ENGI1002 - Topic 2

What are the four mechanical properties?

1. Strength

2. Ductility

3. Toughness

4. Hardness

What are the strain properties (tension and compression)?

Tension gives positive strain, compression gives negative strain.

What is yield strength?

Yield strength is the start of plastic deformation, if unclear, use 0.2%.

What is the ultimate tensile strength (UTS)?

UTS is the maximum on the curve. After UTS, the sample fractures or begins to neck.

What materials have high toughness?

Metals have high toughness, ceramics have low toughness, polymers have the lowest toughness.

Properties of a ductile fracture?

• Necking

• Dull

• Gives warning

Properties of a brittle fracture?

• Shiny

• Flat facets

• Gives no warning

What is strength?

How much load a material can resist.

What is ductility?

How much a material can stretch before failure, on a graph it is the strain at fracture.

What is toughness?

A materials ability to absorb energy before failure.

What is hardness?

A materials resistance to indentation.

How is impact toughness measured?

By breaking the sample under a fast moving pendulum.

How is fracture toughness determined?

Fracture toughness is determined by crack propagation rate.

Which cracks propagate more easily, sharp or blunt?

Cracks with sharp tips propagate easier than cracks with blunt tips because they concentrate stress more strongly.