To investigate the mechanical properties of a dowel action under fatigue loads, nine reinforced concrete specimens were fabricated, and the monotonic and fatigue loadings were performed on these specimens, respectivel...To investigate the mechanical properties of a dowel action under fatigue loads, nine reinforced concrete specimens were fabricated, and the monotonic and fatigue loadings were performed on these specimens, respectively. All of these specimens were divided into two series. Six specimens in SeriesⅠwith different bar diameters of 12, 20 and 25 mm were subjected to monotonic loads and were used to confirm the ultimate bearing capacity. The remaining three specimens in Series Ⅱ were subjected to fatigue loads and were designed to investigate the attenuation character of dowel action and the fatigue failure modes. The test results show that the accumulated fatigue damage due to fatigue loads can reduce the ultimate bearing capacity of specimens. With the increase in fatigue loads, the failure mode can transform to fatigue rupture of the dowel bar under the serviceability loading state,i. e. 55% of the monotonic capacity. The fatigue life is determined by the fatigue properties of steel and concrete.Based on the test data, the failure process of dowel action can be divided into two stages： the accumulation of fatigue damage and the fatigue rupture of dowel bar.展开更多
Before 1980s,the circular suspension spring in automobile subjected to torsion fatigue load,under the cyclic normal tensile stresses,the majority of fatigue fracture occurred was in normal tensile fracture mode（NTFM...Before 1980s,the circular suspension spring in automobile subjected to torsion fatigue load,under the cyclic normal tensile stresses,the majority of fatigue fracture occurred was in normal tensile fracture mode（NTFM）and the fracture surface was under 45°diagonal.Because there exists the interaction between the residual stresses induced by shot peening and the applied cyclic normal tensile stresses in NTFM,which represents as＂stress strengthening mechanism＂,shot peening technology could be used for improving the fatigue fracture resistance（FFR）of springs.However,since 1990s up to date,in addition to regular NTFM,the fatigue fractures occurred of peened springs from time to time are in longitudinal shear fracture mode（LSFM）or transverse shear fracture mode（TSFM）with the increase of applied cyclic shear stresses,which leads to a remarkable decrease of FFR.However,LSFM/TSFM can be avoided effectively by means of shot peening treatment again on the peened springs.The phenomena have been rarely happened before.At present there are few literatures concerning this problem.Based upon the results of force analysis of a spring,there is no interaction between the residual stresses by shot peening and the applied cyclic shear stresses in shear fracture.This;means that the effect of＂stress strengthening mechanism＂for improving the FFR of LSFM/TSFM is disappeared basically.During shot peening,however,both of residual stress and cyclic plastic deformed microstructure are induced synchronously like＂twins＂in the surface layer of a spring.It has been found for the first time by means of force analysis and experimental results that the modified microstructure in the＂twins＂as a＂structure strengthening mechanism＂can improve the FFR of LSFM/TSFM.At the same time,it is;also shown that the optimum technology of shot peening strengthening must have both＂stress strengthening mechanism＂and＂structure strengthening mechanism＂simultaneously so that the FFR of both NTFM and LSFM/TSFM can be improved by shot peening.展开更多
The high cycle fatigue（HCF） tests of modified 9 Cr-1 Mo dissimilarly welded joint were carried out at different elevated temperatures and the fracture mechanism was systematically revealed. The fatigue strength at 1...The high cycle fatigue（HCF） tests of modified 9 Cr-1 Mo dissimilarly welded joint were carried out at different elevated temperatures and the fracture mechanism was systematically revealed. The fatigue strength at 108 cycles based on S-N curve can be estimated as a half of weld joint＇s yield strength for all conducted temperatures, which can be a reliable criterion in predicting the fatigue life. The results show that the inter-critical heat affected zones（IC-HAZs） of both sides are the weak zones due to their low hardness and inferior fatigue resistance property. HAZ of COST-FB2（BM2） is the weakest zone at room temperature due to the existence of numerously distributed defects and the initiation of cracks, either in the surface or interior zone, impacting a crucial effect on the fatigue life of the joint. While at elevated temperatures, fatigue life was controlled mostly by the intrusion-extrusion mechanism at the specimen surface under high stress level and subsurface non-defect fatigue crack origin（SNDFCO） from the interior material under low stress amplitude. With increasing temperature, more and more fatigue failures began to occur at the HAZ of COST-E（BM1） due to its higher susceptibility of temperature. Besides, it is found that the-ferrite in the BM1 has no harm to the HCF behavior of the joint at the conducted temperatures.展开更多
基金The Fund of the National Key Laboratory in China(No.2015-Ky-01)the National Key Technology R&D Program of China(No.2015BAB07B07)
文摘To investigate the mechanical properties of a dowel action under fatigue loads, nine reinforced concrete specimens were fabricated, and the monotonic and fatigue loadings were performed on these specimens, respectively. All of these specimens were divided into two series. Six specimens in SeriesⅠwith different bar diameters of 12, 20 and 25 mm were subjected to monotonic loads and were used to confirm the ultimate bearing capacity. The remaining three specimens in Series Ⅱ were subjected to fatigue loads and were designed to investigate the attenuation character of dowel action and the fatigue failure modes. The test results show that the accumulated fatigue damage due to fatigue loads can reduce the ultimate bearing capacity of specimens. With the increase in fatigue loads, the failure mode can transform to fatigue rupture of the dowel bar under the serviceability loading state,i. e. 55% of the monotonic capacity. The fatigue life is determined by the fatigue properties of steel and concrete.Based on the test data, the failure process of dowel action can be divided into two stages： the accumulation of fatigue damage and the fatigue rupture of dowel bar.
文摘Before 1980s,the circular suspension spring in automobile subjected to torsion fatigue load,under the cyclic normal tensile stresses,the majority of fatigue fracture occurred was in normal tensile fracture mode（NTFM）and the fracture surface was under 45°diagonal.Because there exists the interaction between the residual stresses induced by shot peening and the applied cyclic normal tensile stresses in NTFM,which represents as＂stress strengthening mechanism＂,shot peening technology could be used for improving the fatigue fracture resistance（FFR）of springs.However,since 1990s up to date,in addition to regular NTFM,the fatigue fractures occurred of peened springs from time to time are in longitudinal shear fracture mode（LSFM）or transverse shear fracture mode（TSFM）with the increase of applied cyclic shear stresses,which leads to a remarkable decrease of FFR.However,LSFM/TSFM can be avoided effectively by means of shot peening treatment again on the peened springs.The phenomena have been rarely happened before.At present there are few literatures concerning this problem.Based upon the results of force analysis of a spring,there is no interaction between the residual stresses by shot peening and the applied cyclic shear stresses in shear fracture.This;means that the effect of＂stress strengthening mechanism＂for improving the FFR of LSFM/TSFM is disappeared basically.During shot peening,however,both of residual stress and cyclic plastic deformed microstructure are induced synchronously like＂twins＂in the surface layer of a spring.It has been found for the first time by means of force analysis and experimental results that the modified microstructure in the＂twins＂as a＂structure strengthening mechanism＂can improve the FFR of LSFM/TSFM.At the same time,it is;also shown that the optimum technology of shot peening strengthening must have both＂stress strengthening mechanism＂and＂structure strengthening mechanism＂simultaneously so that the FFR of both NTFM and LSFM/TSFM can be improved by shot peening.
文摘The high cycle fatigue（HCF） tests of modified 9 Cr-1 Mo dissimilarly welded joint were carried out at different elevated temperatures and the fracture mechanism was systematically revealed. The fatigue strength at 108 cycles based on S-N curve can be estimated as a half of weld joint＇s yield strength for all conducted temperatures, which can be a reliable criterion in predicting the fatigue life. The results show that the inter-critical heat affected zones（IC-HAZs） of both sides are the weak zones due to their low hardness and inferior fatigue resistance property. HAZ of COST-FB2（BM2） is the weakest zone at room temperature due to the existence of numerously distributed defects and the initiation of cracks, either in the surface or interior zone, impacting a crucial effect on the fatigue life of the joint. While at elevated temperatures, fatigue life was controlled mostly by the intrusion-extrusion mechanism at the specimen surface under high stress level and subsurface non-defect fatigue crack origin（SNDFCO） from the interior material under low stress amplitude. With increasing temperature, more and more fatigue failures began to occur at the HAZ of COST-E（BM1） due to its higher susceptibility of temperature. Besides, it is found that the-ferrite in the BM1 has no harm to the HCF behavior of the joint at the conducted temperatures.
