X rays and other NDT methods Flashcards
how does an X ray NDT work
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.
how do we calculate intensity at distance x throygh thickness of specimen
l(x) = l0 e^(-μx)
μ is attenuation coefficient (same as absorbtion)
l0 is intencity incident on material
what is photographic density
measure of proportion of light which passes through specimen
formula for photographic density
D = log10 (incident light intensity / transmitted light intensity)
what is exposure
measure of total radiaton incident on film
how do x ray images with too high nd too low exposure look
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
formula for exposure
E = It
I is incident x ray intensity
t is time
formula for relative exposure
Erel = E / Ebase
Ebase is baseline exposure for unexposed film
what is film gradient
measure of films sensitivity to x rays
film gradient formula
Gd = (D - K) / log10(Erel)
what is x ray sensitvity
smallest percentage change in thickness which can be detected
formula for sensitivity
See notes (“Quantifying sensitivity”)
why is damage orientation importantin x rays
- 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
name 2 types of detectors
direct conversion, indirect conversion
explain direct conversion digital detectorsin x rays
- X-ray photoconductor converts incoming x-rays to electric charge.
- Charge build up is proportional to
x-ray intensity. - Array of Thin-film-transistors measures local charge to produce image.
explain indirect conversion digital detectors
- Scintillator converts incoming x-rays to visible light.
- Light detected directly with charged coupled device
- Amorphous silicon converts light to electric charge.
- Array of Thin-film-transistors measures local charge to produce image.
What are CT scans
- 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
Explain x ray set up
see notes
explain CT scans set up
page 49 notes
how does CT scan work
- 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
compare x ray and CT scan NDT methods
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
How does thermography work
detects changes in thermal conductivity due to damage
explain pulsed thermography works
- 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.
explain lock-in thermography
- 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
compare pulse and lock in thermography
- 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
4 advantages of thermography
- fast
- single image covers large areas
- can scan curved surfaces
- better for low heat condictivity materials like CFRP
limitations of thermography
- 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
write out advantages and disadvantages of each of the methods so far
depth (from surface) requirement for thermography
w/d > 2
w is width of defect
explain eddy currrent testing
- 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.
explain eddy current results graph and draw
- Air point - Resistance and inductance of coil when not near sample.
- Lift off - Change when bringing probe coil to sample
- 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
advantages and limitations of eddy current testing
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.
explain Potential Drop Crack Depth Measurement NDT method
- 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.
Explain magnetic particle inspection NDT
- 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.
explain magnetic particle inspection
- 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.
both formulas for sensitivity in x rays
P39 notes
