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Fatigue failure of elastic-plastic materials is the result of damage accumulation. In the initial phase, this process is controlled by plastic deformation. The standard method for the investigation of fatigue phenomena relies on counting the applied load (or strain) cycles to final failure or macro crack initiation. This method does not allow observations of fatigue damage accumulation during testing. Such observations can only be realized when a measurable physical quantity (representing damage) is selected for monitoring. Such a quantity, referred to as the damage indicator, must be proven to change in a monotonic manner with the progress of the damage. In this paper, two damage indicators will be proposed and discussed: local inelastic response and accumulated equivalent plastic strain. Local inelastic response can be referred to as the relative damage indicator when it changes in a monotonic manner during the entire life of the material. In the case of such an indicator, its actual value is compared with its initial value to estimate the amount of accumulated damage. This kind of damage indicator is useless in the case of materials exhibiting more complex behavior (e.g., initial cyclic hardening followed by softening). For such materials, accumulated equivalent plastic strain can be used as the damage indicator. This kind of indicator can be referred to as a cumulative damage indicator because it is necessary to use the cumulated value of such a quantity to estimate the amount of accumulated damage.
fatigue damage, damage indicator, plastic strain
Institute of Aviation, Warsaw,