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Composites > NDT intro and ultrasound > Flashcards

NDT intro and ultrasound Flashcards

1
Q

What is safe life design

A

Design so that damage grows so slowly that no defect becomes dangerous
within lifetime of component. (Very conservative).

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2
Q

Explain role of NDT in safe life design

A

not used!!

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3
Q

What is damage tolerant design

A

Damage tolerant Damage grows slowly enough that can be detected and
repaired/replaced before failure

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4
Q

what is critical damage size

A

size at which damage causes failure in a componeny

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5
Q

what is detection limit

A

smallest damage that can be detected

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6
Q

what is detection window, and symbol

A

Tdw = time between damage being detectable and becoming critical

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7
Q

what is inspection interval

A

the maximum time between inspections

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8
Q

Formula for inspection interval

A

∆t < Tdw / N + 1

N is number of required inspections to be certain of detection

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9
Q

what are the effects of improved inspection techniques

A
  • lower detection limit
  • increased inspection intervals
  • reduced weight with original schedule maintained (by increasing crack growth rate and lowering critical size)
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10
Q

how does visual inspection work

A

Person looks at part, can use tools like boroscopes

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11
Q

What time of defects is visual inspection used for

A

Surface damage

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12
Q

how does dye penetrant testing work

A
  1. clean component
  2. apply die liquid
  3. remove excess liquid
  4. dry (optional)
  5. apply developer (liquid/ solid powder that draws dye from cracks)
  6. inspect
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13
Q

List some types of material defects in composites

A
  • delamination
  • porosity
  • fibre waviness
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14
Q

list some assembly defects in compoistes

A
  • surface scratches
  • bonding faults
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15
Q

list some in service defects with composites

A
  • impact damage
    -water ingress
  • fatigue
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16
Q

explain the process of ultrasonic testing

A
  1. transducer generates ultrasonic pulse which propagates through sample
  2. where no damage, pulse is reflected back by the back of the sample and detected by transducer
  3. damage causes additional reflections which appear in the recieved signal
  4. time tracer gives info on if theres damage, how deep into sample its located, and some of its characteristics
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17
Q

what are the 5 components of an ultrasound NDT system with their purpose

A
  1. computer - controls experiment, stores and analyses data
  2. Pulser/reciever - electrical device generates high voltage pulse fed to transducer. also amplifies and filters signal that comes back from transducer.
  3. transducer - converts electrical pulse into mechanical waves, and converts reflected waves into electrical signal
  4. coupling - a medium for ultrasonic waves to pas from transducer to sampke
  5. ADC - analogue to digital converter. nalogue signals from reciever are digitalised to be input into computer.
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18
Q

how does a transducer work

A
  • piezoceranmic material generates ultrasonic pulse
  • backing material absorbs pulse travelling into transducer
  • wear layer protects element and allows ultrasonic pulse to pass out of transducer
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19
Q

what affects transducer performance

A

transducer performance defined by centre frequency and bandwidth

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20
Q

what does the piezoelectric material do in transducer

A

convert electrical energy into sound waves and vice versa

when a force is applied to material surface, a voltage is created across the electrodes

21
Q

how to calculate number of cycles in signal

A

N = Tp/T

Tp is pulse duration
T is period of oscillation

22
Q

formula for centre frequency

A

fc = 1 / T

23
Q

formula for bandwidth

A

∆f = 1 / Tp

24
Q

formula for longitudinal velocity (longitudinal wave propagation

A

CL = SQRT( (K+4G/3) / ρ ) = f λ

see notes if needed

K is bulk modulus
G is shear stiffness

25
Q

what is tome of flight (TOF)

A

time for pulse to to travel from transducer and back

26
Q

how to calculate time of flight

A

t = 2d / CL

27
Q

Minimum defect size formula

A

w > λ / 2

28
Q

what is depth resolution

A

smallest difference in depth that can be resolved

29
Q

formula for dept resolution

A

∆d = Nλ / 2

30
Q

what is reflection coefficient, R

A

ratio of reflected acoustic power to incident power

31
Q

what us transmission coefficient, T

A

ratio of transmitted acoustic power to incident power

32
Q

Acoustic impedance formula

A

z = p / u

p is pressure amplitude
u is velocity amplitude

33
Q

acoustic impedance formula using material parameters

A

z = ρ CL

34
Q

Formula for reflection coefficient between two materials

A

R = ( (z2/z1 - 1) / (z2/z1 + 1) ) ^2

35
Q

Formula for transmission coefficient between two materials

A

T = (4z2/z1) / (z2/z1 + 1)^2

36
Q

formula for reflected pressure amplitude

A

Pr = SQRT(R) x Pi

pr is reflected pressure amplitude
pi is pressure amplitude of initial wave?

37
Q

formula for transmitted pressure amplitude

A

Pt = SQRT(T) x Pi

pt is transmitted pressure amplitude
pi is pressure amplitude of initial wave?

38
Q

Formula for reflection for two boundary systems

A

check notes

39
Q

name and explain 3 typed of ultrasound scanning methods

A
  1. A scan - single stationary point scan
  2. B scan - transducer moves along straight line
  3. C scan - covers whole area moving in different directions
40
Q

explain the process of B scans

A
  1. transducer moves in straight line taking multiple A scans
  2. A scans plotted next to eachother as colour maps
  3. image produced of reflected signal as function of horizontal position and depth
  4. can recover A scan data to zoom in on particular defects
41
Q

explain the process of C scans

A
  1. transducer scans in 2 dimensions
  2. produces amplitude data in 3D: two translations and horizontal position + depth
  3. requires simplifocation for plotting
  4. apply time window so that front and back reflections are ignored
  5. measure largest reflected amplitude within sample
  6. gives scanned data amplitude value over the 2 scanned dimensions
  7. specific A scans for defects are retrievable
42
Q

what is coupling

A

adding substance, like a gel, to improve the ultrasound passage to the sample by removing air gap

43
Q

list 3 coupling methods

A
  1. suface contact coupling - commonly a gel
  2. water coupling
  3. wheel probes
44
Q

what is typical frequency range for NDT

A

5 - 20 MHz

45
Q

how can we maximise transducer performance

A
  • maximise centre frequency
  • maximise bandwidth

high precision when bandwidth close to centre frequency

46
Q

name two newer transducer developments

A
  1. focussed transducer: curved element focusses at specific distances
  2. phased array - customiseable real time beams - still in development
47
Q

differences between high precision and high power ultrasouns transducers

A

High Precision
- large bandwidth producing short pulses which allow accurate depth measurement
- achieved by adding backing to piezoelectric element to reduce amplitude
High Power:
- undamped transducer
- narrow bandwidth and long pulses, but at high amplitudes

48
Q

how to read T and Tp from graph

A

Tp is time between first and last pulse, T is time between 2 max pulses

49
Q
A

Composites (14 decks)

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