Engineering Sciences Flashcards

What is the difference between engineering stress and true stress?

Answer: Engineering stress uses original area; true stress uses instantaneous area

Engineering stress σ_e = F/A₀ (original area), while true stress σ_t = F/A_i (instantaneous area). True stress is always higher during tension as the cross-section reduces.

The dimensionless Nusselt number in heat transfer represents:

Answer: The ratio of convective to conductive heat transfer

Nu = hL/k represents the enhancement of heat transfer through convection compared to pure conduction. Higher Nu indicates more effective convective heat transfer.

What causes fatigue failure in materials?

Answer: Repeated cyclic loading below the ultimate tensile strength

Fatigue failure occurs from repeated cyclic stresses that individually are below the static failure strength. Microcracks initiate and propagate until sudden fracture occurs, often without warning.

The Prandtl number is the ratio of:

Answer: Momentum diffusivity to thermal diffusivity

Pr = ν/α = μCp/k, the ratio of momentum diffusivity to thermal diffusivity. For air Pr ≈ 0.7, for water Pr ≈ 7, for oils Pr >> 1.

In a stress-strain diagram, the area under the curve up to fracture represents:

Answer: Toughness (energy absorbed per unit volume)

The total area under the stress-strain curve to fracture represents toughness — the total energy per unit volume the material can absorb before fracturing. Ductile materials have higher toughness.

What is the significance of the Mach number in compressible flow?

Answer: It determines whether flow is subsonic, sonic, or supersonic

Mach number Ma = v/c (velocity/speed of sound). Ma 1 is supersonic. Compressibility effects become significant above Ma ≈ 0.3.