Localized Fatigue Response Evaluation of Weld Regions Through Cyclic Indentation Studies
09 Nov 2018-Vol. 12
TL;DR: In this article, 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).
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.
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TL;DR: In this article, a cyclic automated ball indentation (Cyclic ABI) is used to detect fatigue failure in an Inconel 617 alloy using an off-line data analysis.
Abstract: Fatigue has been one of the most researched subjects as most of the critical component failures are traced to fatigue. While fatigue data generation for design purposes is carried out using ASTM or equivalent standard specimens, the use of miniature or small specimens to estimate the fatigue properties is considered as a tool for extending the remaining life of in-service components. Cyclic automated ball indentation (Cyclic ABI) is one of the non-conventional test techniques used for fatigue performance assessment of pristine and in-service damaged materials. This method uses compression-compression cyclic loading of a flat specimen using a tungsten carbide spherical indenter; continuous monitoring of load-displacement (measured close to the indentation location) data provides an idea about the fatigue life of the material. Apart from this, hysteresis in load-displacement is used as an indicative energy parameter to detect fatigue failure in an Inconel 617 alloy using an off-line data analysis. To ensure on-line tracking of failure events, a specially tuned, miniature acoustic emission (AE) sensor was used during cyclic indentation testing. The AE parameters were extracted in the format of counts, absolute energy; the result processed in terms of cumulative counts, cumulative energy as well as first derivative of acoustic emission counts vs. fatigue cycles was used to cross-correlate failure events with other sensor responses. The failure cycles identified from AE were found to be in good agreement with the hysteresis area under the load-displacement curves, as well as extensometer displacement during cyclic loading.
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01 Jan 2019
TL;DR: The developments to estimate the fatigue properties of materials using small volume of sample material—similar to scooped samples are presented and the optimum electrode potential for a fatigue crack growth study has been identified.
Abstract: Accurate and reliable life prediction is one of the challenges faced by engineers working in safety-critical domains such as power plants, transportation, and offshore structures. This paper presents the developments to estimate the fatigue properties of materials using small volume of sample material—similar to scooped samples. Cyclic ball indentation and cyclic small punch testing methods have been developed over a period of nearly two decades and have been demonstrated to predict the fatigue properties of in-service materials. Some salient results are discussed in this paper. In the case of offshore structures, the synergistic effect of mechanical loading at a low frequency combined with the corrosive environment accelerates the damage. Life prediction for such structures requires data on corrosion-fatigue crack growth at low frequencies. This is a time-consuming effort and hence there is a need to estimate the properties through novel test methods. Frequency shedding method is proposed to estimate the fatigue crack growth rate behavior in corrosive environments. Further, designers and operators of offshore equipment resort to avoidance of free corrosion through the use of electrode potentials. However, the choice of electrode potential is dependent on the stress state. Through a systematic study, the optimum electrode potential for a fatigue crack growth study has been identified and the same is discussed here. It is hoped that the results of this study and the directions shown to carry out the data generation under more realistic conditions would help the life prediction and life extension community at large.
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TL;DR: In this paper, experimental fatigue response for selected unirradiated and irradiated reactor materials: AISI 316 stainless steel, ferritic/martensitic MANET and HT9 steels.
Abstract: This paper analyzes experimental fatigue response for selected unirradiated and irradiated reactor materials: AISI 316 stainless steel, ferritic/martensitic MANET and HT9 steels. Available tensile test results on the same or similar materials are used to predict changes in fatigue response using the Universal Slopes method. The predictions are compared with the experimental data to assess the potential for using tensile data to predict reactor component fatigue response. It was found that the effect of irradiation on fatigue life was less severe than on tensile properties. However, tensile properties are useful for qualitative predictions of fatigue response.
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