Papers by Keyword: Cyclic Stress-Strain Response

Abstract: The present work is focused on the study of microstructure and low cycle fatigue behavior of the first generation nickel-base superalloy IN 713LC (low carbon) and its promising second generation successor MAR-M247 HIP (hot isostatic pressing) at 900 °C. Microstructure of both alloys was studied by means of scanning electron microscopy (SEM). The microstructure of both materials is characterized by dendritic grains, carbides and casting defects. Size and morphology of precipitates and casting defects were evaluated. Fractographic observations have been made with the aim to reveal the fatigue crack initiation place and relation to the casting defects and material microstructure. Low cycle fatigue tests were conducted on cylindrical specimens in symmetrical push-pull cycle under strain control with constant total strain amplitude and strain rate at 900 °C in air. Hardening/softening curves, cyclic stress-strain curve and fatigue life data of both materials were obtained. Cyclic stress-strain curve of MAR M247 is shifted approximately to 120 MPa higher stress amplitudes in comparison with IN 713LC. Significantly higher fatigue life of MAR-M247 has been observed in Basquin representation. On the other hand IN 713LC shows prolonged lifetime compared with MAR-M247 in the Coffin-Manson representation. Results obtained from high temperature low cycle fatigue tests are discussed.

Abstract: The present work is focused on the study of low cycle fatigue behavior of grit blasted nickel-base superalloy Inconel 713LC (IN 713LC). Grit blasting parameters are obtained. Button end specimens of IN 713LC in as-received condition and with grit blasted surface were fatigued under strain control with constant total strain amplitude in symmetrical cycle at 900 °C in air. Hardening/softening curves, cyclic stress-strain curve and fatigue life data of both materials were obtained. Both materials exhibit the same stress-strain response. It has not been observed any improvement or reduction of low cycle fatigue life in representation of total strain amplitude versus number of cycles to failure of grit blasted material in comparison with as-received material. Surface relief and fracture surface were observed in SEM. The little effect of surface treatment on fatigue characteristics is discussed.

Abstract: The effect of thermal barrier coating (TBC) on low cycle fatigue behavior of cast superalloy Inconel 713 LC has been studied at 900 °C. The TBC consisting of a CoNiCrAlY bond coat and a zirconia (ZrO₂) top coat stabilized by 8% yttria (Y₂O₃) was deposited on the gauge section of cylindrical specimens using the atmospheric plasma spray technique. Cylindrical specimens of Inconel 713LC in as-received condition and with surface treatment were cyclically strained under strain control with constant total strain amplitude in symmetrical cycle at 900 °C in air. Hardening/softening curves, cyclic stress-strain curve and fatigue life data of coated and uncoated material were obtained. The stress response of the TBC coated specimens is lower in comparison with the uncoated specimens. Detrimental effect of surface treatment on the Basquin curve is documented. Specimen sectioning and fracture surface observations revealed fatigue damage mechanisms and help to discuss differences in fatigue behavior of the coated and uncoated superalloy.

Abstract: A beta phase containing titanium aluminum compound was prepared. Isothermal Fatigue(IF) were subjected at 650 °C at three strain rates, such as 6.67×10^-3 s^-1, 6.67×10^-4 s^-1, 6.67×10^-5 s^-1 to determine the effect of strain rate on cyclic stress-strain response (CSSR) of TiAl alloy during IF tests. The curves of cyclic stress-strain response were discussed and dislocations configuration were also observed by TEM. The results show that strain rates have an apparent effect on CSSR of TiAl alloy during IF tests and CSSR was identified that it had a close relationship with dislocation configuration and deformation twin.

Abstract: Isothermal fatigue (IF) tests were carried out on the gamma-TiAl alloy in the temperature of 500°C, 650°C and 800°C under mechanical strain control in order to evaluate its cyclic deformation behaviors at elevated temperature. Cyclic deformation curves, stress-strain hysteresis loops under different temperature-strain cycles were analyzed and dislocations configurations were also observed by TEM. The mechanism of cyclic hardening or softening during IF tests was also discussed. Results show that during the IF tests, The hysteresis loops were almost symmetrical above 600 °C, such as 650 °C and 800 °C; The hysteresis loops at the temperature of 500 °C generated two apparent asymmetry, one was zero asymmetry, the other was tensile and compressive asymmetry; Dislocation configuration and slip behaviors are contributed to cyclic hardening or cyclic softening.

Abstract: The present paper reports on the effect of texture on the cyclic stress-strain response (CSSR) and the fatigue life of ultrafine-grained (UFG) interstitial-free (IF) steel. Tests in the lowcycle fatigue (LCF) regime were conducted on material that was processed by the equal channel angular extrusion (ECAE) technique along the so called “efficient” route 8E. This route has been shown to result in a homogeneous microstructure with a high fraction of high angle grain boundaries (HAGBs), which are beneficial for a stable CSSR. In addition, the evolution of the microstructure was characterized by means of electron optical techniques, including electron backscattered diffraction, and by X-Ray diffractometry. It was found that the initial texture of specimens cut from the ECAE billet along different orientations with respect to the extrusion direction (ED) has a substantial effect on the CSSR of the UFG IF steel. Furthermore, microscopy results indicated the notable influence of the last ECAE processing step on the evolution of damage in the material.

Abstract: This paper reports on the fatigue performance of an ultrafine-grained (UFG) interstitialfree (IF) steel deformed at various mean stress levels. The UFG microstructure was achieved using equal channel angular extrusion processing at room temperature (RT) and along an “efficient” route, giving way to the formation of high angle grain boundaries (HAGBs) with a high volume fraction. The current study not only confirms the previous finding that a high volume fraction of HAGBs promotes cyclic stability, but also inquires into the role of mean stress level on the cyclic stability. It is shown that the UFG IF steel exhibits a stable cyclic deformation response in the lowcycle fatigue regime within the medium applied mean stress range of -75 to 75 MPa. The corresponding fatigue lives can still be predicted with the Smith-Watson-Topper approach within this range. Furthermore, the present study demonstrates that the evolution of mean strains with cyclic deformation can be linked to the evolution of mean stresses in strain-controlled loading.

Abstract: The tension-tension fatigue tests for SiC/SiC composites were performed under the conditions that the maximum load Pmax was 80-90% to the fracture load of the tensile tests and the stress ratio was Rσ = 0.5. The composites exhibited a width in stress-strain hysteresis loop under one load cycling. In some cases the mean strain εmean gradually increase with increasing in number of cycles. These variations would reflect the developments of the fatigue damage at the fiber/matrix interface during the cyclic loading process. The pull-out lengths of the fibers for the fatigued- and not fatigued-specimens were measured through the SEM observations after the tensile test. In all materials, the average pull-out length of fibers in fatigued material was larger than in not fatigued material because the cyclic loading affected on the fiber/matrix interfacial strength.