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30 Cards in this Set
- Front
- Back
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What inhomogeneities could cause scattering?
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Crystal discontinuities such as grain boundaries or inclusions
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What could happen at inhomogeneities?
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Mode-conversions could happen in mixed grain or anisotropic materials
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What is an anisotropic material?
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A material which is not constant in all directions. (Isotropic)
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What is scattering dependent on?
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The relationship between λ and scatterer size. If scatterer size is less than 0.01λ scattering is negligible
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How can attenuation be expressed in the far field?
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It can be expressed as an attenuation coefficient α.
I=I0 exp(-αL) |
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How is the attenuation coefficient usually expressed?
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In dB/mm according to the amplitude ratio
n= 20log(A1/A2) OR intensity ratio n=10log(I1/I2) |
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What does total attenuation determine?
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The depth to which ultrasonic inspection can be performed which is dependent on frequency
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What is a piezoelectronic material?
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One that changes shape due to an applied electric potential
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What is the near field?
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The area where the waves emanated from the transducer interfere in such a way that there are maxima and minima
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What is the far field?
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This is any distance further than the near field and the wave emitted will have reached a maximum and decreases exponentially
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Sketch the wave amplitude of a piezoelectronic transducer signal
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SKETCH
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To what is the distance of the near field related to?
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Crystal diameter and λ
N~= D^2 / (4λ) |
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What happens in the far field?
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the wave front expands with distance from the transducer by a given angle of divergence
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What is the angle of divergence?
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The angle by which the beam in the far field expands
θ6dB=sin^-1 (0.5λ /D) |
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How can beam spreading be reduced?
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A higher frequency or a larger diameter
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What are the three basic forms of data representation of ultrasonic NDE?
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A-scan (amplitude v time)
B-scan (plane parallel to beam) C-scan (plane perpendicular to beam) |
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What is lateral resolution?
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The ability to discern between features
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What does "gating" mean?
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Look out for signals within a certain time frame because it is expected then due to the mechanical properties of the material
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How can a slanted wave be produced from a straight set of transducers aka Phased arrays?
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Excite at different sources at different times
θ=sin^-1 (VΔt /w) |
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What is the DGS method?
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Distance-gain-size method
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What is the DGS method based on?
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Based on the fact that echo height, which is a measure of intensity, is a direct function of the area of the reflecting surface
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What are the necessary conditions for correct flaw sizing?
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1. Defect smaller than beam
2. Defect parallel to surface 3. Defect is pane shaped 4. Defect must be completely reflective 5. Surface defect must be smooth 6. Defect must be in far field |
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How does the DGS method work?
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1. backwall amolitude is measured
2. Defect amplitude is measured 3. Gain is increased until defect echo has amplitude of backwall echo 4. gain and depth are known therefore size can be read off graph |
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What method is used for defects gibber than the beam?
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the 6dB (20dB) drop method
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How does the 6dB (20dB) drop method work?
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The probe is moved across the defect until the echo amplitude has dropped by half (6dB) or to a tenth (20dB) of the maximum amplitude
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What is the time of flight technique?
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Uses the time difference between pulses arriving from different scattering points
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What assumption does the time of flight technique rely on and when is this valid?
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Relies on the assumption that extremities of a defect act as independent scattering centres. This is valid when d>>λ i.e. high frequency domain
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How does bandwidth depend on pulse length?
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Short pulse --> broad band width
Long pulse --> narrow bandwidth |
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How is bandwidth calculated?
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bw= (f2 - f1)/fc * 100
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Why is frequency of interest during ultrasonic spectroscopy?
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Scatter depends on frequency
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