Localized Fatigue Response Evaluation of Weld Regions Through Cyclic Indentation Studies

Abstract

An experimental investigation of the fatigue response of commonly used structural stainless steel — SS 304 L(N) and SS 316 L(N) — and its weld was carried out through automated cyclic ball indentation (ABI). A Tungsten Carbide (WC) spherical ball indenter of 1.57 mm diameter was used for compression-compression fatigue testing of the specimen under load control at a low frequency of loading (typically 0.1 Hz to 1 Hz). The force-displacement response during fatigue loading was logged continuously during fatigue test and the data was analyzed to extract details such as variations in: total depth of penetration, loading and unloading slopes, loading/unloading intercept, displacement range as a function of number of cycles. From the results, one could identify an unsteady response of material during cyclic loading after some cycles of fatigue loading — typical of failure; this input was used to compare the fatigue response of different zones of the weld. Even though the applied frequency of loading is relatively less (∼ 1 Hz), due to the high levels of plastic deformation that is developed during the indentation process, one could expect an effect of strain rate on the fatigue response during cyclic ball indentation. To verify this, experiments were carried out at three distinct frequencies of 0.1 Hz, 0.5 Hz and 1 Hz for a given loading condition. Further, it was observed that the material response in weld region is the best, followed by the base metal. This can be corroborated with the weld microstructure that is obtained as a consequence of processing. Frequency of loading did not have significant influence on the fatigue failure life. Numerical simulation of cyclic ball indentation was carried out to extract some relevant parameters for failure life such as mean stress and local stress ratio. This will serve as input to correlation of failure life data obtained from conventional specimens.