Creep, Fatigue and Wear Flashcards
What is ‘creep’?
If a material is loaded below its yield stress and held at high temperature it can begin to plastically deform over time - this is due to the process of creep
What is the cause of creep in metals?
Due to movement of dislocations due to high applied load and gradual diffusion of the atoms at high temperature
What is the cause of creep in amorphous ceramics?
Atoms slide past each other – but need high enough temperature for the viscosity to become sufficiently low
What is the cause of creep in thermoplastic polymers?
Molecules slide past each other at high enough temperatures
What are the consequences of creep
Cables lengthening over time Turbine blades lengthening Tungsten light bulb filaments sagging Tennis racket strings require retightening Flat areas on car tyres Need to retighten bolts in service
What occurs at ‘Primary creep’?
Initially rapid but slows as material undergoes work hardening
What occurs at ‘Secondary creep’?
Relatively steady rate of creep
What occurs at ‘Tertiary creep’?
Increasing rate of deformation leading to failure - neck forms.
what is Fatigue in materials
A material may be subjected to repeated application and removal of stress below its yield strength. This can lead after many cycles to failure of the sample -known as fatigue
What do Fatigue tests give?
Provides data on the stress applied to the component versus the number of cycles to failure (called an S-N curve) -depends on component shape and material
What is meant by ‘Fatigue limit’?
Some material types show a levelling out of the applied stress vs cycles to failure curve
What is the ‘Fatigue strength’ of a material?
Maximum stress for which fatigue failure will not occur within a particular number of cycles
What is the cause of wear?
Asperities under high stress that may be greater than the yield stress
Specifically, what is ‘Adhesive wear’?
asperities are plastically deformed and welded together. Motion causes particles to be torn away from the surfaces
Specifically, what is ‘Abrasive wear’?
Hard particles (e.g. grit) between (or attached to) the surfaces cause material to be removed.
