Guided Interface Waves

Many of tomorrow’s technologies are dependent upon the emergence of new advanced materials with superior stiffness and strength but reduced density. Metal matrix composites (MMC’s) consisting of light metal matrices (e. g., aluminum, titanium or magnesium) reinforced with very stiff ceramic fibers or particles (e. g. SiC, AI2O3 or graphite) show considerable promise for satisfying this need. However, the satisfactory performance of these materials has been found to be critically dependent upon the attainment of optimal properties at the metal-ceramic interface; a problem that is compounded by the possibility of chemical reactions between the reactive metal matrix and ceramic reinforcement. Of particular import are the interface adhesion and local elastic properties. Unfortunately no methods exist for the measurement of these quantities even for macroscopic interfaces let alone for the microscopic interfaces occurring within MMC’s

Ultrasonic Methods for Characterizing the Interface in Composites

Micromechanical modeling studies of composite materials have highlighted the need for better characterization of the interface zone in composite materials. Bulk behavior in composites has been predicted to be strongly influenced by the local elastic properties, residual stresses, and adhesion of the interface. Techniques to nondestructively measure these newly perceived quantities of importance do not exist. Thus it is not possible experimentally to (i) confirm the micromechanical model predictions, (ii) explore the relationships between interface properties and processing variables, and (iii) ensure acceptable interface properties in commercially fabricated composites. We report here the current status of a SDIO/ONR funded research program directed at developing experimental techniques for characterizing the interface zone in composites through the use of ultrasonic interface waves [1]

Erosion and Breakup of Polymer Drops Under Simple Shear in High Viscosity Ratio Systems

The deformation and breakup of a single polycarbonate (PC) drop in a polyethylene (PE) matrix were studied at high temperatures under simple shear flow using a specially designed transparent Couette device. Two main breakup modes were observed: (a) erosion from the surface of the drop in the form of thin ribbons and streams of droplets and (b) drop elongation and drop breakup along the axis perpendicular to the velocity direction. This is the first time drop breakup mechanism (a), "erosion," has been visualized in polymer systems. The breakup occurs even when the viscosity ratio (ηr) is greater than 3.5, although it has been reported that breakup is impossible at these high viscosity ratios in Newtonian systems. The breakup of a polymer drop in a polymer matrix cannot be described by Capillary number and viscosity ratio only; it is also controlled by shear rate, temperature, elasticity and other polymer blending parameters. A pseudo first order decay model was used to describe the erosion phenomenon and it fits the experimental data well.link_to_subscribed_fulltex

QUANTUM TUNNELING OF TRAPPED HYDROGEN IN Nb

Quantum tunneling of hydrogen trapped by oxygen in a Nb lattice is found at low temperatures. Information obtained by various measurement techniques will be reviewed and compared. Ultrasonic measurements of attenuation and velocity as a function of polarization, temperature, frequency, defect concentration, isotope and cooling rate provide detailed quantitative information particularly concerning the symmetry and dynamics of isolated systems in Nb where the normal to superconducting transition is used to establish the effect of conduction electrons on the tunneling rate. In the OH system a peak at 2.25 K at 10 MHz in the superconducting state disappears in the normal state. In a second system produced by rapid cooling, a peak at 6 K at 10 MHz also moves dramatically, but in this case the response can be fully measured in the normal state. It is found that a two level system (TLS) formalism which takes into account the relaxation stimulated by inelastic scattering of electrons gives a good quantitative description of the quantum behavior of the OH system. The theory used for analyzing the data is similar to that for metallic glasses but simpler. Ultrasonic experiments, particularly high accuracy velocity measurements, are sufficient for the complete evaluation of all three parameters of the TLS ; Ɗ0 - the minimum gap, α - the coupling to the strain field, and ε0 - the average absolute strain magnitude. For the OH system, the relaxation rate in the normal state cannot be measured. For the quenching peak, the relaxation rate as well as the quantum depletion of the relaxation strength, are directly accessible

Guided Interface Waves

Many of tomorrow’s technologies are dependent upon the emergence of new advanced materials with superior stiffness and strength but reduced density. Metal matrix composites (MMC’s) consisting of light metal matrices (e. g., aluminum, titanium or magnesium) reinforced with very stiff ceramic fibers or particles (e. g. SiC, AI2O3 or graphite) show considerable promise for satisfying this need. However, the satisfactory performance of these materials has been found to be critically dependent upon the attainment of optimal properties at the metal-ceramic interface; a problem that is compounded by the possibility of chemical reactions between the reactive metal matrix and ceramic reinforcement. Of particular import are the interface adhesion and local elastic properties. Unfortunately no methods exist for the measurement of these quantities even for macroscopic interfaces let alone for the microscopic interfaces occurring within MMC’s.</p

Application of Scanning Acoustic Microscopy to Residual Stress Analysis: Theory vs. Experiment

In this project, a new technique for the nondestructive evaluation of residual stress in manufactured components is proposed. Where most approaches to residual stress analysis have been based on monitoring the stress induced changes in sound velocity these techniques are limited in their utility due to use of contact, shear transducers (effectively precluding scanning large areas), the inability to resolve the dependence of residual stresses on depth (important for surface treatments) and the difficulty in making accurate time delay measurements due to the very small acoustoelastic effect observed for most practical materials. Here, we propose to develop a nondestructive test technique suitable for scanning plate structures. An aspherically focused, immersion transducer is used in a scan mode to generate an axially symmetric pulse. We utilize interference phenomena between two shear waves polarized in the directions of the in-plane principal stress axes to increase resolution of the small differences in transit time between the two waves. This technique may become a powerful tool to study actual residual stress distributions in practical engineering materials.</p

UNDERDAMPED DISLOCATION RESONANCE IN Cu

Measurements of dislocation dependent attenuation (αD) in Cu and Al are obscurred at temperatures below 50 K by the attenuation due to electron-phonon interactions (αE). This paper presents the measurements on Al and Cu single crystals in which αE is eliminated by using a magnetic field [MATH] ⊥ [MATH] perpendicular to the direction of propagation of the acoustic waves. Underdamped resonance peak in αD was obtained at 4.2 K in a slightly deformed Cu sample. The results are discussed in the framework of the Granato-Lücke theory

Fast Leaky Modes on Cylindrical Metal-Ceramic Interfaces

In our previous work [1,2,3,], we have studied in detail the radial-axial modes in an infinitely clad isotropic rod. We have shown that in metal matrix composites, where the fibers are stiffer than the matrix, many of these modes are leaky, transmitting energy into the surrounding medium. The existence of this leakage energy offers a potential means for monitoring and imaging the characteristics of the interface zone [2,3,4,]. Detailed numerical methods have been developed for analyzing radial-axial leaky modes in composite systems [1]. The present paper shows an example of these methods applied to determining the sensitivity of the maximum phase velocity to the matrix density changes for leaky modes.</p

Ultrasonic Methods for Characterizing the Interface in Composites

Micromechanical modeling studies of composite materials have highlighted the need for better characterization of the interface zone in composite materials. Bulk behavior in composites has been predicted to be strongly influenced by the local elastic properties, residual stresses, and adhesion of the interface. Techniques to nondestructively measure these newly perceived quantities of importance do not exist. Thus it is not possible experimentally to (i) confirm the micromechanical model predictions, (ii) explore the relationships between interface properties and processing variables, and (iii) ensure acceptable interface properties in commercially fabricated composites. We report here the current status of a SDIO/ONR funded research program directed at developing experimental techniques for characterizing the interface zone in composites through the use of ultrasonic interface waves [1].</p