Guided Wave Energy Distribution Analysis in Inhomogeneous Plates

An analysis of guided wave energy propagation in a inhomogeneous multi-layered composite structure is presented. It has earlier been reported that ultrasonic guided wave energy within an inhomogeneous composite materials tend to follow the orientation of the fibers, even when the plane of the incident wave is not along the fiber direction [1–3]. In this paper, an exact analysis of a plane wave incident onto a generally anisotropic, visco-elastic, multi-layered composite structure is use here to study the energy flow behavior of guided ultrasonic waves in inhomogeneous composite plates [4] is used to predict this behavior. The reflected and refracted coefficients are obtained by using the well know Thomson-Haskell transfer matrix method [5] which transfer boundary conditions from one side of a solid medium to the other. Then, the power flow vector is used to study the energy distributions within the plates as well as the generation mechanism of guided waves

A Study on the Prenatal Zone of Ultrasonic Guided Waves in Plates

Low frequency guided wave based inspection is an extensively used method for asset management with the advantage of wide area coverage from a single location at the cost of spatial resolution. With the advent of high frequency guided waves, short range inspections with high spatial resolution for monitoring corrosion under pipe supports and tank annular plates has gained widespread interest and acceptance. One of the major challenges in the application of high frequency guided waves in a short range inspection is to attain the desired modal displacements with respect to the application. In this paper, an investigation on the generation and formation of fundamental S0 mode is carried out through numerical simulation and experiments to establish a prenatal zone for guided waves. The effect of frequency, thickness of the plate and frequency-thickness (f*d) is studied. The investigation reveals the existence of a rudimentary form with similar modal features to the fully developed mode. This study helps in the design and development of a high frequency guided wave generator for particular applications which demands waves with very less sensitivity to the surface and loading during the initial phase which immediately evolves to a more sensitive wave towards the surface on propagation for the detection of shallow defects

Distributed Temperature Sensing using a SPIRAL Configuration UltrasonicWaveguide

Distributed temperature sensing has important applications in the long term monitoring of critical enclosures such as containment vessels, flue gas stacks, furnaces, underground storage tanks and buildings for fire risk. This paper presents novel techniques for such measurements, using wire in a spiral configuration and having special embodiments such a notch for obtaining wave reflections from desired locations. Transduction is performed using commercially available Piezo-electric crystal that is bonded to one end of the waveguide. Lower order axisymmetric guided ultrasonic modes were employed. Time of fight (TOF) differences between predefined reflectors located on the waveguides are used to infer temperature profile in a chamber with temperature gradients. Both L(0,1) and T(0,1) wave modes were generated and the pros and cons of these two modes are highlighted.The ultrasonic measurements were compared with commercially available thermocouples

Propagation Characteristics of Ao Mode in a Metallic Beam Containing a Semi-infinite Crack

When Ao Lamb wave propagating in a sub-beam encounters the edge of a semi-infinite horizontal crack in a metallic beam, reflection, and transmission into the main beam and sub-beam takes place. Lamb mode transmitted from one sub-beam to the other is termed as "Turning Lamb Mode" (TLM). The variations in reflection and transmission factors of Ao Lamb modes, defined based on Fourier Transform and Wavelet Transform, when a horizontal crack is situated at different positions across the thickness of a beam were studies through numerical simulations. In addition to this, the power reflection and transmission ratios were also estimated. It was observed that the transmission and reflection characteristics of Ao Lamb mode strongly depend on the thickness ratio. It was also observed that the nature of variations in transmission factor, and power transmission coefficient of TLM with thickness ratio is different

Nonlinear Mixing of Laser Generated Narrowband Rayleigh Surface Waves

This study presents the nonlinear mixing technique of two co-directionally travelling Rayleigh surface waves generated and detected using laser ultrasonics. The optical generation of Rayleigh waves on the specimen is obtained by shadow mask method. In conventional nonlinear measurements, the inherently small higher harmonics are greatly influenced by the nonlinearities caused by coupling variabilities and surface roughness between the transducer and specimen interface. The proposed technique is completely contactless and it should be possible to eliminate this problem. Moreover, the nonlinear mixing phenomenon yields not only the second harmonics, but also the sum and difference frequency components, which can be used to measure the acoustic nonlinearity of the specimen. In this paper, we will be addressing the experimental configurations for this technique and characterize the acoustic nonlinearity of the specimen through measuring the nonlinearity parameter ‘β’. The proposed technique is validated experimentally on Aluminum 7075 alloy specimen

Characterization of Tempering Behaviour of Modified 9Cr-1Mo Steel by Ultrasonic Lamb wave Mixing

This work demonstrates the use of mixing of ultrasonic Lamb waves to characterize the tempering behaviour in metallic plate. Lamb wave mixing has been used to measure the generated second harmonic during tempering of mod.9Cr-1Mo steel plate. As-received material is normalized at 1080°C and then tempered in the temperature range of 600 - 850°C with a step size of 50°C for 1.5hrs and followed by furnace cooling. Lamb wave mixing technique has been used to assess the tempering behaviour of this material. Nonlinear ultrasonic parameter β which is the ratio of 2nd harmonic amplitude to the multiplication of the fundamental amplitudes is determined from the mixing wave at each temperature and correlated with microstructural characteristics. It is seen that this nonlinear acoustic parameter (β) is sensitive towards coherency strain generated between precipitate and matrix during tempering

Nonlinear Lamb wave mixing for assessing localized deformation during creep

Nonlinear ultrasonic is known to be a promising technique to characterize the microstructural degradation in engineering materials. This work demonstrates the use of nonlinear Lamb wave mixing technique to assess the localized deformation in modified 9Cr–1Mo steel during creep. Two Lamb wave modes of different frequencies ( ω 1 & ω 2) are allowed to mix within the material under certain resonant condition to generate third type of harmonic waves of frequencies ( ω 1 ω 2). This new generated wave carries the information of material nonline- arity from the mixing site and independent of the other extraneous nonlinear factors. Amplitude of the generated third wave depends on the third order elastic constants of the material. This study reveals that nonlinear Lamb wave mixing technique could be used to assess the localized deformation much prior to its failure

Ultrasonic Wave Propagation Studies in Anisotropic Plates with built-In Material Degratdation

Anomalies of several kind are often found unavoidable during the manufacturing process of 0fiber reinforced composite parts. Several ultrasonic wave propagation and feature based signal analysis techniques can be found for characterizing these defects. Such methods are currently established on a problem to problem basis. Hence, the NDE methods are unable to keep up with the rapidly progressing materials technology and there is a need for a quick turnaround generalized method for anomaly modeling and experimental simulation to physically study the anomaly influence process on the ultrasonic signal. This paper addresses issues regarding the emerging new methods of ultrasonics oblique incidence techniques for the non-destructive evaluation of anisotropic plates. In this paper, efforts on theoretical modeling of imperfect composite structures, with in-built anomalies, have been attempted. Some of the common types of anomalies which can be considered, includes micro and macro porosity, fiber fraction changes, fiber mis-orientation, improper lay-up, interfacial weakness, improper curing, et

Subwavelength Resolution of Cracks in the Metallic Materials

In recent years, various types of acoustic metamaterials have been proposed with capabilities for overcoming the diffraction limit. However, typically such developments only consider the acoustic regime [1] or imaging in liquid media [2]. In this paper we demonstrate the application of a holey structured metamaterial lens for sub-wavelength imaging of defects in a metallic sample, in the ultrasonic regime. Finite Element (FE) simulations are used to study longitudinal wave interaction with ideal cracks in isotropic elastic materials. Holey-structured meta-lenses are then used to transmit the scattered waves. We experimentally demonstrate a subwavelength resolution of λ/7 with a sub-wavelength notch in an aluminium block, which to the best of our knowledge this is the highest resolution achieved in the ultrasonic regime