AM Material Selection & Properties 3 — Questions and Answers
Question 1: What is the function of an alclad coating on aluminum sheet?
- Increase tensile strength
- Provide sacrificial corrosion protection to the core alloy (Correct answer)
- Improve weldability
- Reduce weight
Correct answer: Provide sacrificial corrosion protection to the core alloy
Alclad is a thin layer of pure aluminum bonded to a stronger aluminum alloy core; the pure aluminum corrodes preferentially, protecting the structural core.
Question 2: Which non-destructive testing method is best suited for detecting subsurface cracks in non-ferrous metals?
- Magnetic particle inspection
- Dye penetrant inspection
- Eddy current inspection (Correct answer)
- Visual inspection
Correct answer: Eddy current inspection
Eddy current inspection can detect both surface and subsurface discontinuities in conductive non-ferrous materials without requiring magnetic properties.
Question 3: What does a material's modulus of elasticity (Young's modulus) describe?
- Maximum stress before fracture
- Stiffness—ratio of stress to strain in the elastic range (Correct answer)
- Toughness under impact loading
- Resistance to fatigue
Correct answer: Stiffness—ratio of stress to strain in the elastic range
Young's modulus is the ratio of applied stress to resulting elastic strain, quantifying a material's stiffness or resistance to elastic deformation.
Question 4: Which type of stainless steel is most commonly used for aircraft exhaust components due to its high-temperature oxidation resistance?
- Martensitic stainless steel
- Ferritic stainless steel
- Austenitic stainless steel (Correct answer)
- Precipitation-hardening stainless steel
Correct answer: Austenitic stainless steel
Austenitic stainless steels (300 series) maintain strength and resist oxidation at elevated temperatures, making them suitable for exhaust systems.
Question 5: What is the purpose of shot peening an aircraft component?
- Remove surface corrosion
- Introduce compressive residual stresses to improve fatigue life (Correct answer)
- Increase surface hardness by carburizing
- Reduce component weight by material removal
Correct answer: Introduce compressive residual stresses to improve fatigue life
Shot peening bombards the surface with small steel or glass beads, inducing compressive residual stresses that retard fatigue crack initiation.
Question 6: Carbon fiber reinforced polymer (CFRP) composites are anisotropic. What does this mean for maintenance?
- They corrode uniformly in all directions
- Their mechanical properties differ depending on direction of load application (Correct answer)
- They require the same repair regardless of load path
- They expand equally in all directions with temperature change
Correct answer: Their mechanical properties differ depending on direction of load application
Anisotropic materials have direction-dependent properties, so CFRP repairs must account for ply orientation and load direction to restore structural integrity.
Question 7: What is intergranular corrosion and which aluminum alloy family is most susceptible?
- Surface pitting from chlorides; 7xxx series
- Corrosion along grain boundaries; 2xxx series (Correct answer)
- Uniform surface attack; 1xxx series
- Stress-induced cracking; 6xxx series
Correct answer: Corrosion along grain boundaries; 2xxx series
Intergranular corrosion attacks grain boundaries preferentially; 2xxx series (copper-alloyed) aluminum alloys are particularly susceptible when improperly heat-treated.
AM Material Selection & Properties 3
What is the function of an alclad coating on aluminum sheet?