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Life predictions in fatigue taking into account material memory properties
Processing f a t i g u e test date - t h e A l f a b a t Project.
Cyclic b e h a v i o u r o f steel subeseambleges.
Kosteas, u. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 2 4 6 - 2 5 6
Calado, L. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 9 3 - 1 0 3
The initial development of the AI data bank (ALDABA) covering primarily fatigue beheviour data of AI components has led to the concept of a comprehensive system of information sources linked to design procedures and reliability estimates for AI structures, the AI fatigue behaviour and evaluation task (ALFABET).
The beheviour of structural steel elements under cyclic load based on the ECCS recommended testing procedure is studied. Some experimental studies end numerical models on the cyclic beheviour of steel subelmemblages ere presented. A numerical model for damage accumulation and for predicting the failure of structural subaseemblagee under cyclic loading is also presented. This model is expressed as • function of the inelastic strain and the dissipated hyateratic energy. It is shown that failure of the structural element does not depend on the loading histories but on the accumulation of the inelastic strain.
Fatigue b e h a v i o u r o f AA6005A-T6 (AIMgSiO.7) a l u m i n i u m alloys evaluation o f t h e early f a t i g u e crack propagation stage by using SEM frestogrephy, FCPR curves end LEFM models.
Jaccard, R. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 2 2 9 - 2 4 5 The early stage of fatigue crack propagation, where fatigue cracks initiate at defects and propagate to various crack geometries, can be evaluated by using LEFM models end FCPR data acquired under Kmax constant testing condition. Depending on the physical appearance of the initial defect (a single loose grain, a series of loose grains, inclusions, single and multiple initiation sites, etc), the fatigue crack growth has to be studied in at least two to three phases. This is necessary to apply appropriate LEFM models individually during a specific phase. As the evaluation demonstrates, most fatigue life is consumed in the first phase. /(max 10 const. FCPR should be used for fine-grain material. In the caes of coarse-grain material or coarse-grain surface layers thicker than approx 0.1 ram, FCPR data of Krnax 6.7 const, have to be applied.
A re-analysis o f f a t i g u e test results on w e l d e d steel m e m b e r s : k n o w l e d g e gaps, research needs and standardization requirements.
Castiglioni, C. A. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 2 1 7 - 2 2 8 The existing database of fatigue data is briefly discussed, together with further research needs in the field of long-life fatigue testing of welded structural steel details. As yet no internationally recognized recommendation has been proposed for fatigue testing methods end design of experiments on full-scale members. Some of the topics that the recommendation should cover are discussed. It is hoped that discussions may lead to the formation of a drafting panel for such a document.
Low cycle fatigue fracture l i m i t s o f structural materials and structural elements.
Yamada, M. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 184-192 Low-cycle fatigue fracture limits of structural materials steel elements and members and joints are presented and discussed as the basis of evaluation of the aseismic capacity of structures. The needs of the discussion on the testing methodologies are emphasized not only for structural materials but also and more especially for structural members for the purpose of giving a basis for investigation of the ultimate stages of structures and to establish structural design methods based upon the ultimate states.
Life prediction methods for low-cycle torsional fatigue.
Curioni, S., Freddi, A. and Caligiana, G. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 162-173
Fatigue life prediction of components locally stressed in the plastic field is a very important task for engineers employed in the automotive and electric utility industries both for design and troubleshooting of service failures. The description is given of the utilization of the local strain approach to fatigue life estimates of samples simulating the behaviour of turbine-generator shafts (NiCrMoV steel) subject to torsional oscillations. An outline of the experimental and theoretical work done by the authors is given.
Low cycle fatigue:
characterization earthquake g r o u n d m o t i o n .
of the
plastic
cycles
due t o
Cosenze, E. and Manfredi, G. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 116-131 The effect of destructive earthquake ground motion represents a very important case of low-cycle fatigue; thus the analysis of plastic cycles due to earthquake is the basis for characterizing the inelastic beheviour of structures in seismic areas. With reference to some significant ground motions and to the elastic-perfectly plastic model, the distribution of the plastic cycles is analysed in terms of number of plastic cycles, maximum plastic deformation, mean value and standard deviation of the plastic amount of the other cycles. Different probability models (normal, Iognormal, exponential, beta and gamma) are used to interpolate the real distribution of plastic cycles and, on the basis of the statistical test of Kolmogorov and Smirnov, are compared with each other.
Life predictions
in f a t i g u e
taking
into
account
material
memory
i g l s n e , ~.
Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 104-115 Torsional fatigue life prediction algorithms are considered, with particular reference to memory effect requirements. Local strain approach is employed. Stress concentration is accounted for by utilizing Neuber's rule or torque-local strain curve discretizetion. Mean stress influence is examined. Experimental results (rotor steel) relative to particular samples geometries subject to typical torque-time histories of turbomachineries are compared with numerical predictions.
532
Effect o f stress a m p l i t u d e transient o n f a t i g u e crack initiation and propagation o f h i g h s t r e n g t h steel in synthetic sea w a t e r u n d e r c a t h o l i c
protection. Maruyama, N. and Sumita, M. Tetsu-to-Hagane (J. Iron and Steel Inst. Jpn) (Apr. 1992) 78 (4) 6 4 0 - 6 4 6 (in Japanese) The effect of varying stress on fatigue life under cathodic protection by a Zn sacrificial anode and freely corroding condition in synthetic sea water and in air was investigated using 600 MPa grade high-strength steel notched specimens (Kt = 3.5). The main results obtained are as follows. (1) Under a constant stress amplitude (CSA), fatigue lives in sea water under cathodic protection ere shorter than those in air. However, under two-step stress amplitude (TSSA}, fatigue lives under cathodic protection are rather longer than those in air. (2) In a low-stressamplitude region, fatigue lives under TSSA are shorter than those under CSA in air and under cathodic protection. (3) Even when the higher stress amplitude in TSSA is much lower than the values shown by the S-N curves under CSA, the failure of notched specimens occurs. (4) Under the free corrosion potential, there is no difference in fatigue lives between under TSSA and under CSA, and Miners rule exists between them. (5) Under TSSA. macroscopic crack growth rate is retarded in air and under cathodic protection, and the effect of TSSA on macroscopic crack growth is in reverse to that on crack initiation and microscopic crack growth. Numeric data, graphs, 16 ref. The test m e t h o d of t h e r m a l f a t i g u e w i t h spherical specimen end T-n-d values.
Fang, J., Li, S. and Shi, X. J. Univ. Sci. Technol. Baijing (1992) 14 (4) 4 9 0 - 4 9 5 (in Chinese) A method by which the thermal fatigue resistance of engineering materials can be measured through spherical specimens is given. The parameters of upper temperature T, cycle time n and fracture depth d are recommended as the quantitative indices of measuring the thermal fatigue resistance of engineering materials. Three kinds (cast iron, T8 and 3Cr2W8V tool steels), and 13 specimens were tested using this method and the test results have proved the feasibility of the method. 9 ref. Microstructursl changes in bearing steel d u r i n g rolling contact f a t i g u e test.
Choi, H. C., Nam, WoJ. and Kim, S. W. J. Korean Inst. Met. Mater. (May 1992) 30 (5) 5 0 7 - 5 1 5 (in Korean) Microstructural changes in STB2 bearing steel caused by rolling contact fatigue testing were investigated using optical and scanning electron microecopy. Dark area, butterfly structures and plate-like carbides were found in the area below the flaking. Rolling contact caused the formation of the dark area in the subsurface area of contact in the region of maximum Hertzian stress. Most subsurface cracks were initiated at the region of the dark area. Butterflies found only at the oxide inclusions were frequently associated with cracks. Plate-like carbides acted as the potential crack initiation site responsible for the flaking. The growth of plate-like carbides was caused by carbon diffusion by rolling stress not only between oxide inclusions but also between fine residual carbides. Graphs, photomicrographs, spectra, 18 ref. Italian codes on f a t i g u e testing.
Atzori, B. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 195-202 The various Italian codes (or instructions) dealing with the subject of fatigue are described. The aim of the description is to focus on what has already been done in Italy in the field of high-cycle fatigue testing of materials, joints, end structural components and what, instead, needs to be done. That could be useful as a basis both for a comparison with the existing codes in different countries end for the definition of possible working areas for a technical committee in the field of testing metals (steel and AI) for high-cycle fatigue beheviour of materials, joints end structural components. The analysis performed and the comparison of the various documents considered, taking into account the state of the art of fatigue design, shows clearly the need to improve the existing procedures and the need to assess new procedures to cover existing lacks in testing materials, joints, and structures. Tensile and f a t i g u e fracture b e h a v i o r and w a t e r - e n v i r o n m e n t in a SiC-whiskcr/TO75-AI composite.
effects
Komai, I., Minoshima, K. and Ryoaon, H. Compos. ScL TechnoL (1993) 46 (1) 5 9 - 6 6 An investigation has been carried out of the tensile and fatigue fracture beheviour and of the influence of water envioroments on fatigue strength of a SiC whisker reinforced high-strength AI-Zn-Mg alloy composite (SiCw/7075-T6) fabricated by a powder metallurgy process. Apart from the elongation et failure, the mechanical properties of the composite were superior to those of an unreinforced 7075-T6 alloy. The fatigue strength in air of the composite at a stress ratio of 0.1 was higher than that of an unreinforced AI alloy. At lower stress level, • water envimmant decreased the fatigue strength of the composite, although the strength was superior to that of an unreinforced AI alloy in air. A fatigue crack initiated at a whisker cluster or a crack normal to the loading axis directly initiated at the specimen surface. In water, a fatigue crack was initiated at a corrosion pit. Graphs, photomicrographs, 22 ref.
Int J Fatigue November 1993
Cyclic b e h a v i o u r o f steel subeseambleges.
Kosteas, u. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 2 4 6 - 2 5 6
Calado, L. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 9 3 - 1 0 3
The initial development of the AI data bank (ALDABA) covering primarily fatigue beheviour data of AI components has led to the concept of a comprehensive system of information sources linked to design procedures and reliability estimates for AI structures, the AI fatigue behaviour and evaluation task (ALFABET).
The beheviour of structural steel elements under cyclic load based on the ECCS recommended testing procedure is studied. Some experimental studies end numerical models on the cyclic beheviour of steel subelmemblages ere presented. A numerical model for damage accumulation and for predicting the failure of structural subaseemblagee under cyclic loading is also presented. This model is expressed as • function of the inelastic strain and the dissipated hyateratic energy. It is shown that failure of the structural element does not depend on the loading histories but on the accumulation of the inelastic strain.
Fatigue b e h a v i o u r o f AA6005A-T6 (AIMgSiO.7) a l u m i n i u m alloys evaluation o f t h e early f a t i g u e crack propagation stage by using SEM frestogrephy, FCPR curves end LEFM models.
Jaccard, R. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 2 2 9 - 2 4 5 The early stage of fatigue crack propagation, where fatigue cracks initiate at defects and propagate to various crack geometries, can be evaluated by using LEFM models end FCPR data acquired under Kmax constant testing condition. Depending on the physical appearance of the initial defect (a single loose grain, a series of loose grains, inclusions, single and multiple initiation sites, etc), the fatigue crack growth has to be studied in at least two to three phases. This is necessary to apply appropriate LEFM models individually during a specific phase. As the evaluation demonstrates, most fatigue life is consumed in the first phase. /(max 10 const. FCPR should be used for fine-grain material. In the caes of coarse-grain material or coarse-grain surface layers thicker than approx 0.1 ram, FCPR data of Krnax 6.7 const, have to be applied.
A re-analysis o f f a t i g u e test results on w e l d e d steel m e m b e r s : k n o w l e d g e gaps, research needs and standardization requirements.
Castiglioni, C. A. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 2 1 7 - 2 2 8 The existing database of fatigue data is briefly discussed, together with further research needs in the field of long-life fatigue testing of welded structural steel details. As yet no internationally recognized recommendation has been proposed for fatigue testing methods end design of experiments on full-scale members. Some of the topics that the recommendation should cover are discussed. It is hoped that discussions may lead to the formation of a drafting panel for such a document.
Low cycle fatigue fracture l i m i t s o f structural materials and structural elements.
Yamada, M. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 184-192 Low-cycle fatigue fracture limits of structural materials steel elements and members and joints are presented and discussed as the basis of evaluation of the aseismic capacity of structures. The needs of the discussion on the testing methodologies are emphasized not only for structural materials but also and more especially for structural members for the purpose of giving a basis for investigation of the ultimate stages of structures and to establish structural design methods based upon the ultimate states.
Life prediction methods for low-cycle torsional fatigue.
Curioni, S., Freddi, A. and Caligiana, G. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 162-173
Fatigue life prediction of components locally stressed in the plastic field is a very important task for engineers employed in the automotive and electric utility industries both for design and troubleshooting of service failures. The description is given of the utilization of the local strain approach to fatigue life estimates of samples simulating the behaviour of turbine-generator shafts (NiCrMoV steel) subject to torsional oscillations. An outline of the experimental and theoretical work done by the authors is given.
Low cycle fatigue:
characterization earthquake g r o u n d m o t i o n .
of the
plastic
cycles
due t o
Cosenze, E. and Manfredi, G. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 116-131 The effect of destructive earthquake ground motion represents a very important case of low-cycle fatigue; thus the analysis of plastic cycles due to earthquake is the basis for characterizing the inelastic beheviour of structures in seismic areas. With reference to some significant ground motions and to the elastic-perfectly plastic model, the distribution of the plastic cycles is analysed in terms of number of plastic cycles, maximum plastic deformation, mean value and standard deviation of the plastic amount of the other cycles. Different probability models (normal, Iognormal, exponential, beta and gamma) are used to interpolate the real distribution of plastic cycles and, on the basis of the statistical test of Kolmogorov and Smirnov, are compared with each other.
Life predictions
in f a t i g u e
taking
into
account
material
memory
i g l s n e , ~.
Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 104-115 Torsional fatigue life prediction algorithms are considered, with particular reference to memory effect requirements. Local strain approach is employed. Stress concentration is accounted for by utilizing Neuber's rule or torque-local strain curve discretizetion. Mean stress influence is examined. Experimental results (rotor steel) relative to particular samples geometries subject to typical torque-time histories of turbomachineries are compared with numerical predictions.
532
Effect o f stress a m p l i t u d e transient o n f a t i g u e crack initiation and propagation o f h i g h s t r e n g t h steel in synthetic sea w a t e r u n d e r c a t h o l i c
protection. Maruyama, N. and Sumita, M. Tetsu-to-Hagane (J. Iron and Steel Inst. Jpn) (Apr. 1992) 78 (4) 6 4 0 - 6 4 6 (in Japanese) The effect of varying stress on fatigue life under cathodic protection by a Zn sacrificial anode and freely corroding condition in synthetic sea water and in air was investigated using 600 MPa grade high-strength steel notched specimens (Kt = 3.5). The main results obtained are as follows. (1) Under a constant stress amplitude (CSA), fatigue lives in sea water under cathodic protection ere shorter than those in air. However, under two-step stress amplitude (TSSA}, fatigue lives under cathodic protection are rather longer than those in air. (2) In a low-stressamplitude region, fatigue lives under TSSA are shorter than those under CSA in air and under cathodic protection. (3) Even when the higher stress amplitude in TSSA is much lower than the values shown by the S-N curves under CSA, the failure of notched specimens occurs. (4) Under the free corrosion potential, there is no difference in fatigue lives between under TSSA and under CSA, and Miners rule exists between them. (5) Under TSSA. macroscopic crack growth rate is retarded in air and under cathodic protection, and the effect of TSSA on macroscopic crack growth is in reverse to that on crack initiation and microscopic crack growth. Numeric data, graphs, 16 ref. The test m e t h o d of t h e r m a l f a t i g u e w i t h spherical specimen end T-n-d values.
Fang, J., Li, S. and Shi, X. J. Univ. Sci. Technol. Baijing (1992) 14 (4) 4 9 0 - 4 9 5 (in Chinese) A method by which the thermal fatigue resistance of engineering materials can be measured through spherical specimens is given. The parameters of upper temperature T, cycle time n and fracture depth d are recommended as the quantitative indices of measuring the thermal fatigue resistance of engineering materials. Three kinds (cast iron, T8 and 3Cr2W8V tool steels), and 13 specimens were tested using this method and the test results have proved the feasibility of the method. 9 ref. Microstructursl changes in bearing steel d u r i n g rolling contact f a t i g u e test.
Choi, H. C., Nam, WoJ. and Kim, S. W. J. Korean Inst. Met. Mater. (May 1992) 30 (5) 5 0 7 - 5 1 5 (in Korean) Microstructural changes in STB2 bearing steel caused by rolling contact fatigue testing were investigated using optical and scanning electron microecopy. Dark area, butterfly structures and plate-like carbides were found in the area below the flaking. Rolling contact caused the formation of the dark area in the subsurface area of contact in the region of maximum Hertzian stress. Most subsurface cracks were initiated at the region of the dark area. Butterflies found only at the oxide inclusions were frequently associated with cracks. Plate-like carbides acted as the potential crack initiation site responsible for the flaking. The growth of plate-like carbides was caused by carbon diffusion by rolling stress not only between oxide inclusions but also between fine residual carbides. Graphs, photomicrographs, spectra, 18 ref. Italian codes on f a t i g u e testing.
Atzori, B. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29-31 May 1990, pp 195-202 The various Italian codes (or instructions) dealing with the subject of fatigue are described. The aim of the description is to focus on what has already been done in Italy in the field of high-cycle fatigue testing of materials, joints, end structural components and what, instead, needs to be done. That could be useful as a basis both for a comparison with the existing codes in different countries end for the definition of possible working areas for a technical committee in the field of testing metals (steel and AI) for high-cycle fatigue beheviour of materials, joints end structural components. The analysis performed and the comparison of the various documents considered, taking into account the state of the art of fatigue design, shows clearly the need to improve the existing procedures and the need to assess new procedures to cover existing lacks in testing materials, joints, and structures. Tensile and f a t i g u e fracture b e h a v i o r and w a t e r - e n v i r o n m e n t in a SiC-whiskcr/TO75-AI composite.
effects
Komai, I., Minoshima, K. and Ryoaon, H. Compos. ScL TechnoL (1993) 46 (1) 5 9 - 6 6 An investigation has been carried out of the tensile and fatigue fracture beheviour and of the influence of water envioroments on fatigue strength of a SiC whisker reinforced high-strength AI-Zn-Mg alloy composite (SiCw/7075-T6) fabricated by a powder metallurgy process. Apart from the elongation et failure, the mechanical properties of the composite were superior to those of an unreinforced 7075-T6 alloy. The fatigue strength in air of the composite at a stress ratio of 0.1 was higher than that of an unreinforced AI alloy. At lower stress level, • water envimmant decreased the fatigue strength of the composite, although the strength was superior to that of an unreinforced AI alloy in air. A fatigue crack initiated at a whisker cluster or a crack normal to the loading axis directly initiated at the specimen surface. In water, a fatigue crack was initiated at a corrosion pit. Graphs, photomicrographs, 22 ref.
Int J Fatigue November 1993