L5 - Mechanical Properties: Tensile

What can the mechanical properties of a material be affected by?

Alloying

Temperature

Previous working

Heat Treatments

The Tensile Test

The applied load (or force), F, and the extension dl = l - lo are continuously measured as the test piece is extended at a constant extension rate

What does the extensiometer do?

It measures local specimen strain (extension)

Tensile test graph

Which section of the graph shows Elastic deformation?

The linear part, as at this section, the material can return to its original length when the stress is removed

Which section of the graph shows Plastic deformation?

The section after the linear part, at this section, if the load is removed the material will never return to its original shape

What is the point from which the material switches from elastic to plastic deformation called?

Yield Stress

What is Stress?

It is the force, F, acting during deformation, divided by the cross-sectional area of the material, A, over which the force acts: σ = F/A, units - Pa

What is Strain?

Strain, e, is the fractional amount by which a material deforms (e.g. extends)

ε = (I - Io)/Io = ΔI/Io, no units - its dimensionless

What is Hooke's Law?

The applied load F, is linearly proportional to the extension ΔI, for elastic deformation

σ = E x ε, E = Young's Modulus

What is the Young's Modulus?

It is a measure of a material's stiffness, equal to the gradient of the linear (Hooke's law) region of the stress-strain plot

Like other mechanical properties of a material, is the Young's Modulus affected by alloying or heat treatment?

No.

Unlike strength and ductility, Young's modulus is not affected

What does work-hardening mean?

When a metal is plastically deformed, dislocations in its crystal structure multiply and tangle, making further deformation increasingly difficult.

This makes the metal stronger and harder, but less ductile.

What are Luders bands?

Lüders bands are visible lines or markings that appear on the surface of a metal when it beings to yield.

These are caused by localised plastic deformation spreading across the material, before uniform plastic flow beings

What happens at the Upper Yield Strength in terms of Lüders Bands?

One or more Lüders bands of plastic deformation spread across the specimen.

Outside the band, all deformation remains elastic

What happens at the Lower Yield Strength in terms of Lüders Bands?

Each band spreads sideways until the entire length of the specimen has yielded

What is Proof Stress?

It is the stress necessary to produce a plastic strain

What happens to the specimen after the UTS?

A 'neck' forms

As after the UTS all plastic deformation is concentrated in the neck

Final fracture occurs in the neck

Define Ductility.

A material's ability to be plastically deformed without breaking under tensile loads (e.g. stretching, or in a tensile test)

Define Malleability.

A material's ability to be plastically deformed without breaking under compressive loads (e.g. forging, rolling, or in a compression test)

What does it mean when a material is described as brittle?

It has low ductility, meaning it undergoes a very little amount of plastic deformation before it fractures

How can the ductility and malleability of a material be improved?

By heating - hence forging, rolling etc are often done hot.

What happens to some steels at low temperatures?

Some steels become very brittle, very quickly

What does the area under a stress-strain curve represent?

Energy per unit volume (Jm^-3) required to deform the material.

What mechanical properties do 'tough' materials have then?

They are strong and ductile

What does plastic deformation require?

It requires dislocations to move 'slip' through the crystalline latice.

How can you increase yield strength?

You can change the microstructure of a material in various ways so that it hinders the movement of the dislocations

Alongside work-hardening, what other methods could you do to increase the strength of a material?

Grain size reduction

Solid-solution strengthening

Precipitation strengthening

How does reducing the grain sizes mean stronger materials?

- Dislocations can't slip across grain boundaries

- So, after slipping through the grain lattice, they will get stuck at the first grain boundary.

- They'll meet and pile up and then can't contribute to further plastic deformation shape change.

- Smaller grains means more grain boundaries

- So shorter dislocation-slip distances before they get stuck on a grain boundary.

- Hence the dislocations can't slip as far under stress

- Which means more applied stress is needed to plastically deform the material

- Thus the material is stronger

What is the Hall-Petch equation?

The relationship between yield stress and grain size for some metals.

I.e. the strength is inversely proportional to the square root of average grain size

How does Solid-solution strengthening work?

- A different individual alloy atom is added which causes the lattice around it to become distorted

- The associated stress field around it acts to hinder dislocations moving when a load is applied

- Making the alloy stronger (increasing the yield stress and strength)

How does Precipitation strengthening work?

- The different alloy atoms can react to form chemical compounds

- Creating very small intermetallic compound precipitates of fixed composition within the solid metal

- The distortion and strain fields locally around each precipitate hinders the slip of dislocations during deformation

- This increases the yield stress and strength

What does precipitation strengthening usually require?

Very careful alloying and heat-treatment (sometimes called ageing)