatings

The velocity of Rayleigh waves propagating in a layered medium depends on the frequency of the waves, the thickness of the coating and the properties of both the coating and the substrate materials. Rayleigh waves of various frequencies were generated using a broadband pulse and their velocities were measured as a function of the frequency and compared to the theoretical dispersion curve of the specimen. The thickness of the layer was deduced from comparison between these two curves. A special transducer was designed in order to achieve these measurements. Experiments were carried out on AISI 316L specimens coated with vacuum plasma sprayed (VPS) NiCoCrAlY of various thicknesses (190–330 μm) and various surface states. A good correlation was obtained between the thicknesses measured by means of ultrasonics and those obtained using photomicrographs of the cross-section of the specimens. The elastic characteristics of the coating and the substrate materials were measured using a transmission method for parallel-faced plates (disbonded samples of coating material). The velocity of the shear and compression waves propagating in various directions is obtained by changing the angle of incidence on the sample surface. A small anisotropy of the coating material was found. Finally, we demonstrated the efficiency of the use of surface waves for the detection of surface breaking cracks and disbonding at the interface between the coating and the substrate.     

Preparation and electrochemical characterization of porous SWNT–PPy nanocomposite sheets for supercapacitor applications

Highly porous sheets comprised of single walled carbon nanotubes and doped polypyrrole (SWNT–PPy) were prepared by vacuum filtration of SWNT–PPy methanol dispersions. The employed preparation procedure is an extension of conventional bucky-paper fabrication technique for the multi-component system. A number of nanocomposites with nominal SWNT:PPy compositions ranging from 1:0 to 1:1 were obtained and tested. Electrochemical properties of the nanocomposites were investigated by cyclic voltammetry and galvanostatic spectroscopy technique in aqueous 1 M NaCl electrolyte. The highest specific capacitance of 131 F/g was obtained for nanocomposite with 1:1 SWNT:PPy ratio. Prospective applications of prepared materials range from supercapacitors to electrodes for batteries and electromechanical actuators.     

A mixed numerical/experimental technique for the nondestructive identification of the stiffness properties of fibre reinforced composite materials

The characterization of the stiffness properties of fibre reinforced composite (FRC) materials presents more difficulties than the characterization of traditional isotropic materials. This paper first describes the difficulties that can arise and next presents a nondestructive method that offers a possible solution to the problems. The proposed method is a so-called ‘Mixed Numerical/Experimental Technique’ (MNET). From an experimental point of view, the method requires the measurement of resonant frequencies of freely suspended rectangular test plates. From the numerical side, an accurate model of the test plate must be available. The stress field associated with the different resonant frequencies is multi-axial. The measurement procedure however is very simple and requires little specimen preparation. The equipment is inexpensive and the measurement procedure can be controlled by a PC program. The proposed MNET allows the simultaneous determination of all the anisotropical properties and provides an estimation of their statistical distribution.