2,784 research outputs found
Lamb wave near field enhancements for surface breaking defects in plates
Near field surface wave ultrasonic enhancements have previously been used to detect surface breaking defects in thick samples using Rayleigh waves. Here, we present analogous surface wave enhancements for Lamb waves propagating in plates. By tracking frequency intensities in selected regions of time-frequency representations, we observe frequency enhancement in the near field, due to constructive interference of the incident wave mode with those reflected and mode converted at the defect. This is explained using two test models; a square based notch and an opening crack, which are used to predict the contribution to the out-of-plane displacement from the reflected and mode converted waves. This method has the potential to provide a reliable method for the near field identification and characterisation of surface breaking defects in plates
Laser beam shaping for enhanced Zero-Group Velocity Lamb modes generation
Optimization of Lamb modes induced by laser can be achieved by adjusting the
spatial source distribution to the mode wavelength (). The
excitability of Zero-Group Velocity (ZGV) resonances in isotropic plates is
investigated both theoretically and experimentally for axially symmetric
sources. Optimal parameters and amplitude gains are derived analytically for
spot and annular sources of either Gaussian or rectangular energy profiles. For
a Gaussian spot source, the optimal radius is found to be .
Annular sources increase the amplitude by at least a factor of 3 compared to
the optimal Gaussian source. Rectangular energy profiles provide higher gain
than Gaussian ones. These predictions are confirmed by semi-analytical
simulation of the thermoelastic generation of Lamb waves, including the effect
of material attenuation. Experimentally, Gaussian ring sources of controlled
width and radius are produced with an axicon-lens system. Measured optimal
geometric parameters obtained for Gaussian and annular beams are in good
agreement with theoretical predictions. A ZGV resonance amplification factor of
2.1 is obtained with the Gaussian ring. Such source should facilitate the
inspection of highly attenuating plates made of low ablation threshold
materials like composites.Comment: 11 pages, 12 figure
Phased electromagnetic acoustic transducer array for Rayleigh wave surface defect detection
A phased electromagnetic acoustic transducer (EMAT) array system has been developed for detection and characterisation of surface breaking defects. An array of four linear coils which are individually controlled are used to generate a Rayleigh wave. The high current electronics combined with the coil designs enables the array to generate either narrowband or broadband signals, and controlling the phase delay between the channels makes it possible to change the ultrasound wavelength without requiring the physical separation of the coils to be changed. Experimental results show that the four-coil phased array is able to generate a wavelength range from 3.0 mm to 11.7 mm. Surface breaking defects were characterised using a transmit-receive set-up with a broadband EMAT detector being used to detect the Rayleigh wave. Machined surface slots with different depths were used for technique validation. The results show that the array is sensitive to surface defects and that a wide depth sensitivity range for defect sizing can be easily achieved by applying phasing to tune the wavelength of operation. A large increase in detection flexibility is immediately shown
A noncontact ultrasonic platform for structural inspection
Miniature robotic vehicles are receiving increasing attention for use in nondestructive testing (NDE) due to their attractiveness in terms of cost, safety, and their accessibility to areas where manual inspection is not practical. Conventional ultrasonic inspection requires the provision of a suitable coupling liquid between the probe and the structure under test. This necessitates either an on board reservoir or umbilical providing a constant flow of coupling fluid, neither of which are practical for a fleet of miniature robotic inspection vehicles. Air-coupled ultrasound offers the possibility of couplant-free ultrasonic inspection. This paper describes the sensing methodology, hardware platform and algorithms used to integrate an air-coupled ultrasonic inspection payload into a miniature robotic vehicle platform. The work takes account of the robot's inherent positional uncertainty when constructing an image of the test specimen from aggregated sensor measurements. This paper concludes with the results of an automatic inspection of a aluminium sample
Multiple wavemode scanning for near and far-side defect characterisation
The combination of ultrasonic inspections using different wavemodes can give more information than is available with single mode inspection. In this work, the response of shear and Rayleigh waves to surface-breaking defects propagating on the near-side and far-side of a sample is investigated. The directivity of shear waves generated by a racetrack coil electromagnetic acoustic transducer (EMAT) is identified and used to set an ideal separation for a pair of transmit-receive EMATs. Defects are indicated by a reduction in the transmitted Rayleigh wave amplitude, and by blocking of the shear wave. Used together, these can identify features in the bulk wave behaviour which are due to near-face surface-breaking defects, and give a full picture of both surfaces. By using a combination of the two wavemodes, the angle of propagation and length of any near-side defects can additionally be identified. A scanning method for samples is proposed
Acousto-ultrasonic nondestructive evaluation of materials using laser beam generation and detection
The acousto-ultrasonic method has proven to be a most interesting technique for nondestructive evaluation of the mechanical properties of a variety of materials. Use of the technique or a modification thereof, has led to correlation of the associated stress wave factor with mechanical properties of both metals and composite materials. The method is applied to the nondestructive evaluation of selected fiber reinforced structural composites. For the first time, conventional piezoelectric transducers were replaced with laser beam ultrasonic generators and detectors. This modification permitted true non-contact acousto-ultrasonic measurements to be made, which yielded new information about the basic mechanisms involved as well as proved the feasibility of making such non-contact measurements on terrestrial and space structures and heat engine components. A state-of-the-art laser based acousto-ultrasonic system, incorporating a compact pulsed laser and a fiber-optic heterodyne interferometer, was delivered to the NASA Lewis Research Center
Transient Propagation and Scattering of Quasi-Rayleigh Waves in Plates: Quantitative comparison between Pulsed TV-Holography Measurements and FC(Gram) elastodynamic simulations
We study the scattering of transient, high-frequency, narrow-band
quasi-Rayleigh elastic waves by through-thickness holes in aluminum plates, in
the framework of ultrasonic nondestructive testing (NDT) based on full-field
optical detection. Sequences of the instantaneous two-dimensional (2-D)
out-of-plane displacement scattering maps are measured with a self-developed
PTVH system. The corresponding simulated sequences are obtained by means of an
FC(Gram) elastodynamic solver introduced recently, which implements a full
three-dimensional (3D) vector formulation of the direct linear-elasticity
scattering problem. A detailed quantitative comparison between these
experimental and numerical sequences, which is presented here for the first
time, shows very good agreement both in the amplitude and the phase of the
acoustic field in the forward, lateral and backscattering areas. It is thus
suggested that the combination of the PTVH system and the FC(Gram)
elastodynamic solver provides an effective ultrasonic inspection tool for
plate-like structures, with a significant potential for ultrasonic NDT
applications.Comment: 46 pages, 16 figures, corresponding author Jos\'e Carlos
L\'opez-V\'azquez, [email protected]. Changes: 1st, 4th, 5th paragraphs
(intro), 3rd, 4th paragraphs (sec. 4); [59-60] cited only in appendixes; old
ref. [52] removed; misprints corrected in the uncertainty of c_L (subsec.
3.1), citation to fig. 10 (sec. 4), size of images (caption fig.15);
reference to Lam\'e constants removed in subsec. 3.
Laser induced Zero-Group Velocity resonances in Transversely Isotropic cylinder
The transient response of an elastic cylinder to a laser impact is studied.
When the laser source is a line perpendicular to the cylinder axis, modes
guided along the cylinder are generated. For a millimetric steel cylinder up to
ten narrow resonances can be locally detected by laser interferometry below 8
MHz. Most of these resonances correspond to Zero-Group Velocity guided modes
while a few others can be ascribed to thickness modes. We observe that the
theory describing the propagation of elastic waves in an isotropic cylinder is
not sufficient to precisely predict the resonance spectrum. In fact, the
texture of such elongated structure manifest as elastic anisotropy. Thus, a
transverse isotropic (TI) model is used to calculate the dispersion curves and
compare them with the measured one, obtained by moving the source along the
cylinder. The five elastic constants of a TI cylinder are adjusted leading to a
good agreement between measured and theoretical dispersion curves. Then, all
the resonance frequencies are satisfactorily identified.Comment: 23 pages, 7 figures, submitted to the JAS
A study on the near-field interactions of ultrasonic surface waves with surface-breaking defects
This thesis is concerned with the detection of surface-breaking defects, such
as stress corrosion cracking, using an ultrasonic scanning approach in which a laser
source and detector are scanned over the near-field of a defect. Large increases
in the amplitude and frequency content of an incident ultrasonic wave are present
when either the source or the detector is very close to the defect, leading to a phenomenon
known as ultrasonic near-field enhancement. The extent of the ultrasonic
enhancement varies with defect characteristics such as defect depth and angle to the
surface.
Ultrasonic enhancement is observed in both experiment and finite element
simulations using Rayleigh waves for both scanning laser detection and scanning
laser source methods. The near-field enhancement is shown to vary as a function of
the angle of the defect to the horizontal for Rayleigh wave enhancements, allowing
the positioning and characterisation of artificial angled defects that are similar to
rolling contact fatigue defects in railtrack. The mechanisms behind the near-field
enhancement of Rayleigh waves at angled defects are identified, and this aids in the
understanding of the behaviour of ultrasound as it interacts with surface-breaking
defects.
Ultrasonic enhancements are also reported to be present in individual Lamb
wave modes for interactions with artificial open-mouthed defects in thin plates,
which are similar to the open end of stress corrosion defects. The mechanisms behind
both the scanning laser detection and scanning laser source enhancements are
identified and used to explain the variation in the enhancement as a function of
increasing defect severity. Positioning of these defects is also achieved by identification
of the enhancement location.
Finally, the scanning laser technique is applied to real stress-driven defects,
and both scanning approaches are shown to be capable of detecting partially-closed
defects in a variety of sample geometries. The position, geometric alignment and an
estimate of the defect depth are obtained for real defects in thin plates, pipework
sections and in irregularly shaped engine components
- …