Premature failure of components and structures designed on the basis of monotonic and traditional high cycle life curves has focused the attention of design engineers and scientists to the study of low cycle fatigue of materials. Low cycle fatigue fractures are connected with the infrequent working cycles of equipment or instruments which often result from start-up and shut down operations or interruptions of their function. Important subjects represent also high temperature low cycle fatigue, thermal and thermomechanical fatigue and multiaxial elastoplastic fatigue.
The research of low cycle fatigue started in IPM under the leadership of the deceased Prof. Mirko Klesnil in the sixties when two electrohydraulic testing machines were designed and assembled in the IPM using the components produced by INOVA company. Later two elecrohydraulic computer controlled machines were purchased (see experimental facilities).
The present activities of the group concentrate to the systematic study of the fatigue behaviour of the structural and advanced materials subjected to cyclic elastoplastic loading, mostly under push-pull conditions.
The main subjects pursued:
- cyclic plastic straining the mechanisms, sources of the cyclic stress and relation to the internal structure. Analysis of the hysteresis loop using statistical theory in terms of the internal and effective stress, the relation of the macroscopic response to the internal dislocation structure.
- fatigue damage mechanisms - the mechanisms of cyclic slip localisation, fatigue crack nucleation
- interaction of low cycle fatigue with creep at elevated temperatures, structural changes and damage evolution in high temperature symmetric and asymmetric loading; nickel based superalloys
- fatigue of composite materials - damage evolution, cracking and fatigue fracture of laminate composites (GLARE, ARALL)
- effect of the coatings on the cyclic plasticity and fatigue life of advanced materials - effect of nitride and carbon layers and of the other coating procedures on the individual stages of fatigue process and on the fatigue life.
- effect of depressed and elevated temperatures on the early fatigue damage - study of the surface relief evolution using high resolution techniques (AFM, FESEM, EBSD, FIB) in austenitic, ferritic and austenitic-ferritic duplex stainless steels
- short crack growth kinetics in advanced steels - duplex, Eurofer, effect of mean stress
- study of fatigue damage in TiAl intermetallics
The most important results in the last five years:
- quantitative description of short crack growth regime and its use for the fatigue life prediction
- experimental documentation of fatigue damage evolution in plastic strain amplitude controlled one-step and two-step loading
- separation of the cyclic stress into internal and effective component using loop shape analysis
- measurement of the effective stress and the distribution of the internal critical stresses in stainless austenitic, ferritic and duplex steels
- application of the atomic force microscopy (AFM), high resolution scanning electron microscopy (FESEM) and focused ion beam (FIB) to the study of surface relief evolution and fatigue crack nucleation
- quantitative data on the extrusion and intrusion formation in stainless steel at ambient, depressed and elevated temperatures
- theoretical description of the temperature dependence of the extrusion growth in fatigued single and polycrystals