A series of cumulative damage strain-controlled fatigue tests at 20°C has been carried out on a Type 304 stainless steel with two, three, and five strain levels, both in an increasing and decreasing order. Experimental results show that if the strains are applied in an increasing order, the summation of cycle ratios is greater than unity, whatever the number of applied levels. For a decreasing order, this summation is less than one. However, for the same difference between high and low levels, this summation is closer to unity when the number of applied levels increases. The cumulative damage effect is evaluated using an approach which takes into account the sequence effect of loading. The procedure is based on the modification of the damage evolution with respect to that corresponding to constant amplitude loading. This is explained by an interaction effect due to a previous loading. With the interaction effect parameter suggested, the procedure is generalized to any discrete strain pattern. An application of the method is carried out to estimate the sums of life fractions required for failure for the material investigated. The correlation between predictions and experimental results is then discussed.
The present study aims to assess the additional fatigue life enhancement obtained by coldworking a previously cold expanded plate hole. Two different methods of performing the second coldworking were considered, i.e. moving the mandrel in the same direction as for the first coldworking or moving it in the opposite direction. A three dimensional finite element analysis for establishing the residual stress field induced by two successive coldworkings (5.58% then 4.8%) was carried out. A1 7475-T7351 specimens with a central hole were cold expanded at 5.58%, subjected to cyclic loading at constant amplitude for a predetermined life fraction (on the basis of 5.58% cold expanded hole fatigue life) and then re-coldworked at 4.8%. After this rework, the specimens were again subjected to the same fatigue loading conditions until failure. During cycling, the fatigue crack extension was monitored using a video-camera in order to determine the coldworking effect on both the initiation period and the propagation life.The fatigue test results have shown that a second coldworking may enhance the fatigue life of an already coldworked hole. For a given cyclic loading level, the beneficial effect depends upon the applied life fraction before reworking. The direction of the mandrel movement during the second coldworking has no noticeable influence on the additional fatigue life improvement. These experimental results have been analyzed with reference to the calculated residual stress field. NOMENCLATURE a = crack length d = initial hole diameter D = maximum mandrel diameter e = degree of coldworking h = specimen thickness N = number of applied fatigue cycles N, = number of cycles at failure r =hole radius R = nominal stress ratio (S-/S-) t = sleeve thickness W = specimen width S-, S,,, = minimum and maximum nominal cyclic stress, based on the gross specimen section
A research program has been carried out to establish the low-cycle fatigue and creep-fatigue behaviors of Inconel 625 at elevated temperatures (650 and 815°C). The main observations were related to the effect of temperature and of hold times. Under continuous cycling, a temperature increase from 650 to 815°C caused a reduction in the fatigue life by a factor of 2 at a high strain and by a factor of 3 at a low strain. Tension hold times had little detrimental effect on cyclic life at 650°C concerning the life reduction with respect to continuous cycling; at 815°C, on the other hand, this became more significant. Compressive hold times also had a damaging effect on fatigue life, and this was larger than that associated with tensile hold times; in particular, it was very pronounced for low strain levels at 815°C. An analysis of data using a life prediction method previously suggested for creep-fatigue combination loadings has also been carried out. The method takes into account the damaging effect due to a compression hold time separately from that due to a tensile hold time in interspersed creep-fatigue loadings. The overall correlation between theoretical calculations and experimental results is reasonably good, for strain levels ranging from 1.2 to 0.4%.
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