Ultrasonic characterization of elastic constants and defects in composite materials

This thesis consists of two parts. The first part deals with the determination of anisotropic elastic constants of silicon carbide particulate (SiC[subscript] p) reinforced Al matrix composites using ultrasonic velocity measurements. The composite materials, fabricated in the form of plates by a powder metallurgy extrusion process, included 2124, 6061 and 7091 Al alloys reinforced by 10-30% by volume of [alpha]-SiC[subscript]p. All of the reinforced composite samples exhibited orthotropic behavior, with the maximum elastic anisotropy existing between the extrusion direction and the out-of-plan direction. Microstructure was examined and revealed that the observed elastic anisotropy could be attributed to the preferred orientation distribution of SiC[subscript]p. A theoretical model based on the Eshelby\u27s method and Mori-Tanaka\u27s theory was used to predict the elastic constants of the Al/SiC[subscript]p composites. A 6 x 6 matrix form of effective stiffness expression was developed to investigate the effect of particle characteristics on the anisotropic properties. Ultrasonically measured constants were compared with both the tensile test data and the model prediction; reasonably good agreement was obtained. The second part of the thesis is concerned with the theoretical and experimental studies of the ultrasonic velocity for the nondestructive evaluation of porosity in carbon fiber reinforced plastics (CFRP). A fiber reinforced composite containing voids was used to study the effect of void characteristics and fiber properties on the ultrasonic velocity propagating normal to the fiber. The results showed that the velocity decreased with increasing void content. The void shape was found to have a significant effect on the rate of velocity decrease. However, the volume fraction of transversely isotropic fibers had a negligible effect on the ultrasonic velocity of composites containing voids. Ultrasonic spectroscopy was employed to measure the phase velocity and the attenuation in a through transmission, immersion testing mode. The composite materials studied included carbon (graphite) fiber reinforced epoxy and polyimide laminates containing different level of porosity. Experimental results showed that void content had a correlation with ultrasonic phase velocity. With increasing void content, the velocity decreased substantially. In addition, the velocity of composites containing voids was found to be more dispersive than that of void-free composites. Finally, the relationship between ultrasonic attenuation and velocity dispersion was tested using the local approximation of the exact Kramers-Kronig relation

Measurements of nonlinear harmonic waves at cracked interfaces

Nonlinear harmonic waves generated at cracked interfaces are investigated both experimentally and theoretically. A compact tension specimen is fabricated and the amplitude of transmitted wave is analyzed as a function of position along the fatigued crack surface. In order to measure as many nonlinear harmonic components as possible a broadband Lithium Niobate (LiNbO3) transducers are employed together with a calibration technique for making absolute amplitude measurements with fluid‐coupled receiving transducers. Cracked interfaces are shown to generate high acoustic nonlinearities which are manifested as harmonics in the power spectrum of the received signal. The first subharmonic (f∕2) and the second harmonic (2f) waves are found to be dominant nonlinear components for an incident toneburst signal of frequency f. To explain the observed nonlinear behavior a partially closed crack is modeled by planar half interfaces that can account for crack parameters such as crack opening displacement and crack surface conditions. The simulation results show reasonable agreements with the experimental results

Nonlinear Time Reversal Focusing and Detection of Fatigue Crack

Abstract This paper presents an experimental study on the detection and location of nonlinear scattering source due to the presence of fatigue crack in a laboratory specimen. The proposed technique is based on a combination of nonlinear elastic wave spectroscopy(NEWS) and time reversal(TR) focusing approach. In order to focus on the nonlinear scattering position due to the fatigue crack, we employed only one transmitting transducer and one receiving transducer, taking advantage of long duration of reception signal that includes multiple linear scattering such as mode conversion and boundary reflections. NEWS technique was then used as a pre-treatment of TR for spatial focusing of reemitted second harmonic signal. The robustness of this approach was demonstrated on a cracked specimen and the nonlinear TR focusing behavior is observed on the crack interface from which the second harmonic signal was originated

Focused Ultrasonic Beam Behavior at a Stress-Free Boundary and Applicability for Measuring Nonlinearity Parameter in a Reflection Mode

Measurements of the acoustic nonlinearity parameter β are frequently made for early detection of damage in various materials. The practical implementation of the measurement technique has been limited to the through-transmission setup for determining the nonlinearity parameter of the second harmonic wave. For the purpose of practical applications, a pulse-echo measurement technique is more desirable which enables the single-side access of test components. The issue with using the second harmonic wave reflected from the stress-free interface is that such a boundary destructively alters the nonlinear generation process and consequently makes it difficult to obtain the reliable results of β. In this work, we employ a focused beam theory to modify the phase reversal at the stress-free boundary, and consequently enhance the second harmonic generation during its back-propagation toward the initial source position. We first confirm this concept through experiment by using a spherically focused beam at the water-air interface, and measuring the reflected second harmonic and comparing with a planar wave reflected from the same stress-free or a rigid boundary. In order to test the feasibility of this idea for measuring the nonlinearity parameter of solids in a reflection mode, an array transducer beam is modeled for focusing at and reflection from a stress-free boundary. A nonlinearity parameter expression is then defined together with diffraction and attenuation corrections

Nonlinear beam fields simulation of a mixed wave and definition of nonlinearity parameter with diffraction correction

The acoustic nonlinearity parameter has been frequently measured for early detection of micro damage in various materials. The technique typically employs a toneburst signal of single frequency and measures the second harmonic generation during its propagation in through-transmission mode. In this work, we propose a two wave mixing technique and the use of difference frequency components in determining the nonlinearity parameter. One important advantage of this technique is to use difference frequency components apart from higher harmonics including the second harmonic, therefore effects of source nonlinearity can be minimized and low attenuating nonlinear signal can be acquired. Beam fields radiated from various configurations of radiating transducers are simulated. The fundamental and difference frequency waves are calculated using the multi-Gaussian beam model based on the quasilinear solution for the Westervelt equation. Explicit expressions for diffraction and attenuation corrections are derived, and the nonlinearity parameter is newly defined with these corrections included

Measurement of Elastic Moduli in Ceramic Composites as a Function fo Porosity Content

Longitudinal and transverse ultrasonic velocity measurements were made to obtain elastic moduli of ceramic compacts and continuous fiber ceramic composites (CMCC) as a function of porosity volume fraction. The ceramic compacts were hot pressed silicon carbide and the CMCC were Nicalon fiber reinforced silicon carbide, manufactured using a forced chemical vapor infiltration (FCVI) process developed at Oak Ridge National Laboratory [1]. The purpose of the SiC powder compact study was to obtain experimental results of elastic moduli for various porosity level and to compare the measured results with predictions based on theoretical models. For chemical vapor infiltrated Nicalon/SiC ceramic composites, elastic constants data at different porosity level were not readily available in the literature. The purpose of the study was therefore to generate a more complete set of modulus data as a function of void content. These results can be used for the optimization of the manufacturing process and for comparison with mechanical testing results

Ultrasonic characterization of elastic constants and defects in composite materials

This thesis consists of two parts. The first part deals with the determination of anisotropic elastic constants of silicon carbide particulate (SiC[subscript] p) reinforced Al matrix composites using ultrasonic velocity measurements. The composite materials, fabricated in the form of plates by a powder metallurgy extrusion process, included 2124, 6061 and 7091 Al alloys reinforced by 10-30% by volume of [alpha]-SiC[subscript]p. All of the reinforced composite samples exhibited orthotropic behavior, with the maximum elastic anisotropy existing between the extrusion direction and the out-of-plan direction. Microstructure was examined and revealed that the observed elastic anisotropy could be attributed to the preferred orientation distribution of SiC[subscript]p. A theoretical model based on the Eshelby's method and Mori-Tanaka's theory was used to predict the elastic constants of the Al/SiC[subscript]p composites. A 6 x 6 matrix form of effective stiffness expression was developed to investigate the effect of particle characteristics on the anisotropic properties. Ultrasonically measured constants were compared with both the tensile test data and the model prediction; reasonably good agreement was obtained. The second part of the thesis is concerned with the theoretical and experimental studies of the ultrasonic velocity for the nondestructive evaluation of porosity in carbon fiber reinforced plastics (CFRP). A fiber reinforced composite containing voids was used to study the effect of void characteristics and fiber properties on the ultrasonic velocity propagating normal to the fiber. The results showed that the velocity decreased with increasing void content. The void shape was found to have a significant effect on the rate of velocity decrease. However, the volume fraction of transversely isotropic fibers had a negligible effect on the ultrasonic velocity of composites containing voids. Ultrasonic spectroscopy was employed to measure the phase velocity and the attenuation in a through transmission, immersion testing mode. The composite materials studied included carbon (graphite) fiber reinforced epoxy and polyimide laminates containing different level of porosity. Experimental results showed that void content had a correlation with ultrasonic phase velocity. With increasing void content, the velocity decreased substantially. In addition, the velocity of composites containing voids was found to be more dispersive than that of void-free composites. Finally, the relationship between ultrasonic attenuation and velocity dispersion was tested using the local approximation of the exact Kramers-Kronig relation.</p

Measurements of nonlinear harmonic waves at cracked interfaces

Nonlinear harmonic waves generated at cracked interfaces are investigated both experimentally and theoretically. A compact tension specimen is fabricated and the amplitude of transmitted wave is analyzed as a function of position along the fatigued crack surface. In order to measure as many nonlinear harmonic components as possible a broadband Lithium Niobate (LiNbO3) transducers are employed together with a calibration technique for making absolute amplitude measurements with fluid‐coupled receiving transducers. Cracked interfaces are shown to generate high acoustic nonlinearities which are manifested as harmonics in the power spectrum of the received signal. The first subharmonic (f∕2) and the second harmonic (2f) waves are found to be dominant nonlinear components for an incident toneburst signal of frequency f. To explain the observed nonlinear behavior a partially closed crack is modeled by planar half interfaces that can account for crack parameters such as crack opening displacement and crack surface conditions. The simulation results show reasonable agreements with the experimental results.Copyright 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. This article appeared in AIP Conference Proceedings 1335 (2011): 306–313 and may be found at http://dx.doi.org/10.1063/1.3591869.</p

Ultrasonic Velocity Change and Dispersion Due to Porosity in Composite Laminates

Voids or porosity in carbon fiber reinforced plastics (CFRP) caused by improper curing, moisture in the prepreg and other reasons can degrade the mechanical properties of the composite components [1–3]. Voids caused by trapped air in the layup process or volatile gas released in the curing process tend to occur at the interface between the plies of unidirectional prepregs and are usually elongated along the adjacent fiber directions [4]. On the other hand, voids in woven composites tend to be more spherical. Matrix dominated strengths such as transverse tensile and interlaminar shear strengths are affected the most by the presence of porosity. Quantitative nondestructive evaluation (QNDE) methods for the detection and characterization of porosity in composites are therefore highly desirable.</p

Optimal Design of Annular Phased Array Transducers for Material Nonlinearity Determination in Pulse–Echo Ultrasonic Testing

Nonlinear ultrasound has been proven to be a useful nondestructive testing tool for micro-damage inspection of materials and structures operating in harsh environment. When measuring the nonlinear second harmonic wave in a solid specimen in the pulse&ndash;echo (PE) testing mode, the stress-free boundary characteristics brings the received second harmonic component close to zero. Therefore, the PE method has never been employed to measure the so-called &ldquo;nonlinear parameter (&beta;)&rdquo;, which is used to quantify the degree of micro-damage. When there are stress-free boundaries, a focused beam is known to improve the PE reception of the second harmonic wave, so phased-array (PA) transducers can be used to generate the focused beam. For the practical application of PE nonlinear ultrasonic testing, however, it is necessary to develop a new type of PA transducer that is completely different from conventional ones. In this paper, we propose a new annular PA transducer capable of measuring &beta; with improved second harmonic reception in the PE mode. Basically, the annular PA transducer (APAT) consists of four external ring transmitters and an internal disk receiver at the center. The focused beam properties of the transducers are analyzed using a nonlinear sound beam model which incorporates the effects of beam diffraction, material attenuation, and boundary reflection. The optimal design of the APAT is performed in terms of the maximum second harmonic reception and the total correction close to one, and the results are presented in detail