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Thermomechanical fatigue, oxidation and creep: II. Life production
tensile strength, are affected most strongly by the martanslte percentage, martansita hardness end ferrite grain size being other significant parameters. Based on measurements over a wide range of growth rates, crack growth rates are found to be significantly correlated with the tensile strength. Since RPZS is inversely related to 4 , the mechanism of failure appears to be controlled by the crack-tip plasticity and roughly correlated with the size of the plastic zone, especially in the intermediate growth rate region. It is confirmed that the effect of microstructure and R-ratio of fatigue crack at high and low growth rates is more significant than in the intermediate growth rate. High fatigue threshold levels were obtained by developing dual-phase microstructures. The &Kth values were found in the present case to decrease approximately linearly with increase in both yield and tensile stength. Correlation of A/~h values with microstructurel parameters showed that the principal variables affecting threshold level are martensite percentage, mertensite grain size and martenslte hardness. The strong effect of martensita grain size found is taken to be evidence for the effect of crack path tortunsity on thrsshold levels. As KmJx approached Kic an influence of materiel strength on fracture toughness values KI¢ was observed, due to the occurrence of static fracture modes. Fractogrephic study of the fatigue cracks showed that at near threshold stress intensity the fracture mode was transgrenular with some cleavage fracture for all structures, the fracture surface having a rougher stepped appearance in ferrita matrix structure. C r e e p - f a t i g u e b e h a v i o u r o f d i r e c t i o n a l ~ solidified and single-crystal n i c k e l - e i u m i n i d e intermetellic. (Dissertation). Bellows, R.S. Diss. Abstr. Int. Apr. 1989 49, (10), Pp 83 The creep-fatigue beheviour of Ni3 AI(B,Hf), an ordered nickel aluminide intermetsllic, is investigated using a modified fatigue test that inversely varies the stress range and mean stress. The test matrix allows the cyclic (fatigue) and mean stress (creep) effects to be examined simultaneously. Stress range w cycles-to-failure (S-N) curves and dynamic creep (envelope strain) curves are generated and examined. Directionslly solidified (DS) and single-crystal (SC) forms of the intermstallic are used to reduce the number of transverse grain boundaries, and thus minimize the associated grain boundary effects. The three regimes investigated represent above, near, and below the peak yield strength temperature of approx 760 °C. The addition of thermally activated processes, or creep, causes the S-N curves to invert at higher mean stress levels. At 982 °C, albeit the 0.2% yield strength and ultimate tensile strength (UTS) are low, the relative creep-fatigue resistance is high, possibly as a result of • dynamically generated pinning mechanism. At 760 °C, the peak temperature for yield strength and UTS, the creep-fstigue resistance, which is attributed to the highly ordered lattice inducing homogeneous slip, is found to be superior to that of a conventional wrought Ni-bese superalloy. At 450 °C, the yield strength is low with respect to 760 °C, but the creep-fatigue resistance and UTS remain high, Thus, the creep-fatigue resistance emerges as a function of the UTS, not yield strength, indicating the importance of work hardening at this temperature. Coptanar cracking along { 111} slip bends and TEM results are indicative of intensely planar slip at lower temperatures. At higher temperatures, slip becomes more wavy, although fracture is still crystallographic. Decohesion of parallel grain boundaries limited the lifetimes of several specimens. Although the yield strength decreases severely above and below the peak yield strength temperature, it has been determined that the creep-fatigue resistance of Ni3 AI(B,Hf) remains high over the temperature range investigated and that the intermatellic is suitable for further development as a high-temperature structural candidate for monolithic or fibrereinforced matrices,
fracture mechanics is described. Short cylindrical (ring) specimens have been used as a laboratory simulation of a thick-well cylinder and equipment and experimental techniques have been developed for generating fatigue cracks in these specimens using internal pressure. The experimental programme relates to the case of longitudinal atraight-fronted radial cracks emanating from the cylinder bore. The existence of multiple cracking in thick-wall vessels, particularly gun tubes, is recognised as being of importance. In addition, since many thick-well cylinders are autofrattaged, an attempt has been made to discover the influence of this process on the stress intensity relationship of a cracked cylinder. Experimental stress intensity calibrations have been determined from instrumented fracture tests for ring specimens having diameter ratios of 3.0, 2.0 and 1.75. Measurements of cyclic crack growth rate have been obtained for cylinders with diameter ratios of 3.0, 2,0 and 1.75. These data indicate that the crack growth rate in the 3Ni-Cr-Mo--V gun steel used throughout this investigation is typical of that exhibited by high strength steels. It has been noted that the application of the fracture mechanics concept of stress intensity factor, /~, to the fatigue of thick-well cylinders yields results which are in agreement with established trends. Specifically, it has ben determined that the crack propagation rate in the thick-wall cylinders tested can be described by the relationship d~dN = 5.77 x I0 -s (~KI)2,z. Investigation of t h e f a t i g u e and service conditions o f c o m p o s i t e b e a m s w i t h headed-studs. Roik, K. and Holtkamp, H.-J. Stahlbau Feb. 1989 58, (2), 5 3 - 6 2 (in German) The present fatigue-strength calculations for composite beams are conducted separately for the shear stresses in the studs end for the normal stresses in the flange of the steel beam. Fatigue cracks due to pure shear stresses can occur in a different pattern to those due to the combined shear and normal stresses in the flange. This points to an interactive influence of stress components on the fatigue resistance. Based on existing international investigations and on previously conducted tests, a three-dimensional interaction model is being developed, which can be enlarged to a dimensioning concept of the combination of the above mentioned stresses, This requires further tests to be conducted and evaluated. Graphs, photomicrographs. 24 refs.
T h e r m o m e c h a n i c a l fatigue, o x i d a t i o n and creep: I, Damage mechanisms.
Neu, R.W. and Sehitoglu, H. Metal/. Trans. A Sept. 1989 20, (9), 1755-1767 Isothermal fatigue tests and both out-of-phase and in-phase thermomechanical fatigue tests were performed in air and in helium atmospheres. A wide range of temperatures from 20-700 °C was considered in these tests on 1070 steel specimens. A procedure for inert atmosphere testing using encapsulated specimens is described. Results indicate that the fatigue lives are 2-12 times greater in He than in air. Interrupted tests were performed to characterize the progression of damage in the material. Results indicate that oxidation-induced crack nucleation and crack growth are detrimental at high temperatures for isothermal and out-of-phase thermomechanical fatigue tests. In these tests, transgranular cracking is observed. However, creepinduced intergrsnular cracking is the dominant damage mechanism during in-phass thermomechanical fatigue teats. Graphs, photomicrographs. 51 refs. T h e r m o m e c h a n i c s l fatigue, o x i d a t i o n and creep: II. Life prediction. Neu,
R.W. and Sehitoglu, H. Metal/. Trans. A Sept. 1989 20, (9), 1769-1783 Plastic d e f o r m a t i o n around indentations and t h e i r effect on f a t i g u e cycling. (Dissertation). Abdur-Razzaq Diss. Abstr. Int. Apr. 1989 49, (10), Pp 236 The plastic deformation around indentations and their effect on the low amplitude fatigue cycling of mono- and polycrystalline Cu have been studied. This effect has been studied using both surface and transmission electron microscope (TEM) observations. The project was divided into four main categories: the deformation around identations; the fatigue cycling mechanism; the low strain amplitude fatigue cycling of the indented specimens; and the effect of indentation on the fatigued specimens. The surface topographic studies were made on specially designed polycrystalline and single-crystal specimens of Cu. The transmission electron microscope observations have been done mainly on thin strips sliced and prepared from two bulk single crystals with axial orientation close to (124) and (012). The dislocation structures due to indentations were studied in regions at different positions in relation to the indentations, including the microstructures close to the surface and a p p r o x . : 50 I~m from it. Specimens were fatigued in air, at room temperature, and at total strain amplitudes in the range approx. = 2 x 10-4-9.9 x 10-4for bulk specimens and approx. = 0.8 x 10-3-1.9 x 10- for thin foil specimens. The number of fatigue cycles given to the specimens varied (from specimen to specimen) from a few cycles to several hundred thousand cycles. The Dyer model (1965) has been satisfactorily extended from bali to pyramid indentations to explain the hill formation and the general surface topography around indentations. Dislocation microstructures around indentations in the 'indented' and 'indented and fatigued' specimens of the same orientation have been analysed and compared. Such an analysis has revealed that in the regions of indentations in which PSBs are initiated more readily, the dislocation microstructure due to indentation contained predominantly the Burgers vectors ± a/2 (110) or ± a/2 (011) and +- a/2 (110) or -+ a/2 (011). In contrast to these, the most frequently found Burgers vectors in the same regions of the indentation followed by fatigue were -+ a/2 (101) and ± a/2 (011), which reveals that the Burgers vectors of the dislocations introduced by the indentation in these regions are quite favoureble for the later stage of fatigue cycling, with an enhanced density of dislocations, The application of linear elastic fracture mechanics t o t h e f a t i g u e and fracture o f internally pressurized thick w a l l cylinders. (Dissertation).
Price, H. Diss. Abstr. Int. Apr. 1989 49, (10), Pp 304 The design of internally pressurized thick-wall cylindrical components such as gun tubes, autoclaves end compaction vessels often necessitates the use of very high yield strength steel, typically, of the order of 700-1400 MPa. These high strength steels often exhibit low fracture toughness values and, therefore, the presence of cracks in these cr6)mponents is of overriding concern. In recent years, linear elastic fracture mechanics has been applied with some considerable success to the safe life determination of such components. An experimental programme aimed at quantitatively assessing the fatigue and fracture characteristics of internally pressurized thick-wall cylinders in terms of the parameters provided by linear elastic
144
A life prediction model is developed for crack nucleation and early crack growth based on fatigue, environment (oxidation), and creep damage. The model handles different strein-tsmperature phasings (is in-phase and out-of-phase thermomechanical fatigue, isothermal fatigue, and others, including non-proportional phasings). Fatigue life predictions compare fevourably with experiments in 1070 steel for a wide range of test conditions and strain-tempersture phasings. An oxide growth (oxide damage) model is based on the repeated microrupture process of oxide observed from microscopic measurements. A creep damage expression, which is stress-based, is coupled with a unified constitutive equation, A set of interrupted tests was performed to provide valuable damage progression information. Tests were performed in air and in helium atmospheres to isolate creep damage from oxidation damage. Graphs, photomicrographs. 48 refs. The effects o f hold t i m e and frequency on crack g r o w t h in alloy 800H
at 650 °C. Hour, K. Y. and Stubbins, J.F. Metall. Trans. A Sept. 1989 20, (9), 1727-1734 The heat resistant material Alloy 800H is presently used in petrochemical processes and for steam superheater tubing in power generation systems. In service, the alloy may experience steady loading, cyclic loading, or a combination of the two. Crack growth behaviour under fatigue, creep, and hold time loading conditions have been characterized at a temperature of 650 °C in laboratory air. Fatigue crack growth rate (per second) was observed to increase with increasing frequency. The linear elastic fracture mechanics (LEFM) parameter, ~,K, was found to be a useful parameter to correlate crack growth rates for fatigue. In creep and hold time cases, where significant plasticity was observed, the energy rate line integral parameter, C*, is a useful descriptive parameter. In fact, C* is capable of consolidating all creep and hold-time data into a single straight lins on a logarithmic plot. Both creep and holdtime tests show the same characteristic deflection curves. This demonstrates that hold-time tests are more creep-like than fatigue-like for the test conditions here. Compared to creep loading, fatigue deflection curves show much less induced plasticity, most of which is gathered during the final stages of the experiments at high stress intensity values. Microstructural examination revealed that a transgranular crack growth mode is predominant during fatigue, while both creep and hold-time tests are cavity-controlled crack growth processes with the exception that striations are observed along cavity edges in the hold-time tests. Graphs, photomicrographs. 21 refs.
Grain egression:
a new
mechanism of fatigue crack initiation
in
TI-6AI-4V. Gilbert, J.L. and Piehler, H.R. Metall. Trans. A Sept. 1989 20 (9), 1715-1725 A new mechanism of fatigue crack initiation (FCI), grain egrsssion, was observed in the course of investigating corrosion-fatigue crack initiation in Ti-6AI-4V hip prostheses fabricated using three different processes. Extensive scanning electron microscopy (SEM) was used to document this new mechanism and the other FCI
Int J Fatigue March 1990
fracture mechanics is described. Short cylindrical (ring) specimens have been used as a laboratory simulation of a thick-well cylinder and equipment and experimental techniques have been developed for generating fatigue cracks in these specimens using internal pressure. The experimental programme relates to the case of longitudinal atraight-fronted radial cracks emanating from the cylinder bore. The existence of multiple cracking in thick-wall vessels, particularly gun tubes, is recognised as being of importance. In addition, since many thick-well cylinders are autofrattaged, an attempt has been made to discover the influence of this process on the stress intensity relationship of a cracked cylinder. Experimental stress intensity calibrations have been determined from instrumented fracture tests for ring specimens having diameter ratios of 3.0, 2.0 and 1.75. Measurements of cyclic crack growth rate have been obtained for cylinders with diameter ratios of 3.0, 2,0 and 1.75. These data indicate that the crack growth rate in the 3Ni-Cr-Mo--V gun steel used throughout this investigation is typical of that exhibited by high strength steels. It has been noted that the application of the fracture mechanics concept of stress intensity factor, /~, to the fatigue of thick-well cylinders yields results which are in agreement with established trends. Specifically, it has ben determined that the crack propagation rate in the thick-wall cylinders tested can be described by the relationship d~dN = 5.77 x I0 -s (~KI)2,z. Investigation of t h e f a t i g u e and service conditions o f c o m p o s i t e b e a m s w i t h headed-studs. Roik, K. and Holtkamp, H.-J. Stahlbau Feb. 1989 58, (2), 5 3 - 6 2 (in German) The present fatigue-strength calculations for composite beams are conducted separately for the shear stresses in the studs end for the normal stresses in the flange of the steel beam. Fatigue cracks due to pure shear stresses can occur in a different pattern to those due to the combined shear and normal stresses in the flange. This points to an interactive influence of stress components on the fatigue resistance. Based on existing international investigations and on previously conducted tests, a three-dimensional interaction model is being developed, which can be enlarged to a dimensioning concept of the combination of the above mentioned stresses, This requires further tests to be conducted and evaluated. Graphs, photomicrographs. 24 refs.
T h e r m o m e c h a n i c a l fatigue, o x i d a t i o n and creep: I, Damage mechanisms.
Neu, R.W. and Sehitoglu, H. Metal/. Trans. A Sept. 1989 20, (9), 1755-1767 Isothermal fatigue tests and both out-of-phase and in-phase thermomechanical fatigue tests were performed in air and in helium atmospheres. A wide range of temperatures from 20-700 °C was considered in these tests on 1070 steel specimens. A procedure for inert atmosphere testing using encapsulated specimens is described. Results indicate that the fatigue lives are 2-12 times greater in He than in air. Interrupted tests were performed to characterize the progression of damage in the material. Results indicate that oxidation-induced crack nucleation and crack growth are detrimental at high temperatures for isothermal and out-of-phase thermomechanical fatigue tests. In these tests, transgranular cracking is observed. However, creepinduced intergrsnular cracking is the dominant damage mechanism during in-phass thermomechanical fatigue teats. Graphs, photomicrographs. 51 refs. T h e r m o m e c h a n i c s l fatigue, o x i d a t i o n and creep: II. Life prediction. Neu,
R.W. and Sehitoglu, H. Metal/. Trans. A Sept. 1989 20, (9), 1769-1783 Plastic d e f o r m a t i o n around indentations and t h e i r effect on f a t i g u e cycling. (Dissertation). Abdur-Razzaq Diss. Abstr. Int. Apr. 1989 49, (10), Pp 236 The plastic deformation around indentations and their effect on the low amplitude fatigue cycling of mono- and polycrystalline Cu have been studied. This effect has been studied using both surface and transmission electron microscope (TEM) observations. The project was divided into four main categories: the deformation around identations; the fatigue cycling mechanism; the low strain amplitude fatigue cycling of the indented specimens; and the effect of indentation on the fatigued specimens. The surface topographic studies were made on specially designed polycrystalline and single-crystal specimens of Cu. The transmission electron microscope observations have been done mainly on thin strips sliced and prepared from two bulk single crystals with axial orientation close to (124) and (012). The dislocation structures due to indentations were studied in regions at different positions in relation to the indentations, including the microstructures close to the surface and a p p r o x . : 50 I~m from it. Specimens were fatigued in air, at room temperature, and at total strain amplitudes in the range approx. = 2 x 10-4-9.9 x 10-4for bulk specimens and approx. = 0.8 x 10-3-1.9 x 10- for thin foil specimens. The number of fatigue cycles given to the specimens varied (from specimen to specimen) from a few cycles to several hundred thousand cycles. The Dyer model (1965) has been satisfactorily extended from bali to pyramid indentations to explain the hill formation and the general surface topography around indentations. Dislocation microstructures around indentations in the 'indented' and 'indented and fatigued' specimens of the same orientation have been analysed and compared. Such an analysis has revealed that in the regions of indentations in which PSBs are initiated more readily, the dislocation microstructure due to indentation contained predominantly the Burgers vectors ± a/2 (110) or ± a/2 (011) and +- a/2 (110) or -+ a/2 (011). In contrast to these, the most frequently found Burgers vectors in the same regions of the indentation followed by fatigue were -+ a/2 (101) and ± a/2 (011), which reveals that the Burgers vectors of the dislocations introduced by the indentation in these regions are quite favoureble for the later stage of fatigue cycling, with an enhanced density of dislocations, The application of linear elastic fracture mechanics t o t h e f a t i g u e and fracture o f internally pressurized thick w a l l cylinders. (Dissertation).
Price, H. Diss. Abstr. Int. Apr. 1989 49, (10), Pp 304 The design of internally pressurized thick-wall cylindrical components such as gun tubes, autoclaves end compaction vessels often necessitates the use of very high yield strength steel, typically, of the order of 700-1400 MPa. These high strength steels often exhibit low fracture toughness values and, therefore, the presence of cracks in these cr6)mponents is of overriding concern. In recent years, linear elastic fracture mechanics has been applied with some considerable success to the safe life determination of such components. An experimental programme aimed at quantitatively assessing the fatigue and fracture characteristics of internally pressurized thick-wall cylinders in terms of the parameters provided by linear elastic
144
A life prediction model is developed for crack nucleation and early crack growth based on fatigue, environment (oxidation), and creep damage. The model handles different strein-tsmperature phasings (is in-phase and out-of-phase thermomechanical fatigue, isothermal fatigue, and others, including non-proportional phasings). Fatigue life predictions compare fevourably with experiments in 1070 steel for a wide range of test conditions and strain-tempersture phasings. An oxide growth (oxide damage) model is based on the repeated microrupture process of oxide observed from microscopic measurements. A creep damage expression, which is stress-based, is coupled with a unified constitutive equation, A set of interrupted tests was performed to provide valuable damage progression information. Tests were performed in air and in helium atmospheres to isolate creep damage from oxidation damage. Graphs, photomicrographs. 48 refs. The effects o f hold t i m e and frequency on crack g r o w t h in alloy 800H
at 650 °C. Hour, K. Y. and Stubbins, J.F. Metall. Trans. A Sept. 1989 20, (9), 1727-1734 The heat resistant material Alloy 800H is presently used in petrochemical processes and for steam superheater tubing in power generation systems. In service, the alloy may experience steady loading, cyclic loading, or a combination of the two. Crack growth behaviour under fatigue, creep, and hold time loading conditions have been characterized at a temperature of 650 °C in laboratory air. Fatigue crack growth rate (per second) was observed to increase with increasing frequency. The linear elastic fracture mechanics (LEFM) parameter, ~,K, was found to be a useful parameter to correlate crack growth rates for fatigue. In creep and hold time cases, where significant plasticity was observed, the energy rate line integral parameter, C*, is a useful descriptive parameter. In fact, C* is capable of consolidating all creep and hold-time data into a single straight lins on a logarithmic plot. Both creep and holdtime tests show the same characteristic deflection curves. This demonstrates that hold-time tests are more creep-like than fatigue-like for the test conditions here. Compared to creep loading, fatigue deflection curves show much less induced plasticity, most of which is gathered during the final stages of the experiments at high stress intensity values. Microstructural examination revealed that a transgranular crack growth mode is predominant during fatigue, while both creep and hold-time tests are cavity-controlled crack growth processes with the exception that striations are observed along cavity edges in the hold-time tests. Graphs, photomicrographs. 21 refs.
Grain egression:
a new
mechanism of fatigue crack initiation
in
TI-6AI-4V. Gilbert, J.L. and Piehler, H.R. Metall. Trans. A Sept. 1989 20 (9), 1715-1725 A new mechanism of fatigue crack initiation (FCI), grain egrsssion, was observed in the course of investigating corrosion-fatigue crack initiation in Ti-6AI-4V hip prostheses fabricated using three different processes. Extensive scanning electron microscopy (SEM) was used to document this new mechanism and the other FCI
Int J Fatigue March 1990