CFS Failure Analysis & Prevention 2 — Questions and Answers
Question 1: Which failure mode occurs when a fastener breaks perpendicular to its axis due to repeated cyclic loading?
- Shear fatigue
- Transverse fatigue fracture (Correct answer)
- Stress corrosion cracking
- Hydrogen embrittlement
Correct answer: Transverse fatigue fracture
Transverse fatigue fractures are perpendicular to the fastener axis and result from cyclic tensile loading exceeding the endurance limit.
Question 2: A fastener in a marine environment fails with a brittle, branching crack pattern. What is the most likely failure mechanism?
- Fatigue
- Overload shear
- Stress corrosion cracking (Correct answer)
- Galling
Correct answer: Stress corrosion cracking
Stress corrosion cracking (SCC) produces branching, intergranular cracks in susceptible alloys exposed to aggressive environments like seawater.
Question 3: What does the presence of 'beach marks' (arrest lines) on a fracture surface indicate?
- Hydrogen embrittlement failure
- Overload fracture
- Fatigue crack propagation (Correct answer)
- Intergranular corrosion
Correct answer: Fatigue crack propagation
Beach marks are macroscopic concentric lines on a fracture surface that indicate incremental fatigue crack growth during cyclic loading.
Question 4: A grade 8 bolt fails shortly after installation with no apparent overload. It shows a brittle, intergranular fracture with no plastic deformation. What is the most probable cause?
- Galling during installation
- Hydrogen embrittlement from plating (Correct answer)
- Fatigue from vibration
- Stress corrosion cracking
Correct answer: Hydrogen embrittlement from plating
Hydrogen embrittlement from electroplating (if not properly baked) is a classic cause of delayed brittle fracture in high-strength bolts shortly after installation.
Question 5: Which of the following is the best method to prevent self-loosening in fastener joints subjected to transverse vibration?
- Applying anti-seize lubricant
- Using a higher-grade bolt
- Increasing clamp load above the vibration loosening threshold (Correct answer)
- Replacing bolts with rivets
Correct answer: Increasing clamp load above the vibration loosening threshold
Junker testing shows that maintaining sufficient clamp load prevents the micro-slip between mating threads that causes self-loosening under transverse vibration.
Question 6: Galling between a stainless steel bolt and a stainless steel nut is most effectively prevented by:
- Using a higher clamping torque
- Applying anti-seize compound (Correct answer)
- Using a coarser thread pitch
- Increasing the engagement length
Correct answer: Applying anti-seize compound
Anti-seize compounds create a barrier between similar metallic surfaces, preventing the adhesive welding (galling) that occurs with austenitic stainless steel.
Question 7: A fastener exhibits significant necking and cup-and-cone fracture morphology. This indicates failure by:
- Fatigue
- Ductile overload (Correct answer)
- Stress corrosion cracking
- Hydrogen embrittlement
Correct answer: Ductile overload
Cup-and-cone fracture with significant necking is characteristic of ductile overload failure, indicating the fastener was subjected to a single static load exceeding its tensile strength.
CFS Failure Analysis & Prevention 2
Which failure mode occurs when a fastener breaks perpendicular to its axis due to repeated cyclic loading?