John H. Cantrell

Crystalline structure and symmetry dependence of acoustic nonlinearity parameters

A quantitative measure of elastic wave nonlinearity in crystals is provided by the acoustic nonlinearity parameters. The nonlinearity parameters are defined for arbitrary propagation modes for solids of arbitrary crystalline symmetry and are determined along the pure mode propagation directions for 33 crystals of cubic symmetry from data reported in the literature. The magnitudes of the nonlinearity parameters are found to exhibit a strong dependence on the crystalline structure and symmetries associated with the modal direction in the solid. Calculations based on the Born–Mayer potential for crystals having a dominant repulsive contribution to the elastic constants from the interatomic pair potential suggest that the origin of the structure dependence is associated with the shape rather than the strength of the potential. Considerations based on variations in crystal symmetry during loading along pure mode propagation directions of face‐centered‐cubic solids provide a qualitative explanation for the depe...

Scanning electron acoustic microscopy of indentation‐induced cracks and residual stresses in ceramics

The ability of scanning electron acoustic microscopy (SEAM) to characterize ceramic materials is assessed. SEAM images of Vickers indentations in SiC whisker‐reinforced alumina clearly reveal not only the radial cracks, the length of which can be used to estimate the fracture toughness of the material, but also reveal strong contrast, interpreted as arising from the combined effects of lateral cracks and the residual stress field left in the SiC whisker‐reinforced alumina by the indenter. The strong contrast is removed after the material is heat treated at 1000u2009°C to relieve the residual stresses around the indentations. A comparison of these observations with SEAM and reflected polarized light observations of Vickers indentations in soda‐lime glass both before and after heat treatment confirms our interpretation of the strong contrast.

The Effects of Artificial Aging of Aluminum 2024 on its Nonlinearity Parameter

Precipitation-hardened aluminum alloys have strengthening precipitates that form from the alloy’s solid solution [1]. Depending on the constituents in the alloy, the formation of these precipitates occur during natural aging (aging at room temperature) or artificial aging (aging at an elevated temperature) depending on the time-temperature nucleation characteristics of the precipitate in the alloy [2]. These precipitates greatly influence the dislocation dynamics of the alloy. Engineering properties, such as the ultimate yield strength and the hardness of these materials strongly depend upon the type and size of precipitates that are formed during the aging process.

Materials Characterization Using Acoustic Nonlinearity Parameters and Harmonic Generation: Engineering Materials

Most of the NDE effort using ultrasonics to assess engineering materials has been in the detection of cracks or crack-related phenomena. Other questions involving, for example, NDE measurements of temper or the state of fatigue prior to cack initiation, while very important to material scientists and design engineers, are not easily investigated using ultrasonic techniques based on linear theory. Recent work indicates, however, that the use of ultrasonics based on nonlinear concepts provides potentially useful information about material processing and certain pathological states that develop in materials as they are used.

Signal generation in scanning electron acoustic microscopy

Abstract The thermal conduction and Navier-Stokes equations are used to obtain the stress field generated by an electron beam incident on a disc sample. The piezoelectric equation is then used to obtain the output signal of the transducer coupled to the sample. The results lead to a consideration of the signal generation mechanisms and the spatial resolution in scanning electron acoustic microscopy (SEAM). It is argued that in the near field the spatial resolution is dependent on the distance between the driving source and scatterer and, at best, the lateral resolution is of the order of the diameter of the focal spot of the electron beam. Comparisons with experimental findings in SEAM are presented.

ACOUSTIC HARMONIC GENERATION AND DISLOCATION DYNAMICS OF FATIGUED ALUMINUM ALLOYS

Concern over the aging of the global commercial aircraft fleet has stimulated renewed interest in the development of new methodologies and re-exploring old techniques for the nondestructive evaluation and characterization of metal fatigue. The purpose of the present work is to explore the potential of using bulk acoustic nonlinearity measurements in the determination of metal fatigue with particular emphasis on aluminum alloys (especially A12024-T4).

Scanning electron acoustic microscopy of SiC particles in metal matrix composites

Abstract A scanning electron acoustic microscope operating in the 0.2–1.0 MHz frequency range was used to obtain a series of images of SiC particles embedded in an aluminium matrix. The contrast of the SiC particles was found to change sharply with modulation frequency in very narrow frequency bands. Strong variations with pronounced maxima in contrast were reached at frequencies corresponding to nodes in the sample-transducer vibrational frequency spectrum. Similar variations were found with phase-sensitive detection when the frequency was held fixed at any frequency and the reference phase was varied. Maximum contrast, however, was still found at the nodal frequencies. Such findings are markedly dissimilar to the bright field conditions usually specified for scanning electron acoustic microscopy operation. The role of thermal-to-acoustic wave mode conversion in explaining these results is explored.

MATERIALS CHARACTERIZATION USING ACOUSTIC NONLINEARITY PARAMETERS AND HARMONIC GENERATION: EFFECTS OF CRYSTALLINE AND AMORPHOUS STRUCTURES

The importance of nonlinearity in the description of material behavior is gaining widespread attention. Nonlinearity plays a major, if not dominating, role in a number of material properties. For example, properties that are important in engineering design such as thermal expansion or the pressure dependence of optical refraction are inherently nonlinear [1]. New assembley techniques such as the use of ultrasonic gauges to determine the loading of critical fasteners depend upon nonlinear properties of the fasteners [2]. Areas of considerable fundamental interest in nonlinearity include lattice dynamics [3], radiation stress in solids [4,5], and nonlinear optics [6].

Examination of Fiber-Matrix Interfaces of Composite Materials Using Dual Beam Scanning Acoustic Microscopy

One of the most important factors determining the properties of composite materials is the fiber-matrix interface. The material on either side of the interface may have large differences in mechanical properties which could leave the interface vulnerable to microcrack initiation during loading. Since this may lead to catastrophic failure of the composite, it is essential to understand the mechanical behavior of the fiber-matix interfaces at a microscopic level. It has been observed that in many cases the interface is not discontinuous, but rather forms a continuum of three dimensional interphases wherein the material properties vary gradually over a distance of a few microns. Such interphases have been observed using both electron and acoustic microscopies. The variation of elastic properties in this interphasial region is found to play a dominant role in determining the mechanical properties of the system.

Variation of Sound Velocity in Fatigued Aluminum 2024-T4 as a Function of Hydrostatic Pressure

It is well known that a pressure or stress derivative measurement of the sound velocity gives information about the nonlinear elastic properties of the material under investigation. Over the last several decades numerous experiments on various solids using uniaxial stress apparatus or oil-filled hydrostatic pressure chambers have been reported. In most cases the pressure or stress must be increased to several thousand kilobars to obtain reasonable measurements. The necessity of using such high pressure usually results from the low sensitivity of the devices or techniques used to measure the change in the sound velocity.