Development of Calorimetric Fatigue Gauge.
GRUMMAN AEROSPACE CORP BETHPAGE N Y RESEARCH DEPT
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A research effort to devise a new fatigue gauge for aircraft is described in this report. Differential scanning calorimetry was used to detect microstructural changes resulting from strain-controlled fatigue exposure of aluminum alloy 7050. The calorimetric signature of the microstructure was determined for samples that had been cycled to failure at strains of 0.3, 0.6, and 1.5. Some samples were also fatigued to failure with a small positive or negative mean strain offset, and others were cycled to 30 or 70 of their expected life at 1.5. Thermodynamic and kinetic analyses of the calorimetric results revealed a pronounced effect of fatigue at 1.5 strain on the reaction enthalpy and reaction kinetics of the GP zone dissolution peak. The reaction enthalpy decreased systematically as the number of cycles increased. No effects of fatigue on the calorimetric results were observed in the samples that were fatigued at 0.3 and 0.6. Based upon these results the characteristics of a potential calorimetric fatigue gauge were outlined. Such a gauge would only be sensitive to low cycle plastic fatigue and would produce a cumulative damage parameter, thus indicating the percentage of lifetime expended.