DEVELOPMENT OF A USER FRIENDLY SYSTEM FOR ESTIMATION OF RESIDUAL. FATIGUE LIFE OF A . DYNAMIC " LOADED COMPONENT

Abstract

Fatigue life of a structural component can be evaluated calculating independently ' the initiation and propagation phases. Cyclic loads associated with stress concentration details result in important local stresses and strains responsible for fatigue processes: These fatigue processes are characterized by an initial period of damage accumulation that culminates with a crack initiation. This crack initiation period is followed by a period of stable crack propagation, which terminates when the crack reaches critical dimensions responsible for unstable propagation and consequent failure of the component. Thus, the total number of load cycles that a component can support until failure occurs will be the sum of the number of cycles spent in the crack initiation phase with the number of cycles spent in the crack propagation phase. The crack initiation and propagation phases have different weights in the total life of a component, which can vary in accordance with some factors, such as production processes, service conditions, material quality, etc. In some situations the initiation phase can represent about 90% of total life of a .structural component. An expert system based on interactive model with an user friendly interface was developed for predicting fatigue life, The package is composed of ten modules. The first module covers the evaluation of the initiation phase in structural components containing defects, using local approach methods. Notch load and strain spectra are converted into damage, responsible for the reduction of the initiation fatigue life. Second and third modules allow the evaluation of the propagation phase using linear elastic fracture mechanics concepts. In one of the modules stress intensity factor formulations available in the literature are used; in the other the boundary element method is used to evaluate the stress field at the tip of the notch. The 7th module contains a database on materials properties relevant for fatigue life evaluation. The subsequent modules eight to ten are interlinked and based on earlier models which are required for specific cases.