» Strain : intensity of deformation strain = change in length/original length » Stress : intensity of force Stress = force/ area (MPa, psi) » Strain rate: rate of change in strain with time, units of time -1
» Creep: Time dependent deformation of a material/component under load. Original h F Compressed ∆h ∆h Time Instantaneous elastic deformation Instantaneous elastic recovery force applied force removed Elastic Materials:
» Creep: Time dependent deformation of a material/component under load. Viscoelastic Materials: Original h F Compressed ∆h Instantaneous elastic deformation ∆h Time force applied force removed retarded deformation (creep) Instantaneous elastic recovery retarded recovery permanent deformation
» Where Does it Occur? Combustion chambers (gas & oil fired furnaces) Pressurized piping (steam power plants, nuclear plants, heat exchangers) Blades and components in gas turbines (Jet engines, power plant turbines)
» Creep Curve: 1) Primary: Strain rate slow due to work hardening 2) Secondary: Strain rate constant Balance of work hardening and anealing 3) Tertiary: Strain rate constant Necking Phenomena
» Mechanism of Creep Deformation Creep occurs as a result of the diffusion of vacancies Thermal energy (temperature) is a catalyst for diffusion
» Ageing hardening: Aluminium alloys + Cu, Mg, Zn, Sc warm it to the point that all solute is solubilized Then rapid cooling (quench) In order to reduce the system energy, diffusion of atoms and vacancies occurs
» Creep Age forming (CAF): a process in which a part, usually an aluminium machined plate, is forming by creep on a tool at the same time that an ageing precipitation heat treatment is conducted.