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1.
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
2.
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
3.
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
4.
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
5.
A model is developed for fatigue growth of matrix cracks in metals reinforced with aligned continuous elastic fibers. The mechanics of elastic cracks bridged by frictionally constrained fibers is used to develop the model, which provides estimates of the tip value of the stress intensity factor amplitude, Δ KTIP. It is found that when the applied load amplitude is held fixed during fatigue crack growth, Δ KTIP, and thus the rate of growth approach an asymptotic value independent of crack length. The residual strength after fatigue crack growth is also discussed. In some cases, the residual strength is unaffected by prior fatigue growth. But, in another regime, the matrix crack length allows fibers to begin breaking before the matrix crack grows. The strength is then inversely proportional to the square root of fatigue crack length. 相似文献
6.
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
7.
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
8.
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
9.
In the present test the fatigue crack growth rate in the parent plate, weld and cross-bond regions was measured and the results were correlated with the stress intensity range Δ K and the effective stress intensity range Δ Keff. It is indicated that the welding residual stresses strongly affect the crack growth rate. For the weld metal and cross-bond compact tension specimens in which crack growth is along the weld line the fatigue crack growth rate increases as the crack grows. However, for the T compact tension specimen in which crack growth is perpendicular to the weld line at a constant value of applied Δ K the crack growth rate initially decreases as the crack grows. Particularly, at a low constant value of applied Δ K the crack growth rate obviously decreases and the crack fails to grow after short crack growth. When the crack grows to intersect the welded zone, the fatigue crack growth rate gradually increases as the crack grows further. It is clear that the effect of welding residual stresses on the crack growth rate is related to the position of the crack and its orientation with respect to the weld line. Finally, the models of welding residual stress redistribution in the compact tension specimens with the growing crack and its influence on the fatigue crack closure are discussed. It appears that for a butt-welded joint one of the crack closure mechanisms may be considered by the bend or rotation deformation of crack faces due to the welding residual stress redistribution as the fatigue crack grows in the welded joint. 相似文献
10.
An experimental study of fatigue crack propagation and crack closure behaviour, in compact tension specimens of AISI 316 (N) weld has been conducted. The crack closure load was determined from the changes in the slope of the load–displacement curves using global crack mouth opening displacement (GCMOD) type gauge. The results were compared with those measured by acoustic emission technique which showed good agreement with each other. The experimental data bear clear evidence of fatigue crack closure. The crack opening force was found to increase moderately with crack length and increasing R-ratio, under a constant Pmax of 5 kN. Above a critical R-ratio of 0.45 (approximately), the crack closure load is smaller than the minimum applied load. A good correlation was obtained for Δ Keff/Δ K = 0.6684 – 2.4135 R + 7.0077 R2 in the range 0 R 0.5. The magnitude of crack closure is used to interpret observed crack growth behaviour at different R-ratios. 相似文献
11.
Magnesium alloys are being increasingly used for engineering applications. Fatigue crack-growth data have therefore been obtained for a high strength magnesium-Zr alloy and a medium strength, weldable magnesium-Mn alloy. The results of tests on sheet material are presented in terms of the range of stress intensity factor Δ K. Critical values of Δ K necessary for fatigue crack growth Δ Kc were also obtained. The behaviour of the two alloys was similar; both rates of crack growth and Δ Kc were sensitive to mean stress. Fatigue crack growth was entirely on a 90° plane with no sign of the transition to crack growth or 45° planes usually observed in sheet materials. This was ascribed to the effects of preferred orientation of the crystal structure. 相似文献
12.
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
13.
Room temperature fatigue crack growth rate data were generated for Ni-Mo-V (ASTM A469, Cl-4), Cr-Mo-V (ASTM A470, Cl-8) and Ni-Cr-Mo-V (ASTM A471, Cl-4 and a 156,000 psi yield strength grade) rotor forging steels. Testing was conducted with WOL type compact toughness specimens and the results presented in terms of fracture mechanics parameters. Data show that the Ni-Cr-Mo-V steels exhibit slower fatigue crack growth rates at a given stress intensity range (Δ K) than do the Ni-Mo-V steels. In addition, the Cr-Mo-V steel was found to exhibit slower growth rates than the other alloys at Δ K levels below 40 ksi √in but somewhat foster rates at Δ K levels in excess of 45 ksi √in. The fatigue crack growth rate properties of the alloys studied conform to the generalized fracture mechanics crack growth rate law where d a/d N = C0Δ KR. It was noted that the fatigue crack growth rate parameters n and C0 tend to decrease and increase, respectively, with increasing material toughness, Kic. 相似文献
14.
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
15.
Fibre metal laminates (FMLs) are being used to manufacture many structural components in aerospace industry because of their very high strength to weight ratios, yet the exact model for estimating fatigue crack propagation in FMLs cannot be developed because of many variable parameters affecting it. In this research, tensile strength, fatigue life and fracture toughness values of 2/1 configuration carbon reinforced aluminium laminate (CARALL), aramid reinforced aluminium laminate and glass laminate aluminium reinforced epoxy specimens have been investigated. Mechanical, chemical and electrochemical surface treatments were applied to AA 1050 face sheets to improve the adhesive properties of the laminates. The specimens were prepared using vacuum assisted resin transfer moulding technique and were cut to desired shapes. Fatigue tests were conducted on centre notched specimens according to ASTM Standard E399. Real time material data and properties of adhesive were used in definition of numerical simulation model to obtain the values of stress intensity factor at different crack lengths. It was observed that CARALL shows very superior tensile and fatigue strength because of stress distribution during failure. Numerical simulation model developed in this research accurately predicts fracture toughness of aramid reinforced aluminium laminate, CARALL and glass laminate aluminium reinforced epoxy with less than 2% error. An empirical analytical model using experimental data obtained during research was developed which accurately predicts the trend of FMLs fatigue life. 相似文献
16.
The fatigue crack propagation behavior of Ti–5Al–2.5Fe with various microstructures for biomedical applications was investigated in air and in a simulated body environment, Ringer's solution, in comparison with that of Ti–6Al–4V ELI and that of SUS 316L stainless steel. The crack propagation rate, d a/d N, of Ti–5Al–2.5Fe in the case of each microstructure is greater than that of the Widmanstätten structure in Ti–6Al–4V ELI in air whereas d a/d N of Ti–5Al–2.5Fe is nearly equal to that of the equiaxed structure in Ti–6Al–4V ELI in air when d a/d N is plotted versus the nominal cyclic stress intensity factor range, Δ K. d a/d N of the equiaxed structure and that of the Widmanstätten structure in Ti–5Al–2.5Fe are nearly the same in air when d a/d N is plotted versus Δ K. d a/d N of Ti–5Al–2.5Fe is nearly equal to that of SUS 316L stainless steel in the Paris Law region, whereas d a/d N of Ti–5Al–2.5Fe is greater than that of SUS 316L stainless steel in the threshold region in air, when d a/d N is plotted versus Δ K. d a/d N of Ti–5Al–2.5Fe or Ti–6Al–4V ELI is nearly the same in air and in Ringer's solution when d a/d N is plotted versus the effective cyclic stress intensity factor range, Δ Keff, whereas d a/d N of Ti–5Al–2.5Fe or Ti–6Al–4V ELI is greater in Ringer's solution than in air when d a/d N is plotted versus Δ K. 相似文献
17.
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
18.
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
19.
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
20.
A loading stage for the scanning electron microscope has been used in conjunction with the stereoimaging technique to study fatigue crack closure for a center notched specimen of an aluminum alloy. The results are compared to similar measurements from single edge notched specimens of the same alloy and other materials. The magnitudes of closure loads were found to be different for these two specimen designs, but the dependence on stress intensity factor (Δ K) is the same. Comparison of experiments is made with a finite element model simulation of crack opening, a simple model is developed which simulates the observed opening behavior, and a method is given for estimating threshold Δ K from materials parameters and crack opening behavior in the near threshold region. 相似文献
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