Maximum Crack Growth Rate Research Articles

Modeling of I + II mixed mode crack initiation and growth from the notch

The fatigue crack initiation and growth from the notches under combined loading are modeled by the finite element method. The compact tension shear (CTS) specimens of Q345R steel are used to study the I+II mixed mode crack growth behavior. To consider the effect of the residual stress and loading history induced by fatigue crack growth, the dynamic crack propagation model is employed in the finite element simulation. A multiaxial fatigue damage criterion is employed and an Armstrong–Frederick type cyclic plasticity model was inputted as a UMAT to describe the material behavior. The fatigue crack initiation and growth from the notch root of the CTS specimen is determined by the stabilized stress–strain responses of the material points near the notch root and the fatigue damage of different material planes. The material point with maximum fatigue damage corresponds to the fatigue initiation position, and the crack growth orientation from the initiation position is identical to the material plane on which maximum fatigue damage arises. The fatigue crack propagates on the material plane with maximum crack growth rate. The results of prediction are in excellent agreement with the experimental observations in terms of crack initiation orientation, crack growth rate and crack growth path.

Electrochemical investigation of oxide films formed on nickel alloys 182, 600 and 52 in high temperature water

Nickel-based alloys 182, 600 and 52 were exposed to simulated Pressurized Water Reactor (PWR) primary water (1000 ppm B, 2 ppm Li, O 2 <10 ppb, 325 °C) under different dissolved hydrogen (DH) conditions [0, 2, 25 and 50 cm 3 H 2(STP) kg −1] for times up to 1 month in a recirculating autoclave. The influence of exposure time and DH on oxide films formed on the alloys was evaluated by means of electrochemical tests; electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M–S). The in situ EIS was performed every day, allowing the monitoring of the oxide layer formation and change. M–S was performed at room temperature after the full exposure time. The results showed that the maximum in the defect concentrations obtained via M–S analysis and the maximum crack growth rate are at the same DH content, thus relating electrochemical testing to stress corrosion cracking observations. A conceptual separation between the electrochemical behavior of (1) the oxide layer (visible in the higher frequencies of EIS) and (2) the oxide layer – solution interface (visible in the lower frequencies of EIS) was able to explain the effect of hydrogen on the low-frequency EIS impedance results.

Short-crack-growth-based fatigue assessment of notched components under multiaxial variable amplitude loading

A short crack model originally proposed for multiaxial constant amplitude loading is extended and applied to multiaxial variable amplitude loading. Load sequences have a significant influence on variable amplitude life; they are taken into account using algorithms originally proposed only for uniaxial loading. The estimated fatigue lives of unnotched tubular specimens and notched shafts under different in- and out-of-phase multiaxial constant and variable amplitude load histories are compared with the experimental results. The comparison reveals that the proposed short crack approach enables sufficiently accurate estimation. Moreover, the estimated critical planes, i.e., the planes of maximum crack growth rate or minimum life, are in good agreement with the experimental observations.

Fatigue Crack Propagation Under Complex Loading

The paper describes the recent progress of the understanding of fatigue crack propagation under complex loading. The behavior of fatigue crack propagation is dependent on the macroscopic mode of crack growth : tensile mode and shear mode growth. Tensile mode growth takes place on the plane perpendicular to the maximum tangential stress range, satisfying the local symmetry. Shear mode growth takes place on the plane of the maximum shear stress when a precrack is subjected to high cyclic shear stress. When the shear stress is low, a bent or branched crack is formed at the precrack tip, showing tensile crack growth. Under an intermediate stress, a crack first shows shear mode growth and turns to tensile mode after some extension. This transition is caused by the decrease in the mode II crack-tip stress intensity factor due to increasing crack surface contact. The criterion of the maximum crack growth rate may control the mode of fatigue crack growth. In notched components under complex loading, the life of crack initiation at the notch root is calculated from the critical plane model using the stress and strain at a critical distance from the notch root. The propagation of a fatigue crack from a notch is dependent on the geometry of notched component. The resistance-curve method is applicable for the prediction of fatigue thresholds of thin walled tubular specimens subjected to in-phase and out-of-phase combination of cyclic torsion and tension-compression.

Experimental determination and modelling of long crack crack-face interference-free crack propagation behaviour in normalized SAE 1045 steel

Crack-face interference-free crack growth rate data were obtained from special intermittent overload tests on axial-torsion specimens. Data obtained from these tests and short crack data taken from the literature fell into a single scatter band. These data were used in a short crack growth model to predict the crack length at the changeover from a shear mode to a tensile mode crack plane and the fatigue life for axial-torsional fatigue tests that again used these special overload sequences to achieve crack closure-free/crack-face interference-free crack growth. Model predictions based on two separate criteria for mode change – maximum crack growth rate and maximum strain energy release rate – each predicted reasonably well both the crack length at the change in crack plane and the fatigue life.

A study of dynamic embrittlement in bicrystals of Cu-7%Sn

In a kinetic model [1] for the phenomenon of dynamic embrittlement, the cracking rate is predicted to be proportional to the diffusivity of the embrittling species along the grain boundary. To test this model, bicrystals of Cu-Sn with Σ5 symmetrical tilt boundaries in which tin is the embrittling element are being used. The diffusivities parallel and perpendicular to the tilt axis are expected to be different, therefore, the crack growth rates in these two directions should vary in the same ratio as the diffusivities. Measurements of the crack-growth rate along the [100] direction in the Cu-Sn alloy bicrystal at 265 °C showed that the cracking occurred by decohesion along the grain boundary with almost no observable plasticity at a maximum crack growth rate of approximately 10 −6 m s −1. A model for steady-state crack growth has recently been developed and is discussed here.

Crack growth in the process of unstable brittle fracture

We suggest a model of the transition of a crack from stable to unstable propagation with subsequent growth in the supercritical state. By using the energy balance equation for propagating cracks, we estimate the specific surface energy and crack growth rate under the conditions of unstable propagation. Unstable propagation starts with a rate which is lower than the maximum crack growth rate in the material. The specific surface energy and kinetics of the crack undergoing unstable fracture depend both on the maximum crack growth rate in the material and on the initial rate υ0 of its unstable propagation. The rate υ0 is largely determined by the critical size of the crack (critical stresses).

Environmentally assisted crack growth in a low alloy boiler steel in high temperature water containing oxygen

The crack growth behaviour of the higher strength steel 17 MnMoV 6 4 in deionized high temperature water containing oxygen was investigated with respect to static loading. The tests were performed in an autoclave testing facility at an oxygen content of 8 ppm, a pressure of 70 bar and a temperature of 240°C under quasi-stagnant flow conditions. The stress intensities at the beginning of the tests were 17, 20, 27, 34, 40 and 58 MPa√m. In contrast to the higher loaded specimens no evidence of environmentally assisted cracking was found at stress intensities of 17 and 20 MPa√m. The maximum crack growth rate in the range where only environmetally assisted cracking occured amounted to about 4 x 10 -5 mm/s. The critical stress intensity K IJ for the onset of stable ductile crack growth in air is 100 MPa√m. On the basis of fractographic studies the crack development found can be ascribed to the “Strain Induced Corrosion Cracking (SICC)” mechanism. This mechanism, used as a working hypothesis, gives a satisfactory explanation for the occurence of stress corrosion cracking of unalloyed and low alloyed steels in high temperature water. SICC is particularly characterized by aggravated corrosive attack occuring as soon as the magnetite/haematite protective layer has been locally disturbed. The stress concentration then just becomes so great that in the region of the resulting crack tips, yield/creep deformation within the critical range of strain rates occurs.

Influence of environmental variables and intermittent overloadings on fatigue crack growth in on-board NGV cylinder steel

Fatigue crack growth rates in the on-board Compressed Natural Gas (CNG) steel cylinder (AISI 4130X grade) were obtained in air and in the simulated natural gas environment. Effects of loading frequency (1·0, 0·1 and 0·01 Hz) on corrosion fatigue crack growth rates were investigated. The maximum crack growth rate in the modified solution was about 60 times the rate in air. A maximum corrosion-assisted cracking rate of 2·0 × 10 −7 m/s was obtained. Retardation of fatigue crack growth was observed after the overloadings, equivalent to the present requalification tests of the on-board steel CNG cylinders. Ratio of the experimentally determined overload affected zone to the cyclic plane strain plastic zone varied from 0·85 to 1·91.

Stress corrosion cracking and corrosion fatigue of nuclear reactor pressure vessel steels in hot water

Stress corrosion cracking of quenched and tempered steels in hot water is a phenomenon which has only recently been observed in steels with yield strengths from 230 to 1000 MN/m2. In nuclear reactor pressure vessel (RPV) steels with about 430 MN/m2 yield strength, SCC is transgranular and has a thresholdK,Iscc of about 20 Mn. m−3/2. The maximum crack growth rate is about 7 x 10−8 m/s at 288° C. This threshold and this crack growth rate are identical to the threshold and maximum crack growth rate derived for the time-based component of corrosion fatigue crack growth observed in reactor pressure vessel steels exposed to nuclear reactor coolant at 288° C. The conclusion is drawn that corrosion fatigue crack growth under these conditions can be understood as the superposition of stress corrosion cracking and fatigue crack growth.

Fatigue crack growth behavior under cyclic transient thermal stress.

Thermal fatigue tests were performed using straight pipe specimens subjected to cyclic thermal shocks of liquid sodium, and crack growth behaviors were estimated using striation patterns observed clearly on any crack surface. Crack growth rate under cyclic thermal strain reaches the maximum at one depth, and after that it decreases gradually with crack depth. The peak location of crack growth rate becomes deeper by superposition of constant primary stress. Parallel cracks co-existing in the neighborhood move the peak to shallower location and decrease the maximum crack growth rate. The equivalent stress intensity factor range calculated by Walker's formula is successfully applied to the case of negative stress ratio. Fatigue crack growth rate under cyclic thermal strain agreed well with that under the constant temperature equal to the maximum value in the thermal cycle. Simplified methods for calculating the stress intensity factor and the crack interference factor have been developed. Crack growth behavior under thermal fatigue could be well predicted using numerical analysis results.

Fatigue crack growth behavior under cyclic thermal transient stress.

Thermal fatigue tests were performed using straight pipe specimens subjected to cyclic thermal shocks of liquid sodium, and crack growth behaviors were estimated using striation patterns observed clearly on any crack surface. Crack growth rate under cyclic thermal strain reaches the maximum at one depth, and after that it decreases gradually with crack depth. The peak location of crack growth rate becomes deeper by superposition of constant primary stress. Parallel cracks co-existing in the neighborhood move the peak to shallower location and decrease the maximum crack growth rate. The equivalent stress intensity factor range calculated by Walker's formula is successfully applied to the case of negative stress ratio. Fatigue crack growth rate under cyclic thermal strain agreed well with that under the constant temperature equal to the maximum value in the thermal cycle. Simplifed methods for calculating the stress intensity factor and the crack interference factor have been developed. Crack growth behavior under thermal fatigue could be well predicted using numerical analysis results.

Effect of Nitrogen and Phosphorus on the Stress Corrosion Cracking of Austenitic Steels in MgCl2Solutions

The influence of phosphorus and nitrogen on the stress corrosion cracking behaviour of austenitic stainless steels similar to Type 316 (but with variations in nickel and molybdenum contents) has been investigated. Nitrogen additions decreased the critical stress for stress corrosion crack (SCC) nucleation but did not increase the maximum crack growth rate in Type 316 tested in MgCl2 at 154°c. In a molybdenum-free steel, phosphorus additions were far more deleterious than nitrogen additions and gave increased cracking rates and nucleation frequencies.The stacking fault energy was only decreased a little by the phosphorus addition and nitrogen tended to increase the stacking fault energy, so that changes in SCC cannot be attributed simply to stacking fault energy changes. An ordered carbide type structure was observed in a slow-cooled specimen containing a very high nitrogen content. Only slight freckling contrast and no precipitate diffractions were observed in the range of nitrogen contents usedfor the SCC tests. The strengthening by nitrogen and the slip planarity observed were similar from 13 wt% nickel to 30 wt% nickel. However no cracking was observed in the alloy with 30% nickel. Whilst the change in dislocation behaviour may account for some increase in SCC susceptibility at 13 and 20 wt% nickel, slip planarity is not sufficient to produce crack growth at high nickel contents.The phosphorus addition gave little hardening and little increase in slip planarity. It does, however, have a marked effect on the slip step oxide growth characteristics, producing tunnels rather than slots. It seems, therefore, that the effect of phosphorus is mainly a chemical one, that is, on the rate of dissolution or on the nature of the oxide film, rather than on the dislocation behaviour.

Fracture mechanics and surface chemistry studies of fatigue crack growth in an aluminum alloy

Fracture mechanics and surface chemistry studies were carried out to develop further understanding of the influence of water vapor on fatigue crack growth in aluminum alloys. The room temperature fatigue crack growth response was determined for 2219-T851 aluminum alloy exposed to water vapor at pressures from 1 to 30 Pa over a range of stress intensity factors (K). Data were also obtained in vacuum (at < 0.50 μPa), and dehumidified argon. The test results showed that, at a frequency of 5 Hz, the rate of crack growth is essentially unaffected by water vapor until a threshold pressure is reached. Above this threshold, the rates increased, reaching a maximum within one order of magnitude increase in vapor pressure. This maximum crack growth rate is equal to that obtained in air (40 to 60 pct relative humidity), distilled water and 3.5 pct NaCl solution on the same material. Parallel studies of the reactions of water vapor with fresh alloy surfaces (produced either byin situ impact fracture or by ion etching) were made by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The extent of surface reaction was monitored by changes in the oxygen AES and XPS signals. Correlation between the fatigue crack growth response and the surface reaction kinetics has been made, and is consistent with a transport-limited model for crack growth. The results also suggest that enhancement of fatigue crack growth by water vapor in the aluminum alloys occurs through a “hydrogen embrittle ment” mechanism.

インコネル600の85&amp;deg;C, 50%NaOH溶液中における腐食疲労き裂伝播におよぼす電位の影響

A study has been made of corrosion fatigue crack growth for Inconel 600 solution-annealed at 1100°C for 0.5 h in 85°C 50%NaOH solution with a frequency of 1.1 cycle per minute. The effect of potential on the corrosion fatigue crack growth rate (da⁄dN) as a function of ΔK was examined and the fracture surfaces were observed. The results obtained are as follows:(1) The crack growth rate and the cracking mode were affected by the applied potentials in the anodic polarization curve. The value of da⁄dN was arranged in the following order of the observed potentials: secondary passive region>corrosion potential>primary active region>primary passive region.(2) Intergranular cracking took place at a secondary passive potential (−0.04 V vs SCE) which gave a maximum crack growth rate. Transgranular cracking with fatigue striations occurred at the other potentials.(3) The variation in current with the alternating loading was observed at the potentials where transgranular cracking occurred, whereas no appreciable correlation between current and cyclic loading was shown at a potential of −0.04 V where the intergranular mode cracking occurred.(4) It is pointed out to be helpful in studying the influence of applied potential on the acceleated rate of cracking at the crack tip by the “crack-tip opening displacement” estimated from the stress intensity, as the major mechanical condition.