摘要:

The mechanical properties of mullite (3Al2O32SiO2) environmental barrier coatings are determined using a laser‐based ultrasonic system. It is shown that the laser source generates the two lowest order SAW modes in these systems. Experimental waveforms are generated using a 600ps pulsed Nd:YAG microchip laser and detected using a stabilized Michelson interferometer. The dispersion curves for the generated modes are extracted from the experimental data and the mechanical properties of the coatings are obtained by minimizing the error between the measured and calculated velocity values. In addition, laser ultrasonic characterization of compositionally graded coatings is discussed and preliminary results presented showing the effect of a linear composition profile on the dispersion behavior. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570304

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A nonlinear laser ultrasonic system was developed and used to characterize the fatigue state of a fractured Ti‐6Al‐4V sample with high spatial‐resolution and sensitivity. The measurement system is built around a scanning heterodyne interferometer, which allows detailed displacement field images to be created and visualized for propagating surface and bulk acoustic fields on a material surface. An assessment of the local fatigue damage of the material was made using nonlinear ultrasonic interaction principles, where the local amplitudes of the fundamental and second harmonic displacement fields are monitored simultaneously. This provides a means for evaluating the local acoustical nonlinearity parameter, &bgr;, which can be related to the accumulation of fatigue damage in a material. A large increase in &bgr; was observed between the unfatigued area (near the grip section) and the heavily fatigued area (gauge section) for a fractured dogbone specimen. The measurements show the potential for spatially‐resolving the local fatigue state of a material using laser ultrasonics. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570305

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Ultrasonic diffraction pattern at a crack tip and crack depth sizing based on the diffraction are studied by using laser‐generated bulk waves. Directivity patterns oriented essentially along the normal for the diffracted longitudinal wave and essentially at 45 (deg) for the diffracted shear wave for any incident angles is obtained. A laser‐ultrasonic system combined with time‐of‐flight (TOFD) analysis is produced and demonstrates accurate crack sizing on an artificial slot having a variable depth from 0 mm to 10 mm. An improvement of signal‐to‐noise ratio by split spectrum processing (SSP) is also suggested. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570306

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A new method to obtain a detailed image of surface‐breaking tight cracks by using laser‐ultrasonics is proposed. The surface opposite the cracking is scanned by the generation and detection lasers. Detected signals are processed by the Fourier‐domain Synthetic Aperture Focusing Technique (F‐SAFT). The point of this study is to use the laser‐generated shear waves, which interacts very effectively with the crack roots. A detailed image of the stress corrosion cracks, having depths of typically 0.5 mm and widths of less than 0.05 mm, is successfully reconstructed. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570307

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

In previous work, we showed that laser ultrasonics (photo‐acoustics) is a promising tool for the nondestructive and non‐contact characterization of thin film structures. In particular it was shown that the modulus and residual stresses in two‐layer freestanding Al/Silicon Nitride films could be measured using a narrow‐band laser ultrasonic technique. In this technique, a microchip laser deposits pulsed laser energy as a spatially periodic source on the structure. The resulting narrowband ultrasonic modes are monitored using a broadband Michelson interferometer. By varying the geometry of the spatially periodic source, a wide range of wavenumbers can be probed. For the thin films investigated, which were less than a micron in thickness (300–900 nm), only the two lowest order modes were generated and these in turn can be related to sheet and flexural modes in plates. In this paper we present an extension of this approach. The sensitivity of the photo‐acoustic system is examined by measuring variable residual stresses. Variable residual stresses are obtained in the thin films by two methods: by varying the growth rates for silicon nitride and by varying the membrane size. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570308

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Ultrasonic Lamb waves attenuate when propagating through viscoelastic plates. Experimental attenuation values are usually obtained with transducers by comparing measured time‐domain signals at two different locations. However, this method can measure only a few points on the attenuation‐frequency dispersion curves, and usually only for some specific mode. It has been shown that multi‐mode dispersion curves (plus their associated energy distribution) can be obtained by operating on the transient, time‐domain signals, generated and detected with laser ultrasonics. This technique uses proper signal processing technique, namely, a time‐frequency representation (TFR). The current research uses a similar methodology for Lamb wave attenuation measurements which provides a series of attenuation values for many modes. As a first step, the laser source repeatability (both amplitude and frequency spectrum) must be verified. This research studies the variation of a laser ablation source by considering the surface wave propagating on an isotropic half‐space. A series of experimental time‐domain surface wave signals from different propagation distances are operated on with a TFR and compared in, strictly, propagation‐distance‐independent energy slowness‐frequency domain. The amplitude constant is defined for comparison purposes. Also, the proposed signal processing technique is instituted in an error‐free manner. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570309

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

There is a desire to characterize plastically deformed regions in structures to monitor their integrity. Of particular importance is the accurate prediction of the lifetime of damaged pipelines due to outside force. In order to accurately predict the remaining life it is essential to accurately determine the degree stress and strain in the damaged region for input into fracture mechanics models. Currently, determination of the degree of stress and strain in damaged regions utilizing ultrasonic velocity measurements is complicated by the inherent texture in the materials and the difficulty in separating these effects from the stress and strain contributions. We will report ultrasonic velocity measurements on plastically deformed steel specimens to elucidate the state of damage. Specifically, we have found the shear wave birefringence and SH wave velocity are sensitive to the degree of plastic deformation. Ultrasonic results will be compared with finite element modeling calculation of the stress and strain distributions. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570310

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We have recently initiated the development of a noncontacting thermoelectric method based on magnetic detection of local thermoelectric currents in the compressed near‐surface layer of surface‐treated metals when a temperature gradient is established throughout the specimen. Beside the primary residual stress effect, the thermoelectric method is also sensitive to the secondary “material” effects of shot peening (local texture, increased dislocation density, hardening), but it is entirely insensitive to its “geometrical” by‐product, i.e., the rough surface topography. This method measures only the weighted average of the near‐surface residual stress, which is sufficient for quantitatively evaluating the degree of thermally‐induced stress release, but, in its present form, it is not suitable for detailed mapping of the residual stress profile. Preliminary results are presented for shot‐peened and low‐plasticity‐burnished IN100 nickel‐base superalloy specimens to show that the technique is also applicable to low‐conductivity engine materials. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570311

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Fretting fatigue occurs at locations where the materials are sliding against each other under load. In order to enhance the fatigue life under fretting conditions the surface of the component is shot peened. In general, the shot peening process produces a compressive stress on the surface of the material, thereby increasing the resistance of the material to crack initiation. This paper presents the relaxation of residual stress caused during fretting fatigue. X‐ray diffraction has been utilized as the method to measure residual stress in fretting fatigued samples of Ti‐6Al‐4V. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570312

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Stress is one of the principal external factors affecting the magnetization of materials. A new and improved model equation for interpreting the magnetomechanical effect has been developed based on extension of the previous equation to include the Rayleigh law. The previous theory “law of approach” has been refined by including a new linear term in the model equation in addition to the well‐known quadratic term. It was found that the modified theory provides a much better description of the magnetization changes under stress. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570313

出版商:AIP    

年代:1903

数据来源: AIP