Creep detection of Hastelloy X material for gas turbine components with nonlinear ultrasonic frequency modulation

Creep damage is one of the main failure modes in hot-gas‑leading components in gas turbines, which results from high temperatures along with mechanical loads. The aim of this study is to clarify the metallurgical creep behaviour of the Hastelloy X material and detect and evaluate creep damage at an early stage with a nonlinear ultrasonic modulation technique. For this purpose, multiple samples were examined to demonstrate that pores and microcracks in grain boundaries spread from the outside to the inside. Inside the specimen, molybdenum was identified as the main precipitation element. In addition, the chromium diffusion in the outer areas led to the depletion of this element and favoured the formation of pores and microcracks. Failures were proven with nonlinear dual-frequency ultrasound technology. Moreover, two different longitudinal waves were sent into the samples to use harmonic and modulated response frequencies for evaluation. As a result, harmonic frequencies offered a favourable prediction of pore sizes, whereas defined sideband frequencies reacted very sensitively to the damage density and area distribution of the failures. This study offers a method for detecting creep damage with nonlinear ultrasound techniques at an early stage as well as for differentiating between pores, microcracks, dislocations and precipitation. Therefore, the design of future gas turbine components made of Hastelloy X can be adapted with regard to the shown metallurgical behaviour and damage signatures.</p

Impact detection in anisotropic materials using a time reversal approach

This article presents an in situ imaging method able to detect in real-time the impact source location in reverberant complex composite structures using only one passive sensor. This technique is based on the time reversal acoustic method applied to a number of waveforms stored in a database containing the impulse response (Green's function) of the structure. The proposed method allows achieving the optimal focalization of the acoustic emission source in the time and spatial domain as it overcomes the drawbacks of other ultrasonic techniques. This is mainly due to the dispersive nature of guided Lamb waves as well as the presence of multiple scattering and mode conversion that can degrade the quality of the focusing, causing poor localization. Conversely, using the benefits of a diffuse wave field, the imaging of the source location can be obtained through a virtual time reversal procedure, which does not require any iterative algorithms and a priori knowledge of the mechanical properties and the anisotropic group speed. The efficiency of this method is experimentally demonstrated on a stiffened composite panel. The results showed that the impact source location can be retrieved with a high level of accuracy in any position of the structure (maximum error was less than 3%)

Broadening sound absorption coefficient with Hybrid Resonances

In the last years, a great research effort has been focused on the noise mitigation at low frequencies. Membrane-type acoustic metamaterials (AMM) are one of the most promising solutions to meet the growing demand for low frequency sound absorbers. Typically, acoustic membrane absorbers require large back cavities to achieve low frequency sound absorption, which is usually categorised by a single narrow absorption peak. This paper presents an acoustic resonator unit cell, comprising of a thin elastic silicone plate with an air gap cavity with broadband absorption in a frequency range between 250 and 400 Hz. The broadband and multiple peak sound absorption showed by the proposed resonator is due to hybrid resonances which occur in the frequency range due to coupling of the structural dynamic response of the plate with the acoustic response of the air cavity. A numerical model based on acoustic-structural interaction, validated for experimental data, has been used to explain how the broadening gain in the sound absorption level is strictly related to the hybrid resonances of the unit cell resonator. We demonstrated that hybrid resonances are a function of the geometrical parameters and the ratio between the Young's modulus and the density of the material plate, thus the proposed resonators absorption frequency range is tuneable at low frequencies allowing a wider broadband not achievable with acoustic membrane absorbers.</p

Multifunctional reduced graphene oxide coating on laminated composites

Carbon Fibre Reinforced Plastics (CFRPs) are commonly used for structural applications due to their high specific mechanical properties. CFRPs can also be functionalized exploiting a combination of several materials that introduce multifunctional features to the global performance of a structure and widen their range of operations. This work investigates the use of a Reduced Graphene Oxide (RGO) Film as multifunctional coating on CFRP laminates. Exploiting the inherent properties of these films, surface properties of composite structures such as electrical conductivity and wettability can be improved. Moreover, potential built-in functions, as live strain sensing and DC-biased thermography, are studied. Three point bending tests demonstrated a negligible influence of the RGO films on the flexural properties of the CFRP laminates and confirmed a satisfying adhesion between the coating and the structure

Modelling of multiscale nonlinear interaction of elastic waves with three-dimensional cracks

This paper presents a nonlinear elastic material model able to simulate the nonlinear effects generated by the interaction of acoustic/ultrasonic waves with damage precursors and micro-cracks in a variety of materials. Such a constitutive model is implemented in an in-house finite element code and exhibits a multiscale nature where the macroscopic behavior of damaged structures can be represented through a contribution of a number of mesoscopic elements, which are composed by a statistical collection of microscopic units. By means of the semi-analytical Landau formulation and Preisach-Mayergoyz space representation, this multiscale model allows the description of the structural response under continuous harmonic excitation of micro-damaged materials showing both anharmonic and dissipative hysteretic effects. In this manner, nonlinear effects observed experimentally, such as the generation of both even and odd harmonics, can be reproduced. In addition, by using Kelvin eigentensors and eigenelastic constants, the wave propagation problem in both isotropic and orthotropic solids was extended to the three-dimensional Cartesian space. The developed model has been verified for a number of different geometrical and material configurations. Particularly, the influence of a small region with classical and non-classical elasticity and the variations of the input amplitudes on the harmonics generation were analyzed

Linear and nonlinear ultrasound time reversal using a condensing raster operation

One of the weaknesses of nonlinear methods is the excitation power required to generate nonlinear effects such as higher order harmonics (single frequency) and sidebands (dual excitation). In conjunction with the high-power prerequisite, is the required sensitivity of sensors to capture nonlinear effects which can be multiple orders of magnitude lower than the fundamental/linear response of the system. In this work a semi-air coupled nonlinear ultrasound modulation method combining phase symmetry analysis and time reversal is developed, in the aim of alleviating some of these issues. Phase symmetry analysis is used to characterise the second order and modulated nonlinear responses of a dual frequency excitation signal containing a single frequency (f1) and a sweep burst (f2). Time reversal allows for optimal focusing, in this case at locations on composites panels with barely visible impact damage. Time reversed signals typically focus at single locations at a time, in this work, a raster time reversal methodology is proposed which focuses on multiple locations simultaneously. The raster methodology reduces distortions which may affect air-coupled techniques, reduces inspection time, and shows clear enhancement of damage imaging when compared against the raw fundamental response and standard time reversal.</p