Chapter 5 Flashcards
What is Shape Memory Effect
material recovers to its original size and shape when heated above a certain characteristic transformation temperature
What is Austenite
Heated iron, iron-based metal, or steel to a temperature at which it changes crystal structure from ferrite to austenite
What is Martensite
Martensite is formed in carbon steels by the rapid cooling (quenching) of the austenite form of iron at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure and forms a body centered tetragonal structure (BCT)
What is twinning and de-twinning process
Twinning occurs in austenite or high temperatures where structure is aligned. In lower temperature or martensite the crystal structure become misaligned.
Explain Stress-Strain Relationship of Martensite SMA
Assume the SMA is cooled from austenite w/out stress.
- Starts with elastic deformation with randomly oriented Martensite twins.
- Detwinning begins
- Martensite undergoes elastic deformation
- If stress is continued to be applied plastic deformation occurs and the material will be ruined
Explain Stress-Strain relationship of Austenite SMA
- Material begins fully austenitic
- elastic deformation occurs
- martensite begins to form
- elastic deformation
- plastic deformation occurs if too much stress is applied
Explain Stress-Induced Martnesite
When the material is unloaded with stress induced martnesite, the martensite becomes unstable and the material returns to austenite and its original shape.
What is Superelasticity
the behavior of certain alloys to return to their original shape upon unloading
after a substantial deformation has been applied.
Explain Stress-Induced Martnesite
When the material is unloaded with stress induced martnesite, the martensite becomes unstable and the material returns to austenite and its original shape.
What is Superelasticity
The behavior of certain alloys to return to their original shape upon unloading
after a substantial deformation has been applied.
What is Nitinol?
Nickel titanium alloy
Major advantages of Nitinol?
- Greater ductility
- recoverable motion
- excellent corrosion resistance
- stable transformation temperatures
- ability to be electrically heated for shape recovery
- chemically and biologically compatible with the human body
Explain the major components of the Tanaka constitutive model for an SMA
Exponential expression to describe the stress and strain temperature. It consists of the mechanical stress, thermo-elastic stress, & stress due to phase transformation.
