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1.
Crack propagation velocity in delayed failure under superposed repeating load, (da/dt)R, was compared with that under static load, (da/dt)STwo peaks appear on the relation between decreasing rate of crack propagation velocity, 1-β = 1 ? (da/dt))R/(da/dt)S and frequency, ?, both under sinusoidal and square load. By changing the ratio of holding time at maximum stress intensity factor to that at minimum stress intensity factor in square load, it was deduced that the existence of two peaks on the 1 ? β vs f curve was caused by an asymmetric interaction between hydrogen atoms and cyclic moving of the position with triaxial tensile stress at crack tip. Moreover, the relation between 1 ? β and f under the positive or negative saw tooth load could be well explained by the interaction model.  相似文献   

2.
A study has been made of the influence of load ratio R on fatigue crack propagation behavior and specifically on the value of the fatigue crack growth threshold, ΔK0, in a bainitic 2.25 Cr-1Mo pressure vessel steel tested at 50 Hz in aqueous, and moist and dry gaseous environments. Data are obtained for crack growth in a distilled water environment and are compared to previously published results in air and hydrogen. It is found that in distilled water the dependence of thresholds ΔK0 values on R is far less marked than in moist air and dry hydrogen atmospheres where ΔK0 values decrease sharply with increasing R. Furthermore, whereas in air and hydrogen, the threshold condition is characterized by a constant maximum stress intensity at low load ratios, and a constant alternating stress intensity at high load ratios, no such behavior is observed in water. Based on extensive measurements of crack face oxidation products using scanning Auger speetroscopy and on previous crack closure measurements using ultrasonics techniques, the role of load ratio in influencing near-threshold fatigue behavior is ascribed to mechanisms of crack closure specifically plasticity-induced closure and closure arising from crack face oxide debris. The implications of such plasticity-induced and oxide-induced closure to the load ratio-dependence of near-threshold fatigue behavior in various environments are discussed in detail.  相似文献   

3.
In order to clarify the reason why the stable growth of branched cracks occurs in delayed failure, while not in other subcritical crack propagation process such as fatigue, the stress intensity factor after crack branching in delayed failure was dropped to various values, and the propagation behavior of both cracks was investigated.The well balanced growth of branched cracks in delayed failure occurs only when the crack propagation velocity after crack branching belongs to the region II where the crack propagation velocity is constant independently of K. The fatigue cracks at the tips of artificially branched cracks, on the other hand, can not propagate stably, and only either crack propagates preferentially.The exponent in the crack propagation law (da/dt = c1Km or da/dN = c2(ΔK)m) expresses the degree of unbalance growth of branched cracks. The stable growth of branched cracks occurs only when the crack propagation velocity is constant independently of K or ΔK, i.e. m = 0.  相似文献   

4.
5.
The relation between diffusion behavior of hydrogen atoms and crack branching in delayed failure was discussed.The hypotheses that diffusion paths of hydrogen atoms at crack tip broaden with increase of stress intensity factor K, and that crack branching occurs when hydrogen atoms diffuse through the position where stable cracks are to nucleate, can well explain the facts that crack branching occurs when K reaches to a certain value, KIB, and that KIB increases with increase of temperature. The initial stress intensity factor and loading speed do not actually influence KIB, which can also be explained by the above hypotheses.  相似文献   

6.
The influence of a decreasing rate of stress intensity factor with crack propagation, dK/da, on a stress intensity threshold level, δKth, below which fatigue crack propagation becomes insignificant is investigated. Specimens, 200 mm wide, 10 mm thick with a 40 mm-long central crack, are fatigued at the decreasing rates, d(δK)da∥, of 2,44, 5 and 10 kg/mm5/2 with a peak load control system and a pair of crack followers. In this range of d(δK)/da, the stress intensity threshold levels, δKth, have the same value regardless of dK/da. Therefore, the present method of decreasing the stress intensity factor at a constant rate is suitable for determining the characteristic δKth of materials. Furthermore, the influence of stress ratio, R, is investigated at the decreasing rate, d(δK)/da], of 10 kg/mm5/2.  相似文献   

7.
Two- and three-dimensional thermo-mechanical failure criteria, including the effects of crack/cavity closure, are developed in terms of thermal and mechanical loading by extending the work of McClintock and Walsh. General 2- and 3-D fracture criteria in terms of soley stress intensity factors are developed and it is shown that they are expressed in the single relation, (k2k2c)2 + k1k1c = 1, on the basis of Griffith theory and fracture mechanics. General expressions of stress intensity factors in 3-D crack problems under arbitrary thermo-mechanical loading with the effect of crack closure are also deduced.  相似文献   

8.
Fracture toughness under tensile test and fatigue test on high strength steel at temperature ranging from room temperature to ?180°C were experimentally studied. The value of fracture toughness under fatigue test is considerably tower than that obtained under tensile test.Within the range from room temperature to ?100°C the following results were obtained: the power coefficient δ of the fatigue crack propagation rate [(dc)/(dN)] = AΔK5 is related with [(1)/(T)] as: δ = b1 + [(a1)/(kT)]. [(dc)/(dN)] shows Arrhenius type, and, however, different equation from usual stress dependent rate process equation. The trend is in good agreement with the dislocation dynamics theory of fatigue crack propagation.  相似文献   

9.
The stress distribution is obtained around the tip of a crack running in a brittle material. The stresses are written as the sum of the associated static solution and the wave-effect terms which depend upon the crack speed. The results obtained clearly reduce to the associated static solutions if the crack speed vanishes.Near the tip of the crack, the dynamic stress-intensity factor for the circumferential stress, σθθ, is written as the product of the associated static stress-intensity factor and the dynamic correction factor which is a nondimensional function of the crack speed, V, the angle from the crack plane, θ, and Poisson's ratio, ν. The value of the correction factor is computed for various values of V and θ at ν = 0.25. It is shown that the maximum tensile value of σθθ, occurs on the crack plane for V less than 0.7 time shear wave speed, c2, and suddenly shifts to the plane of θ = 55° for V slightly larger than 0.7 c2. For V > 0.7c2, the angle θ for the maximum σθθ, θ being larger than 55°, varies continuously with the crack speed, V. The results obtained are used to discuss the growth of branching crack.  相似文献   

10.
The fatigue crack propagation characteristics of several rotor and wheel materials that are commonly used in rotating components of steam turbines were investigated. Particular emphasis was placed on the behaviour at near-threshold growth rates, ie below 10?5 mm/cycle, approaching the fatigue-crack propagation threshold, ΔKth. The lifetimes of the cracks of interest lie mostly in this region, and it is also the region where few data are available.The effects of load ratio on the fatigue crack growth rates were examined, as well as the tensile, Charpy V-notch and fracture toughness properties of the rotor and wheel materials. The relationship between fatigue crack propagation behaviour and fractographic features was examined. Fatigue crack growth rate data, da/dN vs stress intesity range ΔK, were fitted with a four parameter Weibull survivorship function. This curve fitting can be used for life estimation and establishment of ΔKth. The results show that load ratio and microstructure play a role in determining the fatigue crack threshold and fatigue crack growth behaviour.  相似文献   

11.
HF1, a hypereutectoid steel which is high in silicon and manganese content, was selected to examine the dependency of Mode III fracture initiation toughness (KIIIQ) on microstructure and strength. Four heat treatments of this steel were investigated and results were compared with those previously reported on a hypoeutectoid steel (4340). It was found that KIIIQ is independent of strength and depends only on microstructure and other metallurgical factors. The KIIIQ value of the hypereutectoid steel and for the nucrostructures examined varied between 35 and 45 MPa.√m. In the higher strength specimens, an initial Mode III crack extension was followed by Mode I crack branching.  相似文献   

12.
A technique for modeling synergistic effects in fatigue crack propagation (FCP) is presented. First, a mission (load/temperature history) is segregated into elemental damage events. A simple three parameter model is then used to describe these events. The model coefficients are seen to be interrelated linear functions of FCP rate controlling variables such as frequency, temperature, stress ratio (σminmax), dwell, overload ratio (Poverload/Pmax) and cycles between overload. Finally, integrating event-by-event crack advance gives the expected component crack propagation life under mission cycling. Results of this procedure applied to gas turbine disk materials IN100 and Waspaloy are discussed to examine the accuracy of the model.  相似文献   

13.
A modification to the model of Weir et al. for surface reaction and transport controlled fatigue crack growth has been developed to explicitly account for the effect of load ratio on environmentally assisted fatigue crack growth. Load ratio was found to affect principally gas transport to the crack tip, and therefore affected only transport controlled crack growth response. Experimental verification of the modified model was made by studying the room temperature fatigue crack growth responses at different load ratios for a 2219-T851 aluminum alloy exposed to water vapor.The results show that the effects of load ratio can be attributed to two different sources—one relating to its effect on local deformation at the crack tip and is reflected through the mechanical component, (da/dN)0 and the other on its role in modifying environmental effect and is manifested through the corrosion fatigue component, (da/dN)cf Furthermore, the results show that the saturation value of corrosion fatigue component, (da/dN)cf,s, is essentially independent of R, and that the exposure needed to produce “saturation response” (P0/2f)s, as a function of load ratio can be predicted from the modified model. The modified model, therefore, allows one to predict the corrosion fatigue crack growth response for any load ratio on the basis of measurements made at a single load ratio, provided that the values of (da/dN), are known.  相似文献   

14.
A model was derived to predict the true threshold value for fatigue crack growth in the absence of crack closure. The model, based only on the tensile and cyclic properties of the material, was successfully verified against a set of experimental data on medium and high strength steels and one aluminium alloy. Good agreement with experimental results was also obtained for Region I of the da/dN vs ΔK curve using a fatigue crack growth rate equation based on the same model.Fatigue crack growth data obtained from the medium strength steel CK45 in the normalized state and two heat-treated conditions were analysed. Good data correlation was shown using a previously developed normalizing parameter, φ = (ΔK2?ΔK2th)/(K2c?K2max), in the entire range of fatigue crack growth rates and for stress ratios ranging from 0.1 to 0.8.  相似文献   

15.
The effects of two aqueous environments, namely chloride and sulphide have been investigated using fracture mechanics approaches in a Ni-Cr-Mo alloy, tempered between 200–600°C temperatures after quenching. The experimental investigation included tensile and fracture toughness tests in the ambient condition, environmental tests to determine the threshold, KISCC and the crack growth rate values dadt and fracture surface studies. An attempt has been made to substantiate the role of microstructure and the source of hydrogen on the susceptibility to failure by computing CcCo ratios for the hydrogen induced cracking process. A crack growth rate expression of the type, dadt = c'(K)n is proposed for Stages I and II to account for the discrepancy between the theoretically calculated and the experimental dadt data. The experimental values of the constants c' and n are determined. For all the tempering conditions investigated, the H2S environment appears to be more hostile than the NaCl medium. However, the susceptibility to both the environments is more pronounced for yield strength values greater than 1500 MPa. The KIfKIC ratio is bound to be less than 1 under the H2S, and greater than 1 under the NaCl solution.  相似文献   

16.
Stress and strain field of a propagating fatigue crack and the resulting crack opening and closing behavior were analysed. It was found that a propagating fatigue crack was closed at tensile external loads due to the cyclically induced residual stresses. Strain range value Δ?y in the vicinity of the crack tip was found to be closely related with the effective stress intensity factor range ΔKeff which was determined on the basts of the analytical crack opening and closing behavior at its tip. Application of this analysis to the non-propagating fatigue crack problem and the fatigue crack propagation problems under variable stress amplitude conditions revealed that both Δ?y and ΔKeff were essential parameters governing fatigue crack growth rate.  相似文献   

17.
A prediction of the direction of incipient crack growth in brittle-like materials and the associated fracture loci under mixed mode loading is proposed. It is postulated that the direction of unstable crack propagation is determined by the “weakest” near-tip element defined as the one which would relax maximum potential energy upon prospective crack extension. Starting from the energy rate principle of crack extension (Eshelby energy-momentum tensor and Rice J-internal vector) it is deduced that a crack will extent in the direction along which the following stress criterion is satisfied, θθ2 ? δrr2) → maximum (for δθθ > 0) The fracture angle in pure Mode II (70.4° away from the original straight path) is shown to be unstable in the sense that any slight tension along the crack (non-singular at the crack tip) affects considerably (up to 22%) the directionality of crack extension. It appears to be sensitive to the extent of the near-tip zone (r0) in which linear elasticity does not hold and the non-singular stress term (squared).The fracture loci in mixed mode loading (generated by projecting the J-integral vector along the prospective fracture path and letting this scalar function attain a critical value) is quadratic in K1 and K2 with an interactive cross product term K1 × K2.The suggested criterion with its implication in predicting critical fracture load, exhibits behavior which is consistent with experimental observations collected from several sources. The common and uncommon features with respect to other known criteria are compared and discussed.  相似文献   

18.
The plastic energy dissipation before crack growth initiation and during stable crack growth was determined in centercracked thin sheet specimens of 2024-T3 aluminum alloy with different width and crack length-to-width ratios. The plastic energy dissipation rate versus stable crack growth curve was found to be approximately linear, but the slope decreased considerably with increase in crack length. No correlation was observed between plastic energy dissipation rate and the linear toughness (G?c), the nonlinear energy toughness (G?c) or the R-curve toughness (GR). The role of net section yielding on the decrease in stable crack growth and toughness values in small specimens is discussed.  相似文献   

19.
The paper deals with an experimental study on the nature of plastic flow at the root of a crack in mild steel beams, for the non-valid KIC test regime, under three point hending loads. Photoelastic coating technique has been used to measure the plasticity spread ahead of the tip in relation to the load-COD record. It is observed that in all cases there is a sudden increase in specimen compliance near the maximum linear load due to an abrupt increase in plastic zone size on some preferential planes ahead of the crack tip. This abrupt increase in plastic flow was seen to occur along the 45° planes (with respect to the plane of the crack) for thicker beams and/or with longer cracks. In contrast, the plastic zone extended more on the plane of the crack for thin section beams with relatively shorter cracks. The stress intensity factor required to cause this sudden loss of resistance to localized deformation is found to be remaining constant beyond a certain crack length for a given specimen thickness. These observations suggest that a critical stress intensity factor (KIpc) concept can be introduced to describe the abrupt flow localization ahead of the crack tip. This (KIpc) can be taken as a new parameter in addition to those commonly used in characterising the overall “fracture” behaviour of large scale yielding materials like mild steel, especially in the non-valid KIC test regime.  相似文献   

20.
Accumulative damage model based on the hysteresis strain energy density is proposed for predicting fatigue crack growth. Investigated is the application of sinusoidal loading on an edge crack whose growth rates are obtained by specifying the number of cycles, ΔN, for each growth step. The corresponding increment of crack growth, Δa, is calculated by having the accumulated local strain energy density to reach certain critical value, (dW/dV)c. As it is to be expected, each growth increment Δa increases up to the point of unstable rapid fracture. The growth rate da/dN versus a data are generated from the nonlinear incremental theory of plasticity. Because of the complexities involved in the stress and subcritical crack growth analysis, the finite element procedure is adopted such that the grid pattern is readjusted for each step of crack growth. Results for the edge crack specimen are displayed graphically and compared with those for the center cracked specimen made of the same material. The different growth characteristics are discussed and expected because material damage by fatigue is sensitive to changes in load history, specimen geometry and crack configuration. Insight into these nonlinear effects provides a means for establishing the range of applicability of the linear fatigue growth models. Discussed in particular are the da/dN vs δk1 and AS relations where the linear theory of elasticity is used to calculate ΔK1 and ΔS.  相似文献   

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