Deformation Processes
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What is creep?
Creep is a slow time-dependent deformation. It occurs at elevated temperatures and allows impurity atoms to diffuse into the grains through the grain boundaries. It involves grains sliding against one each other.
Which materials is creep important for?
Polymers, ice, metals
Give an application in which the consideration of creep is important
Nickel superalloys in turbine blades. Creep causes the component to change with time. Or aluminium 7075 - wingskins in aircraft
What is viscoelasticity?
This is when polymers display both viscous and elastic properties.
The importance of viscoelasticity
When a load is applied to a polymer then the elastic strain will be removed when the load is removed.
What is fatigue?
Fatigue occurs when a component or structure is subject to cyclic stresses. The material may fail at stresses well below the yield stress.
What is a fatigue limit?
Repeated loading below this fatigue limit does not cause fatigue failure.
Which metals show no fatigue limit?
Al and Cu
Which metals show a fatigue limit?
Fe and Ti based alloys.
Examples of fatigue failure
Railway axles and bolts
The main features of a fatigue failure surface.
The surface defect, “beach” marks, region of rapid failure
Solutions to improve the fatigue life
Polishing, avoid notches and radius any corners. Introduce compressive stresses.
What are polymers resistant to?
Attack by acids, alkalis, sea water
What causes polymer degradation?
Ozone, Xrays, UV light, Oxygen, Heat
What leads to polymer degradation because of ozone, xrays, uv light, oxygen, heat?
Bonds rupture and there is surface cracking, discoloration etc. Or breaking of cross links which leads to embrittlement
What is degradation?
The reduction in the performance of a material over its lifetime.