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Fatigue crack growth micromechanisms in commercial high-purity titanium subjected to alternate plane bending
Fatigue Abstracts microstructures was caused in the stages of crack nucleation and/or microcrack growth. Graphs, photomicrographs, 11 ref. Multifactor damage simulation analysis of high temperature low cycle fatigue for superailoy based on the statistical information of crack distribution.
Fujiyama, K., Murakami, 1, Oshioka, Y. and Okabe, N. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42, (481), (in Japanese) 1212-1217 Actual components used under severe conditions often suffer from the complicated material damage affected by multiple influencing factors. 'Multifactor damage simulation analysis' was proposed to solve such complex damage problems. A discrete cluster model of material and the associated damage state matrix [DN] are introduced to solve a high-temperature lowcycle fatigue damage problem of cobalt-base superalloy FSX414. The damage state at the Nth cycle is determined by the damage state at the (N-1)th cycle expressed as follows: [DN] = [ A ] I D N - I ] . As this equation is similar to the discrete dynamical system and operator matrix [A] is non-linear, the chaotic behaviour of damage evolution may occur. Instead of solving the matrix equation directly, simulation was conducted using two-dimensional model of 2500 clusters at 5 × 5 mm area for the dendrite and grain boundary structure of FSX414. Damage of crack initiation and growth was calculated deterministically and the randomness was only introduced at the initial condition of the material. A high-temperature low-cycle fatigue test was conducted to follow up the damage process at the total strain range of 1% and at the temperature of 1123 K. Surface crack morphologies were investigated by the replication technique at 20 and 40% of the failure life. The trends of crack numbers, maximum crack length, and mean crack length against failure cycles were obtained by the simulation. These trends were chaotically complicated due to the interaction of cracks and material structure. The crack length distribution by the simulations agreed well with the experimental results and better agreement was obtained by using intermittent inspection information. These results suggest that the method would be effective for damage prediction of actual components based on inspection informations. Graphs, photomicrographs, 6 ref. Evaluation of fatigue crack propagation by using effective stress intensity factor range. Makabe, C., Kaneshiro, H., Sakihama, H. and Nishida, S. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42(481), 1200-1206 (in Japanese)
The fatigue crack propagation law was investigated under negative stress ratio R (= ~mi,/crmax) condition in the following two cases: crack propagation in the field of welding-residual stress for SM490A welded joint, and after overloading for Fe-3Si alloy. In the former case, the crack propagation rate da/dN depended on the stress ratio R. Owing to the tensile residual stress, the crack propagation rate increased as the stress ratio decreased in the range of 0 ~< R <~ -2. In the latter case, the crack progagation rate da/dN after overloading was higher at first and then became lower than that of the constant stress amplitude. In the present experimental cases, the crack propagation rate was affected by the residual stress and the overloading. However, the crack propagation behaviour had a good correlation with the crack closure behaviour. Therefore, in the present cases, the crack propagation law was reasonably evaluated with the effective stress intensity factor range AK. Graphs, 16 ref. Propagation of microscopic and macroscopic surface fatigue cracks under periodic overstr~-,,sing in Ti-6AI--4V alloy. Fujita, K. and Koterazawa, R. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42(481), 1193-1199 (in Japanese)
Microscopic and macroscopic surface fatigue crack propagation studies were carried out under periodic overstressing with Ti-6AI-4V alloy. Relatively small acceleration of crack propagation (less than ten times) occurred when the crack length was of macroscopic scale > 200 ixm. The fracture surface morphology showed that the crack propagated in a zigzag manner similarly to the cases of macroscopic cracks with large acceleration in carbon steels and aluminium alloys reported previously. The acceleration was compared with that of through-thickness cracks and macroscopic and microscopic surface cracks in steels and AI alloys reported previously. A relatively good correlation was obtained in terms of the tensile strength-Young's modulus ratio era~E, and the acceleration became larger as tra/E became smaller. The surface crack growth rate in Ti-6AI-4V alloy, steels, and Al alloy under constantamplitude loading agreed well with each other and with the through-thicknes crack growth rate in different materials published in literature in the aKcf/E diagram. Graphs, photomicrographs, 24 ref. Fatigue crack growth of Ti-6AI-4V alloy at elevated temperatures. Ogawa, T., Hayashi, Y., Tokaji, K. and Hirose, M. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42 (481), 1186-1192 (in Japanese) Fatigue crack growth has been investigated in Ti--6AI-4V alloy at ambient and elevated temperatures, i.e. 200, 400 and 500~C. Special attention was paid to the nucleation and the morphology of secondary cracks. The roughness of fracture surface was evaluated quantitatively using three-dimensional reconstruction of fracture surface based on a stereo matching technique. The results indicated that the crack growth rates at the elevated temperatures became lower than that at room temperature with increasing stress intensity factor range, AK. This behaviour was attributed to the nucleation of secondary cracks, which led to the increased roughness of fracture surface and the reduction of crack driving force. The nucleation of the secondary cracks was observed more markedly with increasing test temperature. On the other hand, the crack growth rate at 5000C increased compared with that at room temperature in the region of AK ~< 20 MPa m 1/2. This enhanced crack growth may be due to the reduced elastic modulus with increasing temperature.
588 Fatigue, 1994, Vol 16, November
Therefore, the crack growth characteristics at elevated temperatures were controlled by the competing effects between the nucleation of secondary cracks and the reduction of elastic modulus. Graphs, photomicrographs, 11 ref. Fatigue strength and fatigue-crack initiation of Ti-6AI-4V Alloy. Nishida, S., Urashima, C. and Takano, N. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42(481) 1179-1185 (in Japanese)
The fatigue strength in a high-cycle region has been evaluated in this test using the representative titanium alloy Ti-6AI-4V under rotating bending fatigue, and the fractographic consideration is adopted to analyse the testing results. In addition, the fatigue crack initiation process has been investigated with successive observation in the scanning electron microscope (SEM) equipment under out-of-plane bending fatigue. The main results obtained in this test are as follows. (1) In rotating bending fatigue tests in air, fracture was often observed even after 107 cycles. When the specimen was broken by less repetitions that 1 × 106 cycles, the fracture mostly initiated from the outer surface with a facet-like pattern at the origin. On the other hand, in the case of higher repetitions than 2 × 106 cycles, the fracture initiated from the subsurface, where there were neither observed inclusions nor the facetlike fracture pattern. (2)In out-of-plane bending fatigue test in SEM, the ratio of crack initiation life of one grain length to total fatigue life is between 3 and 9% and the fatigue cracks are initiated by 90% from alpha grains and by the residual ratio from alpha/beta boundaries. In addition, the slip bands are at first generated in alpha grains under the cyclic stress, then there appear cross-slip bands and finally those portions become microcracks like cleavage-cracks. Graphs, photomicrographs, 8 ref. The root area parameter model for evaluation of effects of various artillcal small defects and mutual interaction of small defects on fatigue limit. Toriyama,
T. and Murakami, Y. J. Soc. Mater. Sci. Jpn (Oct 1993) 42 (481), 1160-1166 (in Japanese) The fatigue limit of metals containing various small defects can be evaluated by the root area parameter model. The model is constructed with two parameters: a geometrical parameter (root area) and Vickers hardness Hv of matrix. To demonstrate the validity of the root area parameter model, it is experimentally shown that the fatigue limits of hard steel specimens (Fe-0.003C-18.52Ni-7.8Co-4.89Mo maraging steel, H~ = 510) having an initial crack, a two-hole defect connected with fatigue crack and one hole defect with the same value of root area are approximately identical in spite of their big difference in stress concentration factor. In the next step, the method of evaluating the mutual interaction effect of small defects is proposed on the basis of the fatigue test results and three-dimensional numerical stress analysis of crack interaction. Finally, the method is applied to evaluate the mutual interaction effect of spheroidal graphite in a nodular cast iron. The lower bound of scatter in fatigue limit (cr,l) is obtained by using the fatigue limit prediction equation with the expected maximum size of graphite (root area max). The expected value of root area max in a large number of specimens can be estimated by statistics of extreme values. To predict an accurate tr,l, the value of root area max must be estimated by considering the mutual interaction effect of spheroidal graphite. Otherwise, the prediction becomes unconservative. Graphs, photomicrographs, 18 ref. Fatigue crack growth micromechanisms in commercial high-purity titanium subjected to alternate plane bending. Sugano, M. and Satake, T. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42(481), 1146-1152 (in Japanese)
Large grain specimens (0.3 mm thick) with average grain size of 10 mm in diameter were fatigued under alternate plane bending to clarify the micromechanism of fatigue crack growth in high-purity titanium (KS40S). For a crystallographic study of fatigue fracture surface, the stereomicrophotogrammctry technique was incorporated with the X-ray microbeam Laue technique, which was used to determine the orientation of specimen surface. The fatigue fracture surfaces comprised (0001), {10-10}, (10-11}, {10-12}, {11-21}, and {11-20} planes. The (0001) cleavage facet appeared in grains with specimen axes in the angular domain of ~b < 65 °, d~ being the angle between a specimen axis and a basal plane normal. Twinning played an important role in fatigue crack growth behaviour. The formation of a microcrack on { 10-10} and { 11-20) planes was discussed in association with a sessile dislocation a/3 <1-100> resulting from the following reaction of a/ 3 <-1-120< dislocations on the {10-10} primary slip planes: a/312-1-10] + a/3[-1-120] --~ a/3[1-100] + 2/9 a[-1-120] + a/912-1-10]. Crystallographic aspects of fatigue deformation were heterogeneous in the process zone wake of a crack in one grain. Photomicrographs, 17 ref. A contribution to the problem of the endurance limit of nitrided service parts.
Kern, T.-U. and Spies, H.-J. Proc. Conf. Surface Engineering, Bremen, Germany, 1993, pp. 187-189 This paper describes summarily the realized investigations to estimate the endurance limit of nitrided, service part equal samples made at the Institute of Material Engineering of the TU Bergakademie Freiberg. The concept of the local endurance limit was the basis of these investigations, which was extended on points resulting from the speciality of service parts. The relation of Smith, Watson and Topper, modified for stresses near the endurance limit, allows the evaluation of the local endurance limit of nitrided surface layers. A statistical size effect (parameter L) also exists for surface-hardened samples. It is possible to register it with the concept of Kogaev and Serensen. A change in cross-section, stress kind and sample size can be described with this concept in combination with the order parameter relative stress gradient
Fujiyama, K., Murakami, 1, Oshioka, Y. and Okabe, N. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42, (481), (in Japanese) 1212-1217 Actual components used under severe conditions often suffer from the complicated material damage affected by multiple influencing factors. 'Multifactor damage simulation analysis' was proposed to solve such complex damage problems. A discrete cluster model of material and the associated damage state matrix [DN] are introduced to solve a high-temperature lowcycle fatigue damage problem of cobalt-base superalloy FSX414. The damage state at the Nth cycle is determined by the damage state at the (N-1)th cycle expressed as follows: [DN] = [ A ] I D N - I ] . As this equation is similar to the discrete dynamical system and operator matrix [A] is non-linear, the chaotic behaviour of damage evolution may occur. Instead of solving the matrix equation directly, simulation was conducted using two-dimensional model of 2500 clusters at 5 × 5 mm area for the dendrite and grain boundary structure of FSX414. Damage of crack initiation and growth was calculated deterministically and the randomness was only introduced at the initial condition of the material. A high-temperature low-cycle fatigue test was conducted to follow up the damage process at the total strain range of 1% and at the temperature of 1123 K. Surface crack morphologies were investigated by the replication technique at 20 and 40% of the failure life. The trends of crack numbers, maximum crack length, and mean crack length against failure cycles were obtained by the simulation. These trends were chaotically complicated due to the interaction of cracks and material structure. The crack length distribution by the simulations agreed well with the experimental results and better agreement was obtained by using intermittent inspection information. These results suggest that the method would be effective for damage prediction of actual components based on inspection informations. Graphs, photomicrographs, 6 ref. Evaluation of fatigue crack propagation by using effective stress intensity factor range. Makabe, C., Kaneshiro, H., Sakihama, H. and Nishida, S. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42(481), 1200-1206 (in Japanese)
The fatigue crack propagation law was investigated under negative stress ratio R (= ~mi,/crmax) condition in the following two cases: crack propagation in the field of welding-residual stress for SM490A welded joint, and after overloading for Fe-3Si alloy. In the former case, the crack propagation rate da/dN depended on the stress ratio R. Owing to the tensile residual stress, the crack propagation rate increased as the stress ratio decreased in the range of 0 ~< R <~ -2. In the latter case, the crack progagation rate da/dN after overloading was higher at first and then became lower than that of the constant stress amplitude. In the present experimental cases, the crack propagation rate was affected by the residual stress and the overloading. However, the crack propagation behaviour had a good correlation with the crack closure behaviour. Therefore, in the present cases, the crack propagation law was reasonably evaluated with the effective stress intensity factor range AK. Graphs, 16 ref. Propagation of microscopic and macroscopic surface fatigue cracks under periodic overstr~-,,sing in Ti-6AI--4V alloy. Fujita, K. and Koterazawa, R. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42(481), 1193-1199 (in Japanese)
Microscopic and macroscopic surface fatigue crack propagation studies were carried out under periodic overstressing with Ti-6AI-4V alloy. Relatively small acceleration of crack propagation (less than ten times) occurred when the crack length was of macroscopic scale > 200 ixm. The fracture surface morphology showed that the crack propagated in a zigzag manner similarly to the cases of macroscopic cracks with large acceleration in carbon steels and aluminium alloys reported previously. The acceleration was compared with that of through-thickness cracks and macroscopic and microscopic surface cracks in steels and AI alloys reported previously. A relatively good correlation was obtained in terms of the tensile strength-Young's modulus ratio era~E, and the acceleration became larger as tra/E became smaller. The surface crack growth rate in Ti-6AI-4V alloy, steels, and Al alloy under constantamplitude loading agreed well with each other and with the through-thicknes crack growth rate in different materials published in literature in the aKcf/E diagram. Graphs, photomicrographs, 24 ref. Fatigue crack growth of Ti-6AI-4V alloy at elevated temperatures. Ogawa, T., Hayashi, Y., Tokaji, K. and Hirose, M. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42 (481), 1186-1192 (in Japanese) Fatigue crack growth has been investigated in Ti--6AI-4V alloy at ambient and elevated temperatures, i.e. 200, 400 and 500~C. Special attention was paid to the nucleation and the morphology of secondary cracks. The roughness of fracture surface was evaluated quantitatively using three-dimensional reconstruction of fracture surface based on a stereo matching technique. The results indicated that the crack growth rates at the elevated temperatures became lower than that at room temperature with increasing stress intensity factor range, AK. This behaviour was attributed to the nucleation of secondary cracks, which led to the increased roughness of fracture surface and the reduction of crack driving force. The nucleation of the secondary cracks was observed more markedly with increasing test temperature. On the other hand, the crack growth rate at 5000C increased compared with that at room temperature in the region of AK ~< 20 MPa m 1/2. This enhanced crack growth may be due to the reduced elastic modulus with increasing temperature.
588 Fatigue, 1994, Vol 16, November
Therefore, the crack growth characteristics at elevated temperatures were controlled by the competing effects between the nucleation of secondary cracks and the reduction of elastic modulus. Graphs, photomicrographs, 11 ref. Fatigue strength and fatigue-crack initiation of Ti-6AI-4V Alloy. Nishida, S., Urashima, C. and Takano, N. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42(481) 1179-1185 (in Japanese)
The fatigue strength in a high-cycle region has been evaluated in this test using the representative titanium alloy Ti-6AI-4V under rotating bending fatigue, and the fractographic consideration is adopted to analyse the testing results. In addition, the fatigue crack initiation process has been investigated with successive observation in the scanning electron microscope (SEM) equipment under out-of-plane bending fatigue. The main results obtained in this test are as follows. (1) In rotating bending fatigue tests in air, fracture was often observed even after 107 cycles. When the specimen was broken by less repetitions that 1 × 106 cycles, the fracture mostly initiated from the outer surface with a facet-like pattern at the origin. On the other hand, in the case of higher repetitions than 2 × 106 cycles, the fracture initiated from the subsurface, where there were neither observed inclusions nor the facetlike fracture pattern. (2)In out-of-plane bending fatigue test in SEM, the ratio of crack initiation life of one grain length to total fatigue life is between 3 and 9% and the fatigue cracks are initiated by 90% from alpha grains and by the residual ratio from alpha/beta boundaries. In addition, the slip bands are at first generated in alpha grains under the cyclic stress, then there appear cross-slip bands and finally those portions become microcracks like cleavage-cracks. Graphs, photomicrographs, 8 ref. The root area parameter model for evaluation of effects of various artillcal small defects and mutual interaction of small defects on fatigue limit. Toriyama,
T. and Murakami, Y. J. Soc. Mater. Sci. Jpn (Oct 1993) 42 (481), 1160-1166 (in Japanese) The fatigue limit of metals containing various small defects can be evaluated by the root area parameter model. The model is constructed with two parameters: a geometrical parameter (root area) and Vickers hardness Hv of matrix. To demonstrate the validity of the root area parameter model, it is experimentally shown that the fatigue limits of hard steel specimens (Fe-0.003C-18.52Ni-7.8Co-4.89Mo maraging steel, H~ = 510) having an initial crack, a two-hole defect connected with fatigue crack and one hole defect with the same value of root area are approximately identical in spite of their big difference in stress concentration factor. In the next step, the method of evaluating the mutual interaction effect of small defects is proposed on the basis of the fatigue test results and three-dimensional numerical stress analysis of crack interaction. Finally, the method is applied to evaluate the mutual interaction effect of spheroidal graphite in a nodular cast iron. The lower bound of scatter in fatigue limit (cr,l) is obtained by using the fatigue limit prediction equation with the expected maximum size of graphite (root area max). The expected value of root area max in a large number of specimens can be estimated by statistics of extreme values. To predict an accurate tr,l, the value of root area max must be estimated by considering the mutual interaction effect of spheroidal graphite. Otherwise, the prediction becomes unconservative. Graphs, photomicrographs, 18 ref. Fatigue crack growth micromechanisms in commercial high-purity titanium subjected to alternate plane bending. Sugano, M. and Satake, T. J. Soc. Mater. Sci. Jpn (Oct. 1993) 42(481), 1146-1152 (in Japanese)
Large grain specimens (0.3 mm thick) with average grain size of 10 mm in diameter were fatigued under alternate plane bending to clarify the micromechanism of fatigue crack growth in high-purity titanium (KS40S). For a crystallographic study of fatigue fracture surface, the stereomicrophotogrammctry technique was incorporated with the X-ray microbeam Laue technique, which was used to determine the orientation of specimen surface. The fatigue fracture surfaces comprised (0001), {10-10}, (10-11}, {10-12}, {11-21}, and {11-20} planes. The (0001) cleavage facet appeared in grains with specimen axes in the angular domain of ~b < 65 °, d~ being the angle between a specimen axis and a basal plane normal. Twinning played an important role in fatigue crack growth behaviour. The formation of a microcrack on { 10-10} and { 11-20) planes was discussed in association with a sessile dislocation a/3 <1-100> resulting from the following reaction of a/ 3 <-1-120< dislocations on the {10-10} primary slip planes: a/312-1-10] + a/3[-1-120] --~ a/3[1-100] + 2/9 a[-1-120] + a/912-1-10]. Crystallographic aspects of fatigue deformation were heterogeneous in the process zone wake of a crack in one grain. Photomicrographs, 17 ref. A contribution to the problem of the endurance limit of nitrided service parts.
Kern, T.-U. and Spies, H.-J. Proc. Conf. Surface Engineering, Bremen, Germany, 1993, pp. 187-189 This paper describes summarily the realized investigations to estimate the endurance limit of nitrided, service part equal samples made at the Institute of Material Engineering of the TU Bergakademie Freiberg. The concept of the local endurance limit was the basis of these investigations, which was extended on points resulting from the speciality of service parts. The relation of Smith, Watson and Topper, modified for stresses near the endurance limit, allows the evaluation of the local endurance limit of nitrided surface layers. A statistical size effect (parameter L) also exists for surface-hardened samples. It is possible to register it with the concept of Kogaev and Serensen. A change in cross-section, stress kind and sample size can be described with this concept in combination with the order parameter relative stress gradient