Acoustic Emission Basic Process and Definition

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Kelley Andrews
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1 Acoustic Emission Basic Process and Definition Words from the Definition:... transient... elastic... waves... rapid... localized... source M2

2 Many Processes Produce Acoustic Emission Problem or Solution?» Flow Noise / Leak Detection» Impact Noise / Loose Parts Detection» Crack Growth and Plastic Deformation» Friction: Rubbing, Galling, Grinding

3 Some Applications of AE (in nuclear plant) Leak Monitoring» Valves» Primary containment Loose Parts Monitoring Crack Monitoring» IGSCC ISI» Lift Rigs» Power Transformers» Leaks in Fuel Pools Unexpected» Sparger Arms / Irradiation Sample Container

4 AE Signal Formation M44

5 Signal Amplitudes Described in db AE Decibels - a log compression scale for practical use db AE - is made independent of instrument settings as far as possible, by referencing it to the preamplifier input (same as crystal output). M5

6 Scale of AE Source Processes AE Source Amplitude will be Governed by (a) Size, (b) Speed of Source Event M6

7 Types of Crack Growth Intergranular brittle, and transgranular cleavage mechanisms are good AE sources Plastic (i.e. dislocationgoverned) mechanisms are weak AE sources A mixture of mechanisms is often encountered.

8 Crack Growth: Kaiser Effect On a second loading, no AE is seen until the previous maximum load is exceeded M39

9 Wave Propagation Wave Propagation shapes the motion under the sensor, and thus the form of the detected signal Wave Velocity is crucial for source location Attenuation governs detectability at a distance

10 Wave Velocity There are different kinds of wave (compressional, shear, flexural modes etc.) Wave velocities are governed by stiffness and density - material properties Velocity dispersion: in bounded structures (e.g. plates, rods) the velocity also depends on wavelength, frequency. Typical wave velocities in solids range from 1500 to 6000m/s (5000 to 20,000ft/s)

11 Attenuation The AE signal will decay and lose energy, amplitude while traveling through a structure Cracks and leaks have both high and low frequency content ( broadband ) The higher frequency content will decay faster than the low frequency. Lower frequency will carry a greater distance Distant noise sources can usually be reduced by high pass filtering. AE signal attenuates as it travels the length of the pipe

12 Burst Type and Continuous Type AE Burst Type AE Compact Timescale Expanded Timescale Continuous Type AE

13 Burst Type AE: Signal Detection Threshold setting determines sensitivity V O L T S V t Threshold Start of Hit First threshold crossing starts the hit 0 Signal

14 Burst Type AE: Terminating the Hit Hit is terminated by means of a user-set HDT (Hit Definition Time) V O L T S Threshold Start of Hit D End of Hit HDT The hit ends when the HDT has passed without any more threshold crossings. V t 0 Signal HDT

15 Sensitivity is Determined by Gain and/or Threshold Practically All Systems Today» have detection threshold stated in dbae Many Systems» have some kind of built-in gain that affects certain measurements e.g. energy, rms

16 Definition of AE Signal Features (1) Amplitude A - how loud or strong the signal is, in volts or decibels Duration D - how long a signal lasts, in microseconds (cracks are short and friction is a long duration) Volts Threshold and First threshold crossing Amplitude Time Duration

17 Definition of AE Signal Features (2) Counts N - threshold crossings, the first way to measure AE activity Risetime R - a signal shape feature, relates to source-sensor distance Energy E - area under the curve, the best way to measure AE activity Volts 3 counts R 0 Time

18 Continuous Emission Signals The techniques developed for burst type signals are not appropriate So use:» RMS (root mean square, an average derived from the linear voltage)» ASL (Average Signal Level), an average derived from the logarithm of the voltage» Spectral analysis

19 Various Graphing Capabilities

20 Wave Form Analysis FFT and Spectral Wave Form Analysis

21 Noise and its Elimination Noise categories:» Electrical (equipment)» Electromagnetic interference (welding)» Acoustical (pumps & valves, friction) Stages in Elimination» Physical Precautions» Basic Test Setup (sensitivity, frequency)» Front End Filtering (real-time techniques)» Post Test Filtering (data interpretation)

VALVE CONDITION MONITORING BY USING ACOUSTIC EMISSION TECHNIQUE MOHD KHAIRUL NAJMIE BIN MOHD NOR BACHELOR OF ENGINEERING UNIVERSITI MALAYSIA PAHANG

VALVE CONDITION MONITORING BY USING ACOUSTIC EMISSION TECHNIQUE MOHD KHAIRUL NAJMIE BIN MOHD NOR BACHELOR OF ENGINEERING UNIVERSITI MALAYSIA PAHANG VALVE CONDITION MONITORING BY USING ACOUSTIC EMISSION