Research On Determination Of CJP Model Parameters And Characterization Of The Crack Closure Effect On EA4T Axle Stteel

Since the crack closure effect was discovered,it has become a research hotspot of scholars all over the world.The traditional crack tip stress field model can’t describe the impact of crack tip plastic zone and wake zone on crack tip stress field.By contrast,CJP model can make up for the shortcomings of traditional model.CJP model has yielded fruitful results in exploring the formation mechanism of plastic-induced crack closure effect with respect to polycarbonate,industrial pure titanium and aluminum alloy.The load that the axle bears during service is complex,and it is affected by overload caused by factors such as wheel-rail excitation.Therefore,in this thesis,using the high-speed train EA4T axle steel as the test material,the fatigue crack growth test of the compact tension specimen under the overload condition was carried out with the help of digital image related technology.Furthermore,the method of determining the parameters of CJP model and the characterization of the crack closure effect on EA4T axle steel were explored,which provied reference for crack propagation life prediction.The specific research content is:Firstly,the crack propagation and digital image correlation test of EA4T axle steel compact tension specimen was carried out.During the test,the sample was overloaded twice by 1.2 times and 1.5 times,providing support of test data for subsequent research.Secondly,the approximate expression of stress field at the tip of model I crack Westergaard model was derived.At the same time,the shortcomings of Westergaard model in characterizing the stress field at the crack tip and the tip plastic zone were analyzed.Then combined with the analysis results,this thesis deduced the analytical expressions of stress field and stress intensity factor of CJP model in detail.Then combined with the analysis results,this thesis deduced the analytical expression of stress field and stress intensity factor of CJP model in detail.Additionally,a nonlinear iterative method to solve CJP model parameters was proposed.The coordinates of the crack tip were preliminarily determined,using the sudden change feature of vertical displacement field continuity at the crack tip.Besides,the initial parameters of CJP model were solved by linear fitting method.L-M nonlinear iterative method combined with PCG algorithm was adopted to optimize the crack tip position and CJP model parameters.The general selection method of the outer diameter of the fitting area was given,according to the relative error result of Westergaard model and the exact solution of the crack tip stress field.In order to determine the inner diameter of the fitting zone,a two-dimensional Savitzky-Golay digital differentiator was used to solve the strain field,which combined with the generalized Hooke’s law to solve the plastic zone.Furthermore,the difference in shape and size between the experimental plastic zone and the theoretical plastic zone of CJP model was compared under different working conditions.As indicated by the results:the absolute value of the relative error between the theoretical plastic zone area and the experimental plastic zone area is less than 8%,which verifies the accuracy of CJP model in characterizing the crack tip stress field and plastic zone and the effectiveness of the method proposed in this thesis to solve the parameters of CJP model.Finally,the crack opening force of the EA4T steel compact tension specimen under different conditions was obtained,using the parabola-straight line fitting method combined with the experimental displacement field data.The crack opening force of the sample under normal load is maintained at about 470 N,but within 1100 cycles after overload,the opening force is less than 470 N,and the minimum is only 280 N.The CJP model stress intensity factor K_F,K_R change law with external load was explored.Under normal load conditions,the turning point of K_F changing with external load corresponds to a load of 450 N to 500 N,which is in good agreement with the traditional crack closure effect characterization parameter（crack opening force）.However,the load corresponding to the turning point of the K_F change in the1100 cycles after overload is 600 N to 700 N,which is much different from the opening force value of the corresponding working condition.The variation law of K_R with external load has a poor correlation with the opening force.The thesis further studied the variation law of?K_CJP/?K and?K_F/?K with crack length,and combined with the variation of crack growth rate with crack length,indicating that the variation trend of?K_CJP/?K and?K_F/?K both reflect the effect of change of the crack closure effect caused by overload on crack growth rate.What’s more,the relationship between?K_CJP,?K_F and crack growth rate was explored,and the results indicate that?K_CJP,?K_F and the da/d N calculated using the incremental polynomial method showed good linear relationship in the double logarithmic coordinate system,which can provide a reference for the crack propagation life prediction under overload conditions.