Ultrasonic Surface Wave Propagation Research Articles

조직공학용 지지체의 비파괴 분석을 위한 표면 탄성파 시뮬레이션 및 고속초음파 기반 측정법

조직공학용 지지체의 비파괴 분석을 위한 표면 탄성파 시뮬레이션 및 고속초음파 기반 측정법

Experimental ultrasonic characterization of polyester-based materials for cultural heritage applications

For several years, the Réunion des musées nationaux – Grand-Palais has produced polyester resin reproductions in order to replace marble sculptures that have weakened by outdoor exposure. These objects are made of a complex multilayered polyester composite material including reinforcements to ensure the mechanical strength of the final structure and mineral fillers that allow to imitate the original aesthetics. However, the final structure also weakens because of constant outdoor exposure and ageing. This observation leads today to conduct research related to the structural health monitoring of reproductions for preventive conservation of cultural heritage. This paper presents a nondestructive technique to study the properties of the composite material used to produce reproductions of marble sculptures. Firstly, classical ultrasonic contact measurements were performed to estimate bulk properties and Rayleigh wave velocity. Secondly, experimental Rayleigh wave was measured using contact and laser vibrometry methods. The results show the potential of using ultrasonic surface wave propagation and laser vibrometry method to develop a minimum contact technique to study these polyester-based materials. The maximum relative uncertainty with respect to the expected theoretical Rayleigh wave velocity was close to 12%.

An experimental study of EMAT ultrasonic surface waves modes in railhead

A lab-based laser-EMAT (Electromagnetic acoustic transducers) system has been developed to observe the ultrasonic surface wave propagation and interaction with surface breaking defects on the sample rail head surface. The frequency- wavenumber dispersion curves of the dominant surface wave modes were determined using 2D-FFT method based on the measurements of the out-of-plane displacement of the surface wave. A complete picture of the ultrasonic surface wave on the sample surface over time was reconstructed with exceptionally high spatial and temporal resolution. Despite the curvature of the rail head, the ultrasonic surface guided wave propagating down the rail is found to have similar properties to Rayleigh waves by direct comparison to those observed on flat samples using the same technique. Rail breaks caused by rolling contact fatigue (RCF) cracks are of growing concern to the modern high speed railway industry (1). The fatigue crack initiates on or very close to the rail rolling surface, which is not related to any material defect. Its occurrence is increasing on high speed passenger lines, mixed and heavy haul railways and can lead to expensive rail grinding in the attempt to remove it, premature removal of the rails and complete rail failure. The rolling contact fatigue damage on rails can be divided in headchecks, squats and spalling. They are caused by a combination of high normal and tangential stresses between rail and wheel, which cause sever shearing of the rolling surface layer of rail head and either fatigue or exhaustion of ductility of material. The microscopic cracks produced propagations through the heavily deformed surface layer of railhead at a shallow angle to the rail running face (about 10 ◦ ∼15 ◦ ) until it reaches a depth where the steel retains its original isotropic properties (2,3). At this stage the crack is a few millimeters deep into the rail head, and usually the crack may simply lead to spalling of material from the rail surface. However, for reasons still not very clearly understood, isolated oblique crack can down into the rail, and if not to detect by ultrasonic testing car, it may cause the rail to break. RCF damages are much more severe from the point of view of the structure integrity for high speed railway track.

Ultrasonic surface wave propagation and interaction with surface defects on rail track head

Electromagnetic acoustic transducers (EMATs) are non-contact ultrasonic transducers capable of generating wide band ultrasonic surface waves on metallic samples. A lab-based laser-EMAT system has been developed to observe the ultrasonic surface wave propagation and interaction with surface breaking defects on the sample rail head surface. A wide band EMAT generating surface waves with a frequency content between approximately 50 and 500 kHz is used to propagate ultrasonic waves on the surface of a rail head down the length of the sample. A stabilised Michelson interferometer is used to measure the out-of-plane displacement of the surface wave. A complete picture of the ultrasonic surface wave on the sample surface over time is reconstructed using this technique, with exceptionally high spatial and temporal resolution. Despite the curvature of the rail head, the ultrasonic surface wave propagating down the rail is found to have similar properties to Rayleigh waves by direct comparison to those observed on flat samples using the same technique.

Tracking of Cracks in Airplane Components Using Nonlinear Surface Wave Propagation Techniques

Ultrasonic surface waves provide a sensitive means to detect cracks in airplane structures. Until now several obstacles remained to use ultrasonic surface waves for on-line damage detection (i.e. in-flight). In this article a method will be proposed to detect a growing fatigue crack while the aircraft is operating. In contrast to classical ultrasonic measurement methods, that use a high voltage pulse, we applied an optimized multi-sine excitation signal with an amplitude of a few volts only (this agrees better with the applicable safety regulations for aircraft). Furthermore, an indicator quantifying the nonlinearity of the ultrasonic surface wave propagation is used. By using a nonlinearity index the influence of changing operation conditions that can be observed with most linear methods is eliminated. The proposed method is validated on a steel beam that is fatigue loaded with a force signal obtained from in-flight data.

Tunable multiple-scattering system

To study the propagation of multiply scattered waves, it is useful to have a medium in which the scattering properties can be easily controlled. Here, we describe such a system; it involves the propagation of ultrasonic surface waves in a medium with an aligned, disordered pattern of grooves. Waves propagating parallel to the grooves see a homogeneous medium; waves propagating perpendicular to the grooves are strongly scattered. By varying the source–receiver distance and orientation with respect to the grooves, we are able to map out the transition from ballistic to diffusive propagation. In addition, by using an optical detection system we are able to measure the wave motion inside the scattering medium. We measure the angle-dependent macroscopic properties of the medium, such as the group velocity as well as the mean-free path and the diffusion constant in the strong-scattering regime.

Multiple scattering attenuation and anisotropy of ultrasonic surface waves

Multiple scattering of waves induces bulk effects such as attenuation and anisotropy that are important in seismology, optics, medical imaging, and other fields involving propagation in disordered media. Measurements are reported of ultrasonic surface wave propagation in a strong-scattering, quasiperiodic medium consisting of a grooved surface of aluminum. Using noncontacting optical methods we have tracked the evolution of surface wave pulses within the scattering medium. Waves propagating parallel to the grooves propagate nearly attenuation and dispersion free, whereas waves propagating normal to the grooves are dispersed and exponentially attenuated with distance as energy is transferred from the direct pulse into the multiple-scattering coda. We measure this attenuation length and show that there is, in addition, a scattering induced anisotropy in the phase velocity. [This work was partially supported by the U.S. Army Research Office under Grant No. DAAG55-98-1-0070.]

Thermoelastic effects on surface acoustic wave propagation

The effect of thermal stresses on ultrasonic surface wave propagation was investigated. Quasi-static and dynamic thermal stresses and deformations were considered in materials with and without fatigue cracks. The perturbation of ultrasonic surface waves due to these thermal effects were studied both theoretically and experimentally. A long-duration infrared laser pulse was used to irradiate a series of aluminum and titanium specimens with fatigue cracks of known sizes between 0.5 and 1.0 mm. Preliminary numerical results from finite element simulation were found to be in good agreement with the experimentally observed laser-induced modulations of the ultrasonic surface wave. In particularly, both the numerical and experimental results indicated the same characteristic transient behaviors of the thermo-optical modulations for cracked and intact materials. The different behaviors suggest a new promising method for ultrasonic nondestructive evaluation, which cannot only effectively distinguish fatigue cracks from other artifact scatters, but potentially can also provide a method for quantitative evaluation of crack features. [This effort was sponsored by the Defense Advanced Research Project Agency (DARPA) Multidisciplinary University Research Initiatiative (MURI), under Air Force Office of Scientific Research Grant No. F49620-96-1-0442.]

Multiple scattering attenuation and anisotropy of ultrasonic surface waves

Multiple scattering of waves induces bulk effects such as attenuation and anisotropy that are important in seismology, optics, medical imaging, and other fields involving propagation in disordered media. We report here measurements of ultrasonic surface wave propagation in a strong-scattering, quasiperiodic medium consisting of a grooved surface of aluminum. Using noncontacting optical methods we have tracked the evolution of surface wave pulses within the scattering medium. Waves propagating parallel to the grooves propagate nearly attenuation and dispersion free, whereas waves propagating normal to the grooves are dispersed and exponentially attenuated with distance as energy is transferred from the direct pulse into the multiple-scattering coda. We measure this attenuation length and show that there is, in addition, a scattering induced anisotropy in the phase velocity.

Elastic modulus of diamond-like carbon films prepared by pulsed vacuum arc

Amorphous carbon films have been prepared by special pulsed vacuum arc deposition methods allowing high currents up to 1 kA and more. The Young's modulus of these films has been determined with respect to various technological parameters. For these measurements a method based on the propagation of ultrasonic surface waves has been applied which has been specially designed for the investigation of thin films below one micrometer. Large changes of the elastic modulus, depending on the technology, have been observed, contrasting with the common understanding of invariability of the elastic behaviour. Reflecting the large structural variations possible in amorphous carbon, the elastic modulus represents a suitable parameter for characterizing the carbon—carbon network. Furthermore, the elastic modulus of amorphous carbon films may be used for a first estimation of film hardness because of the strong correlation of these two quantities.

Modified Propagation of Ultrasonic Surface Waves by Thin Elastic or Viscous Films

AbstractThe mechanical properties of thin films often differ from those of bulk material and depend on technological parameters. But they can be determined in the coating‐substrate composite directly by means of ultrasonic surface waves. For thin layers in relation to wavelength, the displacement field within the layer is expanded in powers d/Λ. The corresponding equation system given for the general case is evaluated for elastic and viscous coatings in lowest order of d/Λ. An elastic covering layer causes a relative change in velocity proportional to ω η d where the factor is determined by the elastic and inertial differences between film and substrate. By a viscous coating the surface wave is attenuated proportional to ω3ηd. The elastic case confirms in the first order the results of a plate theory approximation given formerly by Achenbach and Keshava. However, their higher order terms prove to be incomplete from the viewpoint of the present systematic expansion.

Propagation of ultrasonic surface waves through molecular films

The propagation characteristics of 75-MHz ultrasonic waves through molecular films of arachidic acid have been investigated. On thermal cycling through the melting temperature the ordered films break up into islands of disordered arachidic acid. With the help of an ultrasonic pulse-difference technique we could detect changes in the transmitted signal when arachidic acid islands as thin as 500 Å changed from the solid to the liquid state. It is suggested that similar experiments at higher frequencies could be of value in the investigation of phase changes and general structural properties of biological membranes.

Investigation of ultrasonic surface wave propagation along the boundary between liquid and solid: Akust. Zh., 9, No. 2, p. 162 (1963)

Investigation of ultrasonic surface wave propagation along the boundary between liquid and solid: Akust. Zh., 9, No. 2, p. 162 (1963)