Contact Ultrasonic Techniques Research Articles

Experimental Determination of Lamb-Wave Attenuation Coefficients

This work determined the attenuation coefficients of Lamb waves of ten engineering materials and compared the results with calculated Lamb-wave attenuation coefficients, α–S and α–A. The Disperse program and a parametric method based on Disperse results were used for calculations. Bulk-wave attenuation coefficients, αL and αT, were required as input parameters to the Disperse calculations. The calculated α–S and α–A values were found to be dominated by the αT contribution. Often α–Ao coincided with αT. The values of αL and αT were previously obtained or newly measured. Attenuation measurement relied on Lamb-wave generation by pulsed excitation of ultrasonic transducers and on surface-displacement detection with point contact receivers. The frequency used ranged from 10 kHz to 1 MHz. A total of 14 sheet and plate samples were evaluated. Sample materials ranged from steel, Al, and silicate glass with low attenuation to polymers and a fiber composite with much higher attenuation. Experimentally obtained Lamb-wave attenuation coefficients, α–S and α–A, for symmetric and asymmetric modes, were mostly for the zeroth mode. Plots of α–So and α–Ao values against frequency were found to coincide reasonably well to theoretically calculated curves. This study confirmed that the Disperse program predicts Lamb-wave attenuation coefficients for elastically isotropic materials within the limitation of the contact ultrasonic techniques used. Further refinements in experimental methods are needed, as large deviations often occurred, especially at low and high frequencies. Methods of refinement are suggested. Displacement measurements were quantified using Rayleigh wave calibration. For signals below 300 kHz, 1-mV receiver output corresponded to 1-pm displacement. Peak displacements after 200-mm propagation were found to range from 10 pm to 1.5 nm. With the use of signal averaging, the point-contact sensor was capable of detecting 1-pm displacement with 40 dB signal-to-noise ratio and had equivalent noise of 4.3 fm/√Hz. Approximate expressions for α–So and α–Ao were obtained, and an empirical correlation was found between bulk-wave attenuation coefficients, i.e., αT = 2.79 αL, for over 150 materials.

Development of a laser-driven ultrasonic technology for characterizations of heated and aged concrete samples

ABSTRACT We have demonstrated a full noncontact laser technology to measure the velocity of ultrasonic waves and their spectra propagated through concrete samples exposed to specified high-temperature conditions with specified durations as models of concrete structures in a severe accident at the Fukushima Daiichi Nuclear Power Station. The velocities and spectra of the ultrasonic waves were strongly dependent on the exposed temperature, i.e. at a high-temperature condition of 400°C, the velocity was 3700 m/s, and at room temperature, the velocity was 5000 m/s. Experimental results are almost comparable to those obtained by the contact ultrasonic technique.

Application of ultrasonic guided waves for non-destructive testing of large and complex geometry engineering structures

The wide group of large engineering constructions made from welded metal components and modern composite materials during daily exploitation under non-stationary loads, continuous influence of vibrations and complicated environmental conditions have demand of periodical tests in order to avoid appearance and evolution of the defects. All of them have one common feature – large areas to be tested and the complex geometry. The objective is to improve air-coupled and contact type testing techniques for excitation and reception of appropriate modes of ultrasonic guided waves in large and complex structures. Also, to develop inspection techniques which enable to detect required types of internal defects. The air-coupled and contact ultrasonic techniques for investigation of large and complex geometry objects, such as composites for aerospace applications and petroleum tanks, have been developed. The investigation of propagation effects of ultrasonic guided (Lamb) waves in the plates of the tank floor has indicated relation between reconstructed properties of guided waves and expected presence of corrosion deposits.

Elastic constant determination of hardwoods using ultrasonic insertion technique

Ultrasonic insertion technique (IT) is an ultrasonic technique which involves sample immersion in a solution to determine its acoustic properties. IT is normally used to determine the acoustic properties of a medical phantom. We proposed the use of IT as an alternative technique to the common contact ultrasonic technique: through-transmission technique (TT) for determining the elastic constant of hardwoods in longitudinal, tangential and radial directions. The elastic constant of twelve rectangular-shaped Malaysian hardwoods from three different categories; heavy, medium and light with the density ranging from 602 to 992kgm−3 were determined using IT and TT. Both techniques were carried out at 24.0°C surrounding temperature and utilized 2.25MHz ultrasonic transducers. Data from both techniques were compared to validate the use of the proposed technique. Findings indicated that IT offers consistent and accurate results for, tangential and radial elastic constants (TEC and REC) within 8.89% and 5.86% differences, respectively compared to TT for all tested hardwoods. IT offers an alternative technique for TEC and REC determinations of precious wood samples.

Evaluation of the Firmness Measurement of Fruit by Using a Non–contact Ultrasonic Technique

Evaluation of the Firmness Measurement of Fruit by Using a Non–contact Ultrasonic Technique

ChemInform Abstract: Non‐contact Ultrasonic Measurements of the Elastic Constants of Magnetic Materials

AbstractReview: 16 refs.

Non-contact ultrasonic measurements of the elastic constants of magnetic materials

Ultrasonic testing using contacting transducers such as quartz or PZT is well established. However, standard measurement techniques used require physical contact of the sample and ultrasonic transducer and some sort of couplant between the two. With this configuration there is a possibility of damaging the sample, transducer or bond during testing, thermal cycling, or removal of the transducer. We present results taken using recent advances in non-contact methods of ultrasound generation and detection using electromagnetic acoustic transducers (EMATs), which offer some significant benefits over contact ultrasonic techniques. Circumventing the need for couplant removes the possibility of contaminating the system, which is an issue for some material property measurements, and allows easier measurements over a wider range of temperatures. An automated data analysis system has been developed which allows the velocity of sound in the sample, and hence the elastic constants, to be determined to a high accuracy. This technique is illustrated using measurements of the alloy Gd64Sc36.

Mechanical and geometric property characterization of dry-coated tablets with contact ultrasonic techniques

A dry-coated tablet is a solid dosage form with a controlled drug-release system, which consists of a core and an outer layer. The accuracy of its geometric (e.g. the outer layer wall and core thicknesses) and mechanical properties (e.g. Young's moduli and mass densities of associated materials) could be crucial to its therapeutic and structural functions. The objective of current study is to develop a non-destructive technique for determining the geometric and mechanical properties of dry-coated tablets. Two contact ultrasonic techniques (i.e. pitch-catch and pulse-echo measurement modes) are employed and the properties of all the structural components of a set of experimental tablets are measured and reported. The thicknesses of the outer layers of the sample tablets are used to obtain the eccentricity of the core tablets. The two approaches are compared for their effectiveness in obtaining these properties of the sample dry-coated tablets. The thicknesses of the outer layers obtained with the proposed approach and with the direct (destructive) measurements are also compared. A good agreement is found; there is an approximately 2% difference. The eccentricity and concentricity of a set of tablets are determined and it is concluded that the observed consistent anomaly in eccentricity can be attributed to the same root cause and its correction can be achieved by control input based on monitoring data. Potential of the approach for in-die real-time monitoring of compaction presses and its PAT (Process Analytical Technology) applications for the pharmaceutical manufacturing are also discussed.

Mechanical Property Characterization of Bilayered Tablets using Nondestructive Air-Coupled Acoustics

A noncontact/nondestructive air-coupled acoustic technique to be potentially used in mechanical property determination of bilayer tablets is presented. In the reported experiments, a bilayer tablet is vibrated via an acoustic field of an air-coupled transducer in a frequency range sufficiently high to excite several vibrational modes (harmonics) of the tablet. The tablet vibrational transient responses at a number of measurement points on the tablet are acquired by a laser vibrometer in a noncontact manner. An iterative computational procedure based on the finite element method is utilized to extract the Young's modulus, the Poisson's ratio, and the mass density values of each layer material of a bilayer tablet from a subset of the measured resonance frequencies. For verification purposes, a contact ultrasonic technique based on the time-of-flight data of the longitudinal (pressure) and transverse (shear) acoustic waves in each layer of a bilayer tablet is also utilized. The extracted mechanical properties from the air-coupled acoustic data agree well with those determined from the contact ultrasonic measurements. The mechanical properties of solid oral dosage forms have been shown to impact its mechanical integrity, disintegration profile and the release rate of the drug in the digestive tract, thus potentially affecting its therapeutic response. The presented nondestructive technique provides greater insight into the mechanical properties of the bilayer tablets and has the potential to identify quality and performance problems related to the mechanical properties of the bilayer tablets early on the production process and, consequently, reduce associated cost and material waste.

Optical and ultrasonic signatures of femtosecond pulse filamentation in fused silica

Millimeter-long filaments and accompanying luminous plasma and defect channels created in fused silica by single, moderately focused femtosecond laser pulses with supercritical powers were probed in situ using optical imaging and contact ultrasonic techniques. Above the threshold pulse energy Eopt=5 μJ corresponding to a few megawatt power, the pulses collapse due to self-focusing and the nonlinear focus moves upstream with increasing pulse energy. Behind the focus, elongated, gradually narrowing awl-shaped channels of electron-hole plasma and luminescent defects are produced. In the channels, whose dimensions generally depend on the pulse energy, supercontinuum emission propagating downstream the channels occurs, although its observation requires elevated pulse energies above 25 μJ in order to compensate energy dissipation in the channels. Ultrasonic side-view imaging of the channels, conducted from a few millimeters distance, reveals predominantly compressive pressure transients. The compressive signals are observed above the same threshold pulse energy Eopt, and their amplitude increases linearly with the laser pulse energy, simultaneously exhibiting significant temporal broadening of the corresponding pulsewidths, reflecting square root dependence of the channel length and sublinear (∝E3/4) dependence of the source pressure on the pulse energy. Altogether, these optical and ultrasonic studies demonstrate filamentary pulse propagation with considerable dissipation (∼10 cm−1) in the awl-shaped subcritical plasma channels rich with generated point defects and optical damage sites.

Optical and ultrasonic monitoring of femtosecond laser filamentation in fused silica

Millimeter-long filaments and accompanying luminous plasma and defect channels created in fused silica (FS) by single focused femtosecond laser pulses with supercritical powers were probed in situ using optical imaging and contact ultrasonic techniques. Above the threshold pulse energy E opt = 5 μJ corresponding to a few megawatt power levels pulses collapse due to self-focusing, producing channels filled by electron–hole plasma and luminescent defects, and exhibits predominantly compressive pressure transients. Analysis of the optical and ultrasonic response versus the laser pulse energy suggests that filamentary pulse propagation in the channels occurs with considerable dissipation of about ∼10 cm −1. The predominant ionization mechanism is most likely associated with avalanche ionization, while the main mechanism of optical absorption is free-carrier absorption via inverse Bremsstrahlung interaction with the polar lattice.

Laser ultrasonic in-process inspection of paper for elastic properties

A laser-ultrasonic (LUS) sensor has been developed that allows measurement of the bending stiffness (BS) and shear rigidity (SR) of paper and paperboard as it is being made on the papermaking machine. A prototype system was recently tested in a paper mill at web speeds up to 5000 ft/min with excellent precision and accuracy. The LUS technique performs well on paper and board with basis weights up to 130 g/m2. Several laboratory methods exist for measuring the bending stiffness in small samples of paper and board. Currently, no commercial method exists for nondestructively measuring this property on the papermaking machine at production speeds. Commercial instruments using contact transducers measure ‘‘tensile strength orientation’’ (TSO) on heavier boards, where marking of the sheet by the contact transducers is not of concern. Unlike contact ultrasonic techniques, LUS does not visibly mark even the lightest grade papers. Contact ultrasonic measurements correlate approximately to the tensile strength of the sheet and can be used to calculate an approximate value for BS. LUS measurements are directly related to BS and should yield more accurate determinations. Optimum use of feed stock, reduced waste, and decreased energy consumption are potential benefits of the LUS technology.

Methods of beam divergence corrections on broadband direct contact ultrasonic technique

The separation of intrinsic attenuation and velocity parameters from extrinsic factors is a complex task in nondestructive ultrasonic characterization of materials. The effect of the ultrasonic beam divergence due to finite source is discussed both theoretically and experimentally in this work. The objective is to evaluate different techniques of attenuation and velocity measurements with beam divergence corrections on direct contact configuration. Both amplitude and phase spectra for signals generated and detected by commercial NDE broadband transducers are to be corrected. The effectiveness of each technique is evaluated with different transducers and sample geometry. The results show that a smaller effective transducer diameter than the nominal must be assumed in theoretical corrections [P. H. Rogers and A. L. Van Buren, J. Acoust. Soc. Am. 55, 724–728 (1974)] and that the experimental determination of calibration correction for a particular transducer by a reference sample can be a good alternative.

Contactless measurement of the elastic Young's modulus of paper by an ultrasonic technique

The use of a non-contact ultrasonic technique for the characterization of the in-plane elastic Young's moduli of paper is considered. The determination of these moduli from A 0 (fundamental antisymmetric plate wave mode) measurements, using the empirical technique introduced by Luukkala et al. (1971), is discussed: it is shown that the common low frequency approximation is not valid for paper, owing to its orthotropic structure. Numerical simulations are used to evaluate the validity domain of Luukkala's method. A new device was developed, and Young's moduli of various paper sheets were determined from A 0 measurements. These moduli values were found to be in good agreement with those measured using a common laboratory contact ultrasonic technique.

Velocity Measurements of Laser-Generated Ultrasound at High Temperatures

The temperature dependence of ultrasonic velocities is determined by a laser-ultrasound technique in a non-contact manner for several kinds of materials. A pulsed Nd:YAG laser is used to generate ultrasound on the surface of a specimen while a He-Ne laser interferometer detects the subsequent displacement on the directly opposite surface. The results are favorably compared with those obtained with a conventional contact ultrasonic technique. The longitudinal velocities of both methods agreed within 0.4% or less in the range from room temperature to 800°C.

Laser-based non-destructive testing techniques for the ultrasonic characterization of subsurface flaws

Laser generation and detection of ultrasound has been used for remote characterization of flat-bottomed holes, acting as subsurface flaws, drilled into an aluminium test sample. A pulsed Nd:YAG laser system was used to generate the ultrasonic source whilst a HeNe laser interferometer detected the subsequent surface displacements. The technique allows both the width and depth of flawa to be assessed. Moreover, an analysis of the data shows that the method is particularly suited to detection of flaws ≤ 4 mm below the surface of the sample, a region which is difficult to inspect by conventional contact ultrasonic techniques.