NDT intro and ultrasound Flashcards
What is safe life design
Design so that damage grows so slowly that no defect becomes dangerous
within lifetime of component. (Very conservative).
Explain role of NDT in safe life design
not used!!
What is damage tolerant design
Damage tolerant Damage grows slowly enough that can be detected and
repaired/replaced before failure
what is critical damage size
size at which damage causes failure in a componeny
what is detection limit
smallest damage that can be detected
what is detection window, and symbol
Tdw = time between damage being detectable and becoming critical
what is inspection interval
the maximum time between inspections
Formula for inspection interval
∆t < Tdw / N + 1
N is number of required inspections to be certain of detection
what are the effects of improved inspection techniques
- lower detection limit
- increased inspection intervals
- reduced weight with original schedule maintained (by increasing crack growth rate and lowering critical size)
how does visual inspection work
Person looks at part, can use tools like boroscopes
What time of defects is visual inspection used for
Surface damage
how does dye penetrant testing work
- clean component
- apply die liquid
- remove excess liquid
- dry (optional)
- apply developer (liquid/ solid powder that draws dye from cracks)
- inspect
List some types of material defects in composites
- delamination
- porosity
- fibre waviness
list some assembly defects in compoistes
- surface scratches
- bonding faults
list some in service defects with composites
- impact damage
-water ingress - fatigue
explain the process of ultrasonic testing
- transducer generates ultrasonic pulse which propagates through sample
- where no damage, pulse is reflected back by the back of the sample and detected by transducer
- damage causes additional reflections which appear in the recieved signal
- time tracer gives info on if theres damage, how deep into sample its located, and some of its characteristics
what are the 5 components of an ultrasound NDT system with their purpose
- computer - controls experiment, stores and analyses data
- Pulser/reciever - electrical device generates high voltage pulse fed to transducer. also amplifies and filters signal that comes back from transducer.
- transducer - converts electrical pulse into mechanical waves, and converts reflected waves into electrical signal
- coupling - a medium for ultrasonic waves to pas from transducer to sampke
- ADC - analogue to digital converter. nalogue signals from reciever are digitalised to be input into computer.
how does a transducer work
- 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
what affects transducer performance
transducer performance defined by centre frequency and bandwidth
what does the piezoelectric material do in transducer
convert electrical energy into sound waves and vice versa
when a force is applied to material surface, a voltage is created across the electrodes
how to calculate number of cycles in signal
N = Tp/T
Tp is pulse duration
T is period of oscillation
formula for centre frequency
fc = 1 / T
formula for bandwidth
∆f = 1 / Tp
formula for longitudinal velocity (longitudinal wave propagation
CL = SQRT( (K+4G/3) / ρ ) = f λ
see notes if needed
K is bulk modulus
G is shear stiffness
what is tome of flight (TOF)
time for pulse to to travel from transducer and back
how to calculate time of flight
t = 2d / CL
Minimum defect size formula
w > λ / 2
what is depth resolution
smallest difference in depth that can be resolved
formula for dept resolution
∆d = Nλ / 2
what is reflection coefficient, R
ratio of reflected acoustic power to incident power
what us transmission coefficient, T
ratio of transmitted acoustic power to incident power
Acoustic impedance formula
z = p / u
p is pressure amplitude
u is velocity amplitude
acoustic impedance formula using material parameters
z = ρ CL
Formula for reflection coefficient between two materials
R = ( (z2/z1 - 1) / (z2/z1 + 1) ) ^2
Formula for transmission coefficient between two materials
T = (4z2/z1) / (z2/z1 + 1)^2
formula for reflected pressure amplitude
Pr = SQRT(R) x Pi
pr is reflected pressure amplitude
pi is pressure amplitude of initial wave?
formula for transmitted pressure amplitude
Pt = SQRT(T) x Pi
pt is transmitted pressure amplitude
pi is pressure amplitude of initial wave?
Formula for reflection for two boundary systems
check notes
name and explain 3 typed of ultrasound scanning methods
- A scan - single stationary point scan
- B scan - transducer moves along straight line
- C scan - covers whole area moving in different directions
explain the process of B scans
- transducer moves in straight line taking multiple A scans
- A scans plotted next to eachother as colour maps
- image produced of reflected signal as function of horizontal position and depth
- can recover A scan data to zoom in on particular defects
explain the process of C scans
- transducer scans in 2 dimensions
- produces amplitude data in 3D: two translations and horizontal position + depth
- requires simplifocation for plotting
- apply time window so that front and back reflections are ignored
- measure largest reflected amplitude within sample
- gives scanned data amplitude value over the 2 scanned dimensions
- specific A scans for defects are retrievable
what is coupling
adding substance, like a gel, to improve the ultrasound passage to the sample by removing air gap
list 3 coupling methods
- suface contact coupling - commonly a gel
- water coupling
- wheel probes
what is typical frequency range for NDT
5 - 20 MHz
how can we maximise transducer performance
- maximise centre frequency
- maximise bandwidth
high precision when bandwidth close to centre frequency
name two newer transducer developments
- focussed transducer: curved element focusses at specific distances
- phased array - customiseable real time beams - still in development
differences between high precision and high power ultrasouns transducers
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
how to read T and Tp from graph
Tp is time between first and last pulse, T is time between 2 max pulses
