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30 Cards in this Set
- Front
- Back
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What is the definition of an NDE technique?
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A procedure which covers the inspection and/or testing of any material, component or assembly by means which do not affect its ultimate serviceability
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What defect information is necessary in order to take full adavantage of stress-analysis?
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Defect type, location, dimension and orientation
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How does one chose an NDE technique?
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Depends on the material and the probability-of-detection curve
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What are the limitations of the NDE capability?
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- Physics of the fundamental interactions
- Instrumentation - Human factors - Defect type, size, population - Base material properties - Component geometry |
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What are the typical operating frequencies of ultrasonic NDE and what effect do different values have?
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The range is between 0.5 - 100MHz.
Higher frequencies have higher resolutions |
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What are the advantages of ultrasonic NDE?
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- Superior penetrating power (7-10m in steel)
- High sensitivity (µm detection) - Accuracy (location, size, orientation, shape, nature) - Only one surface needed - Electronic operation (automatic scanning, in-line production) - Volumetric scanning - Not hazardous to personnel - Portable |
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What can Hooke's law be expressed as?
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F prop Δl
Stress = Young's Modulus x Strain |
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What is mechanical stress?
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F/A
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What is mechanical strain?
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Δl/l
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What types of ultrasonic waves are there?
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- Compressional waves (longitudinal)
- Shear wave (transverse) - Rayleigh wave (surface) - Lamb wave (plate) |
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How do the materials move in Rayleigh and Lamb waves?
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They move in elliptical orbits coutnerclockwise to the direction of propagation
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What are the two lowest-order lamb waves in a plate?
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Symmetrical and asymmetrical
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Why is the wavelength of prime interest in ultrasonic NDE?
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- The sensitivity to detect defects is a function of λ. If d<<λ no siginificant amount of energy will be reflected i.e. no detection
- Penetration is dependent on λ. If λ is large it will be scattered less and more energy will get through |
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What determines the amount of energy reflected at a material boundary?
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The acoustic impedance. The more different the two materials the more reflection
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What is acoustic impedance defined as?
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Z=ρ x Vc
where Vc is the compressional wave speed |
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What does the percentage of reflected energy depend on?
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- Ratio of impedance Z2/Z1
- Angle of incidence |
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If the incidence angle is 90° what is the amplitude reflection coefficient given as?
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R=Ar/Ai=(Z2-Z1)/(Z2+Z1)
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If the incidence angle is 90° what is the transmission reflection coefficient given as?
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T=At/Ai=2Z2/(Z2+Z1)
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What is the definition of intesity?
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The energy transmitted through a unit corss-sectional area and it is proportional to the square of acoustic pressure amplitude
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What is acoustic pressure?
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It is the amplitude of alternating stresses exerted on a material by a propagating wave. It is directly proportional to the product of acoustic impedance and amplitude of particle motion
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What happens if the wave is incident obliquely at the material surface?
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- Refraction: change in direction of propagation
- Mode-conversion: change in nature of wave motion |
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What is snell's law?
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Sinθ1/Sinθ2=V1/V2
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What are the critical angles and why are these of importance?
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There are two critical angles. The first is when the refracted compressional wave disappears. The second is when the refracted shear wave diasppears and is replaced by a Rayleigh wave. This only happens if the incident compressional velocity is smaller than the shear wave velocity
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What are the three main causes of energy loss?
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1. Transmission losses
2. Beam spreading 3. Interference effects |
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What are transmission losses?
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Scattering
Absorption Acoustic impedance |
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What are beam spreading losses?
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Losses due to transition of spherical or cylindrical waves to plane waves depending on shape of transducer
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What are interference effect losses?
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Diffraction
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What is absorption?
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Occurs mainly by conversion of mechanical energy to heat
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How does absorption occur?
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Occurs because during compression and rarefaction the material heats and cools. But because heat flow is slower than ultrasonic waves thermal losses occur. Absorption losses directly increase with frequency
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What is scattering?
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An energy loss because of inhomogeneities in the material which deflect small amounts of energy out of the main beam
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