Ultra-high Cycle Fatigue Research Articles

High-temperature ultra-high cycle fatigue damage of notched single crystal superalloys at high mean stresses

Blade nickel superalloy CMSX-4 widely used in the aero industry and its potential low cost alternative, superalloy CM186LC intended for use in the industrial gas turbines, were subjected to ultra-high-cycle fatigue at high mean stresses to model the effect of vibrations superimposed on sustained load. Circumferentially notched cylindrical specimens of single crystals with the axis orientation of [001] were tested at 850 °C in air. For small amplitudes of the cyclic stress superimposed on the sustained stress the time to fracture is slightly increasing with increasing stress amplitude. This trend is reversed for higher stress amplitudes where the time to fracture quickly decreases with increasing stress amplitude. Fatigue crack initiation and following crack propagation are here the decisive failure mechanisms. Cyclic stress component leads to the formation of persistent slip bands running through the γ matrix and the γ′ precipitates. These bands represent sites for the initiation (in interaction with casting pores and other defects) and early propagation of fatigue cracks. The early crack propagation along the slip planes is later replaced by non-crystallographic propagation of the dominant crack.

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)

435 高強度材の超長寿命疲労特性に及ぼす 2 段変動荷重の影響

435 高強度材の超長寿命疲労特性に及ぼす 2 段変動荷重の影響

On ‘multi‐stage’ fatigue life diagrams and the relevant life‐controlling mechanisms in ultrahigh‐cycle fatigue

ABSTRACT In recent years, several fatigue studies on different metallic materials have indicated that fatigue failures can occur even at amplitudes below the conventional high‐cycle fatigue (HCF) fatigue limit in the gigacycle or ultrahigh‐cycle fatigue (UHCF) range (number of cycles to failure in excess of ca. 107−108). In the latter case, fatigue failures were observed to originate from internal defects (non‐metallic inclusions). The S–N curves displayed a multi‐stage shape, sometimes with a second lower fatigue limit in the UHCF range.The present paper specifies the conditions under which internal and/or surface fatigue failure can occur at low load levels below the conventional HCF fatigue limit. The relevant fatigue thresholds are discussed for two classes of materials, namely pure single‐phase metallic materials (type I) without internal defects and materials such as steels or cast materials (type II) with internal defects (inclusions or pores). In the case of type II materials, the probability of surface versus internal fatigue is discussed in terms of the volume density, size and location of the inclusions and the relevant cracking mechanisms. It is argued that, in both type I and type II materials, multi‐stage S–N curves or Manson–Coffin plots can be expected under certain conditions.

Specific features of high‐cycle and ultra‐high‐cycle fatigue

ABSTRACT Several specific features of high‐cycle fatigue (HCF) and ultra‐high‐cycle fatigue (UHCF) are discussed both on the basis of direct experimental results and on the basis of speculations extrapolating HCF data to the UHCF regions. The following points are dealt with: (i) Extent and distribution of cyclic plastic deformation. (ii) Sensitivity of the fatigue behaviour in the HCF and UHCF regions to the start‐up procedure of the test and to the preceding stress–strain history. (iii) Role of persistent slip bands (PSBs) and the microcrack initiation in the HCF and UHCF regions. (iv) Dependence of the notch sensitivity on the amplitude of cycling. (v) Effect of high number of very small amplitude cycles on creep behaviour of superalloy single crystals.

128 Fretting Fatigue Behavior in the Ultra High Cycle Regime

Research on fatigue behavior in the ultra high cycle regime has been recently focused since the fatigue mechanism and S-N behavior different from those observed less than (10)^7 cycles have been observed. It is also of importance from the viewpoint of the long-term service use of structures and machines. Limited research works have been available on fretting fatigue in the ultra high cycle regime, while lots of studies have been done on plain fatigue even in this regime. In the present study, a new machine for ultra high cycle fatigue test has been developed with an electro-magnetic vibrator. It was confirmed that fretting fatigue strength, lives, frictional force and relative slip amplitude were not significantly influenced by the cyclic loading rate up to 300 Hz, which was the present test condition for ultra high cycle fretting fatigue. The fretting fatigue strengths defined at (10)^7 cycles were effective up to (10)^8 and (10)^9 cycles for titanium alloy and aluminum alloy. The fretting fatigue crack initiated at the end region of fretting contact area even in the ultra high cycle regime, which indicated that the fatigue mechanism would not change even in the ultra high cycle regime. This might result from the singular stress field near the contact edge in fretting fatigue.

On the life‐controlling microstructural fatigue mechanisms in ductile metals and alloys in the gigacycle regime

The high‐cycle fatigue (HCF) behaviour of ductile metals and alloys, and the life‐controlling microstructural fatigue mechanisms known from HCF are reviewed critically with respect to their possible role in the gigacycle or ultra‐high‐cycle fatigue (UHCF) regime. Arguments are presented to support the hypothesis that, at the very low amplitudes of the UHCF regime, fatigue crack initiation, resulting from cyclic strain localization, and slow early Stage I fatigue crack propagation are the life‐controlling mechanisms and that these processes can essentially be described in terms of the microstructurally irreversible portion of the cumulative cyclic plastic strain. Emphasis is placed on the important role of the so‐called slip irreversibility which decreases as the amplitude becomes lower and lower. Finally, the Manson–Coffin law is reformulated for very low amplitudes in terms of microstructurally relevant parameters, and a fatigue life diagram is developed, based on these preceding microstructural considerations. Important features of this diagram are: (i) the plastic strain fatigue limit in the HCF regime which is related to the threshold for cyclic strain localization in persistent slip bands; and (ii) the transition from this plastic strain fatigue limit to a threshold of negligible slip irreversibility at still lower amplitudes in the UHCF regime.