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1.
The behaviour of small fatigue cracks has been studied in the Al---Li---Cu---Mg---Zr alloy 8090. It was found that the crack inclination normal to the surface of the specimen made crack deflections and kinking in the plane of the specimen surface irrelevant to the crack driving force. The low closure levels associated with small fatigue cracks reduce the effect of microstructure on crack growth but this does not affect the ability of Δ K (stress intensity factor range) to detect microstructural influences. The use of Δ J ( J-integral range) as a correlating parameter reduced the differences between the data for long and short fatigue cracks. However, there was no evidence that Δ J was superior at identifying microstructural effects. Similarly the effect of the higher-order terms on the value of Δ K was found to be minor. It is concluded that the use of Δ K is not likely to bias the microstructural effects and so Δ K may be used when examining microstructural effects on small fatigue crack growth. 相似文献
2.
The fatigue threshold, Δ Kth, is strongly influenced by the stress-ratio, ie by the loading conditions. Results for a Ti6A14V alloy show that a Δ K exists for non-propagating fatigue cracks which is independent of loading conditions. This Δ K is called the fatigue tolerance range and is denoted by Δ KK. The fatigue tolerance range corresponds to that part of the Δ Kth during which the fatigue crack is open. Arguments that the fatigue tolerance range has to be explicitly incorporated in equations predicting fatigue crack growth rates are presented. 相似文献
3.
The relationship between fatigue crack propagation rate, da/ dn, and range of stress intensity factor, Δ K, including threshold stress intensity factor, Δ Kth, is analyzed statistically. A non-linear equation, da/ dn = C{(Δ K) m-(Δ Kth) m}, is fitted to the data by regression method to evaluate the 99% confidence intervals. Several experimental results on fatigue crack propagation properties of welded joints are compared by using these confidence intervals. 相似文献
4.
The growth behaviour of small fatigue cracks has been studied in both fine- and coarse-grained versions of a pure titanium under axial loading at stress ratio, R, of −1. The growth behaviour and its statistical properties in a coarse-grained version of a different pure titanium have also been investigated under rotating bending ( R = −1), and the results obtained were compared with those of a fine-grained version of this titanium in a previous report. Under both loading conditions, small cracks grew faster than large cracks. As the growth data were plotted in terms of the effective stress intensity factor range Δ Keff (after allowing for crack closure, the growth rates could be well correlated with large-crack data in a large-crack regime. In a small-crack regime, however, small cracks still grew faster than large cracks. Small cracks in coarse-grained material showed higher growth rates than those in fine-grained material owing to a much smaller effect of microstructure such as grain boundaries and crack deflection. Stage I facets were observed in all the specimens tested, and their depths were less than the maximum grain size estimated by the statistics of the extreme values, but the distribution of stage I facet depths approximately corresponded to the maximum value distributions of grain size of the materials. The growth rates of small cracks followed log-normal distributions independent of grain size. The coefficients of variation, η, of growth rate in coarse-grained material were smaller than those in fine-grained material. The η values were significantly large at a/ d 3 ( a = crack depth, D = grain size), indicating that the relative size of microstructurally small cracks was not dependent on grain size. 相似文献
5.
After cylinder notch fatigue specimens of 40 CrNiMo steel were rolled, their fatigue limit increased by 41%. The rolled specimens did not fracture, even though they had been loaded for 10 7 cycles under fatigue limit stress, but a non-propagating fatigue crack was generated. Thus the value of the fatigue limit depends on the fatigue threshold value Δ Kth of the metal of the rolled layer. Plastic deformation increased Δ Kth in these experiments. It can be inferred that Δ Kth of the rolled layer increases from the occurrence of plastic deformation and microvoids on the layer. Calculation of the effect of residual stress in the crack wake on the stress intensity factor Δ K indicates that residual compression stress decreases Δ K by 21.5 MPa √M. It was calculated that rolling induced both the length of the non-propagating crack and the increase of fatigue limit. The calculated values are in accord with experiment. Analysis and calculations indicate that the non-propagating crack is generated on the rolled layer. Thus the fatigue limit is improved because rolling produces residual compression stress in the layer (which decreases the stress intensity factor), and increases Δ Kth of the layer. 相似文献
6.
This paper deals with the analysis and prediction of a high-cycle fatigue behaviour in notched and damaged specimens, as well as butt-welded joints by using a threshold curve for fatigue crack propagation that includes the short crack regime (a function of crack length, a). The approach regards the effective driving force applied to the crack as the difference between the total applied driving force defined by the applied stress distribution corresponding to a given geometrical and loading configuration, Δ K( a), and the threshold for crack propagation, Δ Kth( a). Chapetti’s model is used to estimate the threshold for crack propagation by using the plain fatigue limit, Δ σeR, the threshold for long cracks, Δ KthR, and the microstructural characteristic dimension (e.g. grain size). Applications, predictions and results, in good agreement with experimental results from the literature, demonstrate the ability of the method to carry out quantitative analyses of the high cycle fatigue propagation behavior (near threshold) of short cracks in different geometrical, mechanical and microstructural configurations. 相似文献
7.
Crack growth rates for large fatigue cracks in 12 variations of particulate silicon carbide reinforced aluminum alloy composites have been measured. Composites with seven different matrix alloys were tested, four of which were of precipitation hardening compositions, and those were tested in both as-extruded and peak aged conditions. Five of the materials were made by casting, ingot metallurgical methods and two of the alloys by mechanical alloying, powder metallurgical methods. For both manufacturing methods, primary fabrication was followed by hot extrusion. The fatigue crack growth curves exhibited an approximately linear, or Paris law, region, fitting the function da/ dN = BΔ Ks, and a threshold stress intensity factor, Δ Kth. As has been found for other materials, the coefficients B and s are correlated; for these composites In B= −16.4−2.1 s. A correlation was also found between Δ Kth and s, and it was found possible to compute the magnitude of Δ Kth using a simple model for the threshold together with yield stress and SiC size and volume fraction. These results were explained using a relationship between Δ Kth and crack closure determined previously for unreinforced aluminum alloys. The path of fatigue crack growth is through the matrix for these composites, and SiC has the effect of altering the slip distance, therefore, the plasticity accompanying fatigue cracks. It was shown that all the crack growth rate curves were reduced to one equation having the form da/ dN = B'Δ Keffs' where B' = 6.5 × 10 -9m/ cy and s' = 1.7. A partly theoretical method for predicting fatigue crack growth rates for untested composites is given. Fatigue crack surface roughness was measured and found to be described by a fractal dimension, but no correlation could be obtained between surface roughness parameters and Δ Kth. 相似文献
8.
Fatigue crack initiation and growth characteristics under mixed mode loading have been investigated on aluminum alloys 2017-T3 and 7075-T6, using a newly developed apparatus for mixed mode loading tests. In 2017-T3, the fatigue crack initiation and growth characteristics from a precrack under mixed mode loading are divided into three regions—shear mode growth, tensile mode growth and no growth—on the Δ KI-Δ KII plane. The shear mode growth is observed in the region expressed approximately by Δ KII > 3MPa√m and Δ KII/Δ KI > 1.6. In 7075-T6, the condition of shear mode crack initiation is expressed by Δ KII > 8 MPa√m and Δ KII/Δ KI > 1.6, and continuous crack growth in shear mode is observed only in the case of Δ KI/Δ KII, 0. The threshold condition of fatigue crack growth in tensile mode is described by the maximum tensile stress criterion, which is given by Δσ θmax √2π r 1.6MPa√m, in both aluminum alloys. The direction of shear mode crack growth approaches the plane in which KI decreases and KII increases towards the maximum with crack growth. da/ dN-Δ KII relations of the curved cracks growing in shear mode under mixed mode loading agree well with the da/ dN-Δ KII relation of a straight crack under pure mode II loading. 相似文献
9.
An analysis is made of shear lip width measurements and the transition of tensile mode fatigue cracks to shear mode fatigue cracks, as observed on fatigue crack surfaces of aluminium alloy sheet material. It could be shown that these phenomena were controlled by Δ Keff, rather than Kmax or Δ K. For crack growth in air the shear lip width was approximately proportional to (Δ Keff) 2, but it was significantly larger than the estimated size of the reversed plastic zone. The initiation of shear lips, the transition from plane stress to plane strain along the crack front and the environmental effect on shear lips are briefly considered in the discussion. 相似文献
10.
A new resistance-curve method was proposed for predicting the growth threshold of short fatigue cracks near the notch root. The resistance curve was constructed in terms of the experimentally determined threshold value of the maximum stress intensity factor which was the sum of the threshold effective stress intensity range Δ Keffth and the opening stress intensity factor Kopth The Δ Keffth value was constant, irrespective of crack length or notch geometry. The relation between Kopth and crack length was independent of notch geometry. The predicted effects of the notch-root radius and the notch depth on the propagation threshold of short fatigue cracks were compared with the experimental data obtained using center-notched specimens with various notch-root radii and single-edge notched specimens with various notch depths. Excellent agreement was obtained between predictions and experiments. 相似文献
11.
A microcomputer-based system for the measurement of fatigue crack growth da/dn versus cyclic stress intensity factor Δ K data using compact-tension test specimens is described. The procedure has been developed to allow automatic measurement of crack growth rate under any specified combination and sequence of load conditions, i.e. Δ K and R (stress ratio) and includes the capability of establishing the threshold cyclic stress intensity factor Δ K0. Crack extension measurement is effected from the elastic compliance evaluated from the AC component of the load and displacement signals to an accuracy of -3 μm every 1000 load cycles. Results from a typical low-alloy-steel rotor forging are presented to illustrate the use of the system. 相似文献
12.
Corrosion fatigue crack growth tests have been carried out at various stress ratios for a low alloy steel SNCM 2 and type 304 stainless steel. Measurements of the effective stress intensity factor range ratio U were performed to explain the effect of stress ratio R. The corrosive environment decreased da/dN at R = 0.1, 0.4 and little affected da/dN at R = 0.9 for SNCM 2 and increased da/dN at all R ratios for SUS 304. It was confirmed that there exists a threshold stress intensity factor ΔKthCF in 3% NaCl solution for both materials tested. The corrosive environment decreased ΔKthCF for all conditions tested except at R = 0.1 and 0.4 for SNCM 2, where ΔKthCF-values were nearly equal to ΔKth-values in air. ΔKthCF/ΔKth was 0.6 at R = 0.9 for SNCM 2 and 0.8, 0.5 and 0.7 at R = 0.1, 0.7 and 0.9 for SUS 304, respectively. It was shown that the complicated effect of stress ratios on crack growth for SNCM 2 can be explained using effective stress intensity factor ΔKeff. 相似文献
13.
A new mathematical model is proposed for measuring fatigue crack propagation (FCP) threshold by using K-decreasing and K-increasing method. The formulae for efficiently selecting load-variation coefficients
and crack growth increments (Δ a)
, (Δ a)
, in the case of K-decreasing and K-increasing tests are given and a correct method for determining FCP rate in the near-threshold region is recommended. On the basis of the above-mentioned work, a personal microcomputer-aided system is set up and successfully used for the FCP rate measurement in Lc9 aluminum alloy in 3.5% NaCI salt water environment. Compared with other systems, this system can increase the measurement accuracy, shorten testing time and obtain more information. Furthermore, the hardware is inexpensive. 相似文献
14.
The near-threshold fatigue crack growth behavior of Ti-6A1-4V alloy has been investigated in low O 2 steam (< 1 ppm), high O 2 steam (40ppm), and boiling water with various concentrations of Nad and/or Na 22SO 4. At load ratio ( R) of 0.5, high O 2 steam increased the crack propagation rates in the threshold region, relative to low O 2 steam. However, at R = 0.8, the near-threshold crack growth rates in low and high O 2 steam were comparable. Values of threshold stress intensity range, Δ Kth, slightly increased with an increase in the concentration of NaCl in the solution. Varying solution pH from 5.0 to 10.0 in a 0.1 g NaCl plus 0.1 g Na 2SO 4 per 100ml H 2O solution had no effect on the rates of near-threshold crack propagation. Increasing the hydrazine level from 30 to 10 7 ppb in the same salt solution also did not change the resistance to crack growth. Comparing the present results with the previous data on 403 stainless steel, the near-threshold crack propagation rate performance in Ti-6Al-4V alloy is superior to that in 403 steel in both the steam and salt solution environments. 相似文献
15.
This paper explores the use of the theory of critical distances (TCD) in the prediction of high-cycle fatigue behaviour in engineering components. Theories of this type have been in use for over 50 years in various forms, and recently have been brought up to date by linking them with linear elastic fracture mechanics (LEFM). The TCD represents a major extension of LEFM, allowing it to be used for short cracks as well as for stress concentrations of arbitrary geometry, using the results of finite element analysis (FEA) or other computer-based methods. A number of different realisations of the TCD exist: several of these express the critical distance as a function of material fatigue limit (Δ σ0) and crack propagation threshold (Δ Kth). This paper illustrates the application of the TCD to a case study on the failure analysis of a large maritime vehicle component in three different designs. Unusually, the increasing of a fillet radius did not prevent fatigue failures from occurring: the TCD was able to explain this, due to its ability to accurately predict notch sensitivity effects. Further applications of the TCD, to problems not only in fatigue but also in brittle fracture in various materials, are also discussed. 相似文献
16.
The plastic work required for a unit area of fatigue crack propagation U was measured by cementing tiny foil strain gages ahead of propagating fatigue cracks and recording the stress-strain curves as the crack approached. Measurements of U and plastic zone size in aluminum alloys 2024-T4, 2219-T861, 2219 overaged, and A1-6.3 wt% Cu-T4, and a binary Ni-base alloy with 7.2 wt% A1 are herein reported. The results are discussed along with previously reported measurements of U in three steels and 7050 aluminum alloy. When U is compared to the fatigue crack propagation rate at constant Δ K along with strength and modulus, the conclusion is drawn that U is one of the parameters which determines the rate of fatigue crack propagation. The relation of U to microstructure is also discussed. “Homogeneous” plastic deformation in the plastic zone ahead of the crack seems desirable. 相似文献
17.
A computational method is described for the determination of Δ Kb, corresponding to a fatigue crack growth rate of b/cyc, where b is the Burgers vector for a monolithic metal alloy. Δ Kb is found to be numerically equal to E√b for the case of closure-free crack growth behavior. Given that the closure-free FCP rate of many monolithic metals varies with Δ K3, the growth rate of metal alloys at Δ K Δ Kb is given by d a/d N = (Δ K/E) 3(1/√b. Excellent agreement is found between experimental and computed FCP data for the case of monolithic metal alloys. The limits of these relations for metal-matrix composites and ceramics are discussed. 相似文献
18.
The development of a fully automated test apparatus for near-threshold fatigue crack growth rate measurements in a liquid helium environment is described, and some initial results for AISI 300 series stainless steels are presented. The experimental apparatus consists of a servohydraulic test machine and a cryostat, complete with a minicomputer, a programmable arbituary waveform generator, a programmable digital oscilloscope and a fully automatic liquid helium refill system. The technique uses 6.4 mm thick compact specimens subjected to systematically decreasing loads, with 24 h operation at 40 Hz, the crack growth being continuously monitored by specimen compliance measurements. The results presented in this study include d a/d N vs Δ K curves and threshold fatigue stress intensity factors, Δ Kth, at 4 K for AISI 304L, 304LN and 316 stainless steels. The near-threshold fatigue behaviours of these materials are similar, and the fatigue crack growth rate trends at intermediate Δ K levels nearly agree with published results. 相似文献
19.
The effect of microstructural characteristics on high-cycle fatigue properties and fatigue crack propagation behavior of welded regions of an investment cast Ti-6Al-4V were investigated. High-cycle fatigue and fatigue crack propagation tests were conducted on the welded regions, which were processed by two different welding methods: tungsten inert gas (TIG) and electron beam (EB) welding. Test data were analyzed in relation to microstructure, tensile properties, and fatigue fracture mode. The base metal was composed of an alpha plate colony structure transformed to a basket-weave structure with thin platelets after welding and annealing. High-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of large micropores formed during welding, although it had the highest yield strength. In the case of the fatigue crack propagation, the EB weld composed of thinner platelets had a faster crack propagation rate than the base metal or the TIG weld. The effective microstructural feature determining the fatigue crack propagation rate was found to be the width of platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold Δ K regime. 相似文献
20.
The objectives of this study were to investigate the effectiveness of a compliance method for analyzing the fatigue crack growth of GLARE3 fiber/metal laminates. The materials tested were GLARE3-5/4 (2.6 mm thick) and GLARE3-3/2 (1.4 mm thick). Centrally notched specimens with two kinds of notch length and two kinds of fiber orientation were fatigue tested under constant amplitude loading. The expression of the experimental stress intensity factor, Kexp, for the 2024-T3 aluminum-alloy layers of a GLARE3 is formulated and Kexp were obtained from the relationship between crack length and specimen compliance. The test results clarified the following: (1) d a/d N–Δ Kexp relationships roughly show the linear relationship independent of the maximum stress level, specimen thickness, notch length, and fiber orientations, (2) the d a/d N–Δ Kexp relationships approximately agree with the linear part and its extension of Paris–Erdogan’s law obtained for the d a/d N–Δ K relationship of the 2024-T3 aluminum-alloy, (3) the compliance method is effective for analyzing fatigue crack growth in GLARE3 laminates. 相似文献
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