To reveal influence factors on the very high cycle fatigue properties of Ti-6Al-4V alloy, uni-axial fatigue tests and fatigue crack propagation tests were conducted. In the uni-axial fatigue tests, specimens with artificial defects of several sizes were used. Both tests were performed in air and vacuum environments. The effects of environments around fatigue cracks on crack propagation properties were investigated. As a results, fatigue crack propagation rates in vacuum were lower than those in air, and the results correspond to the fact that the sub-surface fractures occur in longer life regime than surface fractures. The threshold stress intensity factor range showed size dependencies, in addition, the difference of environments affected the size dependencies. The dependence in vacuum was stronger than in air. The result indicates that a small crack (size of a crack origin) can propagate in lower stress intensity factor ranges in vacuum, and this can relate to the reason why the sub-surface fractures occur in lower stress levels than the surface fractures. Fracture surface observations revealed that fine convex-concave granular pattern (Granular region) was formed around the origins of surface fractures in the vacuum environment, and the aspect resembled the fracture surface of sub-surface fractures. The result showed that the crack propagation mechanism in vacuum and sub-surface of materials were similar. The effects of vacuum environment are almost same as those of environment around the sub-surface crack. The effects of environments around fatigue cracks can well explain the very high cycle fatigue properties of the material.
WC-12Co was thermally sprayed by a high velocity oxygen fuel (HVOF) method on A5052 aluminum alloy as an interlayer with the thicknesses of 25, 70 and 120μm and subsequently diamond-like carbon (DLC) film was deposited with the thickness of 15μm to fabricate DLC/WC-12Co hybrid coating. Rotary bending fatigue tests were conducted using the specimens with DLC/WC-12Co hybrid coating, DLC single coating, WC-12Co single coating and without coating, and the thickness effect of interlayer on the fatigue behavior was investigated. The fatigue strengths of the specimens with DLC single coating and WC-12Co single coating were higher than those of the substrate specimens. The fatigue strengths in the specimens with WC-12Co single coating increased with increasing the thickness of WC-12Co layer. The specimens with hybrid coating exhibited higher fatigue strengths than the specimens with WC-12Co single coating when the thickness of WC-12Co layer was the same. However the thicker WC-12Co interlayer led to the lower improvement of fatigue strength by hybrid coating. In the specimens with WC-12Co layer, the boundary between WC-12Co layer and substrate was rough and uneven due to the thermal spray process and a fatigue crack initiated at the concave site of substrate on the boundary. In the specimens with hybrid coating, cracking in DLC film and WC-12Co interlayer occurred simultaneously and suddenly near the end of fatigue life. A fatigue crack in the substrate had grown up to a large size and the specimen fractured soon after a crack appeared on the surface.
The effects of the cyclic prestrain on the fatigue behavior in type 304 austenitic stainless steel were investigated. Rotating bending fatigue tests have been performed in laboratory air using the specimens subjected to ±5% cyclic prestrain at room temperature (R.T.) and -5°C. Martensitic phase volume fraction of the prestrained specimen at -5°C was 48% and larger than 3.8% at R.T. The prestrained specimens exhibited higher fatigue strengths than the as-received ones, and larger volume fraction of martensitic phase resulted in the higher fatigue limit. EBSD analysis revealed that the martensitic phases were more uniformly distributed in the austenitic matrix in the cyclically-prestrained specimens than in the monotonically-prestrained ones. Fatigue crack initiation from inclusion was observed only in the cyclically-prestrained specimens at -5°C. High volume fraction and uniform distribution of martensitic phase induced the transition of crack initiation mechanism and led to the higher fatigue limit. In type 304 stainless steel with high volume fraction of strain-induced martensitic phase, the prediction of fatigue limit based on Vickers hardness could give unconservative results.
Changes in the surface roughness of SUS316NG during cyclic loadings were investigated, and the relationship between those changes and the fatigue damage processes, including evolution of surface relief due to active slip systems, were discussed on the basis of microscopic observations. Strain-controlled fatigue tests were conducted at three constant strain ranges Δε = 8, 4, and 1%. During the tests, surface roughness was measured periodically at cycles determined with respect to the usage factor UF, and then the surface was observed directly to clarify the surface morphology. As a result, until the middle of fatigue life, the arithmetic mean roughness Ra increased linearly with the number of cycles regardless of the strain range conditions. Clear changes in surface roughness were obtained even at a strain range as small as 1%. The results suggest that the surface roughness measurement can probably be used to assess fatigue damage because it increases linearly with the number of cycles until the middle of fatigue life. The rate at which surface roughness increased with UF became smaller with decreasing applied strain range. In the damage process, the surface became uneven because of active slip systems and the asperity became larger when the number of cycles increased. Surface observations revealed that the number of slip bands decreased and the development of the asperity became more difficult with decreasing applied strain range. The change tendencies of surface roughness were similar to those of the density of slip bands at each strain range. This shows that the rate at which surface roughness increased with UF is affected by the number of slip bands at each strain range condition.
The effect of aging condition on fatigue properties and the mechanism of decrease in fatigue strength in high humidity in 18% Ni maraging steel of grade 350 were investigated under rotating bending in relative humidity of 25% and 85%. Aging conditions investigated were under-, peak- and over-aging ones at the conventional aging temperature of 753K. In addition, double-aging treatments which were under- peak- and over-aging treatments at 673K and under-aging one at 473K after the peak-aging at 753K were also examined. Both of static and fatigue strengths were increased by the double-aging without any decrease in ductility and fatigue fracture toughness. Fatigue strength was markedly decreased by high humidity in all of the steels, and the decrease in fatigue strength was mainly caused by the accelerations of crack initiation and its growth at the early stage of fatigue process. The decrease in fatigue strength in high humidity was also suppressed by the double aging. A few facets comparable to a grain size of a prior austenite were observed at the fracture origins in high humidity but transgranular cracks by slip deformation were in low humidity. However most of the fracture surfaces were covered with lath boundary cracking regardless of the humidity and aging conditions. River pattern was observed in the facets, suggesting that the acceleration of crack growth in high humidity was a behavior related to hydrogen generated in cathode reaction. Based on the results, new aging treatment for improvement of fatigue properties of maraging steel was proposed.
For application of the high-strength steel plate to automobile chassis parts, it is important to reserve the fatigue reliability. In the manufacturing of automotive parts from a steel sheet, the shear cutting process with a die is commonly used. Although the process is economic, it has a drawback that fatigue cracks may initiate from the shearing edges. Therefore, in terms of the reliability of parts, it is necessary to evaluate the influence of shearing edges on the fatigue strength. In this paper, fatigue experiments are conducted on the specimen of high-strength steel sheet with the shearing edge and the influence factors such as the residual stress and surface roughness are examined. As a result, the experiments elucidate that the residual stress has a major influence. The fatigue limit of the shearing edge can be estimated using the residual stress re-distributed by fatigue and fatigue limit diagram, and a method for predicting the S-N curve of shear cutting surface is proposed on the basis of the estimation.
In this paper, fatigue tests and finite element analysis are carried out on spot weld-bonded joints of mild steel (270MPa class) and ultra-high strength steel (980MPa class) in order to investigate influence of strength level of base steels on fatigue strength and fatigue fracture behavior of spot weld-bonded joints. From the fatigue tests and finite element analysis, the following results are obtained : (1) The fatigue strength of the spot weld-bonded specimen is higher than that of the spot welded specimen. (2) The fatigue limit of the spot weld-bonded specimens of the ultra-high strength steel is higher than that of the mild steel. (3) The interfacial debonding propagates from the adhesive edge to a nugget edge, and the fatigue crack initiates at the nugget edge in both steels. (4) The fatigue strength of spot weld-bonded specimens is improved because the stress concentration at the nugget edge is reduced by adhesive bonding during large part of fatigue life.
As the candidate for self-morphing elements and structures, the bending of composite beams including shape memory alloy fibers or plate is examined in terms of simple beam theory. The constitutive relation describing the internal stress/strain arising from the shape memory effect of the fibers is the key point of this design; this study provides the method of dealing with the stress and strain as the initial strain or the initial stress on the basis of the experimental evidences in literatures. This simplified method has advantages in deriving the geometry and statistics of the composite beam in explicit forms useful for designing purposes. The limit of the deflection of the composite beam is determined by the yield strength of the matrix: steel, aluminum and epoxy are considered.