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The influence of δ′ and S′ precipitation on low-cycle fatigue behaviour of an aluminium alloy
annealing. However, the grain boundary position just before the fatigue test was different from that after electrical polishing. This is because the grain boundaries which intersect free surface are liable to move during the hasting-up process prior t o high-temperature fatigue in order to decrease their interfacial energy. This explains the first large displacement of grain boundary migration. A tentative model was proposed to account for the reason why the displacement of grain boundary migration during high-temperature fatigue is large, notwithstanding the small unidirectional deformation. Graphs, photomicrographs. 17 refs. Flexurel w a v e response and failure o f stepped beams. Al-Mousawi,
M. M. and Ghosh, S, K. Int. J. Impact Eng. 1989 8, (1), 53-67 One of the important factors affecting the energy absorption characteristics of structures is the loading duration, particularly when a structure is subjected to impact. The non-uniformities of cross section have a profound effect on the dynamic response of structures. A theoretical solution is presented for the reflection and transmission of flexural waves at a discontinuity of cross section. The solution is based on the Timoshenko beam theory, which takes into account the effects of shear deformation and rotatory inertia, and the numerical results were obtained using the method of characteristics. Experimental results are compared with theoretical predictions. The very good agreement between theory and experiment showed the success of the method used. The present investigation enhances the approach of design against failure for structures (eg AI) under transient impact loading. Graphs. 40 refs. Transverse tensile and f a t i g u e properties of Fe-Mn-AI-C alloys. Jahn,
W. T., Chang, S.-C. and Hsiao, Y. . J. Mater. Sci. Lett. J u n e 1989 8, (6), 723-724 Three Fe-Mn-AI alloys with different carbon contents were examined. They were Fe-29Mn-9.4AI-1.lC (alloy A), Fe-29Mn-9.1AI-0.6C (alloy B) and Fe-29Mn-9.2AI-0.26C (alloy C). After solution treatment, alloy A was fully austenitic, alloy B contained 10% ferrite and 90% austenite and alloy C consisted of 55% ferrite and 45% austenite. The tensile properties of the transverse specimen were close to those of the longitudinal specimen when the specimens had the same composition. The fatigue behaviour of alloys A and C is independent of specimen orientation. The fatigue life of the longitudinal specimen is better than that of the transverse specimen because fatigue crack propagation is impeded by the string arrangement of the ferrite phase. Graphs, photomicrographs. 5 refs.
Amalgam margin breakdown caused by creep fatigue rupture. Williams, P. T. and Cahoon, J. R. J. Dent, Res. July 1989 68, (7), 1188-1193 This study was designed to determine whether the mechanical cyclic stressing that occurs during normal mastication contributes to margin breakdown of dental amalgam restorations. The method used appears to duplicate the mechanical stresses developed in vivo during mastication as the result of tooth flexing. One Iow-Cu alloy (NTD) and three high-Cu alloys (Dispersalloy, Phasselloy and Tytin) were evaluated. Simple amalgam restorations were prepared in a cavity centrally located in an AI beam. Each specimen received five periods of three-point cyclical loading (1.7 Hz, 4200 cycles at 37 °C). The margin area was subjected to SEM examination prior to and at the completion of each period of cycling. At the beginning of each period of cycling, beam deflection was set to establish a maximum theoretical stress of 1, 2, 4, 6, or 8 MPa. For all brands, cycling resulted in margin gap formation and surface wrinkling. Wrinkling in Dispersalloy occurred as a wide band of shallow wrinkles, whereas that in NTD occurred as a narrow band of deep wrinkles. At 21 00O cycles, very little void formation and fracturing had occurred in the Disperselloy or NTD restorations. In contrast, the Phasealloy and Tytin restorations developed extensive fracturing even after 4200 cycles. Fracture surface analyses of Phasealloy and Tytin indicated that creep-fatigue rupture was the fracture mechanism responsible for margin breakdown in these amalgam restorations, when subjected to cyclic compressive loading similar to that experienced during mastication. Graphs, photomicrographs. 31 refs.
A s t u d y o f f a t i g u e crack growth under random st're=ms in t h e aspect
of statlati¢=. Hu, J.-M., Yao, W.-P. and Li, K.-X. Acts Mech. Solids Sin. 1988 1, (1), 51-60 A CCT experiment controlled by pseudo-random signals generated by a white-noise generator with narrow-band filter has been done in the MTS-880 system to compare the effects of different &Kcharon de/dr. A fatigue crack growth life prediction formula which is only dependent on power spectrum density (PSD) of the history under Gaussian narrow-band history is derived. The theoretical results are in good agreement with the experimental ones. Examples of low-carbon steel are given, Graphs, spectra. 11 refs, Effects of eluminium content on t h e e l e v a t e d - t e m p e r e t u r e mechanical properties in t h e F e - A I - C system. Kim, B. G., Kim, D. G., Yim, C. H. and
Ra, H. Y. J. Korean Inst. Met. 1989 27, (2), 157-164 (in Korean) For the purpose of investigating the effects of AI content on the elevsted-temperature mechanical properties of Fe-AI-C alloys, high-temperature tensile tests and thermal fatigue tests were performed using a series of alloys containing various amounts of AI ranging up to approx 4.5 wt.%, including GC25 and GCD45. Thermal conductivity was also measured, At low temperatures, the thermal conductivity of flake graphite cast iron was higher than that of CN graphite cast iron, but both cast irons exhibited a similar value of conductivity at high temperature near 300 °C. The thermal conductivity was decreased with the increase in At content, end the decreasing rate was reduced with the increase in temperature. The tensile strength at 700 °C was increased with the increase in AI content in both cast irons, but tensile strength was higher in the case of the C/V graphite cast iron as compared with the flake graphite cast iron. The thermal fatigue life of CN graphite cast iron was, at temperatures from 300-700 °C, superior to that of flake graphite cast iron. particularly in the case of low AI content. The thermal fatigue life of the CN graphite cast iron deterioriated with increasing AI content, but that of the flake graphite cast iron improved with increasing AI content. Graphs. 33 refs. Fatigue properties o f p r e m i u m q u a l i t y e l u m i n i u m castings. (NTIS Tech. Note.) US Air Force Report No NTN88-O402/XAB M a y 1988 The fatigue crack growth behaviour of two premium quality AI castings, A201-T7 and A357-T6, were compared with those of the wrought alloy 7075-'I"76 in a study described in a recent report from the Air Force Wright Aeronautical Laboratories. Centre-cracked panels for use in this study were obtained from a 12.6 mm thick section of stepped plate castings. The casting method was not reported. 7075-T76 specimens were obtained from a 5 mm thick plate. It was apparent that A357-T6 showed greater crack growth resistance than either A201*T7 or 7050-T6. This material also exhibited the lowest tensile strength.
Constitutive model with damage for high-temperature superalloys. Sherwood, J. A. and Stouffer, D. C. University of Cincinnati Report No N88-21510/8/XAB Apr. 1988 A unified constitutive model is searched for that is applicable for high-temperature superalloys used in modern gas turbines. Two unified inelastic state variable constitutive models were evaluated for use with the damage parameter proposed by Kachanov. The first is a model (Bodner, Partom) in which hardening is modelled through the use of a single state variable that is similar to drag stress. The other (Ramaswamy) employs both a drag stress and back stress. The extension was successful for predicting the tensile, creep, fatigue, torsional and non-proportional response of Rene 80 at several temperatures. In both formulations, a cumulative damage parameter is introduced to model the changes in material properties due to the formation of microcracks and microvoids that ultimately produce a macroscopic crack. A back stress/drag stress/damage model was evaluated for Rene 95 at 649 °C and is shown to predict the tensile, creep, and cyclic loading responses reasonably well. C o n s t i t u t i v e response o f Rene 80 under t h e r m a l mechanical loads. Kim,
Study of t h e applicability o f C* for correlating crack growth rates at elevated t e m p e r a t u r e in alloy 800H. Hour, K. Y. and Stubbins, J. F. Scr. Metall. June 1989 23, (6), 913-918 Creep and hold-time crack growth rates in alloy 800H in a temperature range of 650-900 °C for various loading conditions are correlated by C*, even for the hold period as short as 1 min at 650 °C. The fatigue loading portion of the cycle in holdtime tests has little effect on crack growth mechanisms at 650 °C. Tests on 800H were performed in air at a variety of temperatures, load amplitudes and hold periods. A high-temperature extensometer was used to measure the load-line deflection across the notch. The crack length was monitored using DC electrical potential drop technology. Local stress profiles around crack tips were obtained from microhardness indentation tests. The stress values were derived from a relationship between Vickers hardness number and local stress for Alloy 800H deformed at 650 °C. Graphs, photomicrographs. 15 refs. The influence o f 8' and S' precipitation on low-cycle fatigue behsviour o f an s l u m i n i u m alloy. Khireddine, D. , Rahouadj, R. andClavel, M. Acta Metal/. Jan. 1989 37, (1), 191-201 The low-cycle fatigue properties of an AI-Li alloy (AI-Li-Cu-Mg-Zr) were described in terms of cyclic stress behaviour and fatigue life. The orientation texture has also been considered with regard to the cyclic response of the material. It has been found that at high applied plastic strains, intense coplanar slip bands are created by ~' (AI3Li) shearing, favouring the shearing of S' (AI2CuMg) precipitates. The slip shearing of S' laths has been discussed with respect to their crystallographic structure. Furthermore, a needle-like 8' configuration has been observed for interrupted tests including dwell-time periods at zero strain and at ambient temperature. The formation of these needles has been discussed in relation to thermal activation and applied plastic strain. At low applied plastic strains, microscopic observations have revealed that dislocation veins are formed, these veins involving a regime of steady-state in the stress-number of cycles curves. It has been concluded that the transition in the deformation modes could explain the change in slope observed in the cyclic stress-strain curve. Graphs, photomicrographs, spectra. 53 refs.
Int J Fatigue January 1990
K. S., Cook, T. S. and McKnight, R. L. General Electric Report No N88-21524/9/XAB Apt. 1988 The applicability of a classical constitutive model for stress-strain analysis of a Nibase superalloy, Rene 80, in the gas turbine thermomechanical fatigue (TMF) environment is examined. A variety of tests were conducted to generate basic material data and to investigate the material response under cyclic thermomschanical loading. Isothermal stress-strain data were acquired at a variety of strain rates over the TMF temperature range. Creep curves were examined at two temperature ranges, 871-982 and 760-871 °C. The results provide optimism on the ability of the classical constitutive model for high-temperature applications. Fatigue c r e c k p r o p e g a t i o n in high strength l o w alloy steels. Spies,
H.-J., Pusch, G., Henkel, C. and Rossler, K. Theor. Appl. FracL Mech. May 1989 11, (2), 121-125
The crack propagation behaviour is considered for a bainitic HSLA-steel (eg H60-3, H75-A, H75-B and N-A-XTRA 70) with a minimum yield strength of 600 MPa. As in the welded structures, crack-like defects are present and can affect the resistance to failure by crack growth. Coarsening of the grain corresponded to a higher threshold level &K0 during the early stage of fatigue crack growth. Sensitivity to the mean stress level increases with increasing crack growth rates. Graphs. 14 refs. Fatigue failure initiation analysis of wing/fuselage bolt assembly. Sih,
G. C. and Chao, C. K. Theor. AppL Fract. Mech. M a y 1989 11, (2), 1 0 9 - t 2 0
This work was concerned with predicting the fatigue failure initiation of a wing/ fuselage bolt (eg 4340 steel) assembly. Accounted for in the analysis are both the influence of energy dissipation and damage accumulation as the structure is subjected to repeated cyclic loading. Results involving the location and number of cycles to initiate a fatigue crack 0.25 mm long are obtained. They agreed both qualitatively and quantitatively with the experimental findings. Also discussed is the influence of pre-torque in the bolt which tends to decrease the number of cycles to fatigue crack initiation. Fatigue life may be extended by altering the load path so
77
M. M. and Ghosh, S, K. Int. J. Impact Eng. 1989 8, (1), 53-67 One of the important factors affecting the energy absorption characteristics of structures is the loading duration, particularly when a structure is subjected to impact. The non-uniformities of cross section have a profound effect on the dynamic response of structures. A theoretical solution is presented for the reflection and transmission of flexural waves at a discontinuity of cross section. The solution is based on the Timoshenko beam theory, which takes into account the effects of shear deformation and rotatory inertia, and the numerical results were obtained using the method of characteristics. Experimental results are compared with theoretical predictions. The very good agreement between theory and experiment showed the success of the method used. The present investigation enhances the approach of design against failure for structures (eg AI) under transient impact loading. Graphs. 40 refs. Transverse tensile and f a t i g u e properties of Fe-Mn-AI-C alloys. Jahn,
W. T., Chang, S.-C. and Hsiao, Y. . J. Mater. Sci. Lett. J u n e 1989 8, (6), 723-724 Three Fe-Mn-AI alloys with different carbon contents were examined. They were Fe-29Mn-9.4AI-1.lC (alloy A), Fe-29Mn-9.1AI-0.6C (alloy B) and Fe-29Mn-9.2AI-0.26C (alloy C). After solution treatment, alloy A was fully austenitic, alloy B contained 10% ferrite and 90% austenite and alloy C consisted of 55% ferrite and 45% austenite. The tensile properties of the transverse specimen were close to those of the longitudinal specimen when the specimens had the same composition. The fatigue behaviour of alloys A and C is independent of specimen orientation. The fatigue life of the longitudinal specimen is better than that of the transverse specimen because fatigue crack propagation is impeded by the string arrangement of the ferrite phase. Graphs, photomicrographs. 5 refs.
Amalgam margin breakdown caused by creep fatigue rupture. Williams, P. T. and Cahoon, J. R. J. Dent, Res. July 1989 68, (7), 1188-1193 This study was designed to determine whether the mechanical cyclic stressing that occurs during normal mastication contributes to margin breakdown of dental amalgam restorations. The method used appears to duplicate the mechanical stresses developed in vivo during mastication as the result of tooth flexing. One Iow-Cu alloy (NTD) and three high-Cu alloys (Dispersalloy, Phasselloy and Tytin) were evaluated. Simple amalgam restorations were prepared in a cavity centrally located in an AI beam. Each specimen received five periods of three-point cyclical loading (1.7 Hz, 4200 cycles at 37 °C). The margin area was subjected to SEM examination prior to and at the completion of each period of cycling. At the beginning of each period of cycling, beam deflection was set to establish a maximum theoretical stress of 1, 2, 4, 6, or 8 MPa. For all brands, cycling resulted in margin gap formation and surface wrinkling. Wrinkling in Dispersalloy occurred as a wide band of shallow wrinkles, whereas that in NTD occurred as a narrow band of deep wrinkles. At 21 00O cycles, very little void formation and fracturing had occurred in the Disperselloy or NTD restorations. In contrast, the Phasealloy and Tytin restorations developed extensive fracturing even after 4200 cycles. Fracture surface analyses of Phasealloy and Tytin indicated that creep-fatigue rupture was the fracture mechanism responsible for margin breakdown in these amalgam restorations, when subjected to cyclic compressive loading similar to that experienced during mastication. Graphs, photomicrographs. 31 refs.
A s t u d y o f f a t i g u e crack growth under random st're=ms in t h e aspect
of statlati¢=. Hu, J.-M., Yao, W.-P. and Li, K.-X. Acts Mech. Solids Sin. 1988 1, (1), 51-60 A CCT experiment controlled by pseudo-random signals generated by a white-noise generator with narrow-band filter has been done in the MTS-880 system to compare the effects of different &Kcharon de/dr. A fatigue crack growth life prediction formula which is only dependent on power spectrum density (PSD) of the history under Gaussian narrow-band history is derived. The theoretical results are in good agreement with the experimental ones. Examples of low-carbon steel are given, Graphs, spectra. 11 refs, Effects of eluminium content on t h e e l e v a t e d - t e m p e r e t u r e mechanical properties in t h e F e - A I - C system. Kim, B. G., Kim, D. G., Yim, C. H. and
Ra, H. Y. J. Korean Inst. Met. 1989 27, (2), 157-164 (in Korean) For the purpose of investigating the effects of AI content on the elevsted-temperature mechanical properties of Fe-AI-C alloys, high-temperature tensile tests and thermal fatigue tests were performed using a series of alloys containing various amounts of AI ranging up to approx 4.5 wt.%, including GC25 and GCD45. Thermal conductivity was also measured, At low temperatures, the thermal conductivity of flake graphite cast iron was higher than that of CN graphite cast iron, but both cast irons exhibited a similar value of conductivity at high temperature near 300 °C. The thermal conductivity was decreased with the increase in At content, end the decreasing rate was reduced with the increase in temperature. The tensile strength at 700 °C was increased with the increase in AI content in both cast irons, but tensile strength was higher in the case of the C/V graphite cast iron as compared with the flake graphite cast iron. The thermal fatigue life of CN graphite cast iron was, at temperatures from 300-700 °C, superior to that of flake graphite cast iron. particularly in the case of low AI content. The thermal fatigue life of the CN graphite cast iron deterioriated with increasing AI content, but that of the flake graphite cast iron improved with increasing AI content. Graphs. 33 refs. Fatigue properties o f p r e m i u m q u a l i t y e l u m i n i u m castings. (NTIS Tech. Note.) US Air Force Report No NTN88-O402/XAB M a y 1988 The fatigue crack growth behaviour of two premium quality AI castings, A201-T7 and A357-T6, were compared with those of the wrought alloy 7075-'I"76 in a study described in a recent report from the Air Force Wright Aeronautical Laboratories. Centre-cracked panels for use in this study were obtained from a 12.6 mm thick section of stepped plate castings. The casting method was not reported. 7075-T76 specimens were obtained from a 5 mm thick plate. It was apparent that A357-T6 showed greater crack growth resistance than either A201*T7 or 7050-T6. This material also exhibited the lowest tensile strength.
Constitutive model with damage for high-temperature superalloys. Sherwood, J. A. and Stouffer, D. C. University of Cincinnati Report No N88-21510/8/XAB Apr. 1988 A unified constitutive model is searched for that is applicable for high-temperature superalloys used in modern gas turbines. Two unified inelastic state variable constitutive models were evaluated for use with the damage parameter proposed by Kachanov. The first is a model (Bodner, Partom) in which hardening is modelled through the use of a single state variable that is similar to drag stress. The other (Ramaswamy) employs both a drag stress and back stress. The extension was successful for predicting the tensile, creep, fatigue, torsional and non-proportional response of Rene 80 at several temperatures. In both formulations, a cumulative damage parameter is introduced to model the changes in material properties due to the formation of microcracks and microvoids that ultimately produce a macroscopic crack. A back stress/drag stress/damage model was evaluated for Rene 95 at 649 °C and is shown to predict the tensile, creep, and cyclic loading responses reasonably well. C o n s t i t u t i v e response o f Rene 80 under t h e r m a l mechanical loads. Kim,
Study of t h e applicability o f C* for correlating crack growth rates at elevated t e m p e r a t u r e in alloy 800H. Hour, K. Y. and Stubbins, J. F. Scr. Metall. June 1989 23, (6), 913-918 Creep and hold-time crack growth rates in alloy 800H in a temperature range of 650-900 °C for various loading conditions are correlated by C*, even for the hold period as short as 1 min at 650 °C. The fatigue loading portion of the cycle in holdtime tests has little effect on crack growth mechanisms at 650 °C. Tests on 800H were performed in air at a variety of temperatures, load amplitudes and hold periods. A high-temperature extensometer was used to measure the load-line deflection across the notch. The crack length was monitored using DC electrical potential drop technology. Local stress profiles around crack tips were obtained from microhardness indentation tests. The stress values were derived from a relationship between Vickers hardness number and local stress for Alloy 800H deformed at 650 °C. Graphs, photomicrographs. 15 refs. The influence o f 8' and S' precipitation on low-cycle fatigue behsviour o f an s l u m i n i u m alloy. Khireddine, D. , Rahouadj, R. andClavel, M. Acta Metal/. Jan. 1989 37, (1), 191-201 The low-cycle fatigue properties of an AI-Li alloy (AI-Li-Cu-Mg-Zr) were described in terms of cyclic stress behaviour and fatigue life. The orientation texture has also been considered with regard to the cyclic response of the material. It has been found that at high applied plastic strains, intense coplanar slip bands are created by ~' (AI3Li) shearing, favouring the shearing of S' (AI2CuMg) precipitates. The slip shearing of S' laths has been discussed with respect to their crystallographic structure. Furthermore, a needle-like 8' configuration has been observed for interrupted tests including dwell-time periods at zero strain and at ambient temperature. The formation of these needles has been discussed in relation to thermal activation and applied plastic strain. At low applied plastic strains, microscopic observations have revealed that dislocation veins are formed, these veins involving a regime of steady-state in the stress-number of cycles curves. It has been concluded that the transition in the deformation modes could explain the change in slope observed in the cyclic stress-strain curve. Graphs, photomicrographs, spectra. 53 refs.
Int J Fatigue January 1990
K. S., Cook, T. S. and McKnight, R. L. General Electric Report No N88-21524/9/XAB Apt. 1988 The applicability of a classical constitutive model for stress-strain analysis of a Nibase superalloy, Rene 80, in the gas turbine thermomechanical fatigue (TMF) environment is examined. A variety of tests were conducted to generate basic material data and to investigate the material response under cyclic thermomschanical loading. Isothermal stress-strain data were acquired at a variety of strain rates over the TMF temperature range. Creep curves were examined at two temperature ranges, 871-982 and 760-871 °C. The results provide optimism on the ability of the classical constitutive model for high-temperature applications. Fatigue c r e c k p r o p e g a t i o n in high strength l o w alloy steels. Spies,
H.-J., Pusch, G., Henkel, C. and Rossler, K. Theor. Appl. FracL Mech. May 1989 11, (2), 121-125
The crack propagation behaviour is considered for a bainitic HSLA-steel (eg H60-3, H75-A, H75-B and N-A-XTRA 70) with a minimum yield strength of 600 MPa. As in the welded structures, crack-like defects are present and can affect the resistance to failure by crack growth. Coarsening of the grain corresponded to a higher threshold level &K0 during the early stage of fatigue crack growth. Sensitivity to the mean stress level increases with increasing crack growth rates. Graphs. 14 refs. Fatigue failure initiation analysis of wing/fuselage bolt assembly. Sih,
G. C. and Chao, C. K. Theor. AppL Fract. Mech. M a y 1989 11, (2), 1 0 9 - t 2 0
This work was concerned with predicting the fatigue failure initiation of a wing/ fuselage bolt (eg 4340 steel) assembly. Accounted for in the analysis are both the influence of energy dissipation and damage accumulation as the structure is subjected to repeated cyclic loading. Results involving the location and number of cycles to initiate a fatigue crack 0.25 mm long are obtained. They agreed both qualitatively and quantitatively with the experimental findings. Also discussed is the influence of pre-torque in the bolt which tends to decrease the number of cycles to fatigue crack initiation. Fatigue life may be extended by altering the load path so
77