Year

2018

Degree Name

Doctor of Philosophy

Department

School of Electrical, Computer and Telecommunications Engineering

Abstract

Young’s modulus is one of the most frequently measured mechanical parameters of materials. It can be used for indirect measurement and investigation of compressive strength, flexural strength, and even porosity, texture, intergranular phases and so on. Therefore, it is of significant interest to know the value of this parameter. Traditional methods for determination of Young’s modulus mainly refer to tensile test, flexure test and indentation technique, which are ineffective, destructive with high cost and low accuracy, and inapplicable for a high throughput computer system for measurements of material properties. In recent years, an emerged impulsive excitation technique (IET) has been developed for improvement in Young’s modulus measurement. More gratifying news is that self-mixing interferometry (SMI), as an emerging and promising non-contact sensing technique, has attracted much attention of researchers nowadays and has been employed for multiple engineering applications, such as the measurement of displacement, distance, velocity and surface roughness and so on. In IET technique, as for measurement of Young’s modulus, an empirical equation, building a form relationship between Young’s modulus and resonant frequency, have been proposed and investigated in many standards. Hence, Young’s modulus can be achieved once resonant frequency is obtained. On this basis, a non-destructive SMI measuring system is proposed for measuring the resonant frequency in this thesis.

This basic SMI measuring system consists of a laser diode (LD), a micro-lens and an external target, i.e. the specimen to be tested. The vibration of specimen causes the variation of the length of the external cavity, and accordingly leads to a modulated laser power of the LD, the output signal of the measuring system. This output signal, called as SMI signal, carries the information of vibration which will be applied through fast Fourier transform for obtaining this wanted resonant frequency.

For this purpose, in this thesis, the classic steady mathematical model derived from Lang-Kobayashi equations is firstly applied to establish a connection between the motion of the specimen and the output of the SMI system. Secondly, simulations of damping vibration corresponding SMI signal will have been conducted for discussion on effect of two internal parameters of the laser diode, i.e. the line-width enhancement factor and the feedback level of the self-mixing phenomenon. Thirdly, simulations of extraction of the resonant frequency have been performed in the thesis and show a good result.

A great contribution made in the thesis is that the fiber-coupled system structures are elaborately designed, which consist of mechanical components for generating the input vibrating signal, optical parts for producing the output signal of SMI system, and the processing units for processing the output signal. This provides a specific guidance for achieving high quality measurement. Experiments have been conducted comparing with the traditional tensile tests, and the results show a good agreement and small variation with the data in literature.

In addition, the thesis have addressed and analyzed the influence of the proposed method that may originate from many aspects. First, the accuracy of Fast Fourier Transform (FFT) relies on the resolution of FFT, which determines by the sampling frequency and the data length of SMI signal. Secondly, mode shape of vibration of the specimen varies with the different support location. Thirdly, the manufacturing process that includes surface treatment and discontinuities, shape requirement, edge treatments and the propagation error from measured size and mass are influential to the test results. All of these are detailed and discussed in Chapter 4.

Damping may be caused by friction between moving elements, flow of a fluid through a restriction, or other means, but whatever the source, damping converts kinetic and potential energy into heat. Therefore, the self-mixing interferometry that contains the important information of damping displacement and the resonant frequency can be utilized for evaluation of specimen’s damping. So lastly, in Chapter 5, another typical application of resonant method based on self-mixing interferometry measuring system for evaluation of energy loss is discussed. Fast Fourier transformed has been applied for processing the signal. Both simulations and experiments have been performed as the proof for this application. The results show the effeteness of this proposed method.

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