Failure Analysis & Prevention Flashcards
7 cards from real CFS practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Failure Analysis & Prevention flashcards as text
What is the most common initiation site for fatigue cracks in externally threaded fasteners under axial loading?
Answer: First thread root engaged by the nut
The first engaged thread root experiences the highest stress concentration and carries approximately 30% of the total load, making it the most common fatigue crack initiation site.
A stainless steel fastener in a hot, chloride-rich environment fails with transgranular, branching cracks. The most likely cause is:
Answer: Chloride stress corrosion cracking
Transgranular branching cracking in austenitic stainless steel in chloride environments is the classic presentation of chloride stress corrosion cracking (Cl-SCC).
Hydrogen embrittlement susceptibility in steel fasteners increases with:
Answer: Increasing tensile strength above approximately 150 ksi
Higher-strength steels (above ~150 ksi / ~34 HRC) are significantly more susceptible to hydrogen embrittlement because residual stresses and microstructural trap sites increase with strength.
In a fatigue failure investigation, striations visible under SEM are used to:
Answer: Estimate crack growth rate and number of cycles
Each fatigue striation represents one loading cycle, so counting striations and measuring their spacing allows estimation of crack growth rate and approximate cycle count to failure.
Which prevention method is most effective against crevice corrosion under a fastener head?
Answer: Applying sealant or gasket to eliminate the crevice
Eliminating the crevice geometry with sealants or gaskets removes the occluded, oxygen-depleted zone where crevice corrosion initiates.
A fastener joint experiences embedment relaxation after initial tightening. What is the best corrective action during design?
Answer: Increase the joint grip length to increase bolt elasticity
A longer grip length increases bolt elastic elongation, so the same absolute amount of embedment relaxation results in a smaller percentage loss of preload.
Which non-destructive evaluation (NDE) method is most suitable for detecting internal cracks in a high-strength bolt shank?
Answer: Ultrasonic testing (UT)
Ultrasonic testing can detect subsurface internal discontinuities through the bolt shank, whereas PT only detects surface-open defects and MT is limited to near-surface flaws.