Monitoring particle impact energy using acoustic emission technique

The estimation of energy dissipated during multiple particle impact is a key aspect in evaluating the abrasive potential of particle-laden streams. A systematic investigation of particle impact energy using acoustic emission (AE) measurements is presented in this thesis with experiments carried out over a range of particle sizes, particle densities and configurations. A model of the AE impact time series is developed and validated on sparse streams where there are few particle overlaps and good control over particle kinetic energies. The approach is shown to be robust and extensible to cases where the individual particle energies cannot be distinguished. For airborne particles, a series of impact tests was carried out over a wide range of particle sizes (from 125 microns to 1500 microns) and incident velocities (from 0.9 ms-1 to 16 ms-1). Two parameters, particle diameter and particle impact speed, both of which affect the energy dissipated into the material, were investigated and correlated with AE energy. The results show that AE increases with the third power of particle diameter, i.e. the mass, and with the second power of the velocity, as would be expected. The diameter exponent was only valid up to particle sizes of around 1.5mm, an observation which was attributed to different energy dissipation mechanisms with the higher associated momentum. The velocity exponent, and the general level of the energy were lower for multiple impacts than for single impacts, and this was attributed to particle interactions in the guide tube and/or near the surface leading to an underestimate of the actual impact velocity in magnitude and direction. In order to develop a model of the stream as the cumulation of individual particle arrival events, the probability distribution of particle impact energy was obtained for a range of particle sizes and impact velocities. Two methods of time series processing were investigated to isolate the individual particles arrivals from the background noise and from particle noise associated with contact of the particles with the target after their first arrival. For the conditions where it was possible to resolve individual impacts, the probability distribution of particle arrival AE energy was determined by the best-fit lognormal probability distribution function. The mean and variance of this function was then calibrated against the known nominal mass and impact speed. A pulse shape function was devised for the target plate by inspection of the records, backed up by pencil lead tests and this, coupled with the energy distribution functions allowed the iv records to be simulated knowing the arrival rate and the nominal mass and velocity of the particles. A comparison of the AE energy between the recorded and simulated records showed that the principle of accumulating individual particle impact signatures could be applied to records even when the individual impacts could not be resolved. For particle-laden liquid, a second series of experiments was carried out to investigate the influence of particle size, free stream velocity, particle impact angle, and nominal particle concentration on the amount of energy dissipated in the target using both a slurry impingement erosion test rig and a flow loop test rig. As with airborne particles, the measured AE energy was found overall to be proportional to the incident kinetic energy of the particles. The high arrival rate involved in a slurry jet or real industrial flows poses challenges in resolving individual particle impact signatures in the AE record, hence, and so the model has been further developed and modified (extended) to account for different particle carrier-fluids and to situations where arrivals cannot necessarily be resolved. In combining the fluid mechanics of particles suspended in liquid and the model, this model of AE energy can be used as a semi-quantitative diagnostic indicator for particle impingement in industrial equipments such as pipe bends

Application of acoustic emission to predict corrosion.

Non-destructive testing (NDT) techniques used for petroleum pipelines and offshore windturbines can only detect corrosion after it has occurred. Therefore, intrusive inspections are required regularly, potentially causing disruption to operation and production. Acoustic Emission (AE) is a non-destructive testing (NDT) sensor based technique which measures the detection and the conversion of high frequency (between 100 kHz to 1 MHz) elastic waves generated by the rapid release of energy to electrical signals. AE is released when crack propagates in the specimens during corrosion. This presentation will summarise AE sensor based technique for monitoring corrosion and offer examples of practical applications. Samples tested include aluminium and steel thin plates (rectangular shape) in different corrosive environments. AE from corrosion usually releases much less energy than emission from crack growth, and so is more difficult to detect in the field environment. However, the results present an exponential curve showing a trend between the concentration of the corrosive environment and the energy of the acoustic emission signal

Computational fluid dynamics modelling of multiphase flows in double elbow geometries.

This study investigates the effects of elbow on the transition and development of multiphase flow using computational fluid dynamics modelling techniques. The Eulerian - Multifluid VOF model coupled with an Interfacial Area Transport Equation has been employed to simulate air-water two-phase flow in a pipe with two standard 90 degree elbows mounted in series. Turbulence effects were accounted for by the RNG k-ε model. The effects of separation distance on two-phase flow development have been studied for initial slug and churn flow regimes. Computational fluid dynamics simulation results of phase distribution and time series of void fraction fluctuations were obtained and they showed good agreement with available experimental data. The results show that for initial slug flow regime, there is no flow regime transformation upstream and downstream of the two elbows. While at initial churn flow regime, flow regime transformation occurs at different sections of the flow domain before and after the two elbows. It was noticed that irrespective of the flow regime, the amplitudes and frequencies of void fraction fluctuation become smaller as the fluid flows along the pipe. Changes in the separation distance between the two elbows have larger effects on the flow at churn flow regime

Monitoring tasks in aerospace.

Approximately up to one-fifth of the direct operating cost of a commercial civilian fixed-wing aircraft is projected to be due to inspection and maintenance alone. Managing aircraft health with minimal human intervention and technologies that can perform continuous or on-demand monitoring/evaluation of aircraft components without having to take the aircraft out of service can have a significant impact on increasing availability while reducing maintenance cost. The ambition of these monitoring technologies is to shift aircraft maintenance practice from planned maintenance (PM), where the aircraft is taken out of service for scheduled inspection/maintenance, to condition-based maintenance (CBM), where aircraft is taken out of service only when maintenance is required, while maintaining the required levels of safety. Structural health monitoring (SHM) techniques can play a vital role in progressing towards CBM practice. Therefore, this chapter aims to provide the reader with a brief overview of the different SHM techniques and their use, as well as, challenges in implementing them for aircraft applications

Investigation of sand transport in an undulated pipe using computational fluid dynamics.

A CFD model has been implemented to investigate the effects the pipe undulation on sand transport. Of particular interest of the present study is the sand deposition in small angled V-inclined bend relevant to oil and gas subsea flowlines where sand deposition could be a major problem. The model used is the two-fluid Eulerian-Eulerian model with the granular temperature to tackle the solid phase properties. A number of sub-models for tackling solid-fluid and fluid-fluid interaction has been incorporated in the modelling frame work to capture the transition of flow regimes. The simulation results show that the seemingly small angled V-inclined has significant impact on sand disposition compared to the horizontal section. Sand is deposited at the downstream section of the V-inclined pipe at much higher velocities compared to the minimum transport velocity of the horizontal pipe

Flow noise identification using acoustic emission (AE) energy decomposition for sand monitoring in flow pipeline.

In pipelines used for petroleum production and transportation, sand particles may be present in the multi-phase flow of oil and gas and water. The Acoustic Emission (AE) measurement technique is used in the field of sand monitoring and detection in the oil and gas industry. However, as the AE signals recorded are strongly influenced by flow conditions in the pipe, identification of sand particle related signals or events remain a significant challenge in interpretation of AE signals. Therefore, a systematic investigation of sand particle impact AE energy measurements, using a sensor mounted on the outer surface of a sharp bend in a carbon steel pipe, was carried out in the laboratory to characterise flow signals using a slurry impingement flow loop test rig. A range of silica sand particles fractions of mean particle size (212-710 um) were used in the flow with particle nominal concentration between (1 and 5 wt.%) while the free stream velocity was changed between (4.2 and 14 ms-1). A signal processing technique was developed in which the total AE energy associated with particle-free water impingement was divided into static and oscillated parts and a demodulated frequency analysis was carried out on the oscillated part to identify major spectral components and hence the sources of AE signals. A simple theoretical model for water impingement AE signals was then developed to show the dependence of AE energy components on different flow speeds. A similar decomposition of AE energy into static and oscillatory components was used to analyse AE signals for particle-laden flows. The effect of flow speed on the spectral AE energy for different sand concentrations and particle size fractions was investigated and the results show that the 100 Hz band is attributed to mechanical noise, the 42 Hz band is due to fluid turbulence and the dominant band is broad oscillated component. The AE energy decomposition method together with the water impingement model and coupled with spectral peaks filtering enable isolation of AE energy associated with particle impact from other AE sources and noise and, hence, the proposed decomposition approach can enhance the interpretation of AE data in pipeline flows

Analysis of acoustic emission propagation in metal-to-metal adhesively-bonded joints.

Acoustic emission (AE) monitoring shows promise as one of the most effective methods for condition monitoring of adhesively-bonded joints. Previous research has demonstrated its ability to detect, locate and classify adhesive joint failure, though in these studies little attention appears to have been paid to the differences in AE wave propagation through the bonded and un-bonded sections of the specimens tested, or to the effects of the wave modes excited or the propagation distances. This paper details an experimental study conducted on large aluminium sheet specimens to identify the effects of the presence of an adhesive layer on AE wave propagation. Three specimens are considered; a single aluminium sheet, two aluminium sheets placed together without adhesive, and an adhesively-bonded specimen. A pencil lead break (PLB) is used as a simulated AE source, and is applied to the three specimens at varying propagation distances and orientations. The acquired signals are processed using wavelet-transforms to explore time-frequency features, and compared with modified group-velocity curves based on the Rayleigh-Lamb equations to allow identification of wave-modes and edge-reflections. The effects of propagation distance and source orientation are investigated while comparison is made between the three specimens. It is concluded that while the wave propagation modes can be approximated as being constant throughout all three specimens, there is a significant change in the received waveforms due to the attenuation of high-frequency components exhibited by the bonded specimen. These findings may be utilised to provide a deeper understanding of acquired AE data, improving the current abilities to identify, locate and characterise damage mechanisms occurring within adhesive joints, ultimately improving safety in the use of adhesive bonding for critical applications

Investigation of slug-churn flow induced transient excitation forces at pipe bend.

Numerical simulations of two-phase flow induced fluctuating forces at a pipe bend have been carried out to study the characteristics of multiphase flow induced vibration (FIV). The multiphase flow patterns and turbulence were modelled using the volume of fluid (VOF) method and the k−ϵ turbulence model respectively. Simulations of seventeen cases of slug and churn flows have been carried out showing the effects of superficial gas and superficial liquid velocities. The simulations results show good agreement of the volume fraction fluctuation frequencies of slug and churn flows with the reported experiment. In addition, the vibration characteristics of the excitation force have been accurately captured. The simulation results show that the predominant frequency of fluctuations of force decreases and the RMS of force fluctuation increases with the increase of superficial gas velocity. On the other hand, both predominant frequency and the RMS of force fluctuations increases with the increase of superficial liquid velocity. Increase of gas fraction narrows the range of frequency ranges, while increasing the liquid expands the frequency ranges of force fluctuations

Data reduction strategies.

Based on the variety of methods available for gathering data for the aircraft health status, the challenge is to reduce the overall amount of data in a trackable and safe manner to ensure that the remaining data are characteristic of the current aircraft status. This chapter will cover available data reduction strategies for this task and discuss the data intensity of the SHM methods of Chaps. 5 to 8 and established approaches to deal with the acquired data. This includes aspects of algorithms and legal issues arising in this context

Modal acoustic emission analysis of mode-I and mode-II fracture of adhesively-bonded joints.

Acoustic emission (AE) testing has previously been demonstrated to be well suited to detecting failure in adhesively-bonded joints. In this work, the relationship between the fracture-mode of adhesively-bonded specimens and the acoustic wave-modes excited by their failure is investigated. AE instrumented Double-Cantilever-Beam (Mode-I fracture) and Lap-Shear (Mode-II fracture) tests are conducted on similar adhesively-bonded aluminium specimens. Linear source-location is used to identify the source-to-sensor propagation distance of each recorded hit, theoretical dispersion curves are used to identify regions of the signal corresponding to the symmetric and asymmetric wave modes, and peak wavelet-transform coefficients for the wave-modes are compared between the two fracturemodes. It is demonstrated that while both fracture-modes generate AE dominated by the asymmetric mode, the symmetric mode is generally much more significant during Mode-II fracture than Mode-I. While significant scatter and overlap in results prevents the ratio of peak-wavelet transform coefficients from being a robust single classifier for differentiation between fracture-modes in most cases, other modal analysis methods, or integration of this parameter into multi-parameter methods in future work may result in more reliable differentiation. Understanding of the wave-modes excited by the different fracture-modes also has implications for source-location, as identification of the correct modes is critical for selection of suitable wave velocities