Research program
Methods for fatigue resistance evaluation of notched structural components under multiaxial loading
University Co-ordinator
Università di PISA -
INGEGNERIA MECCANICA,NUCLEARE E DELLA PRODUZIONE - PISA(PI)
Research Unit Leader
Marco BEGHINI
Description
During the operative life, many mechanical and structural components carrying sharp notches can experience overloads that can induce local plastic strain at the notch rooth. This is a typical condition for threaded components as a consequence of the initial static setting that can produce plasticity at the roots of the first threads. However, similar phenomena can be observed also in other components, for instance in shafts with grooves for keys or rings or similar notches.Notch root is a preferential site for the initiation and growth of fatigue defects and the effect of the local plasticity induced by the static load is very important for assessing the fatigue resistance of the whole component.The solution of this problem is challenging as it involves many aspects of the fatigue resistance that are not yet completely understood. For instance:· The presence of an intense residual stress field due to the local plastic zone that strongly modifies the local R ratio· The modification of the local mechanical properties of the material due to the strain hardening and the consequent reduction of ductility· The multiaxiality of the stress field due to the notch; and the characteristics of multiaxiality of the static stress field usually different form those of the cyclic stress field.The notch induces the presence of static stress components that can overcome, for strain hardening materials, the yield stress. In those conditions when the cyclic load is applied, local R-ratios not much less than the unit can be expected. Unfortunately, it appears rather questionable to obtain fatigue strength properties by extrapolating the results of standard tests obtained with smooth specimens at low R ratios. Indeed, the global plastic strain of the specimens makes rather difficult to obtain information on the resistance at high R ratios with standard fatigue tests and specimens.As a consequence, it appears necessary to conduct specific fatigue tests by using properly notched specimens for which the characteristics of the stress field at the notch tip has to be predicted by accurate elastic-plastic analyses (for instance by a Finite Element approach) and, as a consequence, it is necessary to set up a proper experimental apparatus for assessing, at least indirectly, the results of those analyses.The Unit of Pisa has the aim at studying this aspect of the fatigue resistance of the notched specimens by a combined numerical-experimental approach that includes the design, the conduction and the interpretation of fatigue tests on notched specimens.During the first stage of the research, a proper material choice will be performed, in order to select two or three high-strength materials which are interesting for practical applications. On these materials, a complete characterisation of static and cyclic mechanical properties will be performed along with the characterisation of the fatigue resistance.As a second phase, the notched specimens and the related tests will be designed in order to obtain high local R ratios. This phase will end with the definition of the specimen geometry and of the test methodology.An extensive experimental phase is foreseen as a third stage in which the following parameters will be considered:· Mean stress· R-ratio· Fatigue lifeIn parallel, the fourth phase will be promoted consisting in FE analyses of the tests in order to obtain a rational interpretation of the experimental outcomes in terms of local stress and strain parameters.As general results, a criterion for predicting the fatigue resistance of a notched component will be proposed (in particular for the prediciotn of fatigue life and its strength for infinite life). The criterion will be based on: the nominal geometrical notch parameters, the characteristics of the loading cycle (including the possible first overloading conditions) and the material properties which can be obtained by properly designed tests (both static and cyclic).
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