Ultra-high Cycle Fatigue Behavior Research Articles

Ultra-high cycle fatigue and ultra-slow crack growth behavior of additively manufactured AlSi7Mg alloy

PurposeThe purpose of this paper is to determine the ultra-high cycle fatigue behavior and ultra-slow crack propagation behavior of selective laser melting (SLM) AlSi7Mg alloy under as-built conditions.Design/methodology/approachConstant amplitude and two-step variable amplitude fatigue tests were carried out using ultrasonic fatigue equipment. The fracture surface of the failure specimen was quantitatively analyzed by scanning electron microscope (SEM).FindingsThe results show that the competition of surface and interior crack initiation modes leads to a duplex S–N curve. Both manufacturing defects (such as the lack of fusion) and inclusions can act as initially fatal fatigue microcracks, and the fatigue sensitivity level decreases with the location, size and type of the maximum defects.Originality/valueThe research results play a certain role in understanding the ultra-high cycle fatigue behavior of additive manufacturing aluminum alloys. It can provide reference for improving the process parameters of SLM technology.

Assessment of Ultra-High Cycle Fatigue Behavior of EN-GJL-250 Cast Iron Using Ultrasonic Fatigue Testing Machine

High cycle fatigue comprising up to 107 load cycles has been the subject of many studies, and the behavior of many materials was recorded adequately in this regime. However, many applications involve larger numbers of load cycles during the lifetime of machine components. In this ultra-high cycle regime, other failure mechanisms play, and the concept of a fatigue endurance limit (assumed for materials such as steel) is often an oversimplification of reality. When machine component design demands a high geometrical complexity, cast iron grades become interesting candidate materials. Grey cast iron is known for its low cost, high compressive strength, and good damping properties. However, the ultra-high cycle fatigue behavior of cast iron is poorly documented. The current work focuses on the ultra-high cycle fatigue behavior of EN-GJL-250 (GG25) grey cast iron. An ultrasonic (20 kHz) fatigue testing system is developed and instrumented for testing at ambient temperature, and hourglass shaped cast iron specimens with circular cross-section are designed to be excited in resonance. The temperature of the specimen is measured using a pyrometer and controlled by ventilating air. The displacement at the free end of the specimen (which is linked to stress at the reduced section) is measured using an accurate laser sensor, whose analysis requires careful test preparation and data analysis. The high resonance frequency allowed to characterize the S-N behavior of the cast iron of interest within a matter of days, and repeat the tests to quantify the natural scatter in fatigue resistance. The obtained results will be used in a research project that aims to optimize the design of machine components with respect to weight and durability.

Ultra-high cycle fatigue behavior of a novel 1.9 GPa grade super-high-strength maraging stainless steel

Super-high-strength maraging stainless steel (MSS) is of great interest for the next generation aircraft. The ultrasonic fatigue behavior was studied for a 1.9 GPa grade ultra-high-strength MSS with a composition of 13Co-13Cr-3.5Mo-4.5Ni-0.5Ti (at. %). The ultra-high cycle fatigue limit was 560 MPa. Cracking initiated on the surface or at non-inclusion interior sites. A crack initiation and propagation model was proposed through a discussion of the fracture mechanism: (i) the surface defect-induced failure was caused by machining marks with a critical size of 0.6 μm; and (ii) the interior matrix induced failure was caused by a defect larger than 3.3 μm. The fatigue number (Nf) of the surface induced failures ranged from 6 × 104 to 1.4 × 105, while that of the interior induced failures was much longer, ranging from 8.6 × 106 to 3.2 × 108.

The influence of inclusion factors on ultra-high cyclic deformation of a dual phase steel

The ultra-high cycle fatigue test of a dual phase steel was carried out using ultrasonic fatigue vibration system, and the fatigue fracture surface was observed by SEM to reveal crack origin sites that indicated the presence of inclusions. Second, the influence of inclusions on the ultra-high cycle fatigue behavior was studied by considering characteristic parameters of inclusions. Experimental results showed that the tensile strength (Rm) and fatigue limit strength after 109 cycles (σw9) and in the non-failure condition of the dual phase steel were 1603 MPa and 600 MPa, respectively. The position of inclusion in the steel had the greatest influence on the ultra-high cycle fatigue life, and the inclusion size had secondary influence on fatigue property. However, the curvature of inclusion had the least influence on the fatigue behavior. Among all the inclusion parameters, the size and position of inclusion was positively correlated with the fatigue life, but the curvature of inclusion was negatively correlated with fatigue life. Based on the study of inclusion factors, a prediction model of ultra-high cycle fatigue life using the above-mentioned inclusion parameters for the steel is proposed. The significance of the study is that the ultra-high cycle fatigue life can be easily predicted by analyzing inclusion factors.

Ultra-high cycle fatigue property of a multiphase steel microalloyed with niobium

The effect of microstructure on the ultra-high cycle fatigue behavior of a high strength steel microalloyed with niobium (Fe-0.23C-1.73Si-2.24Mn-0.68Cr-0.042Nb) was investigated through ultrasonic fatigue test. The ferrite and martensite (F/M) multiphase structure was obtained by air-cooling treatment, while the bainite and martensite (B/M) multiphase structure was obtained by oil-cooling treatment. The ultra-high cycle fatigue limit (109 cycles, σw9) of steel with F/M multiphase structure was 750 MPa, while the σw9 of steel with B/M multiphase structure was 900 MPa. In the high cycle regime, cracking occurred at the non-inclusion site in the steel with F/M structure. However, half the specimens cracked at the non-inclusion site, the others cracked at inclusions in the steel with B/M structure. The study clearly underscores that the microstructure had a distinct impact on the ultra-high cycle fatigue behavior of high strength steels. Moreover, the present work demonstrates that the fatigue properties of steel can be optimized by using appropriate heat treatment, which is related to the change in the F/M microstructure to B/M microstructure in an advanced Nb-microalloyed steel.

Transition from dislocation glide to creep controlled damage in fatigued thin Cu films

The ultra-high cycle fatigue behavior of supported Cu films with thicknesses between 40 and 360 nm has been investigated using a novel atomic force microscope (AFM)-based resonance method. The damage created under strain controlled fatigue loading is investigated as a function of applied strain, film thickness, and cycle numbers up to 5 × 1010. For films thicker than 100 nm, extrusions and boundary cracks limit the fatigue performance but only appear above a threshold in the applied strain amplitude which scales inversely with the square root of the film thickness. The extrusion formation is attributed to dislocation activation. The grain boundary cracks are replaced by grain boundary grooves in films of 100 nm and thinner. The grooves are believed to form by diffusion mediated creep processes, similar to observations at higher temperatures but here driven by cyclic stresses and capillarity, and become detectable once the accumulated plastic strain exceeds a critical value. These results indicate that due to creep processes, thinner films can be less resistant to fatigue than thicker films, particularly for large cycle numbers.

超音波疲労試験機を用いたZr<sub><font size="-1">55</font></sub>Al<sub><font size="-1">10</font></sub>Ni<sub><font size="-1">5</font></sub>Cu<sub><font size="-1">30</font></sub>金属ガラスの超高サイクル疲労特性

In this work, the ultra-high cycle fatigue behavior of Zr55Al10Ni5Cu30 metallic glass was investigated by using ultrasonic fatigue testing method. Test alloy rods with a diameter of 8mm were prepared by tilt-casting with copper mold. Test specimens were machined to hourglass shape type (the minimum diameter ; 2.0mm), and after machining the surface of specimens was manually polished with polishing paste. The specimens were tested at a frequency of 20kHz under a stress ratio of -1. The fatigue limit (σw) (half a total stress amplitude) and fatigue ratio (σw/σB) of the alloy showed 893MPa and 0.53, respectively. Furthermore, we did not see any fish-eye like crack-growth morphology on fracture surfaces. The initiation region of fatigue crack was always observed at the side surface of the specimens. A viscous-flow fracture region with large dimples between the fatigue crack-growth region and the final unstable fracture region was observed. This peculiar fracture morphology may be due to adiabatic heating of the metallic glass specimens during the fracture, resulting in transition to supercooled liquid, under extremely high frequency of fatigue stress using ultrasonic loading.

Influence of Subjection to Plasma Nitriding Surface Modifications on Ultra-High Cycle Fatigue Behavior of Ti-6Al-4V

Using the ultrasonic fatigue test method, the influence of subjection to plasma nitriding surface modifications on the ultra-high cycle fatigue behavior of Ti-6Al-4V was investigated, then a comparison with corresponding behaviors of the normal Ti-6Al-4V was made. The results show the S-N curve of Ti-6Al-4V with plasma nitriding surface modifications still continuously declines, no conventional fatigue limit exists for Ti-6Al-4V. Plasma nitriding surface modifications enhance surface hardness, but they make the material fatigue strength decrease by about 17 %. The fractography of fatigue failure has been observed by scanning electron microscopy. The observation shows that fatigue failure initiates from internal of specimen after the fatigue life of 108cycles and fatigue cracks mainly initiate from the surface of specimen before the fatigue life of 108cycles.

Ultra-high cycle fatigue behavior of high strength steel with carbide-free bainite/martensite complex microstructure

The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite (CFB/M) complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10 7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10 7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of “soft structure”. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.

Effect of microstructure on ultra-high cycle fatigue behavior of Ti–6Al–4V

The fatigue behavior of Ti–6Al–4V alloy with the bimodal and basketweave microstructures has been investigated. The results show that the S– N curves of Ti–6Al–4V with both microstructures continuously decrease with increasing the number of cycles to failure and have no horizontal asymptote in the regime of 10 5 to 10 9 cycles. SEM observation of fracture surface indicates that the crack initiation sites abruptly shift from surface to interior of the specimen with decreasing the stress amplitude. It is found that the surface crack initiation mainly results from the machining traces. Nevertheless, most of the internal fatigue cracks initiate in the border region of primary α-grains in bimodal microstructure or at α–β interfaces in basketweave microstructure, which induced by inhomogeneity of microstructures and incompatibility of deformation between two phases. The size of the internal crack initiation site was evaluated and the dependence of the fatigue life on the location and size of the initiation sites is discussed.

Staircase testing of a titanium alloy in the gigacycle regime

The objective of this research was to investigate the high cycle fatigue behavior of a titanium alloy using an ultrasonic fatigue system. Fatigue testing from 10 6 to 10 9 cycles under fully reversed cycling ( R = −1) was performed to determine the ultra-high cycle fatigue behavior of Ti–6Al–4V. The staircase test method was employed to obtain accurate values of the mean fatigue limit stress corresponding to fixed numbers of cycles up to 10 9. These results were compared to similar data generated on conventional servohydraulic test systems and electromagnetic shaker systems to determine if there are any frequency effects. In addition, specimens were tested with and without cooling air to determine the effects of temperature on the fatigue behavior. Results indicate that the fatigue strength determined from ultrasonic testing was consistent with conventional testing. In addition, the temperature rise in this material during ultrasonic testing is insignificant and has no apparent effect on the endurance limit.

Fatigue strength of Ti–6Al–4V at very long lives

The objective of this research was to investigate the fatigue strength of Ti–6Al–4V using an ultrasonic fatigue system. Fatigue testing up to 10 9 cycles under fully reversed loading was performed to determine the ultra-high cycle fatigue behavior of Ti–6Al–4V. Endurance limit results were compared to similar data generated on conventional servohydraulic test systems and electromagnetic shaker systems to determine if there are any frequency effects. Fatigue specimens were tested with and without cooling air to determine the effects of increased specimen temperature caused by internal damping due to cycling at a very high frequency. An infrared camera was also used to record specimen temperatures at various load levels. Results indicate that the effects of frequency, including internal heating, on the very high cycle fatigue behavior of Ti–6Al–4V are negligible under fully reversed loading conditions.

Ultrasonic Fatigue Testing of Ti-6Al-4V

Abstract The objective of this research was to investigate the high cycle fatigue behavior of a titanium alloy using an ultrasonic fatigue system. Fatigue testing up to one billion cycles under fully reversed loading conditions was performed to determine the ultra-high cycle fatigue behavior of Ti-6Al-4V. Endurance limit results were compared to similar data generated on conventional servohydraulic test systems and electromagnetic shaker systems to determine if there are any frequency effects. In addition, specimens were tested with and without cooling air to determine the effects of temperature on the fatigue behavior. Results indicate that the fatigue strength determined from ultrasonic testing was consistent with conventional testing. However, preliminary results indicate that cooling air may increase the fatigue limit stress at very long lives.

1345 高強度鋼における2段繰返し変動荷重下の超長寿命疲労特性(S22-1 高強度鋼,S22 ギガサイクル疲労)

1345 高強度鋼における2段繰返し変動荷重下の超長寿命疲労特性(S22-1 高強度鋼,S22 ギガサイクル疲労)