Acoustical methods. Introduction Acoustic waves in solids

Size: px

Start display at page:

Download "Acoustical methods. Introduction Acoustic waves in solids"

Mitchell Flynn
5 years ago
Views:

1 Acoustical methods Introduction Acoustic waves in solids 1

2 Acoustical methods History of acoustics in NDT The early applications of acoustics source: sword-manufacturers-guide.com source: wikipedia.org 2

Acoustical methods History of acoustics in NDT In modern days - ultrasonics came into use: source: wikipedia.org In 1912 a first application was proposed after the "Titanic" had sunk.

3 Acoustical methods History of acoustics in NDT In modern days - ultrasonics came into use: source: wikipedia.org In 1912 a first application was proposed after the "Titanic" had sunk. The Englishman Richardson claimed the identification of icebergs by ultrasound in his patent applications. In France Chilowski and Langevin started their development to detect submarines by ultrasound during World War I Ultrasonics came into industrial use late. The methods of exciting ultrasound were discovered already in 1847 by James Precott Joule and in 1880 by Pierre Curie and his brother Paul Jacques. 3

testing of material surfaces, e.g. the dye penetrant and magnetic particle method.

4 Acoustical methods History of acoustics in NDT In modern days - ultrasonics came into use: source: aws.org At the beginning of the fifties the technician only knew radiography (x-ray or radioactive isotopes) as a method for detection of internal flaws in addition to the methods for nondestructive testing of material surfaces, e.g. the dye penetrant and magnetic particle method. After the Second World War the ultrasonic method, as described by Sokolov in 1935 and applied by Firestone in 1940, was further developed so that very soon instruments were available for ultrasonic testing of materials. The first pulse echo equipment wasn't devised until around Prior to then, the "through transmission" technique had been used but on a limited basis due to the limitations inherent to that technique. In the 1960s, the first battery-operated instruments were introduced and immediately gained wide acceptance because of their more efficient use in the field. 4

5 Basics Longitudinal waves: source: wikipedia/cdang Longitudinal wave velocity (isotropic medium): 5

6 Basics Transverse waves: source: wikipedia/cdang Transverse wave velocity (isotropic medium): 6

7 Basics Reflection and transmission: incident wave Z 1 Z 2 c 1 c 2 Definition of acoustic impedance: 1 R reflected wave 2 transmitted wave T Orthogonal incidence to interface: In general: 7

8 Basics Reflection and transmission: Incoming wave Transmitted wave Incoming wave Transmitted wave Reflected wave Reflected wave 8

9 Basics Interaction with interfaces: Z 1 using: d Z 2 According to B. Bergmann: Z 1 R = 1 4 m 1 m m 1 m sin 2 2 sin 2 2πd λ 2πd λ R R = m 1 m 2 sin 2 2πd λ 9

10 Basics Diffuse reflection: For nearly all acoustic NDT-methods: Intensity/ Amplitude time 10

11 Surface waves Creep waves: 1st critical angle (total reflection condition for longitudinal wave) L L T 11

12 Surface waves Rayleigh waves: 2nd critical angle (total reflection condition of transversal wave) L L T 12

13 Guided waves Interaction with geometric boundary conditions of propagation medium: 13

14 Guided waves Lamb waves (plate waves): H. Lamb Proc. Roy. Soc. London Ser. A 93 (1917) 2 tan(0.5 zd ) 4k tan(0.5 z ) ( k ) d tan(0.5 zd ) ( k ) 2 tan(0.5 z ) 4k d k 2 cl k 2 ct 14

15 Phase velocity [m/s] Group velocity [m/s] Acoustic waves Guided waves Lamb waves (plate waves): S 0 A 0 S 1 A 1 S 2 A S 0 A 0 S 1 A 1 S 2 A Frequency x Thickness [MHzmm] Frequency x Thickness [MHzmm] Characteristic dispersion equation: 1 cg cp ( f zd ) 1 cp 1 dc P fzd d ( fz d ) 15

16 Guided waves Lamb waves (plate waves) anisotropic solids: Wang and Yuan Comp. Sci. Technol. 67 (2007) 16

17 Guided waves Other guided waves: G. Tarantino Diploma thesis (2001) G. Tarantino Diploma thesis (2001) 17

18 Attenuation Definition of lossy medium: For fields with e it time dependence: Thermoelastic dissipation: 18

19 Attenuation Geometric spreading: y u x x 19

20 Attenuation Dispersion: u u 2 u 1 t 20

21 Attenuation Modal dispersion: t t t 21

22 Attenuation [db] Acoustic waves Attenuation Measurement of attenuation: S0 A0 Lineare Reg Lineare Reg 90 For Beert-Lambert behaviour: And attenuation is calculated as: Distance [mm] 22

23 Attenuation Resolution of ultrasonic waves: Penetration depth of ultrasonic waves: 23

24 Signal representation Continuous Fourier Transformation: 24

25 Signal representation Discrete Fourier Transformation: U(t) time Nyquist-Shannon sampling theorem: 25

26 Signal representation Better approaches to visualize signals: U(t) time 26

27 Signal representation Short-time Fourier-Transformation (STFT): 27

28 Signal representation Wavelet-Transformation (WT): 28

29 29 Wavelet-Transformation (WT): 1) Generate mother wavelet: 2) Scale in frequency and shift wavelet in time: Acoustic waves Signal representation Admissibility condition: Y(t) t Y(t) t b Y(t) t a t i t c c c e t Y 2 2 ln( 2) ln(2) 2 ) (

30 Signal representation Wavelet-Transformation (WT): 3) Calculate similarity coefficient: 30

31 Signal representation Quadratic time-frequeny distributions: Cohen s class time-frequency distribution: Ambiguity function: Choi-Williams kernel function: 31

32 Signal representation Choi-Williams-Distribution (CWD): 32

33 Signal representation Comparison to dispersion curve solutions: Aluminum: c L 6320 m/s c T 3100 m/s For known distance of travel r: 33

Professor Emeritus, University of Tokyo, Tokyo, Japan Phone: ;

Professor Emeritus, University of Tokyo, Tokyo, Japan Phone: ; 17th World Conference on Nondestructive Testing, 25-28 Oct 2008, Shanghai, China New Ultrasonic Guided Wave Testing using Remote Excitation of Trapped Energy Mode Morio ONOE 1, Kenji OKA 2 and Takanobu