Yoshikazu Nakai

Modelling of small fatigue crack growth interacting with grain boundary

Abstract The slip band at the tip of a small fatigue crack interacting with grain boundaries is modelled for four cases: a slip band not reaching the grain boundary, a slip band blocked by the grain boundary, a slip band propagated into an adjacent grain, and a slip band propagated through one and then blocked by a second grain boundary. The theory for continuously distributed dislocations is used to calculate the crack-tip sliding or opening displacement and the microscopic stress intensity factor under tensile and shear loading. Assuming that the range of the tip displacement directly determines the propagation rate of both Stage I and II cracks, prediction of the propagation behavior of a small crack is made as a function of the distance between the crack tip and the grain boundary, and of the difficulty to propagate slip into adjacent grains, as well as a function of crack length and stress level. The directions for further development of modelling are discussed.

The effects of stress ratio and grain size on near-threshold fatigue crack propagation in low-carbon steel

Abstract The effects of the stress ratio and the grain size on the fatigue crack growth near the threshold in a low carbon steel were analysed based on the crack-closure measurement and the microscopic observations of cracktip slip deformation and the fracture surface. The low-rate region A was divided into regions A1 and A2 in the relation of the rate against the effective stress intensity range. In regions A2 and B, the rate was expressed in a unique power function of the effective range without respect to the stress ratio and the grain size. In region A1 very close to the threshold, the rate was slower for larger grain sized material, and the effective threshold stress intensity factor increased linearly with the square root of the grain size. The slip-band zone in this region was rather independent of the stress intensity and was sized by the grain dimension. A model of the crack-tip slip bands blocked by the grain boundary was confirmed to be useful for analysing very slow growth as well as the threshold condition. The shear-mode fracture surface observed on the surface in region A1suggests the repeated nucleation mechanism for crack growth. The effects of the stress ratio and the grain size on the crack closure behavior near the threshold was quantified.

Plastic deformation around a fatigue crack near threshold in 3%SiFe

Abstract Fatigue crack propagation tests were conducted on 3%SiFe polycrystals at different stress ratios, and the plastic deformation around fatigue cracks revealed by the etch pit technique is discussed quantitatively in combination with the measurement of crack closure. In the high rate region, region B, the maximum plastic zone size was predictable from the fracture mechanics equation where the friction stress was substituted for the monotonic yield stress. Near the threshold, in region A, it tended to be smaller than that given by the equation established in region B. The increase in the crack closure was remarkable in region A. The size of the highly strained region correlated better with the effective stress intensity range than with the total stress intensity range in both regions A and B. The crack propagation rate was expressed as a power function of the highly strained zone size, and the threshold condition corresponded to the constant size of the highly strained region.

Observation of fatigue damage in structural steel by scanning atomic force microscopy

Abstract Fatigue slip bands and plastic deformation around fatigue microcracks were observed by scanning atomic force microscopy. In fatigue slip bands, extrusions were observed but intrusions were not detected. Large extrusions were found in slip bands whose traces at specimen surface were almost perpendicular to the loading axis. Microcracks propagated under mixed mode condition of Mode I, Mode II, and Mode III.

Fracture mechanics approach to fatigue crack initiation from deep notches

Abstract Fracture mechanics approach is applied to fatigue crack initiation at the tips of deep, blunt notches including those with very small notch-tip radius. The theoretical relations between the stress intensity range ΔKρ and the notch-tip radius ρ for a fixed life for crack initiation were derived based on the models of dislocation-dipole accumulation and blocked slip-band. Those are approximated by a simpler equation: ΔK ρ ΔK o = (1 + ρ/ρ0) 1 2 where ΔK0 and ρ0 are material constants which are related to the fatigue strength of smooth specimens Δρ0 as Δρ0 = 2ΔK 0 (πρ0) 1 2 . The results of experiments done with bluntly notched compact tension specimens of a structural low-carbon steel agree with the above relation between ΔK gr ΔK o and ρ/ρo. The method to predict ΔKo, ρo and Δρo from the fatigue data of cracked and smooth specimens is proposed.

Short-crack growth in corrosion fatigue for a high strength steel

Abstract Fatigue crack growth tests of HY130 steel were conducted under either constant-load-range or constant-stress-intensity-range conditions, by using four-point-bend and compact-tension specimens with a crack of length 0.4–41 mm. The tests were conducted in 3.5% NaCl solution under either the freely corroding condition or with the specimen coupled to a sacrificial zinc anode. Crack growth rates for the zinc-coupled case were higher than those for the freely corroding case, for either the long crack or short crack specimen. The growth rates of short cracks were faster than those of long cracks for the same environment. The maximum amount of crack growth acceleration due to crack size effect was about a factor of two for these alloy-environment combinations. With increasing crack length, the rate for short cracks converges to that for long cracks. Since there was no crack-size effect in air, the additional enhancement in the rate of crack growth for short cracks must be chemical in nature. The upper limit of chemically short cracks (as delineated by the point of convergence of rates) was shorter for the freely corroding case than for the zinc-coupled case, and was longer for the higher stress intensity range, for both the constant-load-range and the constant-stress-intensity-range tests. The observed effect of crack size on crack growth rates is discussed in terms of the hydrogen embrittlement mechanisms.

Effects of loading frequency and environment on delamination fatigue crack growth of CFRP

Abstract The delamination fatigue crack growth was investigated with two kinds of unidirectional CF/epoxy laminates. In the T300/3601 laminate, Mode I crack growths in air were cycle dependent, while it was time dependent in water, and the growth rate in water was lower than that in air. Mode II crack propagation rate either in air or in water was also constant with crack extension, and the crack growth was time-dependent. The crack propagation rate in water was faster than that in air, and it increased with prior-immersion period in water. In the M40J/2500 laminate either in air or in water, Mode I delamination fatigue crack growth was cycle-dependent, and the growth rate in water was lower than that in air. Mode II crack propagation rate either in air or in water was almost constant with crack extension, and the crack growth was cycle-dependent. The crack propagation rate in air was almost identical to that in water.

Effects of frequency and temperature on short fatigue crack growth in aqueous environments

The growth of short fatigue cracks in a NiCrMoV steel forging was examined, under constant applied stress intensity range (ΔK = 31 MPa-m1/2) in deaerated deionized water and 0.3 M Na2SO4 solution, as a function of frequency and temperature. Measurements were also made of the kinetics of electrochemical reactions of bare steel surfaces with the deaerated 0.3 M Na2SO4 solution, under free corrosion, to provide for comparison and correlation. Fatigue crack growth rate increased with reductions in frequency and with increases in temperature. The maximum amount of crack growth enhancement by the different environments appeared to be equal, although the crack growth response in deionized water appeared to be consistent with a faster reaction rate. The temperature and frequency dependence for corrosion fatigue crack growth corresponded directly with that for charge transfer between the “bare” and “filmed” metal surfaces under free corrosion. The results showed that shortcrack growth in the aqueous environments is controlled by the rate of electrochemical reactions, and is thermally activated with an apparent activation energy of about 40 kJ/M.

Measurement of short crack lengths by an a.c. potential method

Abstract An alternating current (a.c.) potential technique was developed for the measurement of short cracks in single-edge-cracked specimens. For a constant crack length, the potential difference ( V ) between two probes located at the crack mouth was found to be directly proportional to the square-root of the operating a.c. frequency ( f ), for the higher values of f . The value of V approached that of the d.c. (direct current) system with decreasing frequency. A linear relationship between the values of V and the crack lengths was obtained at each frequency, so that crack lengths can be evaluated by interpolation from the initial and final values of crack length and potential difference. For real-time crack length measurement, a theoretical equation derived for d.c. potential was adapted, with suitable modifications, for use at an operating frequency of 93 Hz.

Initiation and Growth of Pits and Cracks in Corrosion Fatigue for High Strength Aluminium Alloy Observed by Micro Computed-Tomography Using Ultra-Bright Synchrotron Radiation

Measurement of shape and dimension of pits and cracks formed in corrosion fatigue tests of a high-strength aluminium alloy, 7075-T651 were conducted by computed-tomography using ultra bright synchrotron radiation X-ray. Complex corrosion process could be observed and detail shapes of corrosion pits could be identified. Sometimes, corrosion pit and corroded area were formed under passive film, and those were not observed from the surface.