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Fatigue strength of welded RHS joints in structures and girders
508
Fatigue Abstracts
the load-carrying capacity of the brazed joints only when the brazing temperature was high enough. Fatigue crack initiation and growth under displacement amplitude control were also carried out. Crack closure in the brazed joints was determined by means of back face strain on the compact tension specimen used. The introduction of gap filler was able to increase the fatigue and fracture resistance of the brazed joints when a suitable brazing temperature was used. Crack deflection, branching and uncracked ligament bridging behind the crack tip were observable along the crack paths. Experimental results showed that the gap filler was able to enhance the crack closure by roughness and ligament bridging. Graphs, photomicrographs, 17 ref.
Fatigue performance of friction stir butt welds in a 6000 series aluminum alloy Haagensen, P.J., Midling, O.T. and Ranes, M. Proc. Conf. Computer Methods and Experimental Measurements for Surface Treatment Effects II, Milan, Italy, 7-9 June 1995, pp. 225-237 The present investigation was undertaken to determine the fatigue properties of friction stir welds in 5 mm thick plates in an AA6082 alloy. Extruded plates in the T4 condition were used in the test program. S-N tests in pulsating tension at R = 0.5 were performed on specimens with the weld transverse to the stress direction. Reference tests were made on the base material. Crack growth data were obtained for material in the weld metal, in the HAZ and base material. S-N tests were also made on conventional MIG butt welds from the same batch material to enable a comparison of the two welding methods. The results indicate that the fatigue strength of transverse friction stir welds is approx 50% higher than the fatigue strength of MIG butt welds. The crack growth rates obtained for the weld material were lower than in the base material, probably due to a more fine grained microstructure in the weld region. Graphs, 7 ref.
Fatigue strength of welded RHS joints in structures and girders O'Connor, M. Tube Int. (1995) 14 (66), 165-166 A program of research carried out to determine a design method for the calculation of the fatigue life of structures constructed from rectangular hollow section (RHS) non-alloyed steel members is described. This involved tests on specimens with both fillet and butt welds, configured as X-joints, T-joints and K-joints. The most important local parameter influencing the fatigue life, or number of cycles to failure, of a joint is the maximum or hot spot stress obtained at any point in that joint. These results are shown. There then follows a recommended design procedure which will be useful for designers of these types of hollow joints in latticed structures and Vierendeel girders. Graphs, 1 ref.
Effect of reinforcement on the strength of junctions between cylindrical and conical shells Kalnins, A. and Updike, D.P. J. Pressure Vessel Technol. (Trans. ASME) (1995) 117 (2), 135-141 Two failure modes are addressed for cylinder-cone junctions under internal and external pressure: axisymmetric yielding and low-cycle fatigue. If the junction fails to meet the failure criterion of any one of the two modes, then it must be strengthened by reinforcement. It is shown that the degree to which a junction is strengthened depends on the distribution of the reinforcement. A placement of reinforcement on the cylinder alone, leaving the actual connection between the cylinder and cone unreinforced, adds strength with regard to axisymmetric yielding, but may not strengthen the junction sufficiently with regard to low-cycle fatigue. This means that the junction may appear reinforced, but is not strengthened. It is pointed out that the design rules of Section VIII, Div. 1 of the ASME B&PV code (1992) set the need for reinforcement according to the failure criterion of axisymmetric yielding. There is no assurance that the reinforced junction will meet the failure criterion of low-cycle fatigue. This means that the safety margin on the number of allowed cycles is less than that which is expected, and that the junction may be unfit for cyclic service. It is also shown that a reinforcement distribution that requires minimum thicknesses for sections of both the cylinder and cone near the junction can satisfy criteria for both failure modes. This approach is already used in Code Case 2150 of Section VIII, Div. 1, for half-apex cone angles from 30 to 60 °, and required in Div. 2 for cone angles from 0 to 30 °. Its extension to angles from 0 to 60 ° for both internal and external pressure is recommended. A hypothetical example using steel SA516-70 is used to illustrate the cases discussed. Graphs, 6 ref.
Evaluation of corrosion resistance and corrosion fatigue strength of CrNcoating steel Shiozawa, K., Motobayashi, K. and Sonobe, M. Proc. Conf. Computer Methods and Experimental Measurements for Surface Treatment Effects II, Milan, Italy, 7-9 June 1995, pp. 147-154 Cantilever-type rotating bending fatigue tests were carried out in 3% saline solution by using the round-bar specimen of 0.37 wt.% carbon steel coated with chromium nitride by physical vapor deposition method. Also the corrosion behavior of the coating specimen was determined by electrochemical polarization measurement. Obvious improvement of the corrosion fatigue strength was attained by the coating of electrochemically stable thin film on the steel. The degree of improvement depended on the thickness of the coating film, because the size and amount of open pores in the coating decreased with increasing the thickness. The mechanism of corrosion pit formation during corrosion fatigue was discussed and the same mechanism applied to electrochemical measurement.
Cyclic life of superailoy IN738LC under in-phase and out-of-phase thermomechanical fatigue loading Chen, H., Chen, W., Mukherji, D., Wahi, R.P. and Wever, H. Z. Metallkd. (1995) 86 (6), 423-427 The cyclic life of IN738LC, a widely used nickel base superalloy for blades in stationary gas turbines, was investigated under thermo-mechanical fatigue loading using a temperature variation range of 1023-1223 K, with temperature variation rate in the range of 6 to 15 K min-~. Simple thermo-mechanical cycles with linear sequences corresponding to in-phase (IP) and out-of-phase (OP) tests were performed. Both the IP and OP tests were carried out at different constant mechanical strain ranges varied between 0.8-2.0% and at a constant mechanical strain rate of 10-5 s-~. Thermo-mechanical fatigue lives under both test conditions were compared with each other and with those of isothermal LCF tests at a temperature of 1223 K. The results show that the life under thermo-mechanical fatigue is strongly dependent on the nature of the test, i.e. stress controlled or strain controlled. Photomicrographs, I! ref.
Application of some fatigue criteria to life prediction for a load spectrum composed of periodical blocks of multiaxial stress states [Application des crit~res de fatigue au calcul de la d u r ~ de vie pour un spectre de charge compos~ de blocs p~riodiques d'~tats muitiaxiaux de contraintes] Fogue, M., Djimkouo, Y., Vidal-Salle, E., Robert, J.-L. and Bahuaud, J. Mec. Ind. Mater. (1995) 48 (l), 35-37 The service loadings to which structures are submitted, are generally random. The need to follow as accurately as possible the service loading during a fatigue test is induced by the understanding of the operating fatigue phenomena. It leads us to carry out some tests with different load levels (i.e. the load spectrum or block is composed of periodical stress sequences). This study proposes a method to valuate the predicted life for such multiaxial states of stress with a load spectrum composed of periodical blocks. The damage function proposed in the fatigue criteria and Palmgren-Miner rule are used to express the damage of each cycle. The cumulation of damage and the prediction life are realized also by the way of that damage law. The predicted life is expressed as the number of repetitions of the block until failure. 9 ref.
Effect of hydrogen on the fatigue behavior of alloy 600 at cathodic potential Ho, J.-T. and Yu, G.-P. Script Metall. Mater. (1995) 32 (l 1), 1845-1849 The fatigue behavior of the Alloy 600 was investigated to explore the mechanism of its hydrogen-assisted cracking during fatigue tests. Polished compact tension specimens of Alloy 600 were subjected to a fatigue crack growth (FCG) test in air and 0.1 M NaC1 solution at varying circuit potentials of -123- -1300 VSCE. FCG rates were 1.5-2 times greater in NaCI as compared to that in air. The results indicate that FCG rate increases with increasing load frequency/load ratio. SEM images indicate a transgranular mode of fracture. Many secondary cracks and fatigue striations were found on the fracture surface of samples tested in NaCI at -1300 VSCE, 25°C, and frequency = 0.1 Hz. Enhancement of FCG rates of Alloy 600 under the present experimental conditions is ascribed to hydrogen embrittlement. Graphs, photomicrographs, 11 ref.
Interaction between oxidation and thermo-mechanical fatigue in IN738LC superalloy. I Esmaeili, S., Engler-Pinto, C.C. Jr, llschner, B. and Rezai-Aria, F. Script Metall. Mater. (1995) 32 (l l), 1777-1781 The authors attempted to explore the mechanism of thermo-mechanical fatigue of turbine blades. In this direction, the relationship between thermo-mechanical fatigue and matrix oxidation in a cyclic thermo-mechanical loaded IN738LC superalloy at 400-900°C in air was investigated. SEM images indicated general as well as preferential matrix oxidation of interdendritic and intergranular areas. It is suggested that the mechanical strain range influences the general matrix oxidation, and that the oxidation kinetics of a matrix under thermodynamic fatigue is linear. Graphs, photomicrographs, 10 ref.
Effect of small artificial defects on fatigue strength of Ti-6AI-4V alloy at elevated temperatures Kato, Y., Takafuji, S., Matsuda, T. and Kiriyama, M. J. Soc. Mater. Sci., Jpn. (1995) 44 (499), 477-481 The bending fatigue strength of Ti-6A1-4V alloy was investigated in the temperature range of 20-450°C. A small artificial defect was introduced in the form of a small hole drilled in the surface of an unnotched specimen. The diameter of the hole was approximately 0.1 mm and its depth was virtually the same as the diameter. The fatigue strength ¢y F of the unnotched specimen was compared with the cr FN of specimens with a small hole. The following results were obtained: (1) The fatigue strength is remarkably decreased, even by such a small hole. The decrements of the fatigue strength caused by the hole (¢y F- cr FN)/cy F are approximately 40, 32, 27 and 47% at 20, 250, 350 and 450°C, respectively. The values of ¢y FN predicted by the Murakami's o-a~'~rca parameter model are approximately 17% smaller than the experimental results. The prediction seems to give a conservative estimation. (2) In the temperature range of 350-450°C, both o" F and or FN are increased by solution hardening, dynamic strain aging, and so on. Since the static yield strength is hardly influenced by such a strengthening factor, the temperature dependence of the yield strength may be used as the reference for evaluating the increments in the fatigue strength. It is concluded that the maximum increments of ¢r F and ¢r FN come to approximately 60%. Graphs, photomicrographs, 6 ref.
Fatigue Abstracts
the load-carrying capacity of the brazed joints only when the brazing temperature was high enough. Fatigue crack initiation and growth under displacement amplitude control were also carried out. Crack closure in the brazed joints was determined by means of back face strain on the compact tension specimen used. The introduction of gap filler was able to increase the fatigue and fracture resistance of the brazed joints when a suitable brazing temperature was used. Crack deflection, branching and uncracked ligament bridging behind the crack tip were observable along the crack paths. Experimental results showed that the gap filler was able to enhance the crack closure by roughness and ligament bridging. Graphs, photomicrographs, 17 ref.
Fatigue performance of friction stir butt welds in a 6000 series aluminum alloy Haagensen, P.J., Midling, O.T. and Ranes, M. Proc. Conf. Computer Methods and Experimental Measurements for Surface Treatment Effects II, Milan, Italy, 7-9 June 1995, pp. 225-237 The present investigation was undertaken to determine the fatigue properties of friction stir welds in 5 mm thick plates in an AA6082 alloy. Extruded plates in the T4 condition were used in the test program. S-N tests in pulsating tension at R = 0.5 were performed on specimens with the weld transverse to the stress direction. Reference tests were made on the base material. Crack growth data were obtained for material in the weld metal, in the HAZ and base material. S-N tests were also made on conventional MIG butt welds from the same batch material to enable a comparison of the two welding methods. The results indicate that the fatigue strength of transverse friction stir welds is approx 50% higher than the fatigue strength of MIG butt welds. The crack growth rates obtained for the weld material were lower than in the base material, probably due to a more fine grained microstructure in the weld region. Graphs, 7 ref.
Fatigue strength of welded RHS joints in structures and girders O'Connor, M. Tube Int. (1995) 14 (66), 165-166 A program of research carried out to determine a design method for the calculation of the fatigue life of structures constructed from rectangular hollow section (RHS) non-alloyed steel members is described. This involved tests on specimens with both fillet and butt welds, configured as X-joints, T-joints and K-joints. The most important local parameter influencing the fatigue life, or number of cycles to failure, of a joint is the maximum or hot spot stress obtained at any point in that joint. These results are shown. There then follows a recommended design procedure which will be useful for designers of these types of hollow joints in latticed structures and Vierendeel girders. Graphs, 1 ref.
Effect of reinforcement on the strength of junctions between cylindrical and conical shells Kalnins, A. and Updike, D.P. J. Pressure Vessel Technol. (Trans. ASME) (1995) 117 (2), 135-141 Two failure modes are addressed for cylinder-cone junctions under internal and external pressure: axisymmetric yielding and low-cycle fatigue. If the junction fails to meet the failure criterion of any one of the two modes, then it must be strengthened by reinforcement. It is shown that the degree to which a junction is strengthened depends on the distribution of the reinforcement. A placement of reinforcement on the cylinder alone, leaving the actual connection between the cylinder and cone unreinforced, adds strength with regard to axisymmetric yielding, but may not strengthen the junction sufficiently with regard to low-cycle fatigue. This means that the junction may appear reinforced, but is not strengthened. It is pointed out that the design rules of Section VIII, Div. 1 of the ASME B&PV code (1992) set the need for reinforcement according to the failure criterion of axisymmetric yielding. There is no assurance that the reinforced junction will meet the failure criterion of low-cycle fatigue. This means that the safety margin on the number of allowed cycles is less than that which is expected, and that the junction may be unfit for cyclic service. It is also shown that a reinforcement distribution that requires minimum thicknesses for sections of both the cylinder and cone near the junction can satisfy criteria for both failure modes. This approach is already used in Code Case 2150 of Section VIII, Div. 1, for half-apex cone angles from 30 to 60 °, and required in Div. 2 for cone angles from 0 to 30 °. Its extension to angles from 0 to 60 ° for both internal and external pressure is recommended. A hypothetical example using steel SA516-70 is used to illustrate the cases discussed. Graphs, 6 ref.
Evaluation of corrosion resistance and corrosion fatigue strength of CrNcoating steel Shiozawa, K., Motobayashi, K. and Sonobe, M. Proc. Conf. Computer Methods and Experimental Measurements for Surface Treatment Effects II, Milan, Italy, 7-9 June 1995, pp. 147-154 Cantilever-type rotating bending fatigue tests were carried out in 3% saline solution by using the round-bar specimen of 0.37 wt.% carbon steel coated with chromium nitride by physical vapor deposition method. Also the corrosion behavior of the coating specimen was determined by electrochemical polarization measurement. Obvious improvement of the corrosion fatigue strength was attained by the coating of electrochemically stable thin film on the steel. The degree of improvement depended on the thickness of the coating film, because the size and amount of open pores in the coating decreased with increasing the thickness. The mechanism of corrosion pit formation during corrosion fatigue was discussed and the same mechanism applied to electrochemical measurement.
Cyclic life of superailoy IN738LC under in-phase and out-of-phase thermomechanical fatigue loading Chen, H., Chen, W., Mukherji, D., Wahi, R.P. and Wever, H. Z. Metallkd. (1995) 86 (6), 423-427 The cyclic life of IN738LC, a widely used nickel base superalloy for blades in stationary gas turbines, was investigated under thermo-mechanical fatigue loading using a temperature variation range of 1023-1223 K, with temperature variation rate in the range of 6 to 15 K min-~. Simple thermo-mechanical cycles with linear sequences corresponding to in-phase (IP) and out-of-phase (OP) tests were performed. Both the IP and OP tests were carried out at different constant mechanical strain ranges varied between 0.8-2.0% and at a constant mechanical strain rate of 10-5 s-~. Thermo-mechanical fatigue lives under both test conditions were compared with each other and with those of isothermal LCF tests at a temperature of 1223 K. The results show that the life under thermo-mechanical fatigue is strongly dependent on the nature of the test, i.e. stress controlled or strain controlled. Photomicrographs, I! ref.
Application of some fatigue criteria to life prediction for a load spectrum composed of periodical blocks of multiaxial stress states [Application des crit~res de fatigue au calcul de la d u r ~ de vie pour un spectre de charge compos~ de blocs p~riodiques d'~tats muitiaxiaux de contraintes] Fogue, M., Djimkouo, Y., Vidal-Salle, E., Robert, J.-L. and Bahuaud, J. Mec. Ind. Mater. (1995) 48 (l), 35-37 The service loadings to which structures are submitted, are generally random. The need to follow as accurately as possible the service loading during a fatigue test is induced by the understanding of the operating fatigue phenomena. It leads us to carry out some tests with different load levels (i.e. the load spectrum or block is composed of periodical stress sequences). This study proposes a method to valuate the predicted life for such multiaxial states of stress with a load spectrum composed of periodical blocks. The damage function proposed in the fatigue criteria and Palmgren-Miner rule are used to express the damage of each cycle. The cumulation of damage and the prediction life are realized also by the way of that damage law. The predicted life is expressed as the number of repetitions of the block until failure. 9 ref.
Effect of hydrogen on the fatigue behavior of alloy 600 at cathodic potential Ho, J.-T. and Yu, G.-P. Script Metall. Mater. (1995) 32 (l 1), 1845-1849 The fatigue behavior of the Alloy 600 was investigated to explore the mechanism of its hydrogen-assisted cracking during fatigue tests. Polished compact tension specimens of Alloy 600 were subjected to a fatigue crack growth (FCG) test in air and 0.1 M NaC1 solution at varying circuit potentials of -123- -1300 VSCE. FCG rates were 1.5-2 times greater in NaCI as compared to that in air. The results indicate that FCG rate increases with increasing load frequency/load ratio. SEM images indicate a transgranular mode of fracture. Many secondary cracks and fatigue striations were found on the fracture surface of samples tested in NaCI at -1300 VSCE, 25°C, and frequency = 0.1 Hz. Enhancement of FCG rates of Alloy 600 under the present experimental conditions is ascribed to hydrogen embrittlement. Graphs, photomicrographs, 11 ref.
Interaction between oxidation and thermo-mechanical fatigue in IN738LC superalloy. I Esmaeili, S., Engler-Pinto, C.C. Jr, llschner, B. and Rezai-Aria, F. Script Metall. Mater. (1995) 32 (l l), 1777-1781 The authors attempted to explore the mechanism of thermo-mechanical fatigue of turbine blades. In this direction, the relationship between thermo-mechanical fatigue and matrix oxidation in a cyclic thermo-mechanical loaded IN738LC superalloy at 400-900°C in air was investigated. SEM images indicated general as well as preferential matrix oxidation of interdendritic and intergranular areas. It is suggested that the mechanical strain range influences the general matrix oxidation, and that the oxidation kinetics of a matrix under thermodynamic fatigue is linear. Graphs, photomicrographs, 10 ref.
Effect of small artificial defects on fatigue strength of Ti-6AI-4V alloy at elevated temperatures Kato, Y., Takafuji, S., Matsuda, T. and Kiriyama, M. J. Soc. Mater. Sci., Jpn. (1995) 44 (499), 477-481 The bending fatigue strength of Ti-6A1-4V alloy was investigated in the temperature range of 20-450°C. A small artificial defect was introduced in the form of a small hole drilled in the surface of an unnotched specimen. The diameter of the hole was approximately 0.1 mm and its depth was virtually the same as the diameter. The fatigue strength ¢y F of the unnotched specimen was compared with the cr FN of specimens with a small hole. The following results were obtained: (1) The fatigue strength is remarkably decreased, even by such a small hole. The decrements of the fatigue strength caused by the hole (¢y F- cr FN)/cy F are approximately 40, 32, 27 and 47% at 20, 250, 350 and 450°C, respectively. The values of ¢y FN predicted by the Murakami's o-a~'~rca parameter model are approximately 17% smaller than the experimental results. The prediction seems to give a conservative estimation. (2) In the temperature range of 350-450°C, both o" F and or FN are increased by solution hardening, dynamic strain aging, and so on. Since the static yield strength is hardly influenced by such a strengthening factor, the temperature dependence of the yield strength may be used as the reference for evaluating the increments in the fatigue strength. It is concluded that the maximum increments of ¢r F and ¢r FN come to approximately 60%. Graphs, photomicrographs, 6 ref.