Bruce Maxfield - Academia.edu (original) (raw)

Papers by Bruce Maxfield

Biochemistry, 1992

The reversible folding of cytochrome c in urea at pH 4.0 was investigated by repetitive pressure ... more The reversible folding of cytochrome c in urea at pH 4.0 was investigated by repetitive pressure perturbation kinetics and by equilibrium spectroscopic methods. Two folding reactions were observed in the 1 ms to 10 s time range. The rates and amplitudes of these reactions depend on urea concentration in a complex manner, which is different for each process. The absorbance spectra of the kinetic amplitudes of the two reactions also differ from each other. A model with a three-state mechanism can quantitatively account for all of the kinetic and equilibrium data, and it enables us to determine the rate constants and volume changes of the two steps. If a rapid protonation step is added to the mechanism, the analysis can be extended to calculate the p H dependence of the rate and amplitude of the faster folding step. This pH dependence is in excellent agreement with previously published data [Tsong, T. Y. (1977) J. Biol. Chem. 252, , Kinetic experiments in the 695-nm band show clearly that the axial ligand methionine-80 is involved in the slow folding process and the other axial ligand, histidine-18, is involved in the fast process.

Journal of The Acoustical Society of America, 1993

A new technique is proposed in this paper to measure Rayleigh and Lamb wave speeds in solid half-... more A new technique is proposed in this paper to measure Rayleigh and Lamb wave speeds in solid half-spaces and plates. In this technique two transducers are positioned above the specimen in a pitch-catch orientation. The time of flight of the signal from the transmitter to the receiver is recorded. Then the rate of change of this time as the distance between the reflector and the transducer varies is experimentally determined. This rate remains constant when leaky Rayleigh of Lamb waves are generated but it varies when these waves are not generated. Thus surface waves are detected in an indirect manner. An expression is derived to relate the surface wave speed to the signal flight time change rate with the transducer specimen distance. Using this expression Rayleigh and Lamb wave speeds have been accurately determined in isotropic metals and anisotropic composites.

Erratum: ``A new technique for measuring Rayleigh and Lamb wave speeds

Journal of The Acoustical Society of America, 1994

Generation and detection of higher-order mode clusters of guided waves (HOMC-GW) using meander-coil EMATs

IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2012

This paper reports on a new means of generating higher-order mode clusters of guided waves (HOMC-... more This paper reports on a new means of generating higher-order mode clusters of guided waves (HOMC-GW) using a meander-coil (MC) electromagnetic acoustic transducer (EMAT) in plates at frequencies significantly higher than the lower-order plate modes. These wave modes are considerably less dispersive and they occur at much higher frequency-thickness (f x d) products. Our studies cover the f x d range of 13 to 20 MHz·mm. Experimental measurements were carried out on Al plate samples of different thicknesses using three different EMAT coil periods. To understand the generation and propagation characteristics of HOMC-GW with EMATs, several simulations were carried out using 2-D finite element models at different f x d products. These simulations captured all features observed in the experiments. The time-frequency smoothed pseudo Wigner-Ville distribution (SPWVD) was used to analyze the HOMC-GW modes. Defect detection measurements using HOMC-GW generated using EMATs were made on Al plates with machined defects.

Algorithm for Health Monitoring of Anisotropic Plates Using Flexible Ultrasonic Patches

ABSTRACT. Fiber reinforced composite plate-like structures have been used to achieve substantial ... more ABSTRACT. Fiber reinforced composite plate-like structures have been used to achieve substantial reductions in the structural weight of both military and commercial aircrafts. For large area and layered structures, damage detection using any conventional testing ...

Ndt & E International, 2008

The non-dispersive propagation of ultrasonic guided wave higher order modes cluster (HOMC) travel... more The non-dispersive propagation of ultrasonic guided wave higher order modes cluster (HOMC) traveling along the circumferential direction in a hollow cylinder and its interaction with defects in pipe support regions is reported. These circumferential guided waves were generated in mild steel (MS) pipe specimens containing artificially created axial notches (simulating axial cracks) and pinholes (simulating pinhole-like defects) of different sizes in order to simulate conditions such as cracking and corrosion under pipe supports. The characteristics of these guided waves were also studied as a function of parameters related to how they were generated; namely, using: (a) 2.25 MHz linear phased array transducer, (b) 2.25 MHz conventional circular transducer and (c) 1 MHz conventional circular transducer. These higher frequency modes were explored for their ability to detect and size defects. Because of access limitations to the pipe support region in actual field testing, the transducer was always placed at a fixed circumferential position and moved axially along the length of the pipe. The defect position along the circumference was ascertained from the time of flight while the defect size was estimated using the amplitude data. The signals obtained for all three transducer configurations are compared for their ability to locate, detect and size the above-mentioned defects. It was shown that at these relatively higher frequencies, the guided wave modes exhibit small dispersion and have the ability to provide improved imaging of small size defects throughout the cross-section of the pipe. r

Magnetostrictive Acoustic Transducer Based Torsional Wave Generation in Pipes Using Magnetic Tapes

... E. Kannan, B. Maxfield and K. Balasubramaniam ... The authors thank Mr. Jaya Rao, Mr. Vignesh... more ... E. Kannan, B. Maxfield and K. Balasubramaniam ... The authors thank Mr. Jaya Rao, Mr. Vignesh, Mr. Muniraj and Mr.Vinay Kumar of Center for Nondestructive Evaluation (CNDE), Indian Institute of Technology Madras, for their assistance in conducting the experiments. ...

In-Situ Damage Detection in Plate Structures Using PWAS and Non-Contact Laser Doppler Velocimeter

The capability of embedded piezoelectric wafer active sensors (PWAS) to perform in-situ Nondestru... more The capability of embedded piezoelectric wafer active sensors (PWAS) to perform in-situ Nondestructive evaluation (NDE) for structural health monitoring (SHM) of fiber reinforced polymer (FRP) composite plate like structures is explored. The basic principles of Lamb wave transmission and reception with PWAS transducers were verified with simple laboratory experiments, performed on both isotropic and anisotropic plates. In the second case, Noncontact measurements for Lamb wave sensing using Laser Doppler Velocimeter were explored.