Keiro Tokaji

Effects of humidity on crack initiation mechanism and associated S-N characteristics in very high strength steels

Super-long life fatigue tests were first conducted on Ni–Cr–Mo steel in laboratory air, dry air and distilled water and then performed on high C–Cr bearing steel in controlled humidity for a further understanding of the effect of humidity level. It was indicated that humidity exerted a significant influence on the crack initiation mechanism and associated S–N characteristics in both steels. Based on the experimental results and fractographic analyses of fracture surfaces, the effect of humidity on fatigue life, the humidity dependence of the conventional fatigue limit, the subsurface fracture process, and the presence of the intrinsic fatigue limit are discussed.

The effect of gas nitriding on fatigue behaviour in titanium alloys

Abstract The fatigue behaviour of gas-nitrided Ti6Al4V alloy and Ti15Mo3Al alloy has been studied under rotating bending, and the results obtained were compared with those for annealed or untreated materials on the basis of detailed observations of crack initiation, growth and fracture surfaces. Nitriding was performed using smooth specimens for 4 h and 15 h at 850°C in Ti6Al4V alloy and for 20 h and 60 h at 750°C in Ti15Mo5Zr3Al alloy in pure nitrogen gas. The depths of the nitrided layers obtained were approximately 25 μm and 65μm for the former, and 130 μm and 200 μm for the latter. In Ti6Al4V, the fatigue lives of the material nitrided for 4 h were shorter than those of the corresponding annealed material, but the fatigue limit was increased. However, fatigue strength was reduced by nitriding for 15 h. Similar results were obtained in Ti15Mo5Zr3Al: fatigue strength was decreased by nitriding for 20 h, and the fatigue lives of the material nitrided for 60 h were shorter than those of the annealed material, but the fatigue limit was slightly increased. The reduction in fatigue strength of the nitrided materials in both alloys was primarily attributed to premature crack initiation in the nitrided layer. The role of the nitrided layer in crack initiation is also discussed in terms of results for pure titanium in a previous report.

The effect of grain size on small fatigue crack growth in pure titanium

Abstract The growth behaviour of small fatigue cracks has been studied in both fine- and coarse-grained versions of a pure titanium under axial loading at stress ratio, R, of −1. The growth behaviour and its statistical properties in a coarse-grained version of a different pure titanium have also been investigated under rotating bending (R = −1), and the results obtained were compared with those of a fine-grained version of this titanium in a previous report. Under both loading conditions, small cracks grew faster than large cracks. As the growth data were plotted in terms of the effective stress intensity factor range ΔKeff (after allowing for crack closure, the growth rates could be well correlated with large-crack data in a large-crack regime. In a small-crack regime, however, small cracks still grew faster than large cracks. Small cracks in coarse-grained material showed higher growth rates than those in fine-grained material owing to a much smaller effect of microstructure such as grain boundaries and crack deflection. Stage I facets were observed in all the specimens tested, and their depths were less than the maximum grain size estimated by the statistics of the extreme values, but the distribution of stage I facet depths approximately corresponded to the maximum value distributions of grain size of the materials. The growth rates of small cracks followed log-normal distributions independent of grain size. The coefficients of variation, η, of growth rate in coarse-grained material were smaller than those in fine-grained material. The η values were significantly large at a/d ⩽ 3 (a = crack depth, d = grain size), indicating that the relative size of microstructurally small cracks was not dependent on grain size.

Fatigue crack propagation in spheroidal-graphite cast irons with different microstructures

Abstract Fatigue crack propagation (FCP) and crack closure have been investigated in four spheroidal-graphite cast irons (SGIs) with different microstructures: ferrite, pearlite, bulls eye and aus-ferrite (bainite). The FCP rates plotted against the stress intensity factor range showed a microstructure dependence due to differing contributions of crack closure. Therefore, the effect of microstructure disappeared as crack closure was taken into account, indicating that the intrinsic FCP resistance was the same for all the microstructures studied. Published FCP data on SGIs with a wide variety of microstructures are reviewed, and an overall discussion is developed on the effect of microstructure on FCP behaviour.

The effects of gas nitriding on fatigue behavior in titanium and titanium alloys

Fatigue behavior has been studied on gas-nitrided smooth specimens of commercial pure titanium, an alpha/beta Ti-6Al-4V alloy, and a beta Ti-15Mo-5Zr-3Al alloy under rotating bending, and the obtained results were compared with the fatigue behavior of annealed or untreated specimens. It was found that the role of the nitrided layer on fatigue behavior depended on the strength of the materials. Fatigue strength was increased by nitriding in pure titanium, while it was decreased in the Ti-6Al-4V and Ti-15Mo-5Zr-3Al alloys. Based on detailed observations of fatigue crack initiation, growth, and fracture surfaces, the improvement and the reduction in fatigue strength by nitriding in pure titanium and both alloys were primarily attributed to enhanced crack initiation resistance and to premature crack initiation of the nitrided layer, respectively.

The microstructure dependence of fatigue behaviour in Ti15Mo5Zr3Al alloy

Abstract In order to better understand the fundamental fatigue behaviour of a beta Ti 15Mo 5Zr 3Al alloy, rotating bending fatigue tests have been carried out using materials with three different microstructures, i.e. beta grain sizes, which were solution treated at 735°C (STA735), 850°C (STA850), and 1000°C (STA1000) followed by aging at 500°C. It was found that STA735 showed the highest fatigue strength while the materials solution treated above the beta transus, STA850 and STA1000, had fatigue lives which decreased with increasing solution treatment temperature, i.e. beta grain size. There was no significant difference in fatigue limit between the materials. Fatigue cracks were initiated as a result of cyclic slip deformation in STA735, but the crack initiation site was related to beta grain boundaries in both STA850 and STA1000. In particular, STA850 exhibited an unique initiation behaviour in which cracks were initiated on the specimen surface at higher stress levels, but subsurface crack initiation was observed at lower stress levels. Since STA735 indicated considerably slower crack initiation than STA850 and STA1000 and the growth rates of small cracks were almost the same for all the materials, the excellent fatigue strength of STA735 was primarily attributed to higher crack initiation resistance of its microstructure.

Corrosion fatigue behavior of a steel with sprayed coatings

This paper describes the corrosion fatigue behavior and fracture mechanisms of a steel with different sprayed coatings. Rotating bending fatigue tests were conducted in 3% NaCl solution using specimens of a medium carbon steel with sprayed coatings of a ceramic (Cr2O3), a cermet (WC-12%Co) and two metals (Ni-11 % P and Al-2% Zn). The corrosion fatigue process was basically the same for ceramic, cermet, and Ni-11 % P sprayed specimens. That is, the corrosive media could be supplied from the specimen surface to the substrate through cracks initiated during fatigue cycling and/or pores in the coatings, and thus corrosion pits were generated followed by subsequent crack initiation and growth in the substrate. The corrosion fatigue strength of ceramic sprayed specimens was slightly improved compared to that of the substrate steel because the under-coating (Ni-5%A1) could impede the penetration of the corrosive media although the ceramic coating had a poor resistance to cracking under cyclic loading. Cermet sprayed specimens also exhibited improved corrosion fatigue strength because of the high resistance to cracking and the low volume fraction of pores of the coating. In Ni-11 % Psprayed specimens, cracks were initiated in the coating even at low stress levels; thus the corrosion fatigue strength was the same as that of the substrate. Anodic dissolution took place in Al-2 % Zn coating because the coating was electrochemically poor, and thus the substrate was cathodically protected. Therefore, the corrosion fatigue strength of Al-2 % Zn sprayed specimens was enhanced to as high as the fatigue strength of the substrate in room air. Based on the experimental results, a dual-layer coating consisting of WC-12%Co and Al-2%Zn was fatigue tested. The coating was effective at low stress levels and exhibited long life under conditions where corrosion fatigue strength was critical.

Influence of lamellar orientation on fatigue crack propagation behavior in titanium aluminide TiAl

Abstract Fatigue crack propagation (FCP) behavior of a titanium aluminide (TiAl) with a nearly fully lamellar microstructure has been studied on two different FCP directions relative to the lamellar orientation; i.e. parallel (type A specimen) and perpendicular (type B specimen) to the lamellar orientation, at ambient temperature in laboratory air. It was found that the FCP resistance of the former was considerably lower than that of the latter. Close examinations of crack morphology revealed significant differences between the two FCP directions. In type A specimens, several cracks along lamellae were seen on the surfaces and sections of the specimens, thus uncracked ligaments were formed in the wake of the crack tip. On the contrary, such ligaments were scarcely produced in type B specimens because only the main crack could propagate without remarkable deflections and branching. The FCP rates of type A specimens were decreased gradually with crack extension under constant stress intensity factor range, Δ K , tests, suggesting the role of crack bridging by uncracked ligaments. Finite element method (FEM) analysis indicated considerably reduced Δ K experienced at the crack tip, thus the difference in FCP resistance between two FCP directions based on the actual Δ K at the crack tip after allowing for crack bridging became much larger than that based on the nominal or applied Δ K .

Particle size dependence of fatigue crack propagation in SiC particulate-reinforced aluminium alloy composites

Fatigue crack propagation (FCP) and fracture mechanisms have been studied for two orientations in powder metallurgy 2024 aluminium alloy matrix composites reinforced with three different sizes of silicon carbide particles. Particular attention has been paid to make a better understanding for the mechanistic role of particle size. The FCP rates of the composites decreased with increasing particle size regardless of orientation and were slightly faster in the FCP direction parallel to the extrusion direction. After allowing for crack closure, the differences in FCP rate among the composites and between two orientations were significantly diminished, but the composites showed lower FCP rates than the corresponding unreinforced alloy. Fracture surface roughness was found to be more remarkable with increasing particle size and in the FCP direction perpendicular to the extrusion direction. Taking into account the difference in the modulus of elasticity in addition to crack closure, the differences in FCP rate between the unreinforced alloy and the composites were almost eliminated.

Coaxing Effect in Stainless Steels and High-Strength Steels

The effects of prestrain and strength level on the coaxing behavior were studied in austenitic stainless steels and high strength steels, respectively. The materials used were austenitic stainless steels, SUS304 and SUS316, and high strength steels, SCM435, SNCM439 and SUJ2. Stress incremental fatigue tests were performed using cantilever-type rotating bending fatigue testing machines. It was found that the steels except for SUJ2 showed a marked coaxing effect. Non-propagating cracks were not detected in all the steels examined. Based on hardness test, X-ray diffraction measurement and EBSD analysis, it was indicated that the coaxing effect occurred due to work hardening and strain-induced martensite transformation in austenitic stainless steels and to strain-aging in high strength steels.