Creep Test

It represents one of the main effects of temperature increase in the mechanical behaviour of the materials. Creep test are generally carried out by applying a constant load to conventional test specimen under higher temperature and recording the variation of deformation with time. Figure 2.2 shows a typical creep curve obtained from a uniaxial creep test. It exhibits the characteristic time dependency of creep strain ε, where the instantaneous deformation εo is either elastic or elastic-plastic and the three different regions carrying out the response of the different materials are primary, secondary and tertiary creep regions. In the primary creep region, strain hardening is the governing mechanism and generated by increase of dislocation

3.2 CREEP DUCTILITY
Creep ductility plays a vital role in determining the extent of creep-fatigue interaction and damage. If the creep ductility is high, creep voids to begin its formation because of the presence of the particle matrix decohesion. Creep dominated cracks tends to trans granular rather than intergranular and creep failure due to damage summation with unexpected interactions. The maximum limitation at which creep-fatigue can be high is when the ductility is low and the cavities begin to form grain boundaries. Figure 2.7 [8]: (a) Fatigue dominated trans granular cracking (b) Creep- Fatigue dominated intergranular cracking Figure 2.8 [12]: Influence of creep-ductility on creep-fatigue cracking

This requires a knowledge of the external forces and thermal transients experienced by the structure during service operation and representation of the cyclic and creep deformation properties of the materials of construction in terms of model constitutive equations irrespective of whether the local stress-strain is determined by approximate analytical solutions or finite element analysis, the constitutive model options are generally the same [8] [10]