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1.
| 1. | With thermal cycling loading for coated specimens microracks occur in brittle coating layers, and conditions for their formation depend not only on coating composition, but also on the composition of the metal being protected, and operating conditions (time, temperature). |
| 2. | Propagation of thermal fatigue microcracks into the basic metal is determined not only by the difference in linear thermal expansion coefficients, elasticity moduli of the coating and basic metal, but it also depends on the thickness of the coating, cycle parameters, aggressivity of the corrosive medium, relative position of the cracks formed, and the level of heterogenetiv for the coating itself. |
| 3. | In order to provide reliable protection for turbine blades from corrosion, coating selection should be carried out with reference to actual operating conditions and the grade taking account of chemical composition) of the basic material. |
| 4. | Numerical evaluation of the features of surface microcrack growth from coatings into the basic metal with thermal cycling loading should be carried out on the basis of threshold aggressivity of the corrosive medium. |
2.
| 1. | The structures of the nodal sections of the dished ends designed in accordance with the compensation principle and design restrictions of the standards are charcterized by the permissable level of the maximum working stresses. |
| 2. | In varying the fraction of the reinforcing metal of the nozzle section situated in the wall of the dished end from 50 to 80% the level of the maximum stresses in the section changes only slightly. |
| 3. | The structures containing 70–80% of reinforcing metal in the wall of the nozzle are more rational from the viewpoint of reducing the metal requirement and have sufficient strength at the same time. |
| 4. | The permissible value of the ratio d/D specified by the standards can be increased from 0.3 to 0.5 without reducing the strength of the nozzle section. |
3.
Conclusion Through metallurgical SEM analysis, it was found that 42% of failures occurred due to corrosion plus erosion-corrosion, 16.5%
were due to inclusions and stress gaps that could be correlated oto fatigue, 16.5% had traces of production impurities, and
25% showed fatigue via ductile-type failures. The results/reasons correlation can be summarized as follows:
As a result of the analysis of various implant materials, the main reasons for failure are corrosive attack, manufacturing
defects, and nonstandard-caused failures. Biocompatible inorganic materials, such as hydroxy-apatite coating with some binders,
would make the dissolution of metal ions difficult and so may delay corrosion and wear and also minimize the loosening of
implants from bone. 相似文献
| – | • In the investigation of a total hip prosthesis, the fracture was multiorigined, probably due to the presence of manufacturing defects in both surfaces of the device; was associated with poor design of the component; and introduced points of high stress concentration. |
| – | • The study revealed that the failure of the stainless steel femoral compression plates occurred by a fretting-fatigue mechanism under the action of low nominal stresses and unidirectional bending. |
| – | • The failure of femoral compression titanium plates occurred by corrosion fatigue promoted by the presence of intense localized corrosion and intergranular cracking. |
| – | • The failure of hollow spinal rods occurred by overload caused by the intense formation of pitting corrosion during service. The pitting was associated with the presence of superficial manufacturing routes. |
4.
| 1. | On the basis of an analysis of the equations describing the kinetics of creep crack growth, a new generalizing kinetic equation is suggested; it makes it possible to estimate the effect of the stresses in the cross section, of the SIF, and of the temperature on this process. |
| 2. | It was shown that in dependence on the state of the steel and with the aid of a new kinetic equation it is possible to establish the optimal variant of taking the geometric factor into account in describing the crack growth rate. |
| 3. | It is suggested to compare the creep crack resistance of creep resistant steels according to their resistance to impermissible (impermissible from the point of view of operating conditions) crack growth rate, determined on specimens without initial bending stresses. |
5.
| 1. | The not earlier noted fact of formation in contact impact on the surface of hardened steel of prismatic and segment chipping fragments was established experimentally. The first are formed in intersection of median cracks and the second in intersection of median and circular cracks. |
| 2. | Simple models based on criteria of fracture mechanics and low-cycle fatigue making it possible to analyze the forms of failure considered are proposed. Using the calculation relationships it is possible to determine the order of magnitude of the separate fragment. |
6.
J. Davidov 《Materials Science》1993,28(1):62-67
| 1. | With respect to low-cycle fatigue behaviour the increasing of iron quantity from 0.12% to 0.38% results in a decrease of cyclic ductility of AlSi7Mg alloy but this decrease is not very significant. |
| 2. | The alloy with 0.12% Fe shows better low-cycle fatigue resistance then other materials investigated due to its relatively higher cyclic ductility. |
| 3. | The structure with 0.29% Fe shows the best fatigue crack growth resistance which is due to the best combination of its mechanical properties and relatively ductile type of fracture. |
| 4. | With regard to the low-cycle fatigue behaviour and fatigue crack growth resistance investigations carried out in this work have shown AlSi7Mg alloy with 0.29% Fe seems to be the most appropriate material for manufacturing counterpressure cast car wheels of the particular design investigated because the decrease of cyclic ductility for this structure is not very significant. |
7.
| 1. | The optimum design theory was used to derive equations for optimizing the stress-strain state in the thick-walled pipe with the elasticity modulus varying along the radius and loaded with the internal pressure. |
| 2. | A numerical method of determining the law of the optimum distribution of the elasticity modulus along the radius of the thick-walled pipe was proposed. |
| 3. | The results show that by corresponding changes of the elasticity modulus in the given range along the pipe thickness it is possible to reduce the maximum annular stresses on the internal surface of the pipe. This stress reduction in a ratio expressed in per cent is approximately equal to the variation range of the elasticity modulus. |
| 4. | The minimum annular strains were recorded in the homogeneous pipe produced from the material with the maximum possible elasticity modulus. |
8.
| 1. | High-temperature thermomechanical treatment and microalloying with 0.1% La raise the fatigue and corrosion-fatigue strength of steel 1Kh17N2. |
| 2. | The rise in the fatigue strength is due to an increase in the resistance to crack growth resulting from changes in the structure and substructure brought about in the steel by the high-temperature treatment and microalloying with the rare earth metal. |
| 3. | High-temperature thermomechanical treatment of steel 1Kh17N2 and its alloying with 0.1% La raise the corrosion resistance of the steel and reduce its tendency to intercrystalline corrosion. |
| 4. | The increase in the corrosion resistance of steel 1KM7N2 after the high-temperature treatment and microalloying with the rare earth metal is caused by the structural changes produced in the steel by the treatment and the microalloying. |
9.
| 1. | The structure of the surface layer exerts a significant influence on the fatigue strength of alloy ÉI437BVD. |
| 2. | The fine-grained structure of the surface layer, which is obtained by the method of mechanothermal treatment, is characterized by the complete absence of macro- and microstresses and possesses high thermal stability. |
| 3. | Mechanothermal treatment of the surface layer of components formed from the heat-resisting alloy ÉI437BVD is an effective method of increasing the fatigue strength and operating reserves of components, which can be recommended for turbine and compressor blades. |
10.
S. F. Yur'ev V. F. Luk'yanov V. I. Sheiko V. Ya. Kharchenko V. G. Ovsyannikov A. F. Gorbachev 《Strength of Materials》1976,8(4):441-444
| 1. | Cladding of high-strength steel with type 08KhN2GMTA deposited metal aids in increasing the resistance to failure of plate metal under repeated static biaxial bending in a 3% sodium chloride solution. |
| 2. | With an amplitude of operating stresses of 70% of the yield strength of the high-strength steel the cladding layer at first experiences plastic deformation, which leads to the formation of residual stresses and a change in the degree of asymmetry of the subsequent load cycles of the external layers in a favorable direction. |
| 3. | The appearance in the cladding layer of residual compressive stresses and the decrease on the sample surface of the maximum tensile stresses aids in increasing the resistance to the origin and initial growth of a corrosion-fatigue crack. |
11.
S. B. Kasatkin 《Strength of Materials》1976,8(11):1346-1350
| 1. | Polycarbonate specimens enable direct observation of the stress-strain pattern during the loading process. |
| 2. | It has been shown that under plane strain and plane stress conditions the crack starts at the boundary of the elastic-plastic deformation zone at the moment when the normal stress component reaches the critical value. |
| 3. | Under plane strain conditions the value cr is determined by the yield point and the radius of the notch n. Under plane stress conditions the strip of plastic deformation functions, as a stress concentrator. Fracture occurs when the normal stress component at the boundary reaches the critical value. |
| 4. | During the intervals when the specimen is not loaded the plastically deformed zones act as concentrators of residual stress. When load is again applied there is interaction between the residual stresses and the externally induced stresses. |
12.
E. I. Aleksenko N. M. Grinberg S. I. Aksenova N. A. Grekov G. I. Arkavenko 《Materials Science》1990,26(2):138-144
| 1. | A reduction of the air pressure reduces the rate of fatigue crack growth and increases the threshold range of the SIF in 3M titanium alloy. |
| 2. | A reduction of temperature in vacuum is accompanied by a nonmonotonic variation of the cracking resistance characteristics of the 3M alloy. At 93 K the rate of fatigue crack growth decreases and the threshold range increases. However, a further reduction of temperature to 11 K results in the reversed effect, with the rate of fatigue crack propagation becomming comparable with that in air. |
| 3. | A variation in the duration of the crack initiation stage with a reduction of the air pressure and temperature correlates with the variation of the threshold SIF. |
| 4. | On the basis of changes in the microstructure of the fracture surfaces, it can be concluded that the energy capacity of fatigue failure increases with a reduction of the air pressure and decreases with a reduction of temperature to 11 K. |
13.
M. F. Loshak L. I. Aleksandrova F. P. Smaglenko I. P. Ustimenko E. A. Pikus 《Strength of Materials》1976,8(8):975-977
| 1. | Improved strength and working life in sintered hard alloys can be provided by shot blasting under optimal conditions. |
| 2. | One of the major factors governing the improved strength in such treatment is the production of compressive residual stresses in the surface layer. |
14.
This article presents a failure analysis of 37.5 mW gas turbine third stage buckets made of Udimet 500 superalloy. The buckets
experienced repetitive integral tip shroud fractures assisted by a low temperature (type II) hot corrosion. A detailed analysis
was carried out on elements thought to have influenced the failure process:
The most probable cause of the bucket damage was the combination of increased stresses due to corrosion-induced thinning of
the tip shroud and unfavorable microstructures in the tip shroud region. 相似文献
| a) | the stress increase from the loss of a load bearing cross-sectional area of the bucket tip shroud by the conversion of metal to the corrosion product (scale), |
| b) | influence of the tip shroud microstructure (e.g., a presence of equiaxed and columnar grains, their distribution and orientation), |
| c) | evidence of the transgranular initiation, and |
| d) | intergranular creep mechanism propagation. |
15.
A. V. Prokopenko Yu. K. Petrenya V. N. Ezhov I. A. Makovetskaya M. M. Ivakhov 《Strength of Materials》1989,21(1):39-43
| 1. | At high temperatures the fracture surface changes from being brittle and along crystallographic planes to quasiductile both in polycrystalline, and in monocrystalline alloys. This increases the fatigue crack growth rate. |
| 2. | As the temperature is increased from 1073 to 1273 K, the rate of high-temperature corrosion increases, especially in polycrystalline material. |
| 3. | The fatigue crack growth rate is higher in polycrystalline alloys than in monocrystalline alloys with a <111> orientation, and is lower in monocrystalline alloys with a <001> orientation, i.e., they have an intermediate rate in comparison to specific orientations of the grain. |
| 4. | The advantages of using monocrystalline alloys in increasing the fatigue crack growth resistance are only realized when the orientations of its most resistant planes are advantageously aligned along the direction of highest tensile stress both during brittle shear fracture at 293 K and, during quasiductile fracture at 1073-1273 K. |
16.
| 1. | The fracture of heat-resistant alloys and tool steels under the influence of thermal cycling may be quasistatic, fatigue, or mixed in character. |
| 2. | Quasistatic fracture as a result of thermal cycling takes place with the specimen working portion remaining constant (hard loading mode); it is caused by the accumulation of strains of opposite signs in local material volumes. |
| 3. | The accumulation of residual strains in local specimen volumes for thermoplastic strain materials is due to the mismatch of plastic strain fields along the specimens during the heating and cooling cycles. |
| 4. | Under thermal cycling conditions (as in isothermal low-cycle fatigue) the static damage is measured in terms of the accumulated plastic strain (of a given sign), while the fatigue damage is measured in terms of the magnitude of the plastic strain per cycle. Quasistatic fracture takes place in regions of the maximum accumulated plastic strain which is equal to zero in the zone of fatigue fracture. |
17.
G. S. Neshpor V. P. Gusev G. D. Kudryavtseva M. I. Zhukova A. D. Petrov 《Strength of Materials》1989,21(8):998-1004
| 1. | In a plate of alloy 1163T the mean rates of fatigue crack growth increase from 3.2 to 8.6 mm/kc with when the thickness of the specimens increases from 3 to 15 mm, and the characteristics of static crack resistance attain their maximum when the thickness of the specimens is t* (KIc/0.2)2 10 mm. |
| 2. | With increasing thickness of the specimens, the pitch of the fatigue grooves and the proportion of elements of static failure increase in the fatigue fracture. In a static fracture there is failure by shear, intragranular pitting and ductile grain boundary failure, and with increasing thickness of the specimens the proportion of grain boundary (low-energy) failure increases. |
| 3. | The transition from plane strain to plane state of stress is effected in the region of increase of crack length equal to the thickness of the specimen. |
18.
V. T. Troshchenko S. P. Sinyavskii S. S. Gorodetskii A. P. Gopkalo A. K. Rusanovskii 《Strength of Materials》1976,8(7):747-753
| 1. | The strength characteristics of the piston alloys under consideration, measured under conditions of isothermal and thermal fatigue approaching those encountered in actual practice, are considerably lower than the static strength characteristics. This indicates the importance of allowing for the factors in question when calculating the strength of interual-combustion-engine pistons. |
| 2. | Of the alloy studied in the present investigation, the lowest thermal-fatigue strength characteristics corresponding to approximately normal working conditions are those of aluminum alloy AL25. For temperature cycles of 55330°C the breaking stress on a base of 2·104 cycles of heat exchangers falls to 1 kgf/mm2, as compared with the isothermal fatigue limit (6.1 kgf/mm2) and the static tensile strength (12 kgf/mm2) at the same temperature. |
| 3. | Under conditions of combined loading, the fatigue life of alloy AL25 falls more rapidly than would follow from the linear hypothesis of damage summation, allowing for both mechanical and thermal fatigue. A vital factor in this case is the damage associated with interaction between the fatigue and thermal fatigue processes due to multiple-factor effects. |
| 4. | Criteria for the failure of piston materials under multiple-factor conditions call for urgent and more extensive investigation. |
19.
G. M. Okhrimenko Yu. M. Rodichev T. Sh. Shagdyr V. P. Slin'ko 《Strength of Materials》1976,8(8):915-921
| 1. | It was found that the moire-band method that the stress-strain state in the working part of biaxially compressed samples of glasses and pyroceramics, whose ends are cemented into the grooves of rigid metal bearing races, has satisfactory uniformity throughout the entire stage of loading, whereas the principal stresses coincide in direction with the loads applied to the contour. |
| 2. | The average compressive stresses in a biaxially compressed sample plate whose ends are cemented into the grooves of metal races, loaded over at least 95.5% of the lateral surface, may be calculated by dividing the applied load by the area of the contact surfaces between the sample and races. |
| 3. | The accuracy of experimental deformation diagrams, plotted from the results of interpreting moire-band patterns obtained in systems of parallel lines with 0.02-mm spacing, is higher by a factor of 1.5–2 than that of such diagrams plotted from tensometric data. |
| 4. | When brittle glass- and pyroceramic-type materials are compressed by smooth metal punches, high contact stresses appear, and stress concentrators are formed which cause premature fracture of the samples and understated results. |
| 5. | Owing to the possibility of maintaining the initial end conditions up to high working stresses on compression of glass and pyroceramic samples whose ends are cemented into the grooves of metal races, samples for testing other brittle, high-strength materials, e.g., high-alumina ceramics, porcelain, etc., are promising. |
20.
| 1. | We derived an equation which can be used to determine the endurance in cyclic loading on the basis of the crack initiation criterion in elastoplastic deformation of the material and the triaxial stress state. |
| 2. | Rapid fatigue damage cumulation can take place in the material only if the size of the reversible elastoplastic zone is larger than the grain diameter. |
| 3. | The assumption on the homogeneity of SSS in the structural element makes it possible to describe most adequately the relationship between the strain and fatigue damage of the material. |
| 4. | We derived an analytical expression linking the threshold value of the stress intensity factor \GDKth with the mechanical properties and grain diameter of the material. |
| 5. |
A model of fatigue crack propagation which is based on the approximate analytical solution of the cyclic elastoplastic problem of the SSS in the vicinity of the crack tip was developed. The model takes into account the special features of deformation of the material in the conditions of the triaxial stress state and also uses the assumption on the homogeneity of SSS in the structural element. The main advantages of the model are as follows:
it can be used to determine the crack growth rate in cases in which the variation of the range of the stress intensity factor in the structural members takes place at the variable loading asymmetry; 相似文献
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