Non-contact and non-destructive usages of laser ultrasonic have been researched actively nowadays. Specifically, there are some studies about Lamb wave inverse problems, which is the research about inversely calculating material properties from Lamb wave dispersion curve.
In this study, measurement system of Lamb wave phase velocity and method of inversely calculating elastic modulus of isotropic metal plate from the experimental dispersion curve of A0 mode. Ultrasounds are induced with a pulsed laser and detect them with a laser Doppler vibrometer, and measured the velocities of them in non-contact and non-destructive way. Then, calculating method of phase velocity distribution of A0 mode is introduced.
By using characteristic equation of A0 mode, the sum of squares of residuals, SSR, is defined to represent how close the bulk wave velocities expectations are close to the right values when dispersion curve is experimentally given. Also, this study verified that assuming Poisson's ratio cannot affect inverse calculation results, which allows to roughly assume the ratio for the calculation.
For the aluminum 0.5 mm plate, Lamb wave measurement system, phase velocity calculation method, and inversely calculation method by assuming Poisson's ratio roughly are verified by success of inversely calculating the elastic modulus of aluminum specimen accurately.
Measuring with strain gages has a limitation of consuming time to prepare for the experiment and attaching near the MLCCs, making it does not represent the physical quantities of the MLCCs itself. In this study, we propose a ‘failure criteria for a crack in MLCCs soldered on a Print-Circuit-Board by ball drop test’ based on the acceleration measured by Laser Doppler Vibrometers. By drawing a Shock Response Spectrum, we can find the dominant frequency for the failure of MLCCs. In addition, we present a suitable deformation form of the Print-Circuit-Board by using a high-speed camera.
This paper presents a high repetition rate fiber Bragg grating (FBG) interrogation system that is able to capture the entire
reflection spectrum at a rate of up to 300 kHz. The system uses a high speed MEMS based tunable optical filter that is
driven with a sinusoidal voltage. The time varying FBG reflection spectrum in transmitted through the tunable filter.
The time varying signal is then mapped into time varying reflection spectra. This interrogation system is used during
two dynamic strain tests, in which the reflection spectra are measured at a repetition rate of 100 kHz. The first test is the
impact of a woven carbon composite and the second test is on an electromagnetic railgun.
Various densities of optical fibers are embedded into a total of eighty woven, graphite fiber-epoxy composite laminates,
for which the response to low velocity impacts are evaluated. The goal of this work is to determine the role of hostsensor
interaction on the lifetime of the host material system. The woven composites are subjected to multiple impacts
at 14.5 J until perforation of the specimen. We obtain the energy dissipated by the laminate and the maximum contact
force between the impactor laminate for each strike. From these experimental data we calculate the statistical distribution
of the total energy dissipated at failure as a function of embedded optical fiber density. The total dissipated energy, a
measure of the specimen lifetime, decreased with increasing embedded optical fiber density, however remained constant
after a threshold density was reached. The total maximum contact force per specimen, a measure of the specimen stiffness, continued to decrease with the number of embedded optical fibers.
This paper presents a means for the high repetition rate interrogation of fiber Bragg gratings (FBG's). The new system
highlights a method that allows a tradeoff between the full spectrum capture rate and the wavelength range and/or the
spectral resolution of the technique. Rapid capture of the entire reflection spectrum at high interrogation rates shows
important features that are missed when using methods that merely track changes in the peak location of the spectrum.
The essential feature of the new system is that it incorporates a MEMs tunable filter driven by a variable frequency openloop
sinusoidal source. The paper demonstrates the new system on a laminated composite system under impact loading.
This paper presents experimental measurements of the response of woven composite laminates to multiple low-velocity
impacts. Damage initiation and progression occur at multiple physical and temporal scales in heterogeneous materials,
including fiber breakage, matrix cracking, delamination and matrix relaxation. The sensor/interrogators were therefore
chosen specifically to provide insight into the order and progression of different failure modes. Measurements of the
contact force between the impactor and composite are measured throughout impact. Additionally, the dissipated energy
per impact event is also calculated from the impactor velocity. Surface mounted and embedded fiber Bragg grating
sensors are used for the measurement of the laminate response. Peak wavelength measurements are performed during
impact at 1 kHz, while full-spectral scanning is performed at 5 Hz during relaxation period of the laminate immediately
after impact and quasi-statically to measure post-impact residual strain states within the laminate. The results highlight
the depth of information embedded within the FBG full-spectral data sensors, as well as the added insight to be gained
from combined global-local measurements.
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