Bond Strength Measurements by Ultrasonic Spectroscopy

The goal of this project has been to discover techniques for predicting the strength of a metal-to-metal adhesive bond from nondestructive measurements on the completed structure. Both the cohesive strength of the adhesive material itself and the adhesive strength of the metal-to-adhesive interface must be determined separately. In previous phases of the program, it was demonstrated that the Fourier transform of the ultrasonic echo returned from a metal-adhesive-metal sandwich structure immersed in a water bath contained sufficient information to obtain a prediction of the cohesive strength of the joint. Furthermore, certain features of the Fourier transform were shifted by thin layers of different materials at the metal-to-adhesive interfaces so that detecting poor adhesion was also a possibility. During the current phase of the program, more reliable mechanical tests and more accurate measurement techniques were developed. As a result, measurements of the wave velocity in FM-400 adhesive joints subjected to different degrees of cure correlated with the cohesive shear strength of the joints. Quantitative measurements of the standing wave resonant frequencies in Chemlok 304 adhesive joints showed a correlation with the strength of adhesion at the metal-to-adhesive interface

A Compact EMAT Receiver for Ultrasonic Testing at Elevated Temperatures

For the past several years, the AISI and several national laboratories have cooperated on a program to develop ultrasonic transducers that can be used in steel mills at the highest temperatures encountered during the processing of the solid metal {1}. To date, pulsed laser light focused on the surface appears to make a satisfactory generator or transmitter for ultrasonic pulses while the EMAT or Electromagnetic Acoustic Transducer appears to hold the greatest promise for the receiver {2}. Both of these essentially noncontact devices can be made to withstand the very hot environmnent and they are not very sensitive to the quality of the surface

Effects of Two Energy Scales in Weakly Dimerized Antiferromagnetic Quantum Spin Chains

By means of thermal expansion and specific heat measurements on the high-pressure phase of (VO)$_2$P$_2$O$_7$, the effects of two energy scales of the weakly dimerized antiferromagnetic $S$ = 1/2 Heisenberg chain are explored. The low energy scale, given by the spin gap $\Delta$, is found to manifest itself in a pronounced thermal expansion anomaly. A quantitative analysis, employing T-DMRG calculations, shows that this feature originates from changes in the magnetic entropy with respect to $\Delta$, $\partial S^{m}/ \partial \Delta$. This term, inaccessible by specific heat, is visible only in the weak-dimerization limit where it reflects peculiarities of the excitation spectrum and its sensitivity to variations in $\Delta$.Comment: 4 pages, 4 figures now identical with finally published versio

Electronic interactions in fullerene spheres

The electron-phonon and Coulomb interactions inC$_{60}$, and larger fullerene spheres are analyzed. The coupling between electrons and intramolecular vibrations give corrections $\sim 1 - 10$ meV to the electronic energies for C$_{60}$, and scales as $R^{-4}$ in larger molecules. The energies associated with electrostatic interactions are of order $\sim 1 - 4$ eV, in C$_{60}$ and scale as $R^{-1}$. Charged fullerenes show enhanced electron-phonon coupling, $\sim 10$ meV, which scales as $R^{-2}$. Finally, it is argued that non only C$_{60}^{-}$, but also C$_{60}^{--}$ are highly polarizable molecules. The polarizabilities scale as $R^3$ and $R^4$, respectively. The role of this large polarizability in mediating intermolecular interactions is also discussed.Comment: 12 pages. No figure

Detection and Measurement of Defects in Butt Welds

Sheet metal is produced in rolling mills in the form of coils of finite length. However, stamping and forming plants use these coils as a continuous stream of material to produce all the products we take for granted throughout society. Thus, the middle of this production chain must contain a “shock absorber” where the end of one coil is welded to the beginning of another. If this weld fails, expensive damage can result and productivity is definitely slowed down. Figure 1 shows this process in a schematic form and indicates that the welding is performed in between two clamps that hold the ends of the coils together while the weld is made. Special storage sections are included in the line so that the downstream flow of metal is not interrupted during the time in which the upstream flow is stopped for the weld. This welding machine not only forms the joint but knives clean off the weld bead on the top and bottom of the plate so that a smooth surface is presented to the downstream machines

Solitonic-exchange mechanism of surface~diffusion

We study surface diffusion in the framework of a generalized Frenkel-Kontorova model with a nonconvex transverse degree of freedom. The model describes a lattice of atoms with a given concentration interacting by Morse-type forces, the lattice being subjected to a two-dimensional substrate potential which is periodic in one direction and nonconvex (Morse) in the transverse direction. The results are used to describe the complicated exchange-mediated diffusion mechanism recently observed in MD simulations [J.E. Black and Zeng-Ju Tian, Phys. Rev. Lett. {\bf 71}, 2445-2448(1993)].Comment: 22 Revtex pages, 9 figures to appear in Phys. Rev.

Sliding charge density wave in manganites

The so-called stripe phase of the manganites is an important example of the complex behaviour of metal oxides, and has long been interpreted as the localisation of charge at atomic sites. Here, we demonstrate via resistance measurements on La_{0.50}Ca_{0.50}MnO_3 that this state is in fact a prototypical charge density wave (CDW) which undergoes collective transport. Dramatic resistance hysteresis effects and broadband noise properties are observed, both of which are typical of sliding CDW systems. Moreover, the high levels of disorder typical of manganites result in behaviour similar to that of well-known disordered CDW materials. Our discovery that the manganite superstructure is a CDW shows that unusual transport and structural properties do not require exotic physics, but can emerge when a well-understood phase (the CDW) coexists with disorder.Comment: 13 pages; 4 figure