Degradation Flashcards
Define degradation.
Loss of structural, mechanical or physical properties over long periods.
How can degradation be caused?
Chemical or mechanical processes.
Give an example of chemical degradation.
Corrosion of steel to form rust is a process of chemical degradation.
Give an example of mechanical degradation.
Propagation of micro-cracks during cyclic loads (i.e. fatigue).
Are mechanical and chemical degradation often coupled?
Yes for example steel corrosion crack of metals, where fatigue of fractures in metal is favoured by a corrosive process taking place near the tip of the crack.
Name 3 types of mechanical degradation?
1) Fracture
2) Fatigue
3) Creep
What type of deformation is creep?
Permanent deformation is similar to plastic deformation.
When does creep happen?
In metals at high temperatures (>0.4Tm).
What type of systems experience creep?
Turbine rotors in jet engines and steam generators.
When testing for creep what is kept constant?
Stress and temperature.
When testing for creep what is measured?
The strain is measured and plotted as a function of time.
What are the 3 different areas of a creep curve?
1) Primary
2) Secondary
3) Tertiary
What is the primary area on a creep curve?
Strain hardening takes place and deformation becomes difficult.
What is the secondary area on a creep curve?
Competition between strain hardening and recovery. Longest duration.
What is the tertiary area on a creep curve?
Strain rate increases as failure or rupture occurs.
