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Increasing fatigue strength by water peening
588
Fatigue Abstracts
Influence of shot peening on the fatigue fife and subsurface deformation phenomena of torsionally loaded ainmininm alloy specimens. Jaensson, B. and Ahmad, M. Proc. Fourth Int. Conf. on Residual Stresses, Baltimore, MD, USA (8-10 June 1994) 1031-1037
Surface and subsurface deformation phenomena in torsion fatigue cycled A1 alloy (7010) specimens have been studied by scanning electron microscopy. The deformation mechanisms found have been used in an attempt to rationalize the influence of shot peening on the torsion fatigue properties which have been reported earlier by the authors. Graphs, photomicrographs, 7 ref. Estimation of Kmax and AK~ff on fatigue fracture in several structural steels by X-ray fractography. Akita, K., Kodama, S. and Misawa, H. Proc. Fourth Int. Conf. on Residual Stresses, Baltimore, MD, USA (8-10 June 1994) 1024-1030 X-ray fractography was applied to the fatigue fractured surfaces produced by variable-amplitude loading using several structural steels. The crack propagation rates on the variable-amplitude loading fatigue fractured surfaces could be successfully estimated from the relationship between residual stress and stress intensity factor obtained by constant-amplitude fatigue tests using SNCM439. Graphs, 1 ref. An understanding of the effect of residual stress induced by shot peening on fatigue strength. Yao, M., Li, J., Shao, P., Wang, S., Wang, R. and Li, X. Proc. Fourth Int. Conf. on Residual Stresses, Baltimore, MD, USA (8-10 June 1994) 1002-1007 In relationship with technological conditions, the fatigue crack of a shotpeened specimen may initiate either at the surface or in the subsurface zone beneath the hardened layer. The residual stress induced by shot peening should play different roles when the crack source is located at different positions. Based on an analysis of experimental results obtained on specimens of a medium-carbon steel 40Cr, it is believed that, for specimens with a surface crack source, the compressive residual stress at the surface plays a role of lowering the mean stress of the sustained stress cycle and thus results in an increasing of their apparent fatigue strength (in this case, the effect of shot peening on surface roughness, which may have some influence on fatigue strength, should be taken into account too); while for specimens with a subsurface crack source, the compressive residual stress in the surfacehardened layer only plays a role of pushing the weakest link to the region beneath the hardened layer, and the apparent fatigue strength of shot-peened specimens is mainly related to the local fatigue strength of metals at the position where the fatigue crack initiates. In this case, the improvement of the apparent fatigue strength of shot-peened specimens is mainly related to the depth of the crack source from the surface, the value of the tensile residual stress and the local fatigue strength of metals at the position where the fatigue crack initiates. In this case, the improvement of apparent fatigue strength of shot-peened specimens is attributed to a higher value of local fatigue strength (which is called by us 'internal fatigue strength') when the crack initiates in the internal region. According to our work, the internal fatigue strength is approx 35% higher than the 'surface fatigue strength' of the same metal. Graphs, 24 ref. Shot peening and overload residual stresses to improve fatigue life of the mniti-hig breech system. Lee, S.L. Proc. Fourth Int. Conf. on Residual Stresses, Baltimore, MD, USA (8-10 June 1994) 894-903
In the design of the advanced multi-lug breech (4340 steel) system, a series of lugs replace the conventional slide breech mechanism to diffuse the concentrated load at the block-ring interface. In this work, shot peening and overload cold-working operations were performed to induce favourable residual stresses in the multi-lug breeches. The breeches were then fatigue tested to failure. X-ray diffraction stress analysis in the lugs of the breech ring demonstrated that the shot-peening residual stresses were more uniformly distributed and the overload residual stresses were location- and geometrydependent. The magnitudes of the maximum compressive residual stresses induced by the two processes were comparable, while the overload process produced much deeper zones of compressive residual stresses than the shotpeening process. The shot-peening process is known to retard crack growth
and improve the fatigue life of engineering components. Analysis showed that in the multi-lug breech system, the overload process is preferred over the shot-preening process in further improving fatigue life. Graphs, 24 ref. Increasing fatigue strength by water peening. Tonshoff, H.K. and Kroos, F. Proc. Fourth Int. Conf. on Residual Stresses, Baltimore, MD, USA (8-10 June 1994) 615-623
The properties of surface layers strongly influence the fatigue strength of dynamically loaded components (16MnCr5). The increase of fatigue strength as a result of mechanical, thermal or thermochemical surface treatments is based on an increase of compressive residual stress and hardness in the surface layers. Investigations on the influence of water peening parameters on residual stresses are presented in this paper. X-ray measurements proved that high compressive residual stresses can be produced in surface layers by water peening even in hard materials. The results are compared with those achieved by shot-peening experiments. Graphs, photomicrographs, 8 ref. Fatigue damage control of mechanical systems. Ray, A. and Wu, M.-K. Smart Mater. Struct. (Mar. 1994) 3 (1), 47-58
This paper presents the concept and architecture of a fatigue damage control system for mechanical structures. In contrast to the conventional cycle-based approach, fatigue damage is represented via non-linear differential euqations with respect to time in the state-variable setting.This damage model is compatible with the dynamic model of the plant, i.e. the process under operation and control, and the instantaneous damage rate depends on the current level of accumulated damage. The objective is to achieve an optmized trade-off between dynamic performance and structural durability of the plant. This interdisciplinary effort requires augmentation of the system-theoretic techniques for decision making and control, with governing equations and inequality constraints representing the fatigue damage properties of structural materials. The major challenge in the reported work is to characterize the fatigue damage generation process in mechanical structures and then utilize this information for synthesizing algorithms of performance optmization, robust control and risk assessment for plant operation. Graphs, 36 ref. Effect of aging on the fatigue crack growth kinetics of an AI-Zn-Mg-Cu alloy in two directions. Alpay, S.P. and Gurbuz, R. Scr. Metall. Mater. (15 Feb. 1994) 35 (4), 423-427
Fatigue tests on 7050 aluminium alloy after four different heat treatments show that the optimum temper for fatigue crack growth resistance is the double ageing T73651 while there was no significant difference in stage If fatigue crack growth rates when solution treated, under-aged, or over-aged CT-type specimens cut from cold-rolled 7050 T73651 plate were machined in the L-S and S-L directions and solution treated for 20 rain at 480 °C. One set was then under-aged for 30 h at 121 °C, another over-aged for 16 h at 160 °C and one double aged for 24 h at 121 °C plus 24 h at 163 °C. The equation constants of the Paris-Erdegan relation are tabled and show that the material obeys the relationship in stage II of crack growth. Graphs compare the four ageing conditions in the L-S and S-L orientations and compare the two orientations for each ageing condition. The optimum orientation was the L-S direction and differences were less marked in the S-L direction. Graphs, 16 ref. Cyclic strain hardening in polycrystniline copper. Liu, C.D., You, D.X. and Bassim, M.N. Acta. Metall. Mater. (May 1994) 42 (5), 1631-1638 Polycrystals of pure Cu were cyclically deformed, at room temperature under symmetric tension-compression fatigue at constant total strain amplitude control with an approximate constant plastic strain rate of 10-4 . The relationship between the saturation stress amplitude and strain amplitude over a range of plastic strain from 2×10 -7 to 10-2 reveals three regions of cyclic hardening. A quasi-plateau, where the stresses show a slow constant increase, was observed in the intermediate region extending in the plastic strain range Vp~, of 1.5x10 -5 to 7.5x10 -4. In this region, persistent slip bands (PSBs) which consist of ladder structures, similar to the case of single crystals, were found in the bulk of the fatigued polycrystals. The fatigue limit was found to be A~rs/2/=73 MPa, which corresponds to the plastic strain A%~/2=1.5×10 -5, where PSBs do not form. Graphs, photomicrographs, 62 ref.
Fatigue Abstracts
Influence of shot peening on the fatigue fife and subsurface deformation phenomena of torsionally loaded ainmininm alloy specimens. Jaensson, B. and Ahmad, M. Proc. Fourth Int. Conf. on Residual Stresses, Baltimore, MD, USA (8-10 June 1994) 1031-1037
Surface and subsurface deformation phenomena in torsion fatigue cycled A1 alloy (7010) specimens have been studied by scanning electron microscopy. The deformation mechanisms found have been used in an attempt to rationalize the influence of shot peening on the torsion fatigue properties which have been reported earlier by the authors. Graphs, photomicrographs, 7 ref. Estimation of Kmax and AK~ff on fatigue fracture in several structural steels by X-ray fractography. Akita, K., Kodama, S. and Misawa, H. Proc. Fourth Int. Conf. on Residual Stresses, Baltimore, MD, USA (8-10 June 1994) 1024-1030 X-ray fractography was applied to the fatigue fractured surfaces produced by variable-amplitude loading using several structural steels. The crack propagation rates on the variable-amplitude loading fatigue fractured surfaces could be successfully estimated from the relationship between residual stress and stress intensity factor obtained by constant-amplitude fatigue tests using SNCM439. Graphs, 1 ref. An understanding of the effect of residual stress induced by shot peening on fatigue strength. Yao, M., Li, J., Shao, P., Wang, S., Wang, R. and Li, X. Proc. Fourth Int. Conf. on Residual Stresses, Baltimore, MD, USA (8-10 June 1994) 1002-1007 In relationship with technological conditions, the fatigue crack of a shotpeened specimen may initiate either at the surface or in the subsurface zone beneath the hardened layer. The residual stress induced by shot peening should play different roles when the crack source is located at different positions. Based on an analysis of experimental results obtained on specimens of a medium-carbon steel 40Cr, it is believed that, for specimens with a surface crack source, the compressive residual stress at the surface plays a role of lowering the mean stress of the sustained stress cycle and thus results in an increasing of their apparent fatigue strength (in this case, the effect of shot peening on surface roughness, which may have some influence on fatigue strength, should be taken into account too); while for specimens with a subsurface crack source, the compressive residual stress in the surfacehardened layer only plays a role of pushing the weakest link to the region beneath the hardened layer, and the apparent fatigue strength of shot-peened specimens is mainly related to the local fatigue strength of metals at the position where the fatigue crack initiates. In this case, the improvement of the apparent fatigue strength of shot-peened specimens is mainly related to the depth of the crack source from the surface, the value of the tensile residual stress and the local fatigue strength of metals at the position where the fatigue crack initiates. In this case, the improvement of apparent fatigue strength of shot-peened specimens is attributed to a higher value of local fatigue strength (which is called by us 'internal fatigue strength') when the crack initiates in the internal region. According to our work, the internal fatigue strength is approx 35% higher than the 'surface fatigue strength' of the same metal. Graphs, 24 ref. Shot peening and overload residual stresses to improve fatigue life of the mniti-hig breech system. Lee, S.L. Proc. Fourth Int. Conf. on Residual Stresses, Baltimore, MD, USA (8-10 June 1994) 894-903
In the design of the advanced multi-lug breech (4340 steel) system, a series of lugs replace the conventional slide breech mechanism to diffuse the concentrated load at the block-ring interface. In this work, shot peening and overload cold-working operations were performed to induce favourable residual stresses in the multi-lug breeches. The breeches were then fatigue tested to failure. X-ray diffraction stress analysis in the lugs of the breech ring demonstrated that the shot-peening residual stresses were more uniformly distributed and the overload residual stresses were location- and geometrydependent. The magnitudes of the maximum compressive residual stresses induced by the two processes were comparable, while the overload process produced much deeper zones of compressive residual stresses than the shotpeening process. The shot-peening process is known to retard crack growth
and improve the fatigue life of engineering components. Analysis showed that in the multi-lug breech system, the overload process is preferred over the shot-preening process in further improving fatigue life. Graphs, 24 ref. Increasing fatigue strength by water peening. Tonshoff, H.K. and Kroos, F. Proc. Fourth Int. Conf. on Residual Stresses, Baltimore, MD, USA (8-10 June 1994) 615-623
The properties of surface layers strongly influence the fatigue strength of dynamically loaded components (16MnCr5). The increase of fatigue strength as a result of mechanical, thermal or thermochemical surface treatments is based on an increase of compressive residual stress and hardness in the surface layers. Investigations on the influence of water peening parameters on residual stresses are presented in this paper. X-ray measurements proved that high compressive residual stresses can be produced in surface layers by water peening even in hard materials. The results are compared with those achieved by shot-peening experiments. Graphs, photomicrographs, 8 ref. Fatigue damage control of mechanical systems. Ray, A. and Wu, M.-K. Smart Mater. Struct. (Mar. 1994) 3 (1), 47-58
This paper presents the concept and architecture of a fatigue damage control system for mechanical structures. In contrast to the conventional cycle-based approach, fatigue damage is represented via non-linear differential euqations with respect to time in the state-variable setting.This damage model is compatible with the dynamic model of the plant, i.e. the process under operation and control, and the instantaneous damage rate depends on the current level of accumulated damage. The objective is to achieve an optmized trade-off between dynamic performance and structural durability of the plant. This interdisciplinary effort requires augmentation of the system-theoretic techniques for decision making and control, with governing equations and inequality constraints representing the fatigue damage properties of structural materials. The major challenge in the reported work is to characterize the fatigue damage generation process in mechanical structures and then utilize this information for synthesizing algorithms of performance optmization, robust control and risk assessment for plant operation. Graphs, 36 ref. Effect of aging on the fatigue crack growth kinetics of an AI-Zn-Mg-Cu alloy in two directions. Alpay, S.P. and Gurbuz, R. Scr. Metall. Mater. (15 Feb. 1994) 35 (4), 423-427
Fatigue tests on 7050 aluminium alloy after four different heat treatments show that the optimum temper for fatigue crack growth resistance is the double ageing T73651 while there was no significant difference in stage If fatigue crack growth rates when solution treated, under-aged, or over-aged CT-type specimens cut from cold-rolled 7050 T73651 plate were machined in the L-S and S-L directions and solution treated for 20 rain at 480 °C. One set was then under-aged for 30 h at 121 °C, another over-aged for 16 h at 160 °C and one double aged for 24 h at 121 °C plus 24 h at 163 °C. The equation constants of the Paris-Erdegan relation are tabled and show that the material obeys the relationship in stage II of crack growth. Graphs compare the four ageing conditions in the L-S and S-L orientations and compare the two orientations for each ageing condition. The optimum orientation was the L-S direction and differences were less marked in the S-L direction. Graphs, 16 ref. Cyclic strain hardening in polycrystniline copper. Liu, C.D., You, D.X. and Bassim, M.N. Acta. Metall. Mater. (May 1994) 42 (5), 1631-1638 Polycrystals of pure Cu were cyclically deformed, at room temperature under symmetric tension-compression fatigue at constant total strain amplitude control with an approximate constant plastic strain rate of 10-4 . The relationship between the saturation stress amplitude and strain amplitude over a range of plastic strain from 2×10 -7 to 10-2 reveals three regions of cyclic hardening. A quasi-plateau, where the stresses show a slow constant increase, was observed in the intermediate region extending in the plastic strain range Vp~, of 1.5x10 -5 to 7.5x10 -4. In this region, persistent slip bands (PSBs) which consist of ladder structures, similar to the case of single crystals, were found in the bulk of the fatigued polycrystals. The fatigue limit was found to be A~rs/2/=73 MPa, which corresponds to the plastic strain A%~/2=1.5×10 -5, where PSBs do not form. Graphs, photomicrographs, 62 ref.