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Biomedical subjects

B T Smith

Publications and source records attributed to B T Smith.

At least 19 recordsLinked to original sources
PubMed2003

Implanted functional electrical stimulation: an alternative for standing and walking in pediatric spinal cord injury.

STUDY DESIGN: Post intervention, repeated measures design, comparing two interventions. SETTING: Orthopedic pediatric hospital specializing in spinal cord injury. METHODS: Nine subjects, ages 7-20 years, received an eight-channel implanted lower extremity functional electrical stimulation (FES) system for standing and walking. Electrodes were placed to stimulate hip and knee extension, and hip abduction and adduction. Standing and walking were achieved through constant stimulation to the implanted muscles, allowing a swing through gait pattern with an assistive device. After training with FES and long leg braces (LLB), subjects were tested in eight upright mobility activities, which were scored based upon completion time and level of independence. RESULTS: Seven subjects completed data collection. These subjects completed four activities faster (P<0.02) and five activities more independently (P<0.025) with FES as compared to LLB. Transitions between sitting and standing, which were scored in isolation for two mobility activities, were achieved faster and with more independence with FES. In addition, subjects reported preferring FES for the majority of activities. No activity required more time or more assistance to complete with FES as compared to LLB. CONCLUSION: The implanted FES system provided these subjects with enhanced functional abilities over traditional LLB and decreased the need for physical assistance by a caregiver, suggesting that it is a realistic alternative for upright mobility in a pediatric population with spinal cord injury.

Adolescent↗
PubMed2001

Visualization of mismatch repair in bacterial cells.

We determined the localizations of mismatch repair proteins in living Bacillus subtilis cells. MutS-GFP colocalized with the chromosome in all cells and formed foci in a subset of cells. MutL-GFP formed foci in a subset of cells, and its localization was MutS dependent. The introduction of mismatches by growth in 2-aminopurine caused a replication-dependent increase in the number of cells with MutS and MutL foci. Approximately half of the MutS foci colocalized with DNA polymerase foci. We conclude that MutS is associated with the entire chromosome, poised to detect mismatches. After detection, it appears that mismatch repair foci assemble at mismatches as they emerge from the DNA polymerase and are then carried away from the replisome by continuing replication.

2-Aminopurine↗
PubMed2001

An implantable upper extremity neuroprosthesis in a growing child with a C5 spinal cord injury.

OBJECTIVES: To implement a functional electrical stimulation (FES) hand neuroprosthesis called the Freehand System in a growing child with spinal cord injury (SCI) using extra lead wire to accommodate limb growth, and to evaluate the performance of the Freehand System during the subject's maturation. SETTING: Pediatric orthopedic hospital specializing in SCI rehabilitation. SUBJECT: Ten-year-old female patient with a C5 level SCI. METHOD: The Freehand System was implanted. Eight electrodes were implanted to targeted forearm and hand muscles to provide grasp and release function. The lead wire associated with each electrode was pathed subcutaneously up the arm with 4 cm of extra lead distributed throughout the path to accommodate expected limb growth. All leads were attached to a stimulator placed in the upper chest. Measures of lead unwinding, limb growth, stimulated muscle strength, and hand function were made at 6 and 16 months after implant. RESULTS: By 16 months post implant, the upper limb growth plates were closed and humeral and radial bone growth combined was 2.7 cm from the time of surgery. For all eight leads, lead unwinding in the upper arm was approximately 1.2 cm and was comparable to humeral bone growth (1.4 cm). Lead unwinding in the lower arm was also measurable for the two electrodes in hand muscles. Six of eight electrodes maintained grade 3 or better stimulated muscle strength throughout the growth period according to a manual muscle test. Of the two other electrodes, one appeared to have lost function due to depletion of excess lead. However, hand function with FES was comparable at 6 and 16 months post implant suggesting that growth did not negatively impact performance with the FES system. Hand function with FES was improved over voluntary hand function as well. Using the Freehand System, a pinch force of approximately 15 N was achieved compared to 1.3 N of voluntary tenodesis pinch force. Scores on the Functional Independence Measure (FIM) increased by 9 points when FES was used as compared to voluntary function. Improvements occurred primarily in eating and grooming. Independence in writing was achieved only with FES. CONCLUSIONS: For this child, hand function with the Freehand System was sustained over the growth period and was a significant functional improvement over voluntary hand function. By using excess lead wire, the Freehand System was successfully implemented before skeletal maturity, affording the child improved hand function earlier than would be otherwise indicated.

Arm↗
PubMed2000

Regiochemical studies of the ring expansion reactions of hydroxy azides with cyclic ketones.

The regiochemistry of ring expansions of 2-substituted cyclic ketones using 1,2-azidoethanol and 1,3-azidopropanol was examined. It was determined that the reactions of ketones with an adjacent methyl or ethyl group are generally unselective, but that bulkier substituents lead to preferential migration of the more highly substituted carbon. In addition, it was found that ketones bearing inductively electron-withdrawing substituents (OMe, Ph, Br) undergo selective migration of the less highly substituted carbon. For some substrates, alternative reaction pathways were also identified.

Azides↗
PubMed2000

Intraperoxisomal localization of very-long-chain fatty acyl-CoA synthetase: implication in X-adrenoleukodystrophy.

X-adrenoleukodystrophy (X-ALD) is a demyelinating disorder characterized by the accumulation of saturated very-long-chain (VLC) fatty acids (>C(22:0)) due to the impaired activity of VLC acyl-CoA synthetase (VLCAS). The gene responsible for X-ALD was found to code for a peroxisomal integral membrane protein (ALDP) that belongs to the ATP binding cassette superfamily of transporters. To understand the function of ALDP and how ALDP and VLCAS interrelate in the peroxisomal beta-oxidation of VLC fatty acids we investigated the peroxisomal topology of VLCAS protein. Antibodies raised against a peptide toward the C-terminus of VLCAS as well as against the N-terminus were used to define the intraperoxisomal localization and orientation of VLCAS in peroxisomes. Indirect immunofluorescent and electron microscopic studies show that peroxisomal VLCAS is localized on the matrix side. This finding was supported by protease protection assays and Western blot analysis of isolated peroxisomes. To further address the membrane topology of VLCAS, Western blot analysis of total membranes or integral membranes prepared from microsomes and peroxisomes indicates that VLCAS is a peripheral membrane-associated protein in peroxisomes, but an integral membrane in microsomes. Moreover, peroxisomes isolated from cultured skin fibroblasts from X-ALD patients with a mutation as well as a deletion in ALDP showed a normal amount of VLCAS. The consequence of VLCAS being localized to the luminal side of peroxisomes suggests that ALDP may be involved in stabilizing VLCAS activity, possibly through protein-protein interactions, and that loss or alterations in these interactions may account for the observed loss of peroxisomal VLCAS activity in X-ALD.

Adrenoleukodystrophy↗
PubMed2000

Endotoxin induces structure-function alterations of rat liver peroxisomes: Kupffer cells released factors as possible modulators.

We report that endotoxin treatment results in decreased amounts of peroxisomes as well as changes in structure and function of peroxisomal membranes. Peroxisomes isolated from the liver of control and treated animals showed a marked decrease in total protein, but no significant alteration in the sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) protein profile. However, the Western blot study of the peroxisomal beta-oxidation enzymes and catalase showed an increase in those enzymes in the peroxisomal peak of normal density in endotoxin-treated rats. Disintegration of peroxisomal membranes by carbonate treatment from endotoxin-treated liver and change in the fluidity of peroxisomal membranes suggests alterations in peroxisomal membrane structure. No such alterations were found in mitochondrial or microsomal membranes of endotoxin-treated livers. The lipid analysis of these organelles showed that the only organelle affected was the peroxisome, with a significant decrease in the phospholipid and cholesterol concentrations. To understand the mechanism of endotoxin-mediated alterations in peroxisomes, we studied the possible role of Kupffer cell secreted soluble factors (tumor necrosis factor alpha [TNF-alpha]) on the peroxisomal structure/function. Inactivation/elimination of Kupffer cells by gadolinium chloride before endotoxin treatment did not normalize the overall peroxisomal protein amount and the lipid composition of isolated peroxisomes. However, the levels of individual protein amount in remaining peroxisomes were normalized. Endotoxin also decreased peroxisomal beta-oxidation, and this was partially restored with gadolinium treatment. These results clearly show that peroxisomes are severely affected by endotoxin treatment and suggest that the damage to this organelle may contribute, at least in part, to endotoxin-induced hepatic cytotoxicity.

Animals↗
PubMed2000

Mechanism of spreading of the highly related neurofibromatosis type 1 (NF1) pseudogenes on chromosomes 2, 14 and 22.

Neurofibromatosis type 1 (NF1) is a frequent hereditary disorder that involves tissues derived from the embryonic neural crest. Besides the functional gene on chromosome arm 17q, NF1-related sequences (pseudogenes) are present on a number of chromosomes including 2, 12, 14, 15, 18, 21, and 22. We elucidated the complete nucleotide sequence of the NF1 pseudogene on chromosome 22. Only the middle part of the functional gene but not exons 21-27a, encoding the functionally important GAP-related domain of the NF1 protein, is presented in this pseudogene. In addition to the two known NF1 pseudogenes on chromosome 14 we identified two novel variants. A phylogenetic tree was constructed, from which we concluded that the NF1 pseudogenes on chromosomes 2, 14, and 22 are closely related to each other. Clones containing one of these pseudogenes cross-hybridised with the other pseudogenes in this subset, but did not reveal any in situ hybridisation with the functional NF1 gene or with NF1 pseudogenes on other chromosomes. This suggests that their hybridisation specificity is mainly determined by homologous sequences flanking the pseudogenes. Strong support for this concept was obtained by sequence analysis of the flanking regions, which revealed more than 95% homology. We hypothesise that during evolution this subset of NF1 pseudogenes initially arose by duplication and transposition of the middle part of the functional NF1 gene to chromosome 2. Subsequently, a much larger fragment, including flanking sequences, was duplicated and gave rise to the current NF1 pseudogene copies on chromosomes 14 and 22.

Base Sequence↗
PubMed2000

A role for the umuDC gene products of Escherichia coli in increasing resistance to DNA damage in stationary phase by inhibiting the transition to exponential growth.

The umuDC gene products, whose expression is induced by DNA-damaging treatments, have been extensively characterized for their role in SOS mutagenesis. We have recently presented evidence that supports a role for the umuDC gene products in the regulation of growth after DNA damage in exponentially growing cells, analogous to a prokaryotic DNA damage checkpoint. Our further characterization of the growth inhibition at 30 degrees C associated with constitutive expression of the umuDC gene products from a multicopy plasmid has shown that the umuDC gene products specifically inhibit the transition from stationary phase to exponential growth at the restrictive temperature of 30 degrees C and that this is correlated with a rapid inhibition of DNA synthesis. These observations led to the finding that physiologically relevant levels of the umuDC gene products, expressed from a single, SOS-regulated chromosomal copy of the operon, modulate the transition to rapid growth in E. coli cells that have experienced DNA damage while in stationary phase. This activity of the umuDC gene products is correlated with an increase in survival after UV irradiation. In a distinction from SOS mutagenesis, uncleaved UmuD together with UmuC is responsible for this activity. The umuDC-dependent increase in resistance in UV-irradiated stationary-phase cells appears to involve, at least in part, counteracting a Fis-dependent activity and thereby regulating the transition to rapid growth in cells that have experienced DNA damage. Thus, the umuDC gene products appear to increase DNA damage tolerance at least partially by regulating growth after DNA damage in both exponentially growing and stationary-phase cells.

Bacterial Proteins↗
PubMed2000

The SOS response: recent insights into umuDC-dependent mutagenesis and DNA damage tolerance.

Be they prokaryotic or eukaryotic, organisms are exposed to a multitude of deoxyribonucleic acid (DNA) damaging agents ranging from ultraviolet (UV) light to fungal metabolites, like Aflatoxin B1. Furthermore, DNA damaging agents, such as reactive oxygen species, can be produced by cells themselves as metabolic byproducts and intermediates. Together, these agents pose a constant threat to an organism's genome. As a result, organisms have evolved a number of vitally important mechanisms to repair DNA damage in a high fidelity manner. They have also evolved systems (cell cycle checkpoints) that delay the resumption of the cell cycle after DNA damage to allow more time for these accurate processes to occur. If a cell cannot repair DNA damage accurately, a mutagenic event may occur. Most bacteria, including Escherichia coli, have evolved a coordinated response to these challenges to the integrity of their genomes. In E. coli, this inducible system is termed the SOS response, and it controls both accurate and potentially mutagenic DNA repair functions [reviewed comprehensively in () and also in ()]. Recent advances have focused attention on the umuD(+)C(+)-dependent, translesion DNA synthesis (TLS) process that is responsible for SOS mutagenesis (). Here we discuss the SOS response of E. coli and concentrate in particular on the roles of the umuD(+)C(+) gene products in promoting cell survival after DNA damage via TLS and a primitive DNA damage checkpoint.

Adaptation, Physiological↗
PubMed1999

A model for a umuDC-dependent prokaryotic DNA damage checkpoint.

The products of the Escherichia coli umuDC operon are required for translesion synthesis, the mechanistic basis of most mutagenesis caused by UV radiation and many chemicals. The UmuD protein shares homology with LexA, the repressor of SOS-regulated loci, and similarly undergoes a facilitated autodigestion on interaction with the RecA/single-stranded DNA nucleoprotein filaments formed after a cell experiences DNA damage. This cleavage, in which Ser-60 of UmuD acts as the nucleophile, produces UmuD', the form active in translesion synthesis. Expression of the noncleavable UmuD(S60A) protein and UmuC was found to increase survival after UV irradiation, despite the inability of the UmuD(S60A) protein to participate in translesion synthesis; this survival increase is uvr(+) dependent. Additional observations that expression of the UmuD(S60A) protein and UmuC delayed the resumption of DNA replication and cell growth after UV irradiation lead us to propose that the uncleaved UmuD protein and UmuC delay the resumption of DNA replication, thereby allowing nucleotide excision repair additional time to repair the damage accurately before replication is attempted. After a UV dose of 20 J/m(2), uncleaved UmuD is the predominant form for approximately 20 min, after which UmuD' becomes the predominant form, suggesting that the umuDC gene products play two distinct and temporally separated roles in DNA damage tolerance, the first in cell-cycle control and the second in translesion synthesis over unrepaired or irreparable lesions. The relationship of these observations to the eukaryotic DNA damage checkpoint is discussed.

Bacterial Proteins↗
PubMed1999

Comparison of functional electrical stimulation to long leg braces for upright mobility for children with complete thoracic level spinal injuries.

OBJECTIVE: To prospectively compare functional electrical stimulation (FES) to long leg braces (LLB) as a means of upright mobility for children with motor-complete thoracic level spinal cord injuries (SCIs). DESIGN: Intrasubject group comparison of two interventions. SETTING: Nonprofit pediatric orthopedic rehabilitation facility specializing in SCI. PATIENTS OR OTHER PARTICIPANTS: Convenience sample of five children between 9 and 18 years old with motor-complete thoracic level SCI. The hip and knee extensors were excitable by electrical stimulation. INTERVENTIONS: The FES system consisted of percutaneous intramuscular electrodes implanted to the hip and knee extensors and a push-button activated stimulator worn about the waist. Standing was accomplished by simultaneous stimulation of all implanted muscles. For foot and ankle stability, either ankle-foot orthoses (AFO) or supramalleolar orthoses were used. The LLB system consisted of a custom knee-ankle foot orthosis (KAFO) for four subjects and a custom reciprocating gait orthosis (RGO) for one subject who required bracing at the hip. For both interventions, either a front-wheeled walker or Lofstrand crutches were used as assistive devices. Each subject was trained in the use of both FES and LLB in seven standardized upright mobility activities: stand and reach, high transfer, toilet transfer, floor to stand, 6-meter walk, stair ascent, and stair descent. MAIN OUTCOME MEASURES: For each mobility activity, five repeated measures of level of independence, using the 7-point Functional Independence Measure (FIM) scale, and time to completion were recorded for each intervention. Subjects were also asked which intervention they preferred. RESULTS: For 94% of comparisons, subjects required equal (70%) or less (24%) assistance using FES as compared with LLB. Six of the seven mobility activities required less time to complete using FES, two activities at significant levels. The FES system was preferred in 62% of the cases, LLB were desired 27% of the time, and there was no preference in 11% of the cases. CONCLUSIONS: The FES system generally provided equal or greater independence in seven mobility activities as compared with LLB, provided faster sit-to-stand times, and was preferred over LLB in a majority of cases. Follow-up evaluations of both modes of upright mobility are needed to compare long-term performance and satisfaction.

Activities of Daily Living↗
PubMed1999

Evaluation of the lower motor neuron integrity of upper extremity muscles in high level spinal cord injury.

PURPOSE: To evaluate the lower motor neuron (LMN) integrity of upper extremity muscles of persons with high tetraplegia (C1-C4) in order to determine muscles available for stimulation. METHODS: Fourteen subjects (23 arms) were evaluated for LMN integrity. Muscles that elicited a functional response (grade 3 or better) to surface electrical stimulation were considered to have intact LMN and good candidates for FES. Strength-duration (S-D) curves were generated on muscles that showed weak (less than grade 3) or no response to surface stimulation. Muscles were considered denervated if S-D curves were discontinuous or depicted steep, increasing amplitude for pulse durations greater than 1 m. RESULTS: Muscles for grasp and release had intact LMN in 19 of 23 (83%) arms. The wrist extensors and flexors and pronator were excitable in 17 (74%), 20 (87%) and 19 (83%) arms, respectively. The supinator demonstrated LMN lesion in 80% of the arms. Over 90% of the biceps muscles were unresponsive to electrical stimulation and 85% and 87% of the deltoid and supraspinatus muscles, respectively, were not electrically excitable. The latissimus dorsi and triceps muscles were typically innervated (78% and 91%, respectively) and slightly more than half (52%) of the pectoralis major muscles were excitable. CONCLUSION: These data suggest that application of FES in high tetraplegia for hand and arm function would require augmentation because of the inability to stimulate the elbow flexors, deltoid and rotator cuff muscles. These data also show that several paralyzed proximal muscles with intact LMN that have been historically transferred to address shoulder paralysis in other patient populations are available for transfer and stimulation in the population with high level spinal injuries.

Adolescent↗
PubMed1999

Outcome of functional electrical stimulation in the rehabilitation of a child with C-5 tetraplegia.

Hand function was provided for a six-year-old child with C-5 American Spinal Injuries Association (ASIA) classification-A tetraplegia through a percutaneous intramuscular (i.m.) functional electrical stimulation (FES) system. In conjunction with implantation of 10 percutaneous i.m. electrodes for provision of grasp and release of her right hand, reconstructive surgery was performed to provide upper extremity positioning to optimize hand use. The subject participated in FES training over a nine-week period for approximately five hours weekly, with an additional five hours each week dedicated to exercise and conditioning of her arm muscles. Physical and functional assessments included range of motion (ROM), manual muscle testing (MMT), activities of daily living (ADL) abilities and the Canadian Occupational Performance Measure (COPM), used to evaluate the effect of stimulated hand function and surgical reconstruction on functional ability. These were conducted prior to FES and surgery and repeated after rehabilitation training. With rehabilitation and training, the child was able to control her FES system. Physical assessments revealed increased strength of both shoulders and more useful range of arm movement. Functional assessments show that the FES system enabled her to perform age-appropriate ADL that previously were achievable only with physical assistance. Her overall level of independence in ADL ability increased, as did self-rated levels of satisfaction and performance on chosen activities. Positive gains demonstrated here suggest the need for further studies of FES systems in young children with SCI.

Activities of Daily Living↗
PubMed1999

A comparison of FES with KAFO for providing ambulation and upright mobility in a child with a complete thoracic spinal cord injury.

This study compared functional and physiologic measures of ambulation and upright mobility with functional electrical stimulation (FES) versus knee-ankle-foot-orthoses (KAFO) in an 11-year-old boy with a T-10 level spinal cord injury. The child was a limited community ambulator with bilateral KAFO and loftstrand crutches. The FES system consisted of percutaneous intramuscular electrodes controlled by a portable stimulator and thumbswitch, an AFO for ankle and foot support, and loftstrand crutches. The subject used a swing-through gait pattern with both modes of mobility. The Functional Independence Measure scoring system and time to completion were used to compare performance in 6 standardized activities: donning, high transfer, inaccessible toilet transfer, ascend/descend stairs, and floor-to-standing transfer. Ten repeated measures were performed for each mode. Physiologic measures included energy expenditure, postural stability using forceplates, and a Functional Standing Test (FST). The subject performed all 6 mobility activities independently with FES and KAFO. In 4 of 6 activities, there was a trend toward faster times with FES, but this was not statistically significant. Toilet transfers and stair descent were performed significantly faster with KAFO. There was no difference in completion times on the activities of the FST. Measures of postural sway suggested that the subject was more stable with KAFO during quiet standing, while the modes were equal during a dynamic activity (raising arm for functional use). Energy expenditure results revealed no significant difference in oxygen cost per meter but a significantly higher oxygen consumption rate per minute for FES. Ambulation with both modes was performed at levels consistent with strenuous exercise. Maximum ambulation distances were relatively equal while the subject's velocity was significantly faster with FES. Of note, the subject reported ceasing ambulation during maximum distance trials due to general fatigue when using FES and due to shoulder pain with KAFO ambulation. For this subject, FES provided a means of performing upright mobility tasks independently, comparable with that of KAFO, while providing a faster ambulation velocity and a potential means of cardiovascular training.

Activities of Daily Living↗
PubMed1999

A study of shoulder motions as a control source for adolescents with C4 level SCI.

This study quantitatively examined and compared the shoulder motions of C4 level spinal cord injury (SCI), C5 level SCI, and able-bodied persons as a command source. The study was motivated by both the success of shoulder control in functional electrical stimulation (FES) systems designed for C5 level SCI people and the lack of quantitative information on the shoulder motion of persons with C4 level SCI. A dual-axis transducer was used to monitor the elevation/depression and protraction/retraction angles of each subject's shoulder while they performed three experimental sections which examined: the range of active shoulder motion; the ability to move incrementally to discrete positions with the aid of visual feedback; and the ability to hold discrete shoulder positions for an extended period without visual feedback. Results indicated that each group had the largest average shoulder displacements (abled = 23 degrees +/- 4 degrees, C5's = 14 degrees +/- 3 degrees, and C4's = 9 degrees +/- 3 degrees) while attempting to elevate and that on average the C4 group had the smallest range of active shoulder motion. No statistically significant differences between the groups were found in either the accuracy or stability of reaching discrete positions with the aid of visual feedback or in the accuracy of holding discrete shoulder positions for an extended period without visual feedback. The results suggest that within their limited range of motion the individuals with C4 level SCI retained shoulder control sufficient for use as an neuroprosthetic command interface.

Adolescent↗
PubMed1999

Implantable electrode lead in a growing limb.

An implantable electrode leadwire system used to provide limb function for individuals with spinal cord injuries (SCI's) was evaluated in a series of growing dogs to determine whether it could maintain its performance in the presence of growth. Thirty implantable electrodes (15 epimysial and 15 intramuscular) were implanted in the forelimb muscles of six young dogs. The electrodes' leads were tunneled subcutaneously and anchored proximally in the shoulder with excess lead incorporated into the subcutaneous space to accommodate growth. Six of the leads had some of this excess placed in pouches made from surgical membrane while the other 24 leads had excess placed freely within the subcutaneous space. Motor responses to the electrodes were tested before and after growth with tendon force transducers and were compared to the performance of new electrodes implanted to the same muscles of the mature dog during the explant procedure. Measured were the pulse duration at which a measurable force is first produced (threshold) and the percentage of the maximum force that could be attained from the target muscle before activation of adjacent muscles (usable force range). An analysis of variance indicated that there was no difference in the usable force range (p = 0.62) of the original electrodes before and after growth and that of the new electrodes placed at maturity. There was a difference in the threshold (p = 0.001) which can be attributed to an increase in the values measured from the original electrodes after growth. However, the increase in threshold with growth averaged 6 micros which is not clinically significant and can be accommodated through stimulation programming. Growth of the limb and unwinding of excess lead were quantified by radiograph. Extension of the freely placed excess lead was comparable to growth so that the pouch enclosures were found to be unnecessary for facilitating lead expansion. By radiograph and surgical observations, only two of 30 electrodes (both intramuscular) appeared to have been subjected to lead tension, although they continued to provide adequate motor responses. Insufficient excess lead was judged to be the cause of dislodgment for one of these electrodes. Results of this study suggest that for this implantable leadwire system, excess lead placed in the subcutaneous space can unwind on demand with limb growth such that an electrode will remain in position and provide a stable motor response.

Analysis of Variance↗
PubMed1999

Time-frequency analysis of the dispersion of Lamb modes.

Accurate knowledge of the velocity dispersion of Lamb modes is important for ultrasonic nondestructive evaluation methods used in detecting and locating flaws in thin plates and in determining their elastic stiffness coefficients. Lamb mode dispersion is also important in the acoustic emission technique for accurately triangulating the location of emissions in thin plates. In this research, the ability to characterize Lamb mode dispersion through a time-frequency analysis (the pseudo-Wigner-Ville distribution) was demonstrated. A major advantage of time-frequency methods is the ability to analyze acoustic signals containing multiple propagation modes, which overlap and superimpose in the time domain signal. By combining time-frequency analysis with a broadband acoustic excitation source, the dispersion of multiple Lamb modes over a wide frequency range can be determined from as little as a single measurement. In addition, the technique provides a direct measurement of the group velocity dispersion. The technique was first demonstrated in the analysis of a simulated waveform in an aluminum plate in which the Lamb mode dispersion was well known. Portions of the dispersion curves of the A0, A1, S0, and S2 Lamb modes were obtained from this one waveform. The technique was also applied for the analysis of experimental waveforms from a unidirectional graphite/epoxy composite plate. Measurements were made both along and perpendicular to the fiber direction. In this case, the signals contained only the lowest order symmetric and antisymmetric modes. A least squares fit of the results from several source to detector distances was used. Theoretical dispersion curves were calculated and are shown to be in good agreement with experimental results.

Acoustics↗