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Correlation between Strain-Based Low-Cycle Fatigue and Energy-Based Linear Damage Models

Although the potential for cumulative damage of structures during long duration earthquakes is generally recognized, most design codes do not explicitly takes into account the damage potential of such events. In this paper, a strain-based low-cycle fatigue model commonly used for the prediction of fatigue life in metals is adapted for cumulative damage assessment of structures under seismic conditions. By defining the number of load cycles in terms of the total plastic strain energy dissipated by the structure, the model is presented in a form capable of predicting the plastic strain energy capacity of the structure at the ultimate limit state. The plastic strain energy is expected to decrease rapidly with increased displacement in the small displacement range and to decrease gradually in a near linear manner with increased displacement in the large displacement range. The model is shown to calibrate reasonably well with small-scale aluminum cantilever specimens tested under large-amplitude reversed cyclic loading. At the ultimate limit state, the modified Park and Ang damage model may be considered as a linear approximation to the low-cycle fatigue model in the large displacement range.