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Fatigue crack tip deformation processes as influenced by the environment
Fatigue crack tip deformation processes as influenced by the environment. Diss. Abstr. /nL Dec. 1990 51, (6), 147 pp The process of stage-II fatigue crack tip deformation as influenced by the environment was studied in austenitic stainless steels and the process of stage-I fatigue crack propagation was studied in a single crystal of a Ni-based superaltoy. A new technique was developed to observe the crack tip region under load in plane strain conditions. The effect of environment on the dislocation substructure under the fracture surfaces was studied as well. It was found that in air, environmentally assisted cracking led to strain localization, resulting in the formation of sharper cracks and faster crack growth rates than in vacuum. In vacuum the deformation was widespread and slip reversal was extensive, resulting in slow crack growth rates. A mechanism, based upon environmentally assisted cracking and strain localization, for the formation of striations in air and their absence in vacuum is proposed. In metastable austenitic stainless steels, the extent of the stress-induced transformation in vacuum was greater with respect to air owing to the greater crack tip deformations. In tests in air, strain localization kept the deformation confined to a narrow volume next to the crack tip so the extent of the stress-induced transformation was less than in vacuum tests. The dependence of the crack opening level in type-304 stainless steel tested in air is explained in terms of the dependence of the amount of transformed martensite on the stress intensity at the crack tip and the gradient in the residual volume expansion behind the crack tip, which affected the crack closure distances behind the crack tip. More evidence that supports the cross-slip extrusion mechanism of crack growth in stage-I fatigue crack growth was obtained. The evidence consisted of the verification of the existence of extrusions and two slip systems with their Burger's vectors on the plane of fracture, consistent with pure mode-II deformation characteristics as proposed in that mechanism.
Short and long fatigue crack growth in 2024-T3 under Fokker 100 spectrum loading. Wanhill, R.J.H. and Schra, L. National Aerospace Laboratory Report No N91-11248/2/XAB Aug. 1990 26 pp The behaviour of short and long fatigue cracks in the widely used damage-tolerant AI alloy 2024-T3 was compared using flight simulation loading representations for the Fokker 100 wing/fuselage structure. The results showed that the apparently anomalous behaviour of short cracks is not significant for the durability analysis of the current wing/fuselage structure. In addition, the data provide a reference for evaluating new candidate materials for durable wing/fuselage structures in transport aircraft,
Fatigue propagation of t w o parallel cracks. Jiang, Z.D., Petit, J. and Bezme, G. Eng. Fract. Mech. 1990 37, (5), 1139-1144 The objective is to study the fatigue propagation behaviour of two parallel edge cracks in a finite-width sheet. By using the stress intensity factor relation proposed (e.g., 7475-T7351 All experimental data are analysed and the influence of the geometry of edge-cracked sheets on the crack growth rate is discussed. The applicability of the maximum dilatational strain energy density criterion, the Tcriterion, as proposed by Theocaris and Andrianopoulos, is investigated for use in a crack propagation angle study. Graphs. 16 refs. C u m u l a t i v e damage model for mean fatigue crack l~lrowth based on the kinetic theory of thermally activated fracture. Kozm, F. and Bogdanoff,
J.L. Eng. Fract. Mech. 1990 37, (5), 9 9 5 - 1 0 1 0 The objective is to show how the physical concepts developed by Tobolsky-Eyring for the description of atomic (micro) bond fracture can be extended to the macroproblem of the mean fatigue crack growth, which is of engineering interest. The parameters of the model have a physical meaning. The estimation of parameters is studied with the aid of four data sets (e.g. 10Ni-8Co-IMo steel, 2024-T3 and 7075-T6 AI) on fatigue crack growth. Because of the fundamental physical concepts employed, the model provides new insights into the fatigue crack growth process. Graphs. 18 refs. Probabilistic fracture mechanics application to highway bridges, Yazdani,
N. and Albrecht, P. Eng. FracL Mech. 1990 37, (5), 9 6 9 - 9 8 5 A probabilistic fracture mechanics model was developed for determining the risk of fatigue failure of steel highway bridges. A deterministic fracture mechanics calculation of crack growth with stochastic inputs for crack growth rate, fracture toughness, initial crack size and load history was used. Stress intensity expressions were compiled from the literature for details of typical AASHTO category bridges. Statistical distributions for the input variables were derived from the data reported in the literature. The model was found to predict the short service life of the Yellow Mill Pond Bridge in Connecticut well. The materials investigated were the steels A36, A588, and A514. Graphs. 22 refs.
Near-threshold fatigue crack growth behaviour of a t i t a n i u m alloy: T i - 6 A I - 4 V . Ravichandran, K.S. Acta Metal/. Mater. Mar. 1991 39, (3), 4 0 1 - 4 1 0 Near-threshold fatigue crack growth behaviour of Ti-6AI-4V alloy was investigated as a function of Widmanstatten microstructures. In particular, the effect of colony size on AKth, AKeff,th and Kckth has been studied. It has been found that crack growth rates are strongly affected by the size of microstructural features such as colonies and (x-laths. However, the microstructural units controlling crack growth are colonies in fast-cooled microstructures consisting of fine Widmanstatten colonies while there are (x-laths in relatively slow cooled ones with coarse colonies. As a result, the results in the literature about the increased fatigue crack growth resistance at large colony sizes in Ti alloys cannot be generalized, This distinction appears to be brought about by the thick continuous interplatelet ~-phase present in slow-cooled structures. In fast-cooled structures, the thin discontinuous ~-phase is seen to be ineffective in arresting slip or crack. However, in slow-cooled ones the thick ~-phase appears to effectively retard the slip~crack in fatigue. The thickness, composition, intrinsic properties such as modulus and ductility of the ~-phase and an added environmental effect have been suggested to be important in this respec¢. The crack growth rates and the magnitudes of ~Kth and ~'Keff.th can be uniquely ordered when
434
compared in terms of the controlling microstructural units in the respective microstructures. However, crack closure levels at threshold appear to be dependent on colony size. In addition, an increase in the intrinsic crack growth resistance ~Keff,th appears to exist when the cyclic plastic zone size approaches the thickness of Q-laths in the microstructure. Graphs, photomicrographs. 31 refs.
Dislocation emission and fatigue crack growth threshold. Pippan, R. Acta Metal/. Mater. Mar. 1991 39, (3), 2 5 5 - 2 6 2 The stress intensity range ~K below which no cyclic plastic deformation at the crack tip, and hence, no fatigue crack propagation occurs is investigated. The emission of dislocations from the crack tip is assumed as the mechanism for the dislocating generation. For a mode-Ill crack a computer simulation is carried out to study the influence of the number of dislocations, the friction stress and the critical stress intensity, ke, to emit a dislocation. If during loading only one dislocation is emitted, the return of this dislocation to the crack tip and the emission of a dislocation with an opposite sign and the recombination with the first dislocation are possible during unloading. The ~.K necessary for both mechanisms is approximately 2Ke. If during loading more than one dislocation is emitted, during unloading at first a certain number of dislocations return to the crack tip before a dislocation of opposite sign is emitted. The value of AK necessary to move one dislocation back to the crack tip during unloading decreases with increasing number of dislocations and reaches a constant value of approx 1.1 ke. This value of ~.K is then roughly independent of the friction stress and /re. The materials investigated were AI and Fe. Graphs. 19 refs.
A unified approach to fatigue crack propagation in metals and polymers. Chow, C.L. and Lu, TJ. J. Mater. Sci. Lett. Dec. 1990 9, (12), 1427-1430 This review covers the development of a unified approach to characterizing the fatigue crack propagation (FCP) of both metals and non-metals. The applicability of the equations discussed for producing a master FCP equation has been examined by Chow and his co-workers for AI alloys (2024-T3 and 7075-T6), mild steel, PMMA, PVC and a short glass fibre reinforced nylon 66. The application of the unified FCP law to characterize both metals and polymers with a master diagram can be concluded to be successful, despite the fact that the mechanical properties of the materials may differ by one order of magnitude. This result supports an earlier conclusion of Cherepanov and Halmanov that the fatigue crack growth is governed by the same mechanism for all materials manifesting plastic properties. Graphs. 30 refs.
The effect of martensite content on the fatigue behaviour of a f e r r i t i c - m a r t e n s i t i c steel. Aran, A. and Turker, H. J. Mater. Sci. Lett. Dec. 1990 9, (12), 1407-1408 Results are reported from a study on the effect of martensite content on the high cycle fatigue performance in which possible correlations between the quasi-static and fatigue properties have been sought. It was shown that the yield, and tensile and high-cycle fatigue strength of ferritic-martensitic steels (e.g., 5115) increase with increasing martensite content continuously. The correlation between the yield strength and the fatigue strength can be expressed quantitatively and used to obtain approximate data for design purposes. Graphs. 8 refs.
Further development of the helical extension spring. Huhnen, J. Wire Nov. 1990 40, (6), 6 2 2 - 6 2 4 Extension springs have advantages over compression springs for some types of construction but have a fatal weak point at the loop root often leading to fracture. In an attempt to improve this situation, the author introduced shot peening of extension springs in the company R. Bosch in about 1956, peening mainly on the coil outer suface and loop root. "Bowing" and relaxation at the loop root under high stress can of course not be reduced in this way, and the process cannot be used for wires less than about 1.5 mm in diameter with index numbers larger than eight or nine and large spring lengths, due to excessive deformations, among other reasons. With new ends the fatigue strength of the spring material, as given in DIN 2089 Sheet 1 for the unpeened condition, for example, can be fully utilized for extension springs. Graphs. 5 refs.
Damage to and defects in structural elements due to design and fabrication not being adapted to the heat treatment. III, Uhlig, W. Metal/. Nov.-Dec. 1990 44, (6), 175-177 (in German) Fabrication must be adapted to the changed processing properties of the heattreated material. The state of the material will to some extent at least be influenced during trueing, grinding, erosion, electroplating, blasting and cleaning after heat treatment. Thus on chromizing steel 2OMoCr5 case-hardened pinion shaft butt ends, irrespective of the extreme H risk to the high-C martensite structure, no heat treatment (e.g., at 200°C) to remove the H was undertaken. Cracking of the chromized zone eventually resulted in fatigue failure at the shaft-pinion cross-section enlargement zone. Graphs, photomicrographs. 6 refs.
Fatigue crack speed of materials with linear hardening. Li, Y.C. and Huang, N.C. Int. J. Solids and Structures 1991 27, (7), 8 6 5 - 8 8 3 This investigation deals with the steady propagation of a fatigue crack in a thin plate, subjected to cyclic tensile remote loading. The problem is analysed according to a Dugdale model of the fracture process zone. The material stretches periodically in the fracture process zone prior to its failure. The effect of crack closure is considered in the analysis. Unlike previous work the study is based on a constitutive relation of the linear work-hardening for material in the fracture process zone. The criterion of total accumulative plastic work is adopted for the determination of the growth rate of the fatigue crack. The theoretical reau,ltsthus obtained are compared with experimental data, on AI alloy 2024. 16 ~efs.
Int J Fatigue September 1991
Short and long fatigue crack growth in 2024-T3 under Fokker 100 spectrum loading. Wanhill, R.J.H. and Schra, L. National Aerospace Laboratory Report No N91-11248/2/XAB Aug. 1990 26 pp The behaviour of short and long fatigue cracks in the widely used damage-tolerant AI alloy 2024-T3 was compared using flight simulation loading representations for the Fokker 100 wing/fuselage structure. The results showed that the apparently anomalous behaviour of short cracks is not significant for the durability analysis of the current wing/fuselage structure. In addition, the data provide a reference for evaluating new candidate materials for durable wing/fuselage structures in transport aircraft,
Fatigue propagation of t w o parallel cracks. Jiang, Z.D., Petit, J. and Bezme, G. Eng. Fract. Mech. 1990 37, (5), 1139-1144 The objective is to study the fatigue propagation behaviour of two parallel edge cracks in a finite-width sheet. By using the stress intensity factor relation proposed (e.g., 7475-T7351 All experimental data are analysed and the influence of the geometry of edge-cracked sheets on the crack growth rate is discussed. The applicability of the maximum dilatational strain energy density criterion, the Tcriterion, as proposed by Theocaris and Andrianopoulos, is investigated for use in a crack propagation angle study. Graphs. 16 refs. C u m u l a t i v e damage model for mean fatigue crack l~lrowth based on the kinetic theory of thermally activated fracture. Kozm, F. and Bogdanoff,
J.L. Eng. Fract. Mech. 1990 37, (5), 9 9 5 - 1 0 1 0 The objective is to show how the physical concepts developed by Tobolsky-Eyring for the description of atomic (micro) bond fracture can be extended to the macroproblem of the mean fatigue crack growth, which is of engineering interest. The parameters of the model have a physical meaning. The estimation of parameters is studied with the aid of four data sets (e.g. 10Ni-8Co-IMo steel, 2024-T3 and 7075-T6 AI) on fatigue crack growth. Because of the fundamental physical concepts employed, the model provides new insights into the fatigue crack growth process. Graphs. 18 refs. Probabilistic fracture mechanics application to highway bridges, Yazdani,
N. and Albrecht, P. Eng. FracL Mech. 1990 37, (5), 9 6 9 - 9 8 5 A probabilistic fracture mechanics model was developed for determining the risk of fatigue failure of steel highway bridges. A deterministic fracture mechanics calculation of crack growth with stochastic inputs for crack growth rate, fracture toughness, initial crack size and load history was used. Stress intensity expressions were compiled from the literature for details of typical AASHTO category bridges. Statistical distributions for the input variables were derived from the data reported in the literature. The model was found to predict the short service life of the Yellow Mill Pond Bridge in Connecticut well. The materials investigated were the steels A36, A588, and A514. Graphs. 22 refs.
Near-threshold fatigue crack growth behaviour of a t i t a n i u m alloy: T i - 6 A I - 4 V . Ravichandran, K.S. Acta Metal/. Mater. Mar. 1991 39, (3), 4 0 1 - 4 1 0 Near-threshold fatigue crack growth behaviour of Ti-6AI-4V alloy was investigated as a function of Widmanstatten microstructures. In particular, the effect of colony size on AKth, AKeff,th and Kckth has been studied. It has been found that crack growth rates are strongly affected by the size of microstructural features such as colonies and (x-laths. However, the microstructural units controlling crack growth are colonies in fast-cooled microstructures consisting of fine Widmanstatten colonies while there are (x-laths in relatively slow cooled ones with coarse colonies. As a result, the results in the literature about the increased fatigue crack growth resistance at large colony sizes in Ti alloys cannot be generalized, This distinction appears to be brought about by the thick continuous interplatelet ~-phase present in slow-cooled structures. In fast-cooled structures, the thin discontinuous ~-phase is seen to be ineffective in arresting slip or crack. However, in slow-cooled ones the thick ~-phase appears to effectively retard the slip~crack in fatigue. The thickness, composition, intrinsic properties such as modulus and ductility of the ~-phase and an added environmental effect have been suggested to be important in this respec¢. The crack growth rates and the magnitudes of ~Kth and ~'Keff.th can be uniquely ordered when
434
compared in terms of the controlling microstructural units in the respective microstructures. However, crack closure levels at threshold appear to be dependent on colony size. In addition, an increase in the intrinsic crack growth resistance ~Keff,th appears to exist when the cyclic plastic zone size approaches the thickness of Q-laths in the microstructure. Graphs, photomicrographs. 31 refs.
Dislocation emission and fatigue crack growth threshold. Pippan, R. Acta Metal/. Mater. Mar. 1991 39, (3), 2 5 5 - 2 6 2 The stress intensity range ~K below which no cyclic plastic deformation at the crack tip, and hence, no fatigue crack propagation occurs is investigated. The emission of dislocations from the crack tip is assumed as the mechanism for the dislocating generation. For a mode-Ill crack a computer simulation is carried out to study the influence of the number of dislocations, the friction stress and the critical stress intensity, ke, to emit a dislocation. If during loading only one dislocation is emitted, the return of this dislocation to the crack tip and the emission of a dislocation with an opposite sign and the recombination with the first dislocation are possible during unloading. The ~.K necessary for both mechanisms is approximately 2Ke. If during loading more than one dislocation is emitted, during unloading at first a certain number of dislocations return to the crack tip before a dislocation of opposite sign is emitted. The value of AK necessary to move one dislocation back to the crack tip during unloading decreases with increasing number of dislocations and reaches a constant value of approx 1.1 ke. This value of ~.K is then roughly independent of the friction stress and /re. The materials investigated were AI and Fe. Graphs. 19 refs.
A unified approach to fatigue crack propagation in metals and polymers. Chow, C.L. and Lu, TJ. J. Mater. Sci. Lett. Dec. 1990 9, (12), 1427-1430 This review covers the development of a unified approach to characterizing the fatigue crack propagation (FCP) of both metals and non-metals. The applicability of the equations discussed for producing a master FCP equation has been examined by Chow and his co-workers for AI alloys (2024-T3 and 7075-T6), mild steel, PMMA, PVC and a short glass fibre reinforced nylon 66. The application of the unified FCP law to characterize both metals and polymers with a master diagram can be concluded to be successful, despite the fact that the mechanical properties of the materials may differ by one order of magnitude. This result supports an earlier conclusion of Cherepanov and Halmanov that the fatigue crack growth is governed by the same mechanism for all materials manifesting plastic properties. Graphs. 30 refs.
The effect of martensite content on the fatigue behaviour of a f e r r i t i c - m a r t e n s i t i c steel. Aran, A. and Turker, H. J. Mater. Sci. Lett. Dec. 1990 9, (12), 1407-1408 Results are reported from a study on the effect of martensite content on the high cycle fatigue performance in which possible correlations between the quasi-static and fatigue properties have been sought. It was shown that the yield, and tensile and high-cycle fatigue strength of ferritic-martensitic steels (e.g., 5115) increase with increasing martensite content continuously. The correlation between the yield strength and the fatigue strength can be expressed quantitatively and used to obtain approximate data for design purposes. Graphs. 8 refs.
Further development of the helical extension spring. Huhnen, J. Wire Nov. 1990 40, (6), 6 2 2 - 6 2 4 Extension springs have advantages over compression springs for some types of construction but have a fatal weak point at the loop root often leading to fracture. In an attempt to improve this situation, the author introduced shot peening of extension springs in the company R. Bosch in about 1956, peening mainly on the coil outer suface and loop root. "Bowing" and relaxation at the loop root under high stress can of course not be reduced in this way, and the process cannot be used for wires less than about 1.5 mm in diameter with index numbers larger than eight or nine and large spring lengths, due to excessive deformations, among other reasons. With new ends the fatigue strength of the spring material, as given in DIN 2089 Sheet 1 for the unpeened condition, for example, can be fully utilized for extension springs. Graphs. 5 refs.
Damage to and defects in structural elements due to design and fabrication not being adapted to the heat treatment. III, Uhlig, W. Metal/. Nov.-Dec. 1990 44, (6), 175-177 (in German) Fabrication must be adapted to the changed processing properties of the heattreated material. The state of the material will to some extent at least be influenced during trueing, grinding, erosion, electroplating, blasting and cleaning after heat treatment. Thus on chromizing steel 2OMoCr5 case-hardened pinion shaft butt ends, irrespective of the extreme H risk to the high-C martensite structure, no heat treatment (e.g., at 200°C) to remove the H was undertaken. Cracking of the chromized zone eventually resulted in fatigue failure at the shaft-pinion cross-section enlargement zone. Graphs, photomicrographs. 6 refs.
Fatigue crack speed of materials with linear hardening. Li, Y.C. and Huang, N.C. Int. J. Solids and Structures 1991 27, (7), 8 6 5 - 8 8 3 This investigation deals with the steady propagation of a fatigue crack in a thin plate, subjected to cyclic tensile remote loading. The problem is analysed according to a Dugdale model of the fracture process zone. The material stretches periodically in the fracture process zone prior to its failure. The effect of crack closure is considered in the analysis. Unlike previous work the study is based on a constitutive relation of the linear work-hardening for material in the fracture process zone. The criterion of total accumulative plastic work is adopted for the determination of the growth rate of the fatigue crack. The theoretical reau,ltsthus obtained are compared with experimental data, on AI alloy 2024. 16 ~efs.
Int J Fatigue September 1991