The influence of time-temperature effects on performance of the fatigue gage



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This thesis describes the influence of time temperature effects on performance of the S/N Fatigue Life Gage, a recent and remarkable development in the field of electric resistance strain gages. The sensitive element of the gage is made of a completely annealed copper-constantan alloy which has been subjected to temperatures in excess of 400°F during the manufacturing process. In use, the gage is cemented in place on a machine or structural element, and any fatigue stresses affecting the element also affect the gage. As damage accumulates in the structure, it also accumulates in the gage, and the soft sensitive element, of the gage under goes strain aging or work hardening which is accompanied by a change in resistance. At room temperature, there is an excellent correlation between this change in resistance and the pre-cracking fatigue life of the metal on which the gage is cemented. When the gage is subjected to temperatures in excess of the usual room temperatures, however, the annealing process involved in manufacture of the gage is initiated again, and the gage loses some of its memory (change in resistance) of the accumulated fatigue damage to which it has been subjected. The study of this phenomenon is the subject of this thesis. Percentage losses in memory of 6.2, 10.6, 19.1, and 62.8 were found for temperatures of 250°F, 300°F, 350°F, and 400°F, respectively. It was found also that the behavior of the gage, after being subjected to such temperatures, was essentially the same as for room temperature use only.