CFS Failure Analysis & Prevention 3 — Questions and Answers
Question 1: Which environmental condition most accelerates galvanic corrosion between steel fasteners and aluminum structural members?
- Dry desert heat
- Presence of an electrolyte such as saltwater (Correct answer)
- Vacuum conditions
- Elevated temperatures above 200°F
Correct answer: Presence of an electrolyte such as saltwater
Galvanic corrosion requires an electrolyte to conduct ionic current; saltwater dramatically increases conductivity and accelerates the corrosion cell between dissimilar metals.
Question 2: What is the primary purpose of performing a torque audit on in-service fasteners?
- To verify correct material grade
- To detect loss of preload and potential loosening (Correct answer)
- To measure surface finish quality
- To check thread engagement length
Correct answer: To detect loss of preload and potential loosening
A torque audit checks whether fasteners still have adequate clamp load by measuring the torque required to continue turning them, revealing any relaxation or loosening.
Question 3: Fretting wear on fastener contact surfaces is characterized by:
- Smooth polished bearing surfaces
- Red-brown iron oxide debris and surface pitting (Correct answer)
- Uniform material loss across the shank
- White crystalline corrosion deposits
Correct answer: Red-brown iron oxide debris and surface pitting
Fretting produces fine metallic debris that oxidizes to form characteristic red-brown iron oxide powder, along with surface pitting at the contact interface.
Question 4: A fastener assembly in a chemical plant fails due to dealloying. Which alloy is most susceptible to dezincification?
- 304 stainless steel
- Grade 8 alloy steel
- Yellow brass (Cu-Zn) (Correct answer)
- Titanium alloy
Correct answer: Yellow brass (Cu-Zn)
Brass (copper-zinc alloys) is highly susceptible to dezincification, where zinc is selectively leached, leaving a weak, porous copper structure.
Question 5: During failure analysis, scanning electron microscopy (SEM) is primarily used to:
- Measure bulk hardness of the fastener
- Examine fracture surface morphology at high magnification (Correct answer)
- Determine chemical composition via XRF
- Measure residual thread stresses
Correct answer: Examine fracture surface morphology at high magnification
SEM provides high-magnification, high-depth-of-field images of fracture surfaces, enabling identification of fatigue striations, intergranular patterns, and other failure signatures.
Question 6: What thread form defect most significantly reduces a fastener's fatigue life?
- Slightly oversized pitch diameter
- Sharp thread root radius (under-radius) (Correct answer)
- Coarser than specified pitch
- Minor surface oxidation on threads
Correct answer: Sharp thread root radius (under-radius)
A sharp or undersized thread root radius creates a severe stress concentration, dramatically reducing fatigue life since cracks typically initiate at the first thread root.
Question 7: Which proof load test result indicates a fastener has undergone permanent set and should be rejected?
- Fastener returns to original length after load removal
- Fastener elongates 0.0005 in. permanently after load removal (Correct answer)
- Fastener shows no surface cracking under load
- Fastener achieves specified break load
Correct answer: Fastener elongates 0.0005 in. permanently after load removal
A fastener that does not return to its original length after proof load removal has undergone permanent set, indicating the material yielded and the part fails the proof load requirement.
CFS Failure Analysis & Prevention 3
Which environmental condition most accelerates galvanic corrosion between steel fasteners and aluminum structural members?