Papers by Keyword: Low Cycle Fatigue

Abstract: Additive Manufacturing enables the production of intricate geometries and products with improved strength-to-weight ratios, driving its applications in defence, aerospace, and automobile sectors. Maraging steel and Superalloy Inconel 625 are renowned for their excellent mechanical properties and are candidate materials for high performance applications. It is essential to study the fatigue behaviour of additively manufactured samples prior to their deployment in real working environments, as their mechanical properties and fatigue behaviour differ from those of conventionally manufactured materials. In the present study, low cycle fatigue (LCF) behaviour of Maraging steel (M300) and Inconel 625, at different strain amplitudes and heat treatment conditions were evaluated. Fractographic characterization was conducted using scanning electron microscopy (SEM). To understand the effect of build orientation on LCF behaviour, the maraging steel samples were also tested in different build orientations (0^o and 90^o). It was found that build orientation significantly affected the fatigue life of additive manufactured maraging steel samples. The LCF study was also done at different strain amplitudes for Inconel 625 and results indicated that there was drastic decrease in fatigue life at higher strain amplitudes. It was observed that defects introduced due to layer wise processing of additive samples have adverse effect on fatigue life. Ultrasonic shot peening was also applied to the additively manufactured fatigue samples to examine its impact on fatigue life.

Abstract: Mechanical fatigue is an essential phenomenon that occurs when the structures are exposed to dynamic, fluctuating loadings. Especially automotive components are regularly exposed to random vibration loadings. Vibration fatigue failures may arise even in components that meet static requirements and are stable and robust, because of dynamic and fluctuating loadings. The primary focus of this study is the research of the vibration fatigue. Hence, in addition to the analyses, the tests are conducted. In order to study the analyses and tests, aluminum cross-section beams are designed and manufactured. The notched sections added to the beam geometry to acquire a more distinct fatigue life compared to other parts of the beams. The results obtained from the experimental tests are used to correlate the effect of notch parameters on fatigue life.

Abstract: Anisotropy in tensile behaviour, plastic flow behaviour, and low cycle fatigue (LCF) behaviour of additively manufactured (AM) maraging steel in different build orientations are presented and compared with conventionally manufactured maraging steel. Also, the effect of heat treatment (namely, solution treatment and ageing) on tensile behaviour and low-cycle fatigue behaviour were studied. The AM maraging steel showed more anisotropy in as-built (AB) condition and moderate anisotropy in heat-treated (HT) condition. Experimental engineering stress-engineering strain and true stress-true strain data of AB AM maraging steel and HT conditions have been analysed using Hollomon, Ludwik, Swift, Ludwigson, and Voce plastic flow relationships. It is also observed that the 0° oriented specimen exhibits better tensile and LCF behaviour as compared to the 90° oriented specimen in AB and HT conditions.

Abstract: Low Cycle Fatigue (LCF) is a prominent failure mechanism in many design components; therefore, an evaluation of cycles to failure in this regime is of high importance. Most international standards recommend a closed loop strain-controlled mode specimen testing in this regime. However, the ꜪN data obtained from this test is not suitable for life evaluation of parts enduring force-controlled history during actual service without correction for control mode. Many existing procedures, which accounts for cyclic strain stabilization during force-controlled loading may significantly complicate the finite elements analysis (FEA) at solving or post processing stages and are often an inherent source of uncertainty. A heuristic, cost effective and sufficiently accurate approach for LCF life estimation is advocated. The method involves only two force loading FEA simulations, one of the actual parts and the other of the test specimen, using initial unstabilized stress strain curve, followed by a limited number of force-controlled specimens testing. Actual part and specimen life correlation performed using first loading unstabilized equivalent plastic strain value Ꜫ_p1 under locality and similitude assumptions. Unstabilized strain vs. number of cycles to failure curve Ꜫ_p1N is constructed and discussion regarding specimen geometry considerations for providing sufficient accuracy is included. Method validation and crack propagation study are provided.

Abstract: The main originality of this work consists in investigating low cycle fatigue of cylindrical test piece with wings under imposed constraint and for the temperature 20°c, 200°c, 400°c. Based on a combination between the fatigue parameter of Jiang-Sehitoglu and the relationship of Coffin-Manson, a numerical model for the prediction of the number of cycles at break. It was found that the CuCrZr cylindrical test piece showed a reduction in fatigue life with increasing temperature.

Abstract: Components of gas turbines must be extremely resistant to high temperatures, high stresses, high-temperature corrosion, and erosive environments. The materials used in these environmental conditions are mainly nickel-based superalloys. In this study, the low-cycle fatigue of the nickel-based superalloy Inconel 792 was examined. The total strain range of a gas turbine between 760 °C and 870 °C was considered as the parameter representing the actual gas turbine operation. In addition, tests were performed using a trapezoidal waveform of the total strain to reflect the operation-stop conditions of a gas turbine with frequent shutdowns. The results of the fatigue test were compared with the Coffin–Manson method and energy method. The fractured surface was analyzed using a scanning electron microscope (SEM).

Abstract: β-Ti (Ti–13Nb–13Zr) alloy was subjected to ultrasonic shotpeening (USSP) and a nanocrystalline layer of ~60 µm thickness was developed on the metastable Ti-13-Nb-13Zr alloy. In this investigation, the surface hardening and low cycle fatigue (LCF) behavior of the alloy were studied after USSP treatment. Compared to the un-shotpeened samples, the shotpeened specimens exhibit high surface hardness and an enhancement in fatigue life. A notable impact of USSP on the fatigue crack initiation and growth of the alloy was also observed. The results show that the crack initiation at free-surface was suppressed due to the formation of a nanograined microstructure and fatigue crack initiation site shifts from surface to inside of the material. Further, the microstructural analysis proves that the nanograin formation and compressive stresses imparted by ultrasonic shot peening treatment are helpful in significant improvement of fatigue life.

Abstract: Low cycle fatigue (LCF) tests are performed on CP-Tiat different temperatures (293K,423K and 523K). It is found that the fatigue life of CP-Tidecreases with temperature. A short cycle hardening phenomenon occurs at the beginning of cyclic deformationat 293K and 423K, followed by cyclic softening untilfailure. At 523K, cycle hardening isexhibited throughout the entire cycle until thefracture. The fatigue-life curves obtained from the tests are constructed using Coffin-Manson-Basquin model. According to the relationship between the four parameters of Coffin-Manson-Basquin model and temperature, the temperature-based life prediction model is further proposed. Scanning electron microscopy observation of fatigue fractures showsthat the fatigue cracks of CP-Tiat 423K and 523K under different strain amplitudes initiate on the surface of fatigue specimens and extend to the fracture zone by the transgranular mode.

Abstract: The service conditions of thermal recovery wells make the casing repeatedly bear the tension and compression load and form low cycle fatigue. Meanwhile, many factors, such as pre-strain and creep, lead to the formation of asymmetrical low cycle fatigue (R≠-1), which is the low cycle fatigue behavior under the influence of mean strain. This work studied the effect of mean strain on low cycle fatigue behavior of N80Q steel. Different strain amplitude conditions were selected for low cycle fatigue test, which were 0.5%, 0.7%, 1.0%, 1.5% and 2.0% respectively. Then tests at mean strains of-0.8%, 0%, 0.5% and 1.0% were conducted under constant strain amplitude. And the microstructure and fracture surface of the material after the tests were characterized using scanning electron microscopy and transmission electron microscopy, respectively. The results show that the mean strain makes the fatigue life reduce significantly under the condition of constant strain amplitude, and is related to the amplitude of the mean strain. The value of the mean strain and the strain amplitude will ultimately affect the fatigue life. And the fatigue life is related to the maximum absolute value of strain and has a linear relationship in the double logarithmic coordinate system. The SEM results of fracture morphology show that the brittleness feature of the crack growth area with high mean strain decreases significantly. And the fracture cross-section observation shows that the crack propagation is transgranular propagation. The TEM results show that a large number of dislocations pile-up is formed at lath subgrain boundary.

Abstract: The main originality of this work consists in investigating low cycle fatigue of AISI 316L cardiovascular stents under hypertensive loading. For this purpose, two geometries of stents are expanded to various diameters and subjected to hypertensive blood pressure. Based on a combination between the fatigue parameter of Jiang-Sehitoglu and the relationship of Coffin-Manson, a numerical model for the prediction of the number of cycles to crack failure is developed. The stent is found to exhibit a fatigue life reduction with the increase of the expansion diameter due to ratchetting strain. In addition, the location of the failure is independent on the design. However, the U-shape strut permits a better distribution of pressure over the stent strut resulting in a longer fatigue life as compared to the Ω-shape.