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Modeling high crack growth rates under variable amplitude loading
Modeling high crack growth rates under v a r i a b l e a m p l i t u d e loading. Dougherty, D.J., de Koning, A.U. and Hi//berry, B.M. Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 2 1 4 - 2 3 3 A two-component crack-growth model has been developed to treat fatigue crack growth under constant and variable amplitude loading. The model is based on a concept introduced by Paris which relates the crack growth to the instantaneous plastic zone size ahead of the crack tip. This concept was apphed to describe crack growth in the presence of cyclic and primary plastic flow. The model was formulated on an incremental basis in which a particular component of the model is integrated about a discrete portion of the load excursion. The model includes the effects of cyclic and monotonic crack growths and crack closure. The results for fatigue crackgrowth data reported in the literature for multiple overload and severe flight sequences are compared with the model predictions. The model was shown to describe accelerated crack-growth behaviour typically observed during these sequences. Accelerstmn effects occur in the presence of primary plastic flow which IS related to the ductile tearing mechanism and is not dependent upon crack closure or other load-history effects. Moreover, the model describes the phenomenological variation of the acceleration effects over such sequences. The predicted crack growth agrees well with the expenmental data (for AI alloys)
"'~ue crack growth from narrow-bend Gaussien spectrum loading in a l u m i n u m alloy. Veers, P.S. and Van Den Avyle, J.A. Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, Cafifornia, USA, 24 Apr. 1990 1 9 1 - 2 1 3 Constant amphtude and narrow-band Gaussian Ioadmgs are applied to extruded 6063 AI crack-growth specimens in an effort to characterize the effective stress intensity levels during random loading. Crack-growth rates are determined for constant amplitude Ioadings at stress ratios (R) of 0.09, 0.3, and 0.5, and for a variable-amplitude loading simulated to match a narrow-band Gaussian spectrum. Crack-opening stress levels measured by the compliance method during the constant amplitude loading are found to differ substantially for - T 5 and - T 6 heat treatments due to a change from intergranular to transgranular crack growth. Crack-opening load ratios correlate well with the maximum apphed stress intensity factor, Kmax, for the - T 5 material. The Krnax dependence leads to an effective halwng of the crack-growth exponent. Calculated variable amplitude lives are much shorter when this correlation is taken into account (an acceleration effect) and show a greater dlfferencs between loading blocks condensed by racetrack filtering at threshold levels of two and four standard deviations, similar to what was observed in the tests. Crack-opening-load measurements in one specimen with the narrow-band Gaussian (variable amplitude) loading failed to detect any closure. A substantial difference in the closure behaviour of nominally Identical R = 0.3 tests indicates that closure may occur irregularly in the extruded AI. Calculated crack-growth lives, assuming no closure in the variable-amplitude tests, are much shorter than the test results. Including closure in the variable-amphtude IOadlngs greatly tmproves the predictions.
Contribution of individual load cycles to crack growth under aircraft
s p e c t r u m loading. Sunder, R.
Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 1 7 6 - 1 9 0 Fatigue crack growth in airframe materials under a variety of aircraft wing load spectra is numerically analysed using a simple crack-closure-based model An mtegrsted Ioad/damage-exceedance diagram is used to describe graphically the load spectrum and crack growth damage distributton. Characteristic features of crack-growth damage contrlbutton from individual load cycles are associated with the nature of the load spectrum. The significance of fatigue crack closure and qts impact on damage distribution are studied (for AI L73, 2219 and 8090).
development of an advanced thermal fatigue life prediction method. Strain-controlled thermomechanical and load-controlled, strain-ltmited, bithsrmal fatigue tests were used to determine the fatigue crack initiation and cyclic etrees-strain response characteristics of two superalloys, cast Ni-bese B1900 + Hf and wrought cobalt base Haynes 188. Both in-phase and out-of-phase thsrmomechanicsl and bithsrmal fatigue experiments were conducted using computer-controlled, servohydraulie uniaxtal fatigue machines equipped with dlametral extenaometry. The thermomechanical tests were conducted with triangular-wave diametral strain vs. time and temperature vs. time histories, with cycle times of 3 or 4 rain. Bitharmal experiments utdized a trapezoidal-wave temperature vs. time profile wherein mechanical straining was imposed only during the isothermal portions at the maximum and minimum temperatures. Temperature was changed while the specimen was held at zero load, thus excluding mechanical strain applicetmn during thermal expansion straining. This testing procedure permits significantly more accurate partitioning of the mechanmal and thermal expansion components of strata compared with conventional thermomechamcst straining experiments. Thcs is especially important when it is desired to separate the mechanical strain further into its elastic and inelastic components, the latter of whmh may be further partitioned into time-dependent and time-independent (creep and plasticity) components. Bitherm81 temperatures of 483 and 871 °C were employed for the the cast B1900 + Hf Ni base alloy and 316 and 760°C for the wrought Haynes 188 Co base alloy. Thermochemical fatigue tests were also conducted using maximum and minimum temperatures corresponding to those for the blthermal experiments. Lives cover the range from approx 10 to 3000 cycles to failure. Comparisons are made with isothermal fatigue results obtained previously
Application of a thermal fatigue life prediction model to high-temperature aerospace alloys B1900 + Hf and Haynes 188. Ha/ford, G.R., Saltsman, J.F., Verrilli, M.J. and Arya, V. Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 1 0 7 - 1 1 9 The results are presented of the application of a newly proposed thermomechanical fatigue hfe prediction method to a series of laboratory TMF results on two hightemperature aerospace engine alloys: cast B1900 + Hf and Haynes 188. The method, referred to as TMF/TS-SRP, ~s based upon three relatwely recent developments: the total strain version of the method of strata-range partitioning (TS-SRP), the bitharmal testing technique for measuring TMF behaviour, and advanced vlscoplastic constitutive models. The high-temperature data reported m a companion paper are used to evaluate the constants In the model and to provide the TMF verlflCetlon data to check its accuracy. Predicted lives are in agreement with the experimental lives to within a factor of approximately two. C u m u l a t i v e creep-fatigue d a m a g e evolution in an austenitic stainless
steel. McGaw, M.A. Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 8 4 - 1 0 6 A model of cumulative creep-fatigue damage has been developed which is based on the use of damage curve equations to describe the evolution of creep-fatigue damage for four basic creep-fatigue cycle types. These cycle types correspond to the four fundamental cycles of the strain range partitioning hfe prediction approach of Manson, Halford, and Hirschberg A concept referred to as damage coupling is introduced to account analytically for the differences in the nature of the damage introduced by each cycle type For applicstmn of this model, the cumulative creep-fatigue damage behaviour of Type 316 stemless steel at 816°C has been experimentaity established for the two-level loading cases revolving fatigue and creep-fatigue, in various permutations The tests were conducted such that the lower-life (high strata) cycling was applied first, for a controlled number of cycles, and the higher-life (lower strain) cycling was conducted at the second level, to failure. The proposed model correlated the majority of the observed cumulative creep-fatigue data.
Near-threchold f a t i g u e crack g r o w t h prediction u n d e r s p e c t r u m loading.
Sunder, Ft, Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 1 6 1 - 1 7 5 A constant crack closure stress-based model assuming the absence of a threshold stress intensity range under spectrum loading forms the basis for a simple procedure to predict crack growth under spectrum loading. The procedure requtres material constants that can be derived from constant-amplitude fatigue crack growth tests at two stress ratios: at a high stress ratio in the absence of fatigue crack closure, and at a stress ratio corresponding to the most severe cycle under the toad spectrum of interest. Crack-growth estimates were compared with available experimental data for a variety of materials and load spectra (for AI L73, 2219, 8090) Elevated t e m p e r a t u r e crack growth in aircraft e n g i n e materials. Nicholas,
T. and Mall, S. Proc. Conf. Advances m Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 143-157
Crack growth rate charactenstics m Inconel 718 and Ti3AI titanium alummlde alloy are compared at 650 °C under conditions of cyclic loading and superimposed hold times at maximum load. Whereas a decrease in frequency increases the growth rate m both materials, addition of hold times has different effects in the two materials. Hold times increase the growth rate in Inconel 718, but cause anythtng from a shght Increase to a very slight decrease in growth rate in TI3AI, depending on the cyclic frequency. A simple emplncal model is proposed which accounts for crack-growth retardation due to creep blunting from hold-time effects and considers the environmental degradation of the material with exposure time. The model is seen to provtde reasonable capability to reproduce most of the growth-rate charactertstlcs observed expenmentally while using the stress intensity factor, K, as the correlating parameter
Evaluation of t h e effect of creep and m e a n stress o n f a t i g u e life using a d a m a g e mechanics approach. Abuelfoutouh, N.M. Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, Califorma, USA, 24 Apr. 1990 7 7 - 8 3 A method for fatigue-life prediction of high-temperature polycrystalhne materials under high-rate fully reversed strain cycles (HRSC) was previously derived using a continuum damage mechanics approach. The method is generalized to predict the fatigue life of balanced cyclic creep rupture (BCCR), tensile cyclic creep rupture (TCCR), and compressive cyclic creep rupture (CCCR). It is assumed that creep deformation mechanisms dominate dunng the creep portion of the fatigue cycle and that plasticity mechanisms dominate during the plastic portion of the cycle. The interaction of plasttcity and creep with the mean apphed stress and the effect of thts interaction on the fatigue life are investlgatsd. A sample calculation is applied to Rene at 1255 K T h e r m o - m e c h a n i c a l f a t i g u e life p r e d i c t i o n m e t h o d s . Sehitoglu, H.
Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, Cahfornia, USA, 24 Apr 7990 4 7 - 7 6 Synergistic effects of oxidation on high-temperature isothermal and thermomechanical fatigue behavlour are studied Experiments were conducted under different strata-temperature variations and atmospheres to reveal the varying severities of oxide-induced failure. Conditions conductive to intergranular creep damage under thermomechanical fatigue are identified, and the competing damage mechanisms are classified for steels, Nt-based superalloys, and AI alloys. Requirements and a solutton procedure are outlined for a thermomechamcal fatigue tile prediction model which is then applied to 1070 steel, MarM-247 Ni-based superalloy, and A12xxx-T4 AI alloy.
Thermomschanicsl and b i t h e r m a l f a t i g u e behavior of cast B1900 + Hf and wrought Haynes 188. Halford, G.R., Verrilli, M.J., Kalluri, S., Ritzert, F.J,, Holland, F.A. and Duckert, R.E. Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 120-142
A fracture mechanics based m o d e l f o r c u m u l a t i v e d a m a g e assessment as p a r t o f f a t i g u e life prediction. Vormwald, M., Hauler, P. and Seeger,
The high-temperature, low-cycle, thermal and mechanical fatigue behawour has been mvesttgated for two high-strength, oxtdatlon-resistant, superalloys used tn aerospace propulston systems Experimental results were generated to support
Most fetlgue-hfe prediction concepts are based on the transfer of some characteristic material data to the component under consideration. The implicitly assumed equivalence can be limited by various factors including different surface conditions,
Int J Fatigue January 1993
7.
Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 2 8 - 4 3
67
"'~ue crack growth from narrow-bend Gaussien spectrum loading in a l u m i n u m alloy. Veers, P.S. and Van Den Avyle, J.A. Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, Cafifornia, USA, 24 Apr. 1990 1 9 1 - 2 1 3 Constant amphtude and narrow-band Gaussian Ioadmgs are applied to extruded 6063 AI crack-growth specimens in an effort to characterize the effective stress intensity levels during random loading. Crack-growth rates are determined for constant amplitude Ioadings at stress ratios (R) of 0.09, 0.3, and 0.5, and for a variable-amplitude loading simulated to match a narrow-band Gaussian spectrum. Crack-opening stress levels measured by the compliance method during the constant amplitude loading are found to differ substantially for - T 5 and - T 6 heat treatments due to a change from intergranular to transgranular crack growth. Crack-opening load ratios correlate well with the maximum apphed stress intensity factor, Kmax, for the - T 5 material. The Krnax dependence leads to an effective halwng of the crack-growth exponent. Calculated variable amplitude lives are much shorter when this correlation is taken into account (an acceleration effect) and show a greater dlfferencs between loading blocks condensed by racetrack filtering at threshold levels of two and four standard deviations, similar to what was observed in the tests. Crack-opening-load measurements in one specimen with the narrow-band Gaussian (variable amplitude) loading failed to detect any closure. A substantial difference in the closure behaviour of nominally Identical R = 0.3 tests indicates that closure may occur irregularly in the extruded AI. Calculated crack-growth lives, assuming no closure in the variable-amplitude tests, are much shorter than the test results. Including closure in the variable-amphtude IOadlngs greatly tmproves the predictions.
Contribution of individual load cycles to crack growth under aircraft
s p e c t r u m loading. Sunder, R.
Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 1 7 6 - 1 9 0 Fatigue crack growth in airframe materials under a variety of aircraft wing load spectra is numerically analysed using a simple crack-closure-based model An mtegrsted Ioad/damage-exceedance diagram is used to describe graphically the load spectrum and crack growth damage distributton. Characteristic features of crack-growth damage contrlbutton from individual load cycles are associated with the nature of the load spectrum. The significance of fatigue crack closure and qts impact on damage distribution are studied (for AI L73, 2219 and 8090).
development of an advanced thermal fatigue life prediction method. Strain-controlled thermomechanical and load-controlled, strain-ltmited, bithsrmal fatigue tests were used to determine the fatigue crack initiation and cyclic etrees-strain response characteristics of two superalloys, cast Ni-bese B1900 + Hf and wrought cobalt base Haynes 188. Both in-phase and out-of-phase thsrmomechanicsl and bithsrmal fatigue experiments were conducted using computer-controlled, servohydraulie uniaxtal fatigue machines equipped with dlametral extenaometry. The thermomechanical tests were conducted with triangular-wave diametral strain vs. time and temperature vs. time histories, with cycle times of 3 or 4 rain. Bitharmal experiments utdized a trapezoidal-wave temperature vs. time profile wherein mechanical straining was imposed only during the isothermal portions at the maximum and minimum temperatures. Temperature was changed while the specimen was held at zero load, thus excluding mechanical strain applicetmn during thermal expansion straining. This testing procedure permits significantly more accurate partitioning of the mechanmal and thermal expansion components of strata compared with conventional thermomechamcst straining experiments. Thcs is especially important when it is desired to separate the mechanical strain further into its elastic and inelastic components, the latter of whmh may be further partitioned into time-dependent and time-independent (creep and plasticity) components. Bitherm81 temperatures of 483 and 871 °C were employed for the the cast B1900 + Hf Ni base alloy and 316 and 760°C for the wrought Haynes 188 Co base alloy. Thermochemical fatigue tests were also conducted using maximum and minimum temperatures corresponding to those for the blthermal experiments. Lives cover the range from approx 10 to 3000 cycles to failure. Comparisons are made with isothermal fatigue results obtained previously
Application of a thermal fatigue life prediction model to high-temperature aerospace alloys B1900 + Hf and Haynes 188. Ha/ford, G.R., Saltsman, J.F., Verrilli, M.J. and Arya, V. Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 1 0 7 - 1 1 9 The results are presented of the application of a newly proposed thermomechanical fatigue hfe prediction method to a series of laboratory TMF results on two hightemperature aerospace engine alloys: cast B1900 + Hf and Haynes 188. The method, referred to as TMF/TS-SRP, ~s based upon three relatwely recent developments: the total strain version of the method of strata-range partitioning (TS-SRP), the bitharmal testing technique for measuring TMF behaviour, and advanced vlscoplastic constitutive models. The high-temperature data reported m a companion paper are used to evaluate the constants In the model and to provide the TMF verlflCetlon data to check its accuracy. Predicted lives are in agreement with the experimental lives to within a factor of approximately two. C u m u l a t i v e creep-fatigue d a m a g e evolution in an austenitic stainless
steel. McGaw, M.A. Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 8 4 - 1 0 6 A model of cumulative creep-fatigue damage has been developed which is based on the use of damage curve equations to describe the evolution of creep-fatigue damage for four basic creep-fatigue cycle types. These cycle types correspond to the four fundamental cycles of the strain range partitioning hfe prediction approach of Manson, Halford, and Hirschberg A concept referred to as damage coupling is introduced to account analytically for the differences in the nature of the damage introduced by each cycle type For applicstmn of this model, the cumulative creep-fatigue damage behaviour of Type 316 stemless steel at 816°C has been experimentaity established for the two-level loading cases revolving fatigue and creep-fatigue, in various permutations The tests were conducted such that the lower-life (high strata) cycling was applied first, for a controlled number of cycles, and the higher-life (lower strain) cycling was conducted at the second level, to failure. The proposed model correlated the majority of the observed cumulative creep-fatigue data.
Near-threchold f a t i g u e crack g r o w t h prediction u n d e r s p e c t r u m loading.
Sunder, Ft, Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 1 6 1 - 1 7 5 A constant crack closure stress-based model assuming the absence of a threshold stress intensity range under spectrum loading forms the basis for a simple procedure to predict crack growth under spectrum loading. The procedure requtres material constants that can be derived from constant-amplitude fatigue crack growth tests at two stress ratios: at a high stress ratio in the absence of fatigue crack closure, and at a stress ratio corresponding to the most severe cycle under the toad spectrum of interest. Crack-growth estimates were compared with available experimental data for a variety of materials and load spectra (for AI L73, 2219, 8090) Elevated t e m p e r a t u r e crack growth in aircraft e n g i n e materials. Nicholas,
T. and Mall, S. Proc. Conf. Advances m Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 143-157
Crack growth rate charactenstics m Inconel 718 and Ti3AI titanium alummlde alloy are compared at 650 °C under conditions of cyclic loading and superimposed hold times at maximum load. Whereas a decrease in frequency increases the growth rate m both materials, addition of hold times has different effects in the two materials. Hold times increase the growth rate in Inconel 718, but cause anythtng from a shght Increase to a very slight decrease in growth rate in TI3AI, depending on the cyclic frequency. A simple emplncal model is proposed which accounts for crack-growth retardation due to creep blunting from hold-time effects and considers the environmental degradation of the material with exposure time. The model is seen to provtde reasonable capability to reproduce most of the growth-rate charactertstlcs observed expenmentally while using the stress intensity factor, K, as the correlating parameter
Evaluation of t h e effect of creep and m e a n stress o n f a t i g u e life using a d a m a g e mechanics approach. Abuelfoutouh, N.M. Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, Califorma, USA, 24 Apr. 1990 7 7 - 8 3 A method for fatigue-life prediction of high-temperature polycrystalhne materials under high-rate fully reversed strain cycles (HRSC) was previously derived using a continuum damage mechanics approach. The method is generalized to predict the fatigue life of balanced cyclic creep rupture (BCCR), tensile cyclic creep rupture (TCCR), and compressive cyclic creep rupture (CCCR). It is assumed that creep deformation mechanisms dominate dunng the creep portion of the fatigue cycle and that plasticity mechanisms dominate during the plastic portion of the cycle. The interaction of plasttcity and creep with the mean apphed stress and the effect of thts interaction on the fatigue life are investlgatsd. A sample calculation is applied to Rene at 1255 K T h e r m o - m e c h a n i c a l f a t i g u e life p r e d i c t i o n m e t h o d s . Sehitoglu, H.
Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, Cahfornia, USA, 24 Apr 7990 4 7 - 7 6 Synergistic effects of oxidation on high-temperature isothermal and thermomechanical fatigue behavlour are studied Experiments were conducted under different strata-temperature variations and atmospheres to reveal the varying severities of oxide-induced failure. Conditions conductive to intergranular creep damage under thermomechanical fatigue are identified, and the competing damage mechanisms are classified for steels, Nt-based superalloys, and AI alloys. Requirements and a solutton procedure are outlined for a thermomechamcal fatigue tile prediction model which is then applied to 1070 steel, MarM-247 Ni-based superalloy, and A12xxx-T4 AI alloy.
Thermomschanicsl and b i t h e r m a l f a t i g u e behavior of cast B1900 + Hf and wrought Haynes 188. Halford, G.R., Verrilli, M.J., Kalluri, S., Ritzert, F.J,, Holland, F.A. and Duckert, R.E. Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 120-142
A fracture mechanics based m o d e l f o r c u m u l a t i v e d a m a g e assessment as p a r t o f f a t i g u e life prediction. Vormwald, M., Hauler, P. and Seeger,
The high-temperature, low-cycle, thermal and mechanical fatigue behawour has been mvesttgated for two high-strength, oxtdatlon-resistant, superalloys used tn aerospace propulston systems Experimental results were generated to support
Most fetlgue-hfe prediction concepts are based on the transfer of some characteristic material data to the component under consideration. The implicitly assumed equivalence can be limited by various factors including different surface conditions,
Int J Fatigue January 1993
7.
Proc. Conf. Advances in Fatigue Lifetime Predictive Techniques, San Francisco, California, USA, 24 Apr. 1990 2 8 - 4 3
67