Internal Fracture Mode Research Articles

쇼트피닝 처리를 한 베어링강의 고사이클 피로수명 및 파괴거동

Shot-peening effects on the fatigue behavior of bearing steel were investigated under the high cyclic loading. Hourglass shape specimens were made of bearing steel(JIS-SUJ2) for rotary bending fatigue tests. Two kinds of treatments were performed : a heat treatment and a shot-peened surface treatment after the heat treatment. The fracture surfaces of specimens were classified into two types of fracture mode : the surface fracture mode induced by a surface defect and the internal fracture mode induced by a nonmetallic inclusion. Inclusion depth and shape affected considerably the fatigue life. Shot-peening treatment improved much the fatigue life of the bearing steel under low and high levels of cyclic loads. Probabilistic-stress-life (P-S-N) curves were suggested for the reliable fatigue life estimation of the improved bearing steel.

Effect of Tempering Temperature on Very High Cycle Fatigue Behavior in Low-Alloy Steel, SNCM439

In order to investigate the influence of the tempering temperature on a S-N characteristic in very high cycle fatigue regime, cantilever-type rotary bending fatigue tests up to 109 cycles were performed in air at room temperature using hour-glass shaped specimens heat treated at four kinds of tempering temperature in low-alloy steel, JIS SNCM439. Specimens tempered at low temperature of 433K and 573K showed a step-wise S-N curve, and were ruptured by internal fracture mode at high cycle fatigue regime above 106cycles. On the other hand, surface fracture mode only appeared on the specimens tempered at high temperature of 773K and 893K, and the fatigue limit existed clearly. Granular bright facet (GBF) area was observed in the vicinity of a non-metallic inclusion which was origin of an internal fracture. Properties of the GBF area were verified from the experimental facts that the roughness of the GBF area was larger than that outside the GBF in a fish-eye, the size and the distribution of convex particles on the GBF area were same as those of spherical carbide particles in the matrix, and a high density of carbon was detected on the GBF area by EPMA. It was suggested that a decohesion of spherical carbide from the matrix for the formation of the GBF area was affected by the size of the carbide which became large with decreasing the tempering temperature.

Gigacycle Fatigue Behavior and Fracture Morphology of High Speed Tool Steel, JIS SKH 51.

Gigacycle fatigue characteristics of high speed tool steel, JIS SKH 51, was investigated under cantilever-type rotary bending fatigue tests in an open environment at room temperature. As a result of fatigue tests, S-N curve was clearly classified into two types of fracture modes by the different crack origins. One fracture mode occurred at the surface inclusion in the region of short fatigue life and high stress amplitude level. The other was at the subsurface inclusion in the region of long fatigue life and low stress amplitude level. The transition of crack initiation site from surface to subsurface appeared at about 2×104 cycles and the stress amplitude for the transition was affected by the tempering temperature of the specimen. On the other hand, difference on fatigue life for internal fracture mode between specimens treated by different tempering temperature did not observed. Fatigue fracture mechanisms were discussed through the detailed observation of subsurface crack initiation site.

Very high‐cycle fatigue behaviour of shot‐peened high‐carbon–chromium bearing steel

ABSTRACTEffect of shot‐peening on fatigue behaviour in the gigacycle regime was investigated in order to clarify the duplex S–N curve characteristics. Cantilever‐type rotary bending fatigue tests were performed in laboratory air at room temperature by using hourglass‐shaped specimens of high‐carbon–chromium bearing steel, JIS SUJ2. Fatigue crack initiation site changed from the surface of untreated specimen to the subsurface of the specimen because of hardening and compressive residual stress with shot‐peening in the region of high‐stress amplitude. On the other hand, no difference in fatigue life controlled by the subsurface crack initiation between untreated specimen and shot‐peening one was observed in high‐cycle region. It was suggested that the S–N curve corresponding to the internal fracture mode is inherent in the material, as compared with the S–N curve of surface fracture mode, which is affected by surface conditions, environmental conditions and so on. Subsurface crack initiation and propagation behaviour were discussed under the detailed measurement of crack initiation area and shape of the fish‐eye fracture surface.

S–N curve characteristics and subsurface crack initiation behaviour in ultra‐long life fatigue of a high carbon‐chromium bearing steel

The S–N curve obtained from cantilever‐type rotary bending fatigue tests using hour‐glass‐shaped specimens of high carbon‐chromium bearing steel clearly distinguished the fracture modes into two groups each having a different crack origin. One was governed by crystal slip on the specimen surface, which occurred in the region of short fatigue life and a high stress amplitude level. The other was governed by a non‐metallic inclusion at a subsurface level which occurred in the region of long fatigue life and low stress amplitude. The inclusion developed a fish‐eye fracture mode that was distributed over a wide range of stress amplitude not only below the fatigue limit defined as the threshold for fracture due to the surface slip mode but also above the fatigue limit. This remarkable shape of the S–N curve was different from the step‐wise one reported in previous literature and is characterized as a duplex S–N curve composed of two different S–N curves corresponding to the respective fracture modes. From detailed observations of the fracture surface and the fatigue crack origin, the mechanisms for the internal fracture mode and the characteristics of the S–N curve are discussed.

Fatigue strength and fracture mechanism of steel modified by super-rapid induction heating and quenching

Four kinds of surface hardened-specimens (ordinary structural steel with carbon content of 0.45% C) having hardened thicknesses of 0.7–1.8 mm were prepared using a ‘super-rapid induction heating (SRIH) system’. Rotation bending fatigue tests were performed with special focus on the effect of a hardened thickness on fatigue properties. Measurement of residual stress and observation of the fracture surface were also carried out to investigate the fracture mechanism of the specimen with a shallow hardened layer. It was found that there is not much improvement of fatigue strength at 10 7 cycles for specimens with shallow hardened layers in spite of having a high compressive residual stress of about 1000 MPa. This is because the fatigue crack originating from inside the hardened layer leads to the final fracture of the specimen (internal fracture mode). Improvement of fatigue strength has been achieved on the specimen with thick hardened layers, such as those about 1.8 mm thick. In this case, fatigue cracks originate from inclusions located in hardened layers, which leads to final fracture (hardened-layer fracture mode).

Fatigue Strength and Fracture Mode of Superrapid Induction Hardened Steel with Shallow Hardened Layer.

Four kinds of surface hardened specimens having hardened thicknesses of 0.7∼1.8mm were prepared using a"Micropulse induction heating system". Rotation bending fatigue tests were performed with special focus on the effect of hardened thickness on fatigue properties. Measurement of residual stress and observation of the fracture surface were also carried out to investigate the fracture mechanism of the specimen with a shallow hardened layer. It was found that there is not much improvement of fatigue strength at 107 cycles for specimens with shallow hardened layers in spite of having a high compressive residual stress of about 1000MPa. This is because the fatigue cracks originating from inside the hardened layer leads to final fracture of the specimen (internal fracture mode). Improvement of fatigue strength has been achieved by forming thick hardened layers. In this case, fatigue cracks originate from inclusions located in hardened layers, which leads to the final fracture (hardened layer fracture mode).

The Effect of Fracture Mechanism on Low-Cycle Fatigue Life of Pure Iron at Low Temperature.

Low-cycle fatigue tests were performed at -140°C using commercial-grade pure iron having an average ferrite grain size of 400 μm. The effect of fracture mechanism on fatigue life properties was investigated, especially on the dispersion of fatigue lives in the high plastic strain range (Δεp) where fracture occurs in an internal fracture mode. Fracture surfaces were observed using a scanning electron microscope. The dispersion of fatigue lives is due mainly to the difference of internal fracting mode. Specimens fractured by intersection of a deformation twin and a grain boundary (T-B type) show longer fatigue lives than those fractured by intersection of deformation twins (T-T type) or a deformation twin and an inclusion (T-I type). The effect of grain size at the internal fracture origination site was also discussed.

進行性引張り塑性ひずみを伴う塑性疲労特性と表面き裂進展挙動

Plastic strain-controlled low cycle fatigue tests and cyclic tension (C.T.) tests with various plastic strain ratio Re (Re=Δem/Δep: Δem being a tensile plastic strain per cycle and Δep a controlled plastic strain range) were carried out on rod specimens with a small hole which were machined from austenitic stainless steel 316L. The effects of the Re value on fatigue deformation properties, final fracture mode and surface crack growth behavior were investigated from the C.T. test results.In the C.T. tests (Re=0-0.667), the stress amplitude in the cyclic work hardening curve showed a notable decrease after a peak stress, and a cyclic strain exponent n' in the cyclic stress-strain curve was lower (about n'=0.17) than that of a constant amplitude test (n'=0.50). In the case of the low Re value (Re<0.25), the specimen was fractured with the surface fracture mode by fatigue, while in the high Re value (Re≥0.25) it was fractured with the internal fracture mode by the effect of accumulative tensile deformation, and the estimated Δep-Nf relation from the linear accumulation of the fatigue damage Df and the tensile damage Dt indicated fairly good agreement with the experimental data for the range of these C.T. tests. Moreover, in the range of the low Re value, the surface crack growth rate da/dN was evaluated successfully by plotting against the parameter of modified J integral range ΔJ*(ΔJ*=ΔJ/(1-Re)n, where n is a constant), after considering the effect of Re value.

Effect of repeated annealing on the fatigue life in extremely low cycle fatigue.

The effect of repeated annealing during course of a fatigue test on a fatigue behavior was studied on two kinds of low-carbon steel having fine- and coarse-grained microstructures, paying special attention to relationship between fracture modes and fatigue life in extremely low-cycle fatigue regime (Nf≤102). Results show that in a coarse-grained steel, (i) repeated annealing has beneficial effect of repairing damage inside material; (ii) in this case, final fracture occurs in a surface fracture mode instead of internal fracture mode which normally appears in extremely low cycle fatigue regime. Such a change occurs after strain cycles obtained by extrapolation from fatigue life data for a surface fracture mode at low plastic strain range; and (iii) these findings can be clearly explained by concept of the competition of two failure limit lines which correspond to each fracture mode.

Grain size effect in the low cycle fatigue of a steel under mean strain.

Low cycle fatigue tests and small crack propagation tests were conducted on two kinds of annealed low carbon steel having fine and coarse grained microstructures with a special interest in the influence of grain refinement and mean strain on the low cycle fatigue life and on the fracture mode transition. Results show that (i) a difference in the fatigue lives for the fine and coarse grained steels under mean strain e=0 is caused by a difference in a crack initiation life for both materials, (ii) a surface to internal fracture mode transition occurs easily with an increase in mean strain and grain size and, (iii) the fatigue life of the specimen failed in the surface fracture mode at a small plastic strain is not primarily controlled by the exhaustion of ductility. In such a regime, the characteristics of fatigue life under mean strain for both materials can be predicted by the calculation of a low cycle fatigue life on the basis of the law of small crack growth.