- Email: [email protected]
Synthesis of realistic loading specifications
Fatigue Fatigue behaviour of 8090-AI-Li alloy. in chloride .contahdug environments. ~~nm~l~gsrerhalten der 8090 AI-L1 legterung m chlormhaltlgen Umge-
..
Bertolini, Mmrinls
L., Cipla, A.. Don:el/i, G., Postore, T. and Ronucci, 7: cmd Corrosion ( 1996) 47( 8). 446-45 I
The effect of an environment containing chloride on the fatigue behavior of a 8090 Al-Li-M&u alloy was studied. Results were compared with those obtained on a traditional 2024 Al-Co-Mg alloy. Fatigue and corrosion-fatigue tests were carried out both on smooth specimens and revited samples. In order to ashess the corrosion behavior of the materials and their susceptibility to stress corrosion cracking, potentiodynamic polarization and slow strain rate tests were carned out. Results showed a remarkable effect of the aggressive environment on the fatigue behavior both of the innovative 8090 alloy (in T8 ageing conditions) and on the traditional clad 2024 alloy (in T3 natural ageing conditions), though the former showed shghtly better behavior. Nevertheless, in the presence of rivets the reduction in fatigue strength in the aggressive environment was negligible. Slow strain rate test showed premature fractures under anodic polarization above the pitting potential and with a strain rate of IO - 6 \-I, only for the 8090 TX alloy. Graphs, 16 ref. Synthesis of realistic loading specifications Drrssler, K.. Kott~m. V.B. and Kot:le. H. Europmrr Journnl of Mechmicol En,yinrrrin,q
( 1996)
41(3),
The aspect ratio data were determined from the measurements of crack compliance. made using a laser interfere metric system, and the measurements of surface crack length (2c), made using a photomicroscopic system. The variations in aspect ratio were found to be large at small crack sizes of the order of a few grain diameters. The experimental n/c data were compared with the patterns of crack aspect ratio variation, obtained from theoretical simulations. The simulated data were generated by assuming alternate crack propagation at the surface and at the depth, the detail\ of which are presented in Part I of the study accompanying this article. A good agreement was found between the simulated and the experimentally observed variation\. After incorporating the u/c variations in AK calculations, the scatter in the growth data of small cracks was significantly reduced and uas found to be of the \ame order as in large cracks. Additionally, it i\ shown in this study that the conventional methods of analysis of small-crack data, performed with an assumption of (I/C = I, can result in significant errors in AK calculation and an increased level of scatter in small-crack growth data. Small cracks al\o were found to exhibit low closure levels relative to large cracks. The results of the study strongly indicate that characteristics of small cracks. often referred to as anomalous, are due to the assumption of u/c= I in Gtuntiom of large variations in aspect ratio. the use of conventlonal methods of data analysis, and the lower levels of crack closure found naturally in small cracks. Graphs, photomicrographs, 39 ref.
153-168
Fatigue evaluation in the automotive industry cannot rely on numerical procedures only. Analyai\ and synthesis of customer- and product-specific loading both for testing and numerical life prediction is a major part of the fatigue evaluation process. Certain mathematical tasks are often solved more easily in a domain different from the original one, and the solution is then transformed back into the original domain. Similarly, solving tasks of fatigue loading analysis and synthesis like monitoring, editing, superposition and extrapolation in the ‘rainflow domain’ and then transforming the results back into the time domain by reconstructing load histories from the raintlow matrix. was successful for uniaxial loading. It is now also possble for multiaxial, non-proportional loading. Graph\. 26 ref. Bayesian neural network analysis of fatigue crack growth base superalloys. Fujii. H.. Mackay. D.J.C. curd Bhrrdeshicr, H.K.D.H. /SIJ Ir~tenmrionrrl (1996) 36( I I ), 1373-1382
rate in nickel
The fatigue crack growth rate of nickel base superalloys has been modelled using a neural network model within a Bayesian framework. A ‘commit& model was also introduced to increase the accuracy of the predictions. The rate was modelled as a fun&on of some 5 I variables, including stress intensity temperature, grain size, heat treatrange AK, log AK. chemical composition, ment, frequency, load waveform, atmosphere. R-ratio, the distinction between short crack growth and long crack growth, sample thickness and yield strength. The Bayeaian method puts error bars on the predicted value of the rate and allows the significance of each individual factor to be estimated. In addition, It was possible to estimate the isolated effect of particular variables such a\ the grain bize. which cannot in practice be varied independently. This demonstrates the ability of the method to investigate new phenomena in cases where the information cannot be accessed experimentally. Astrology, CMSX4, and lnconel 718 are mentioned. Graphs, SO ref. Effect of shot peening on the fatigue and fracture titanium alloys. Sridhrrr, B. R.. Rorrrczc~htmdr~r, K. rrnd Padmnncrbhm. K.A. Joumol of Motrricrk Scirncr ( 1996) 31(22). 5953-5960
Asbstracts
behavior
of two
The fatigue and fracture behavior of two titanium alloys, the near-a IMI-685 and w-p IMI-318. were studied in the machined and polished (MP) and the machined, polished and shot (glass-bead) peened (MPS) conditions. Glass-bead peening reduced the room-temperature and the high-temperature (450°C) fatigue life of alloy IMI-685 at high stress amplitudes, g,,, approaching the proof \tres?l. m,,>, of the material (LCF region). When the applied stress amplitude (O-770 MPa, HCF region) was comparable to the peen-induced peak longitudlnal residual stress. n,.,,, that is (rr,.Jcr,,)=0.92, an improvement in the roomtemperature fatigue life of IMI-685 was observed. When the (cr,Jq,) ratio was less than this value, decreases in the fatigue life were seen. The roomtemperature fatigue behavior of IMI-318 at high stress amplitudes was similar to that of IMI-685. The decreases in the fatigue life of this alloy, at a stress amplitude (770 MPa) where improvement was observed for IMI-685 could be attributed to the higher relaxation of peen-induced residual stresses in IMI-31X compared with IMI-685. Class-bead peening improved the high-temperature (450°C) fatigue life of IMI-685 at a low stress amplitude (465 MPa; (rr,jcr,,) = 0.87). The crack-initiation sites in the MP and the MPS conditions were at the surface for both the alloys. However, fracture in the surface layer\ of the alloys appeared more brittle in the peened (MPS) rather than in the unpeened (MP) condition. Graphs, photomicrographs, 32 ref. Effects of crack aspect ratio on the behavior of small surface cracks fatigue. II. Exnrrimmtc- O!I o titanrum (Ti-8AI) U//OY. R~~vichnndrcrn. K.S. and Lmsen, J.h. Mrtollurgicnl md Mnteriols Tronsocrions A (I 997) 28A(I). I S7- I69A
in
The continuous variations in crack shape or aspect ratio, a/c ((I is the crack depth and c IS the half-surface length), of small surface cracks, induced by grain boundaries, have been investigated during the fatigue crack growth of Emall cracks in a titanium (Ti-SAI) alloy. The significance of the aspect ratio variations in explaming the ‘anomalous’ small-crack behavior was evaluated.
Effects of crack aspect ratio on the behavior of small surface cracks fatigue. I. Simulation. Rm~ichnrrdrm. K.S. Metrrllurgiccrl and Moferirds 7’rmscrcrior~r A ( I997 1 28A( I ), 14% IShA
in
A simple simulation of alternate growth of a small surface crack in the \urfaace and depth directions WBL performed to illustrate the changes in crack aspect ratio, induced by grain boundaries, as a function of crack File. It 15 shown that at small crack sizes, large variations in aspect ratio, (r/c (o is the crack depth and c IS the half-surface length), occur. due to local crack front perturbations induced by grains that are oriented for crack growth. At the\e crack sizes, the assumption of a semicrcular crack shape (u/c = I .O) was found to cause errors in stress intensity range (0 calculations. This. in turn, led to significant scatter or ‘anomaly’ in small crack growth rates relative to large cracks. At large crack siLe$, the effects of local crack front perturbations on crack aspect ratio and AK were found to be insignificant. As a result, the scatter in crack growth data was found to decrease to a negligible level at large crack sizes. It is suggested that the limiting crack size above which the small crack behaves as a large crack, /?= IOd (d-grain \tze), proposed by Taylor and Knott. is related to the crack size above uhich the effect\ due to aspect ratio variations are small. Graphs. 46 rel
Fatigue and fracture behavior of a fine-grained lamellar TiAl alloy. Chon, K.S. cmd Shih, D.S. Mrmllurgiccrl mrd Mnwricrl~ Trcrn.w~io~~~ A ( I997 128A( I ). 79-90A The fatigue and fracture resistance of a TIAI alloy, fi37A1-2Nb-20. with 0.2 at.% boron addition was studied by performing tensile. fracture toughnea\, and fatigue crack growth tests. The material was heat treated to exhibit a fincgrained, fully lamellar microstructure with -150 pm grain size and I-@rn lamellae \pacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile transition temperature (BDTT). while fracture and fatigue terts were preformed at 2.5 and 8lYC. Fracture toughness tests were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, and using ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained. fully lamellar microstructure were identified and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture toughness and fatigue crack growth resistance, despite low tensile ductility. The bources of ductility. fracture toughness, and fatigue resistance were identified and related to pertinent microstructural varlablec. Graphs. photomicrograph\, 59 ref.
Crain failures due to wind induced vibrations. [Kranschaden duzierte Schwingungen.]. Klinger, C.. Hortmunns, M., Ruschunyh, H. (ml Wohlrr. H. Stcrhlbnu (1996) 6% IO). 377-39 I
durch windin-
Self-excited vibrations with large amplitudes in natural wind may occur at slender structural elements with low damping. Because of the different design\ in the past and today (e.g. using solid rectiona insteAd of cables for tension elements) the \uaceptibility to galloping oscillations has increased. Especially systems with high prestresbed element\, like hangers of bridges or temion bars of cranes, are sensitive to wind induced vibrations. Therefore more and more fatigue problems caused by galloping oscillations have occurred in the la\t years. This paper describes exemplary collapses of two cranes caused by galloping-vibrations, showing which parameters lead to galloping-vibrations and how their amplitudes can be estimated with more accuracy by a nonlinear and nonstationary approach. Furthermore it is shown that those dynamic Qresseb caused fatigue failure. To prevent loss, knowledge and results gained by these investigations should be put at the disposal of engineers working on this field design. The research works on galloping will contribute to the development of appropriate technical standards.
..
Bertolini, Mmrinls
L., Cipla, A.. Don:el/i, G., Postore, T. and Ronucci, 7: cmd Corrosion ( 1996) 47( 8). 446-45 I
The effect of an environment containing chloride on the fatigue behavior of a 8090 Al-Li-M&u alloy was studied. Results were compared with those obtained on a traditional 2024 Al-Co-Mg alloy. Fatigue and corrosion-fatigue tests were carried out both on smooth specimens and revited samples. In order to ashess the corrosion behavior of the materials and their susceptibility to stress corrosion cracking, potentiodynamic polarization and slow strain rate tests were carned out. Results showed a remarkable effect of the aggressive environment on the fatigue behavior both of the innovative 8090 alloy (in T8 ageing conditions) and on the traditional clad 2024 alloy (in T3 natural ageing conditions), though the former showed shghtly better behavior. Nevertheless, in the presence of rivets the reduction in fatigue strength in the aggressive environment was negligible. Slow strain rate test showed premature fractures under anodic polarization above the pitting potential and with a strain rate of IO - 6 \-I, only for the 8090 TX alloy. Graphs, 16 ref. Synthesis of realistic loading specifications Drrssler, K.. Kott~m. V.B. and Kot:le. H. Europmrr Journnl of Mechmicol En,yinrrrin,q
( 1996)
41(3),
The aspect ratio data were determined from the measurements of crack compliance. made using a laser interfere metric system, and the measurements of surface crack length (2c), made using a photomicroscopic system. The variations in aspect ratio were found to be large at small crack sizes of the order of a few grain diameters. The experimental n/c data were compared with the patterns of crack aspect ratio variation, obtained from theoretical simulations. The simulated data were generated by assuming alternate crack propagation at the surface and at the depth, the detail\ of which are presented in Part I of the study accompanying this article. A good agreement was found between the simulated and the experimentally observed variation\. After incorporating the u/c variations in AK calculations, the scatter in the growth data of small cracks was significantly reduced and uas found to be of the \ame order as in large cracks. Additionally, it i\ shown in this study that the conventional methods of analysis of small-crack data, performed with an assumption of (I/C = I, can result in significant errors in AK calculation and an increased level of scatter in small-crack growth data. Small cracks al\o were found to exhibit low closure levels relative to large cracks. The results of the study strongly indicate that characteristics of small cracks. often referred to as anomalous, are due to the assumption of u/c= I in Gtuntiom of large variations in aspect ratio. the use of conventlonal methods of data analysis, and the lower levels of crack closure found naturally in small cracks. Graphs, photomicrographs, 39 ref.
153-168
Fatigue evaluation in the automotive industry cannot rely on numerical procedures only. Analyai\ and synthesis of customer- and product-specific loading both for testing and numerical life prediction is a major part of the fatigue evaluation process. Certain mathematical tasks are often solved more easily in a domain different from the original one, and the solution is then transformed back into the original domain. Similarly, solving tasks of fatigue loading analysis and synthesis like monitoring, editing, superposition and extrapolation in the ‘rainflow domain’ and then transforming the results back into the time domain by reconstructing load histories from the raintlow matrix. was successful for uniaxial loading. It is now also possble for multiaxial, non-proportional loading. Graph\. 26 ref. Bayesian neural network analysis of fatigue crack growth base superalloys. Fujii. H.. Mackay. D.J.C. curd Bhrrdeshicr, H.K.D.H. /SIJ Ir~tenmrionrrl (1996) 36( I I ), 1373-1382
rate in nickel
The fatigue crack growth rate of nickel base superalloys has been modelled using a neural network model within a Bayesian framework. A ‘commit& model was also introduced to increase the accuracy of the predictions. The rate was modelled as a fun&on of some 5 I variables, including stress intensity temperature, grain size, heat treatrange AK, log AK. chemical composition, ment, frequency, load waveform, atmosphere. R-ratio, the distinction between short crack growth and long crack growth, sample thickness and yield strength. The Bayeaian method puts error bars on the predicted value of the rate and allows the significance of each individual factor to be estimated. In addition, It was possible to estimate the isolated effect of particular variables such a\ the grain bize. which cannot in practice be varied independently. This demonstrates the ability of the method to investigate new phenomena in cases where the information cannot be accessed experimentally. Astrology, CMSX4, and lnconel 718 are mentioned. Graphs, SO ref. Effect of shot peening on the fatigue and fracture titanium alloys. Sridhrrr, B. R.. Rorrrczc~htmdr~r, K. rrnd Padmnncrbhm. K.A. Joumol of Motrricrk Scirncr ( 1996) 31(22). 5953-5960
Asbstracts
behavior
of two
The fatigue and fracture behavior of two titanium alloys, the near-a IMI-685 and w-p IMI-318. were studied in the machined and polished (MP) and the machined, polished and shot (glass-bead) peened (MPS) conditions. Glass-bead peening reduced the room-temperature and the high-temperature (450°C) fatigue life of alloy IMI-685 at high stress amplitudes, g,,, approaching the proof \tres?l. m,,>, of the material (LCF region). When the applied stress amplitude (O-770 MPa, HCF region) was comparable to the peen-induced peak longitudlnal residual stress. n,.,,, that is (rr,.Jcr,,)=0.92, an improvement in the roomtemperature fatigue life of IMI-685 was observed. When the (cr,Jq,) ratio was less than this value, decreases in the fatigue life were seen. The roomtemperature fatigue behavior of IMI-318 at high stress amplitudes was similar to that of IMI-685. The decreases in the fatigue life of this alloy, at a stress amplitude (770 MPa) where improvement was observed for IMI-685 could be attributed to the higher relaxation of peen-induced residual stresses in IMI-31X compared with IMI-685. Class-bead peening improved the high-temperature (450°C) fatigue life of IMI-685 at a low stress amplitude (465 MPa; (rr,jcr,,) = 0.87). The crack-initiation sites in the MP and the MPS conditions were at the surface for both the alloys. However, fracture in the surface layer\ of the alloys appeared more brittle in the peened (MPS) rather than in the unpeened (MP) condition. Graphs, photomicrographs, 32 ref. Effects of crack aspect ratio on the behavior of small surface cracks fatigue. II. Exnrrimmtc- O!I o titanrum (Ti-8AI) U//OY. R~~vichnndrcrn. K.S. and Lmsen, J.h. Mrtollurgicnl md Mnteriols Tronsocrions A (I 997) 28A(I). I S7- I69A
in
The continuous variations in crack shape or aspect ratio, a/c ((I is the crack depth and c IS the half-surface length), of small surface cracks, induced by grain boundaries, have been investigated during the fatigue crack growth of Emall cracks in a titanium (Ti-SAI) alloy. The significance of the aspect ratio variations in explaming the ‘anomalous’ small-crack behavior was evaluated.
Effects of crack aspect ratio on the behavior of small surface cracks fatigue. I. Simulation. Rm~ichnrrdrm. K.S. Metrrllurgiccrl and Moferirds 7’rmscrcrior~r A ( I997 1 28A( I ), 14% IShA
in
A simple simulation of alternate growth of a small surface crack in the \urfaace and depth directions WBL performed to illustrate the changes in crack aspect ratio, induced by grain boundaries, as a function of crack File. It 15 shown that at small crack sizes, large variations in aspect ratio, (r/c (o is the crack depth and c IS the half-surface length), occur. due to local crack front perturbations induced by grains that are oriented for crack growth. At the\e crack sizes, the assumption of a semicrcular crack shape (u/c = I .O) was found to cause errors in stress intensity range (0 calculations. This. in turn, led to significant scatter or ‘anomaly’ in small crack growth rates relative to large cracks. At large crack siLe$, the effects of local crack front perturbations on crack aspect ratio and AK were found to be insignificant. As a result, the scatter in crack growth data was found to decrease to a negligible level at large crack sizes. It is suggested that the limiting crack size above which the small crack behaves as a large crack, /?= IOd (d-grain \tze), proposed by Taylor and Knott. is related to the crack size above uhich the effect\ due to aspect ratio variations are small. Graphs. 46 rel
Fatigue and fracture behavior of a fine-grained lamellar TiAl alloy. Chon, K.S. cmd Shih, D.S. Mrmllurgiccrl mrd Mnwricrl~ Trcrn.w~io~~~ A ( I997 128A( I ). 79-90A The fatigue and fracture resistance of a TIAI alloy, fi37A1-2Nb-20. with 0.2 at.% boron addition was studied by performing tensile. fracture toughnea\, and fatigue crack growth tests. The material was heat treated to exhibit a fincgrained, fully lamellar microstructure with -150 pm grain size and I-@rn lamellae \pacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile transition temperature (BDTT). while fracture and fatigue terts were preformed at 2.5 and 8lYC. Fracture toughness tests were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, and using ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained. fully lamellar microstructure were identified and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture toughness and fatigue crack growth resistance, despite low tensile ductility. The bources of ductility. fracture toughness, and fatigue resistance were identified and related to pertinent microstructural varlablec. Graphs. photomicrograph\, 59 ref.
Crain failures due to wind induced vibrations. [Kranschaden duzierte Schwingungen.]. Klinger, C.. Hortmunns, M., Ruschunyh, H. (ml Wohlrr. H. Stcrhlbnu (1996) 6% IO). 377-39 I
durch windin-
Self-excited vibrations with large amplitudes in natural wind may occur at slender structural elements with low damping. Because of the different design\ in the past and today (e.g. using solid rectiona insteAd of cables for tension elements) the \uaceptibility to galloping oscillations has increased. Especially systems with high prestresbed element\, like hangers of bridges or temion bars of cranes, are sensitive to wind induced vibrations. Therefore more and more fatigue problems caused by galloping oscillations have occurred in the la\t years. This paper describes exemplary collapses of two cranes caused by galloping-vibrations, showing which parameters lead to galloping-vibrations and how their amplitudes can be estimated with more accuracy by a nonlinear and nonstationary approach. Furthermore it is shown that those dynamic Qresseb caused fatigue failure. To prevent loss, knowledge and results gained by these investigations should be put at the disposal of engineers working on this field design. The research works on galloping will contribute to the development of appropriate technical standards.