Materials Science Study Guide_ Strengthening in Metals & Fatigue

What are dislocations in materials science?

Crystal defects that enable plastic deformation.

What is plastic deformation?

Occurs when dislocations move through a crystal.

What is the strengthening principle in metals?

Increasing the force required to move dislocations increases material strength.

What is strain (work) hardening?

A process where dislocation movement creates more dislocations, which interact and impede further movement.

How does strain hardening affect yield strength and ductility?

Increases yield strength and hardness; decreases ductility.

What is solid solution strengthening?

A process of adding impurity atoms to create a solid solution that impedes dislocation movement.

What is the Hall-Petch equation?

σYS = σi + ky·d^(-1/2), relating yield strength to grain size.

What does the fatigue definition entail?

Failure occurring under cyclic loading at stresses below the static yield strength.

What percentage of service failures in metals are caused by fatigue?

Approximately 90%.

What are the stages of fatigue failure?

1. Crack Nucleation, 2. Slow Crack Growth, 3. Sudden Fracture.

What do striations on a fracture surface represent?

The position of the crack front at each cycle.

What is the formula for mean stress?

σm = (σmax + σmin)/2.

What is a fatigue limit?

Stress below which failure doesn't occur in some materials.

Why do fatigue cracks usually initiate at the surface?

Due to the highest stresses at the surface during bending/torsion.

What does the stress ratio R = -1 indicate?

Completely reversed loading.

What does a fatigue strength represent?

Stress at which failure occurs after a specific number of cycles.

What is the effect of increasing hardness on fatigue resistance?

Increased hardness improves fatigue limit.

What do beach marks on a fatigue fracture surface indicate?

Position of the crack front at different stages (not individual cycles).

How is yield strength affected by grain size according to the Hall-Petch equation?

As grain size decreases, yield strength increases.

What is the role of grain boundaries in strengthening?

Grain boundaries act as barriers to dislocation movement.

What does σYS = 3τy represent?

Calculating polycrystalline yield strength.

What is one common requirement for fatigue failure?

Cyclic loading.

What occurs during slow crack growth?

Progressive crack propagation with each cycle.

How does solid solution strengthening affect dislocation movement?

Impurity atoms create lattice distortions that impede dislocation movement.