脉冲激光激发表面声波在金属工件上的研究

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摘要

表面声波技术已经广泛应用于传感器产业。基于表面声波技术的激光声学技术是

一种新型的有前途和创造性的测试方法。随着激光声学技术的发展，由于应用在

材料特性和非破坏性试验的潜力，激发表面声波的激光结构装置受到了越来越多

的关注。

在研究的第一部分，采用经典测试方法。建立了叉指换能器作为激发源的实验装

置，并且检测接受信号。表面声波在以下不同的条件下研究：基于不同材料的金

属基片，基于不同的金属基片厚度和基于不同的测试距离（介于叉指换能器和聚

偏氟乙烯薄膜之间的距离）。

在研究的第二部分,安装了适合于脉冲激光激发的表面声波的氮气激光装置，取

代叉指换能器作为激发源且应用于实验。此外使用聚偏二氟乙烯薄膜和叉指换能

器作为接受信号的传感器，用以检测表面声波脉冲在金属基片上的传播。在这种

情况下，我们测量了在不同的测试距离下（介于激光聚焦线和接受传感器之间的

距离）表面声波信号。

经过这两种不同类型的表面声波激发的测试方法的对比,可以看出激光激发产生

而接受到的信号不如叉指换能器激发而接受到的信号来的稳定。对此，激光激发

而用来检测表面声波的方法还需要在未来的研究中改进。

关键字：表面声波激光声学技术叉指换能器聚偏二氟乙烯薄膜

ABSTRACT

Surface acoustic wave (SAW) technology has been widely used in sensor industry. The

laser-acoustic technique based on SAWs is a promising and creative test method. With

the development of laser-acoustic technique, more and more attention is attached to the

SAWs generated by laser beam on structures for its potential applications in material

characterization and non-destructive testing.

In the first part of research, the classic test method which used an interdigital transducer

(IDT) as excitation source was built and measured. The surface acoustic waveform is

detected at different metal substrate, different thickness of substrate and different

distances between the IDT and Polyvinylidene fluoride (PVDF) film.

In the second part of research, a nitrogen laser device which is suitable for pulse laser

excitation of surface acoustic waves was brought into operation and tested. The SAW

pulses propagating on the metal substrate are detected with PVDF film and interdigital

transducer. The surface acoustic waveform is detected at different distances between the

laser focus line and the transducer.

Compared with these two different kinds of measurements with two different types of

SAW excitation, the received signal excited by laser is not as stable as the received

signal excited by IDT. The method of detecting SAWs which were excited by laser still

needs to improve in future’s research.

Key Words: SAW, laser-acoustic technique, IDT, PVDF film

Table of contents

ABSTRACT

1. Introduction ............................................................................................................... 1

1.1 Background ..................................................................................................... 1

1.2 About my work ............................................................................................... 1

2. Theoretical foundation ............................................................................................... 3

2.1 Surface Acoustic Waves ................................................................................. 3

2.1.1 Rayleigh wave ...................................................................................... 3

2.1.2 Lamb wave ........................................................................................... 4

2.2 Piezoelectricity ................................................................................................ 5

2.2.1 The history of piezoelectricity .............................................................. 6

2.2.2 The properties of piezoelectricity ......................................................... 6

2.2.3 The applications of piezoelectricity ...................................................... 8

2.3 Laser acoustic technique ................................................................................. 8

3. Material and apparatus ............................................................................................. 10

3.1 Interdigital transducer (IDT) ......................................................................... 10

3.2 Polyvinylidene fluoride film sensor .............................................................. 10

3.3 High pass filter .............................................................................................. 11

3.4 Function Genetator Agilent 33220A ............................................................. 12

3.5 Oscilloscope .................................................................................................. 13

3.6 M-511. HD Nano-precision .......................................................................... 14

3.7 N2 laser ......................................................................................................... 16

4. Set up and general signal processing ....................................................................... 17

4.1 IDT exciting .................................................................................................. 17

4.2 Laser exciting ................................................................................................ 20

4.3 Signal processing in Matlab .......................................................................... 21

4.3.1 Band pass filter design ........................................................................ 21

4.3.2 Fast Fourier Transform (FFT) ............................................................ 27

5. Experiment results and discussion ........................................................................... 29

5.1 Results (signal by IDT exciting) ................................................................... 29

5.1.1 Different substrate .............................................................................. 29

5.1.2 Different thickness .............................................................................. 39

5.1.3 Different distance ............................................................................... 40

5.2 Results (signal by laser exciting) .................................................................. 41

5.2.1 With aluminum substrate .................................................................... 41

5.2.2 With steel substrate ............................................................................. 45

6. Summary and conclusions ....................................................................................... 48

Reference ........................................................................................................................ 49

Acknowledgements ........................................................................................................ 53

1. Introduction

1.1 Background

Surface acoustic waves (SAWs) are elastic waves propagating on the surfaces of solid

media. With the development of laser ultrasonic technique, more and more attention is

attached to the surface acoustic waves generated by laser beam on structures for its

potential applications in material characterization and non-destructive testing [1]. SAW

have been utilized in electronic signal processing since the 1980s [2]. In 1965 when

White and Voltmer at the University of California developed the interdigital transducer

(IDT) which enables the generation of surface acoustic waves via electric excitation on

piezoelectric crystals [3]. Mechanically testing of near surface region is a challenge of

practical interest. Therefore, for instance the potential of the laser-acoustic technique

was investigated for testing diamond-like-carbon films down to 5 nm thickness. The

films were deposited with the pulsed arc technology on pure silicon and on silicon with

80 nm metal layer. Film thickness was measured with ellipsometry to be in the range

from 5 to 30 nm [4]. Based on acoustic waves, the material characteristics can be

measured. Usually piezoelectric interdigital transducer has to be attached at the outside

surface. Pulsed laser excitation of surface acoustic waves is a non-contact investigation

method of non-destructive testing.

The use of optical technique for the generation and detection of ultrasonic waves has

attached considerable interest in the past decade [5]. Ultrasonic methods are not limited

by the types of material under study and can be utilized for residual stress

measurements on thick samples [6]. Thus ultrasonic methods are very promising. Most

ultrasonic methods are based on the effect of acoustoelasticity [7-11]-the dependency of

the acoustic wave velocity on the value of applied stresses. There are many examples of

experimental work that demonstrate the possibility of residual stress reconstruction with

using of longitudinal [12], shear [13–14] and Rayleigh waves [15].

1.2 About my work

“Pulse laser excitation of surface acoustic waves on metallic work pieces” is the theme

of my master thesis, which was given by the Institute of Sensor and Actuator

Technology (ISAT). The aim and challenge of this topic is to build up a testing system

to excite and detect the SAWs. In the first step of research, the IDT was taken as an

excitation source for the generation of SAWs. The classic technology is one IDT as an

Pulse laser excitation of surface acoustic waves on metallic work pieces

excitation of signal source, the other IDT as a receiving transducer. There is an

important difference from the previous technology in my study, the polyvinylidene

fluoride (PVDF) film was selected as the receiving sensor. Then in the second step, a

laser was chosen as the signal source. It is a creative and novel method, which also has

wide perspective in the future.

The ISAT, which belongs to the Coburg University of Applied Sciences, was found by

a group of best students, working on projects with a wide variety of expert knowledge

in different fields. The main competence of ISAT is the research and development of

sensor and actuator technology based on micro-surface acoustic waves. By means of

this technology numerous applications have already been successfully implemented by

the ISAT, such as a sensor for the detection of liquid properties. The chairmen of the

ISAT are Professor Gerhard Lindner and Professor Martin Springer.

In the first chapter of this thesis, the background of surface acoustic wave technology

and laser-acoustic technology are introduced briefly. The primary theoretical

foundations are detailed presented in the second chapter. Chapter 3 shows the main

material and apparatus. In chapter 4, two set ups about SAW excitation and the general

signal processing are presented. In chapter 5 the experimental results by using two

different systems and the discussion following every experiment are showed. At the end

the conclusions and the prospect are provided in chapter 6.

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摘要：

摘要表面声波技术已经广泛应用于传感器产业。基于表面声波技术的激光声学技术是一种新型的有前途和创造性的测试方法。随着激光声学技术的发展，由于应用在材料特性和非破坏性试验的潜力，激发表面声波的激光结构装置受到了越来越多的关注。在研究的第一部分，采用经典测试方法。建立了叉指换能器作为激发源的实验装置，并且检测接受信号。表面声波在以下不同的条件下研究：基于不同材料的金属基片，基于不同的金属基片厚度和基于不同的测试距离（介于叉指换能器和聚偏氟乙烯薄膜之间的距离）。在研究的第二部分,安装了适合于脉冲激光激发的表面声波的氮气激光装置，取代叉指换能器作为激发源且应用于实验。此外使用聚偏二氟乙烯薄膜和叉指换能器...

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