| Cast steel joints have been widely used in bridges,offshore platforms and high-rise structures with good weldability,fabricability and beautiful appearance.Fatigue crack generally initiates in fillets or defects of a joint suffering to cycle loading from vehicle,wave,wind etc,which cause fatigue problem.In this paper,the fatigue tests of notched specimens and T-joints of G20Mn5 QT cast steel were conducted,and the fatigue failure mechanism and failure mode were revealed by analyzing the fatigue performance in depth.A new strain-based fatigue crack initiation life evaluation method was proposed,of which effectiveness,applicability and accuracy were verified.Finally,study on the influence of various factors on the fatigue performance of G20Mn5 QT cast steel joints is carried out,which provides guidance for the application of G20Mn5 QT cast steel joint in engineering.(1)Study on fatigue life evaluation method.A new strain-based fatigue crack initiation life evaluation method,strain field intensity(SNFI)method,was proposed.By analyzing the effective distance,fatigue damage region and weight function,the three-dimensional fatigue damage region of the semi-ellipsoid is established.Comparing the prediction results of modified Neuber’s rule and equivalent strain energy density(ESED)method on the basis of fatigue test results of G20Mn5 QT cast steel notched specimens,it can be seen that predictions by strain field intensity method are located in a scatter batter with a factor of 2,while predictions by modified Neuber’s rule and ESED method are located in a scatter batter with a factor of 5,which verified the effectiveness,applicability and accuracy of SNFI method.Comparing predictions of SNFI method with degenerating into one-dimensional fatigue damage region(SNFI-1D),it can be seen that predictions by strain field intensity method with threedimensional fatigue damage region are located in a scatter batter with a factor of 2,while predictions by SNFI-1D are located in a scatter batter with a factor of 5,which explains clearly the necessity of three-dimensional fatigue damage region for evaluating fatigue crack initiation.Besides,based on fatigue tests of G20Mn5 QT cast steel notched specimens,the fatigue crack growth analysis was carried out,and then the stress intensity factors(SIFs)were obtained based on J-integral method,extrapolation method and node displacement method,by comparison on SIFs,the J integral method is recommended to calculate the SIF of cast steel specimens.(2)Experimental study on fatigue performance of double-edged notched specimens of G20Mn5 QT cast steel.Fatigue tests of double-edged notched specimens of G20Mn5 QT cast steel under stress ratios with-1 and 0.1 were conducted,by which the stress-fatigue life(S-N)curves is obtained.By comparison with smooth standard specimens of G20Mn5 QT cast steel,the fatigue performance of notched specimens of G20Mn5 QT cast steel is significantly decreased.Comparing the S-N curves based on three mean stress correction rules,suggestions on how to choose mean stress correction rules for fatigue crack initiation life evaluation are presented: Goodman rule and Gerber rule are the first recommended,which provide essential support for the fatigue evaluation of cast steel members under non-full reverse loading.Fracture analysis was performed on the fatigue failure specimens under different stress ratios,and the microscopic morphology of the fatigue fracture surface was obtained.Casting defects can be seen at the notch root of the G20Mn5 QT cast steel notch specimen;clear fatigue striations and secondary cracks can be seen in the fatigue crack propagation region;large and small dimples can be seen in the instantaneous fracture region.For cast steel notched specimens,the direct reason of crack initiation is the notch effect caused by geometric mutation,casting defects amplify the notch effect.Thus,the fatigue failure of G20Mn5 QT cast steel notch specimen is the result of the notch effect and casting defects.(3)Experimental study on fatigue performance of tubular T-joint of G20Mn5 QT cast steel under in-plane bending moment.The fatigue tests of tubular T-joint of G20Mn5 QT cast steel under in-plane bending moment were carried out with stress ratio of 0,by which the S-N curves,loading-displacement curves and strain responses are obtained.Analyzing the loading-fatigue life curves and stiffness degeneration curves,three stiffness degeneration laws,completely degeneration region,platform region +uniform degeneration region and platform region + slowly degeneration region +rapidly degeneration region,are presented.Comparing the fatigue failure location and process of tubular T-joints of G20Mn5 QT cast steel,two fatigue failure modes,failure at chamfer and failure at core support,were obtained.By fracture analysis of tubular Tjoints of G20Mn5 QT cast steel under two kinds of fatigue failure modes,the fracture morphologies are obtained.On the fracture surface,the fatigue steps caused by fatigue origins on different planes can be seen.Comparing the fatigue crack growth zone and the instantaneous fracture zone,it can be seen that the former exists finer grains and smoother facture surface due to the constantly opening,closing and friction of the crack surfaces;the latter exists coarser grains and brighter facture surface due to shorter contact time with air.(4)Fatigue strength of tubular T-joint of G20Mn5 QT cast steel and its influence factors.The proposed strain field intensity method in this paper is used to evaluate fatigue life of tubular T-joint of G20Mn5 QT cast steel.By comparing its predictions with the test results,predictions of modified Neuber’s rule and ESED method,the applicability and accuracy of SNFI method were further verified.Based on the strain field strength method,the relationship between the strain concentration factor and the strain field intensity is established,and the influence regular pattern of each geometric parameter is obtained.And then the formula of the strain concentration factor on the geometric parameter is proposed,based on which the S-N curves under different geometric parameters is obtained.Meanwhile,the scope of application of each geometric parameter was pointed out,which provides theoretical basis and data support for anti-fatigue design of cast steel nodes in China.This paper proposes a validated strain-based fatigue crack initiation life evaluation method with good generalizability,which also has been applied in cast steel joints.The approach can overcome the shortcomings of the traditional fatigue crack initiation life evaluation approaches effectively and serve for the development and application of cast steel joints in China. |
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