AAE552 - NDE FINAL EXAM REVIEW UPDATED ACTUAL
QUESTIONS AND CORRECT ANSWERS
Question:
1. What is meant by 'probability of detection'?
Answer:
(1) NDE is inherently probabilistic. Even with consistent detection methods, human operators are
inconsistent. (2) i) Myth: Passing NDE inspection guarantees no flaws. ii) Fact: All materials contain flaws
(Mordfin's 1st axiom), and many are below detection capability. (3) Largest flaw missed is much more
important than smallest flaw detected.
Reasons for Type I Errors (good part rejected):
- False indications
- Poor cleaning/preparation
- Wrong test applied
- Human factors
- Prior knowledge (expecting to find something)
Reasons for Type II Errors (bad part accepted):
- Wrong test applied (not sensitive to defect of interest)
- Correct test applied incorrectly
- Human factors
- Prior knowledge (ignoring non-hotspots)
Question:
2. What is a 90/95 crack and why is this metric important?
Answer:
(1) 90/95 crack is a statistical term that quantifies uncertainty: i) A crack (a*) can be detected 90% of the
time with 95% confidence (2) Imagine 100 tests performed on multiple specimens with know flaws, this
yields: i) Each test may have a crack of Xmm that is detected 90% of the time, where Xmm is different (ie.
4mm on one test, 3.5mm on another) ii) Observing all tests provides the 95% confidence; ie. for each of
the 100 tests we have different crack lengths detected 90% of the time. iii) If 95 of those 100 tests have a
90% crack length that is < 6mm, then 6mm = 90/95 crack length:
a) A crack length that can be detected 90% of the time with 95% confidence iii)
Question:
3. What is a proof test , when may it be useful, ad what information can it provide?
Answer:
(1) Process of over-loading a structure or component. i) IF failure (fracture) does not occur it is known that
any cracks must be below critical size. ii) If failure does occur a critically-sized crack must have been
present. (2) Example: i) Material designed to support 7MPa is then proof loaded to 10MPa. ii) At 10MPa,
if a 10mm crack exists, the component will fracture. iii) If no fracture, cracks are smaller than 10mm.
Question:
4. What are the tools commonly used for visual inspection?
,Answer:
(1) For visual inspection: i) Human eye, mirror, magnifying devices, certain light waves (ie. certain light in
visible spectrum like green light). ii) Borescopes, fiber-optic cameras, charge-coupled devices. (2) For
defect evaluation: i) Linear measurements: vernier caliber and micrometer. ii) Diameter gauges (circular
measurements), fillet and radius gauges. iii) Welds: palmgren gauges
Question:
5. What are the desirable properties of a liquid penetrant?
Answer:
(1) Needs to wet surface and be able to enter crack via capillary action. i) Wetting: ability to spread over
surface
a) If adhesion forces (liquid to solid) > cohesive forces (liquid to liquid), liquid will wet ii) Capillary
action: liquid can enter crack without air of external agents (pressure/gravity) iii) NOTE: Viscosity is not
in expression for capillary action. Viscosity only influences the time for capillary action to take place, not
the magnitude.
Question:
6. Describe the steps for a liquid-penetrant test.
Answer:
(1) Clean surface (2) Apply liquid penetrant (3) Allow penetration into discontinuities (dwell-time) (4)
Remove excess penetrant (5) Apply developer (6) Developer draws liquid out from cracks (bloom) (7)
Locate/interpret discontinuities (8) Clean surface of residual developer/penetrant (9) (Image of 6 steps)
Question:
7. What is the purpose of a developer?
Answer:
(1) Acts as a blotter to draw out trapped penetrant from flaws (2) Spreads penetrant over surface,
increasing visibility (3) Provides high-contrast background to increase visibility i) Fluorescent: Blue-Black
ii) Red-Dye: White (4) Forms of developers: DWWN - Don't Wade in Water Nude i) Form A: Dry Powder
a) Part exposed to cloud of developer. Part must be thoroughly dried before application.
b) Insufficient contrast for use with non-fluorescent (type II penetrants) ii) Form B: Water Soluble
a) Used with both fluorescent and non-fluorescent. Applied as dry powder concentrate, mixed with water.
iii) Form C: Water Suspendable
a) Used in type I and type II (fluorescent and non-fluoro)
b) Supplied as dry concentrate, dispersed in water iv) Form D: Non Aqueous Solvent Suspendable
a) Used for both types. Sold in ready-to-use form
Question:
8. What is the difference between penetrant types?
Answer:
(1) Penetrant types: i) Type I (Fluorescent):
a) Level 1/2: Ultralow sensitivity
b) Level 1/2/3/4 --> Low/Medium/High/Ultrahigh Sensitivity
c) Higher sensitivity --> detects smaller cracks ii) Type II (Non-fluorescent):
a) Have approximately Level I sensitivity
Question:
9. What are the penetrant removal methods?
, Answer:
WOWS - World of Warcraft, Stupid (1) Note that phraseology categorized these as 'Methods'
(2) Water Wash ('Method A') i) Formulated for easy removal by rinsing with water spray
(3) Oil-Based Emulsifier ('Method B') i) Lipophilic
(4) Water-Based Emulsifier ('Method D') i) Hydrophilic
(5) Solvent ('Method C') i) Excess penetrant is removed with solvent-dampened rag ii) Common in field
work since solvent is often same as components standard cleaning solvent
(6) NOTES: i) Methods A and C are easiest to remove ii) Methods B and D:
a) Penetrants can be impervious to water wash, therefore emulsifying agent is added after dwell time
Question:
10. What are the advantages and limitations of visual and liquid-penetrant inspection methods?
Answer:
Visual: (1) Advantages: i) Easy to implement in large, open assemblies ii) All NDE intrinsically
incorporates this as a 'first line of defense' iii) Greatly aided by advances in technology (tools listed
previously) (2) Limitations: i) Difficult to implement in complex assemblies with limited access to surface
ii) Limited to surface discontinuity iii) Highly dependent on external conditions (lighting) iv) Highly
dependent on inspector skill Liquid Penetrant: (1) Advantages: i) Portable for field work ii) Inexpensive
iii) Minimal skill iv) Sensitive to small defects v) Can be used for high-volume production vi) Insensitive
to flaw orientation vii) Can spot check for large components viii) Most materials can be tested (2)
Limitations: i) Limited to surface defects ii) Requires surface access iii) Penetration can be inhibited by
residual compressive stress iv) Surface prep necessary v) Cleanliness required vi) Chemicals must be
compatible vii) Prone to false positives
Question:
11. How are magnetic flux intensity and magnetic field intensity related?
Answer:
(1) They are related via material permeability (2) B_i = mu_ij*H_j B_i : magnetic flux density (vector)
mu_ij : permeability (material property) H_j : magnetic field intensity
Question:
12. Describe why some materials are 'magnetic' and others are not.
Answer:
(1) Materials which have electron orbitals with unpaired electrons are ferro-magnetic (iron, cobalt, nickel).
(2) Due to random orientation of each atomic magnetic direction, material will not act like a magnet,
however under the presence of an electric field, these atomic level unpaired electrons will align, resulting
in magnetization. (3) NOTE: mu >> 1 for magnetic materials, ~= 1 for non-magnetic.
Question:
13. What is the primary difference between permeability and relative permeability?
Answer:
In a vacuum, EQ 1 can be written as: B_i = mu_0 * H_i where mu_0 = 4*pi E-7 H/m (Henry/m) is the
permeability of free space.
In other medium, B_i = mu_0 * mu_r * H_i where mu_r is the relative permeability of the medium
(dimensionless, mu_air ~=
1).
QUESTIONS AND CORRECT ANSWERS
Question:
1. What is meant by 'probability of detection'?
Answer:
(1) NDE is inherently probabilistic. Even with consistent detection methods, human operators are
inconsistent. (2) i) Myth: Passing NDE inspection guarantees no flaws. ii) Fact: All materials contain flaws
(Mordfin's 1st axiom), and many are below detection capability. (3) Largest flaw missed is much more
important than smallest flaw detected.
Reasons for Type I Errors (good part rejected):
- False indications
- Poor cleaning/preparation
- Wrong test applied
- Human factors
- Prior knowledge (expecting to find something)
Reasons for Type II Errors (bad part accepted):
- Wrong test applied (not sensitive to defect of interest)
- Correct test applied incorrectly
- Human factors
- Prior knowledge (ignoring non-hotspots)
Question:
2. What is a 90/95 crack and why is this metric important?
Answer:
(1) 90/95 crack is a statistical term that quantifies uncertainty: i) A crack (a*) can be detected 90% of the
time with 95% confidence (2) Imagine 100 tests performed on multiple specimens with know flaws, this
yields: i) Each test may have a crack of Xmm that is detected 90% of the time, where Xmm is different (ie.
4mm on one test, 3.5mm on another) ii) Observing all tests provides the 95% confidence; ie. for each of
the 100 tests we have different crack lengths detected 90% of the time. iii) If 95 of those 100 tests have a
90% crack length that is < 6mm, then 6mm = 90/95 crack length:
a) A crack length that can be detected 90% of the time with 95% confidence iii)
Question:
3. What is a proof test , when may it be useful, ad what information can it provide?
Answer:
(1) Process of over-loading a structure or component. i) IF failure (fracture) does not occur it is known that
any cracks must be below critical size. ii) If failure does occur a critically-sized crack must have been
present. (2) Example: i) Material designed to support 7MPa is then proof loaded to 10MPa. ii) At 10MPa,
if a 10mm crack exists, the component will fracture. iii) If no fracture, cracks are smaller than 10mm.
Question:
4. What are the tools commonly used for visual inspection?
,Answer:
(1) For visual inspection: i) Human eye, mirror, magnifying devices, certain light waves (ie. certain light in
visible spectrum like green light). ii) Borescopes, fiber-optic cameras, charge-coupled devices. (2) For
defect evaluation: i) Linear measurements: vernier caliber and micrometer. ii) Diameter gauges (circular
measurements), fillet and radius gauges. iii) Welds: palmgren gauges
Question:
5. What are the desirable properties of a liquid penetrant?
Answer:
(1) Needs to wet surface and be able to enter crack via capillary action. i) Wetting: ability to spread over
surface
a) If adhesion forces (liquid to solid) > cohesive forces (liquid to liquid), liquid will wet ii) Capillary
action: liquid can enter crack without air of external agents (pressure/gravity) iii) NOTE: Viscosity is not
in expression for capillary action. Viscosity only influences the time for capillary action to take place, not
the magnitude.
Question:
6. Describe the steps for a liquid-penetrant test.
Answer:
(1) Clean surface (2) Apply liquid penetrant (3) Allow penetration into discontinuities (dwell-time) (4)
Remove excess penetrant (5) Apply developer (6) Developer draws liquid out from cracks (bloom) (7)
Locate/interpret discontinuities (8) Clean surface of residual developer/penetrant (9) (Image of 6 steps)
Question:
7. What is the purpose of a developer?
Answer:
(1) Acts as a blotter to draw out trapped penetrant from flaws (2) Spreads penetrant over surface,
increasing visibility (3) Provides high-contrast background to increase visibility i) Fluorescent: Blue-Black
ii) Red-Dye: White (4) Forms of developers: DWWN - Don't Wade in Water Nude i) Form A: Dry Powder
a) Part exposed to cloud of developer. Part must be thoroughly dried before application.
b) Insufficient contrast for use with non-fluorescent (type II penetrants) ii) Form B: Water Soluble
a) Used with both fluorescent and non-fluorescent. Applied as dry powder concentrate, mixed with water.
iii) Form C: Water Suspendable
a) Used in type I and type II (fluorescent and non-fluoro)
b) Supplied as dry concentrate, dispersed in water iv) Form D: Non Aqueous Solvent Suspendable
a) Used for both types. Sold in ready-to-use form
Question:
8. What is the difference between penetrant types?
Answer:
(1) Penetrant types: i) Type I (Fluorescent):
a) Level 1/2: Ultralow sensitivity
b) Level 1/2/3/4 --> Low/Medium/High/Ultrahigh Sensitivity
c) Higher sensitivity --> detects smaller cracks ii) Type II (Non-fluorescent):
a) Have approximately Level I sensitivity
Question:
9. What are the penetrant removal methods?
, Answer:
WOWS - World of Warcraft, Stupid (1) Note that phraseology categorized these as 'Methods'
(2) Water Wash ('Method A') i) Formulated for easy removal by rinsing with water spray
(3) Oil-Based Emulsifier ('Method B') i) Lipophilic
(4) Water-Based Emulsifier ('Method D') i) Hydrophilic
(5) Solvent ('Method C') i) Excess penetrant is removed with solvent-dampened rag ii) Common in field
work since solvent is often same as components standard cleaning solvent
(6) NOTES: i) Methods A and C are easiest to remove ii) Methods B and D:
a) Penetrants can be impervious to water wash, therefore emulsifying agent is added after dwell time
Question:
10. What are the advantages and limitations of visual and liquid-penetrant inspection methods?
Answer:
Visual: (1) Advantages: i) Easy to implement in large, open assemblies ii) All NDE intrinsically
incorporates this as a 'first line of defense' iii) Greatly aided by advances in technology (tools listed
previously) (2) Limitations: i) Difficult to implement in complex assemblies with limited access to surface
ii) Limited to surface discontinuity iii) Highly dependent on external conditions (lighting) iv) Highly
dependent on inspector skill Liquid Penetrant: (1) Advantages: i) Portable for field work ii) Inexpensive
iii) Minimal skill iv) Sensitive to small defects v) Can be used for high-volume production vi) Insensitive
to flaw orientation vii) Can spot check for large components viii) Most materials can be tested (2)
Limitations: i) Limited to surface defects ii) Requires surface access iii) Penetration can be inhibited by
residual compressive stress iv) Surface prep necessary v) Cleanliness required vi) Chemicals must be
compatible vii) Prone to false positives
Question:
11. How are magnetic flux intensity and magnetic field intensity related?
Answer:
(1) They are related via material permeability (2) B_i = mu_ij*H_j B_i : magnetic flux density (vector)
mu_ij : permeability (material property) H_j : magnetic field intensity
Question:
12. Describe why some materials are 'magnetic' and others are not.
Answer:
(1) Materials which have electron orbitals with unpaired electrons are ferro-magnetic (iron, cobalt, nickel).
(2) Due to random orientation of each atomic magnetic direction, material will not act like a magnet,
however under the presence of an electric field, these atomic level unpaired electrons will align, resulting
in magnetization. (3) NOTE: mu >> 1 for magnetic materials, ~= 1 for non-magnetic.
Question:
13. What is the primary difference between permeability and relative permeability?
Answer:
In a vacuum, EQ 1 can be written as: B_i = mu_0 * H_i where mu_0 = 4*pi E-7 H/m (Henry/m) is the
permeability of free space.
In other medium, B_i = mu_0 * mu_r * H_i where mu_r is the relative permeability of the medium
(dimensionless, mu_air ~=
1).
