X rays and other NDT methods COPY

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

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

Answer 5

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 6

formula for relative exposure

Answer 6

Erel = E / Ebase

Ebase is baseline exposure for unexposed film

Question 7

film gradient formula

Answer 7

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

Question 8

name 2 types of x ray detectors

Answer 8

direct conversion, indirect conversion

Question 9

explain direct conversion digital detectorsin x rays

Answer 9

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 10

explain indirect conversion digital detectors

Answer 10

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 11

What are CT scans

Answer 11

• 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 12

explain CT scans set up

Answer 12

page 49 notes

Question 13

how does CT scan work

Answer 13

• 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 14

compare x ray and CT scan NDT methods

Answer 14

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 15

compare pulse and lock in thermography

Answer 15

• 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 16

4 advantages of thermography

Answer 16

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

Question 17

limitations of thermography

Answer 17

• 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 18

depth (from surface) requirement for thermography

Answer 18

w/d > 2

w is width of defect

Question 19

explain eddy currrent testing

Answer 19

• 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 20

advantages and limitations of eddy current testing

Answer 20

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 21

explain Potential Drop Crack Depth Measurement NDT method

Answer 21

• 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 22

Explain magnetic particle inspection NDT

Answer 22

• 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 23

explain magnetic particle inspection

Answer 23

• 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 24

both formulas for sensitivity in x rays

Answer 24

P39 notes