EXPERIMENTS1)A coil of solder will be suspended from a clamp into a glass.

Boiling water will then be added.

You must visually inspect the coil every 30 seconds for 10 minutes and note any changes.

2)An aluminium rod will be handed to each pair.

In an orderly manner, the class will make their way to the mechanical workshop and place the rod in a vice.

Using pliers, the aluminium rod should be bent or twisted moderately but repeatedly to mimic cyclic loading until the rod fails. One person should visually inspect the rod and note changes after each cycle and feed back to the class at the end of the experiment.

CREEP AND FATIGUE FAILUREProperties and applications of engineering materials

AIMS

• To explore creep within metals.

• To explore fatigue within metals.

• To define the terms creep and fatigue.

OBJECTIVES

By the end of this session the learners should be able to:•Define the terms creep and fatigue.•Describe the changes observed in a coil of solder suspended in boiling water and relate this to the creep mechanism.•Describe the changes observed in an aluminium rod when repeatedly loaded or strained and relate this to the fatigue mechanism.

FAILURE MODES

Brittle/ Ductile fracture

Creep

Fatigue

Degradation

CreepCreep occurs in materials that have been subjected to high temperatures and forces causing deformation. For example, in a jet engine, the blades may creep as the temperatures and forces acting on the blades cause deformation. This can lead to the blades extending, rubbing the engine housing , leading to engine failure.

More about creep• Definition: Creep is the gradual extension of

a material under a constant applied load.• Three stages: Primary, secondary and

tertiary• The primary stage commences at a rapid

rate but slows due to work hardening.• The Secondary stage is much slower and

commences at a constant rate.• The tertiary stage is again rapid and is

where the failure of the material occurs. Necking can occur, similar to a ductile fracture. It is in the tertiary stage where a material surpasses its elastic point.

THE WORLD TRADE CENTRE• 11th September 2001

• Terrorist attack in which 2 planes were flown into the towers.

• The south tower fell first; followed by the north.

• The way in which the towers collapsed raised questions; conspiracy theories thrived.

• The creep mechanism can help to explain the way in which the towers fell.

Fatigue failureFatigue failure is caused by cyclic stress, the stress may not be applied in all instances but is repeated over and over again. There are three main stages to fatigue failure• Crack initiation• Crack propagation• Failure

More about fatigue• Most common failure mode for components.• Fatigue is defined as the alternating removal

and application of stresses and loads. This is repeated and is ‘cyclic’. The material fractures and eventually fails.

• There are 3 stages: Crack initiation, crack propagation and full failure/ complete fracture.

• The different stages of fatigue is evident on the cross sectional area of the component where the fracture occurred.

More about fatigue

Initiation

PropagationFull failure and fracture

THE BOSTON MOLASSES DISASTER• 15th January 1919

• 21 fatalities and 150 injured

• Molasses was used in the production of ethanol. Consistency of syrup.

• Fact: Killed humans and animals by blocking their airways.

• Fatigue failure of the steel tank.

Creep and fatigue.

Complete your work sheets.

Creep in materialshttp://www.youtube.com/watch?v=jreYtvQusKo

Fatigue Failurehttp://www.youtube.com/watch?v=7nyQ3G74GFw

Brittle Fracture http://www.youtube.com/watch?v=0N9gEPZP1g8

BRAINSTORMING

In pairs, brainstorm the numerous ways in which fatigue failure can occur.

Brainstorm the effects that creep may have on a bridge.

AIMS

• To explore creep within metals.

• To explore fatigue within metals.

• To define the terms creep and fatigue.

OBJECTIVES

By the end of this session the learners should be able to:•Define the terms creep and fatigue.•Describe the changes observed in a coil of solder suspended in boiling water and relate this to the creep mechanism.•Describe the changes observed in an aluminium rod when repeatedly loaded or strained and relate this to the fatigue mechanism.

ANY QUESTIONS?

Further reading

Please see me for photocopies of what the engineering materials book has to say on creep and fatigue.