API 571 Mechanical and Metallurgical Failures 1 — Questions and Answers
Question 1: Fatigue crack initiation preferentially occurs at which locations in process equipment?
- At the center of solid cross-sections
- At stress concentrations such as notches, surface defects, or sharp geometry changes (Correct answer)
- At locations of maximum compressive stress
- Only in the heat-affected zones of welds
Correct answer: At stress concentrations such as notches, surface defects, or sharp geometry changes
Fatigue cracks initiate at stress concentration points where local stress amplification exceeds the material's fatigue threshold, even when nominal stresses are within design limits.
Question 2: Which characteristic feature is found on fatigue fracture surfaces that distinguishes them from other failure modes?
- Shear lips and ductile dimples throughout
- Cleavage facets indicating brittle fracture
- Beach marks or clamshell patterns showing crack progression (Correct answer)
- Intergranular fissures parallel to the surface
Correct answer: Beach marks or clamshell patterns showing crack progression
Beach marks on fatigue fracture surfaces record the progressive growth of the fatigue crack, with each mark corresponding to a change in loading frequency or amplitude.
Question 3: Brittle fracture in carbon steel is most likely to occur under what temperature condition?
- At high temperatures above 300°F where steel is ductile
- At or below the ductile-to-brittle transition temperature (DBTT) (Correct answer)
- At any temperature under very slow loading rates
- Only in austenitic materials at cryogenic temperatures
Correct answer: At or below the ductile-to-brittle transition temperature (DBTT)
Carbon steel undergoes a ductile-to-brittle transition, and below the DBTT, fracture occurs by cleavage with little energy absorption and no plastic deformation.
Question 4: A fatigue fracture surface characteristically shows which combination of features?
- Uniform gradual wall thinning across the section
- A crack growth zone with progression marks and a final rapid fracture zone (Correct answer)
- Uniform plastic elongation before fracture
- Pitting and corrosion products on the fracture face
Correct answer: A crack growth zone with progression marks and a final rapid fracture zone
Fatigue fractures show a smooth crack propagation zone with beach marks and a final rough rapid fracture zone, recording the progressive crack growth history.
Question 5: Thermal shock in process equipment is caused by which mechanism?
- Sustained uniform high-temperature operation
- Rapid, extreme temperature changes creating severe thermal gradients that exceed material limits (Correct answer)
- Radiant heat exposure from nearby fired equipment
- Creep deformation at sustained high temperature
Correct answer: Rapid, extreme temperature changes creating severe thermal gradients that exceed material limits
Thermal shock results from rapid temperature changes inducing stresses that exceed the tensile or fatigue strength of the material due to steep thermal gradients.
Question 6: How is low-cycle fatigue distinguished from high-cycle fatigue?
- Low-cycle fatigue involves fewer than approximately 10,000 cycles with high strain amplitudes (Correct answer)
- Low-cycle fatigue occurs at higher frequencies than high-cycle
- Low-cycle fatigue only affects high-alloy materials
- Low-cycle fatigue requires temperatures above 500°F
Correct answer: Low-cycle fatigue involves fewer than approximately 10,000 cycles with high strain amplitudes
Low-cycle fatigue involves fewer than ~10,000 cycles with relatively high plastic strain amplitudes, while high-cycle fatigue involves millions of cycles at nominally elastic stresses.
API 571 Mechanical and Metallurgical Failures 1
Fatigue crack initiation preferentially occurs at which locations in process equipment?