X rays and other NDT methods

Question 1

how does an X ray NDT work

Answer 1

X-ray Penetration: High-energy X-rays pass through the composite, with varying absorption by different materials and defects.

Image Formation: The composite’s absorption rates of X-rays are captured on a detector, creating a contrasted image that reveals structures and potential defects.

Defect Identification: Defects in the composite, like cracks or voids, show up as contrasted areas on the X-ray image, due to density differences affecting X-ray absorption.

Question 2

how do we calculate intensity at distance x throygh thickness of specimen

Answer 2

l(x) = l0 e^(-μx)

μ is attenuation coefficient (same as absorbtion)
l0 is intencity incident on material

Question 3

what is photographic density

Answer 3

measure of proportion of light which passes through specimen

Question 4

formula for photographic density

Answer 4

D = log10 (incident light intensity / transmitted light intensity)

Question 5

what is exposure

Answer 5

measure of total radiaton incident on film

Question 6

how do x ray images with too high nd too low exposure look

Answer 6

too high (overeposure) - Excessive exposure leaves much of image black, obscuring
detail at lightly absorbing regions.
too low (underexposure) - Insufficient exposure leaves much of image white, obscuring
detail at highly absorbing regions

Question 7

formula for exposure

Answer 7

E = It

I is incident x ray intensity
t is time

Question 8

formula for relative exposure

Answer 8

Erel = E / Ebase

Ebase is baseline exposure for unexposed film

Question 9

what is film gradient

Answer 9

measure of films sensitivity to x rays

Question 10

film gradient formula

Answer 10

Gd = (D - K) / log10(Erel)

Question 11

what is x ray sensitvity

Answer 11

smallest percentage change in thickness which can be detected

Question 12

formula for sensitivity

Answer 12

See notes (“Quantifying sensitivity”)

Question 13

why is damage orientation importantin x rays

Answer 13

• Need change in material present on path of x ray to exceed sensitivity for
  system and sample
• Delaminations and cracks involve minimal change in material thickness if
  viewed perpendicularly
• Delaminations due to impact damage often accompanied by cracking that
  can be easily seen, but not always. Limitation of simple radiography

Question 14

name 2 types of detectors

Answer 14

direct conversion, indirect conversion

Question 15

explain direct conversion digital detectorsin x rays

Answer 15

1. X-ray photoconductor converts incoming x-rays to electric charge.
2. Charge build up is proportional to
   x-ray intensity.
3. Array of Thin-film-transistors measures local charge to produce image.

Question 16

explain indirect conversion digital detectors

Answer 16

1. Scintillator converts incoming x-rays to visible light.
2. Light detected directly with charged coupled device
3. Amorphous silicon converts light to electric charge.
4. Array of Thin-film-transistors measures local charge to produce image.

Question 17

What are CT scans

Answer 17

• Imaging a slice of an object by using a rotating x-ray imaging system.
• By imaging from many directions avoids problem of defect orientation
• 2D slices can be stacked to give 3D images

Question 18

Explain x ray set up

Answer 18

see notes

Question 19

explain CT scans set up

Answer 19

page 49 notes

Question 20

how does CT scan work

Answer 20

• collimator shield only allows s rays in single plane to pass
• line detector measures intensity along line - 1D image
• fan shaped beam illustrates a slice of the target, absorption leads to intensity variations in 1D image
• individual 1D images are projected back across a 2D plane
• summation reproduces features which changed the amplitude of x rays
• more 1D images give a better 2D reconstruction

Question 21

compare x ray and CT scan NDT methods

Answer 21

X ray:
- 2D image produced by exposure of 2D film
- Image due to absorption along path from source to image point
- Structure effectively flattened into 2D plane
- Portable systems
- Large (approx. 1 m) areas scanned quickly (a few seconds)
- Used for quality control and in-situ NDT

CT Scan:
- 2D image produced by combining many 1D images taken around the target
- Image is slice of target with each point representing absorption of that point in the object
- Large fixed location equipment
- Limited size of sample (typically up to 1m) can fit in scanner. takes up to a minute for 2D slice.
- Used for research and (some) quality control

Question 22

How does thermography work

Answer 22

detects changes in thermal conductivity due to damage

Question 23

explain pulsed thermography works

Answer 23

• flash from lamp rapidly raises material surface temp
• heat is diffused through the material
• defects act as insulators, trapping heat near the surface
• hot regions in image reveal damage locations.

Question 24

explain lock-in thermography

Answer 24

• similar set up to pulsed
• continuously varied lamp heat output rather than one short burst
• Surface temp monitored with an IR camera
• time lag between lamp and surface temp is present
• thinner areas heat quicker so less lag
• delaminations trap heat near surface so heater region is thin

Question 25

compare pulse and lock in thermography

Answer 25

• lock-in requires lower instantaneous power
• lock-in has better heat distribution accoss sample - no hot spots
• lock in you can vary frequency to get more info
• lock in has more complicated electronics and processing
• sensetivity of each method depends on the defect type and depth

Question 26

4 advantages of thermography

Answer 26

• fast
• single image covers large areas
• can scan curved surfaces
• better for low heat condictivity materials like CFRP

Question 27

limitations of thermography

Answer 27

• cant detect defects perpendicular to the surface
• cant detect narrow cracks, which do not prevent heat transfer
• can only do defects near surface.
• easier to detect large defects near surface

Question 28

write out advantages and disadvantages of each of the methods so far

Question 29

depth (from surface) requirement for thermography

Answer 29

w/d > 2

w is width of defect

Question 30

explain eddy currrent testing

Answer 30

• An alternating current in a coil generates a time-varying magnetic field.
• A time-varying magnetic field will cause a current in a nearby electrical conductor: eddy currents.
• The eddy currents generate their own magnetic field, which opposes the original current. - - This causes a measurable change in the coil’s electrical impedance.
• Cracks act as electrical insulators, restricting current flow in the sample and reducing the effect of the eddy currents on coil impedance.

Question 31

explain eddy current results graph and draw

Answer 31

1. Air point - Resistance and inductance of coil when not near sample.
2. Lift off - Change when bringing probe coil to sample
3. Balance point - Resistance and inductance of coil when next to undamaged
   sample

Plot stays at balance point when move near to sample, until it a crack is nearby
when there is a characteristic change in both inductance and resistance.
Eddy current measurement link

Question 32

advantages and limitations of eddy current testing

Answer 32

advantages:
- Very portable.
- Detects surface cracks in metals very well.
- Eddy currents have limited penetration depth.

disadvantages:
- Only works for conductive materials.
- Eddy current not generally applied to CFRP, but ongoing research for specific detection of fibre breakage.

Question 33

explain Potential Drop Crack Depth Measurement NDT method

Answer 33

• Constant current applied across two outer electrodes in contact with the sample.
• Voltage across two probe electrodes measured.
• Change in resistance (due to surface crack) causes change in measured voltage.
• Simple method of detecting cracks in conducting materials.
• No current application to composites.

Question 34

Explain magnetic particle inspection NDT

Answer 34

• Method of detecting defects in ferromagnetic materials (Iron, nickel, cobalt, and alloys thereof).
• Magnetic field lines concentrated in ferromagnetic material.
• Defects are discontinuities in the material that cause a distortion in the field.
• If the defect is close to the surface, the field lines pass outside the sample.

Question 35

explain magnetic particle inspection

Answer 35

• Ferromagnetic particles (usually iron oxide) trapped by exposed field at defects.
• Particles usually suspended in liquid for easy application.
• Good method for defects perpendicular to surface, i.e., fatigue cracks.
• Applied to individual components, removed from assemblies.
• No usage with CFRP, ferromagnetic materials only.

Question 36

both formulas for sensitivity in x rays

Answer 36

P39 notes