Study On Fatigue Crack Growth Rate Of High Strength Steel Under Different Stress Ratio

Fracture and failure accidents of engineering components that Caused by metal fatigue often occur,resulting in serious economic losses and casualties.Therefore,the issue of fatigue fracture has been concerned and studied by the engineering and majority of scholars.Fatigue crack growth rate da/dN is the main content of fatigue fracture,which is an important mechanical index of material resistance to crack propagation and an important basis for fatigue damage tolerance design and safety assessment of engineering components.Domestic and foreign scholars have done a lot of study on the fatigue crack growth rate,and put forward a lot of da/dN expression.There are many factors that affect da/dN,where stress ratio R is one of the main factors affecting da/dN.the change of stress ratio R will also cause the change of stress intensity factor?K and mean stress?_m At present,the study about the effect of stress ratio R on da/dN is mostly based on the Paris formula,and the effect of stress ratio R on da/dN under the same?K is studied.This is only considering the effect of average stress on da/dN.And does not take into account the?K is also affected by the stress ratio R.In this thesis,the maximum stress intensity factor K_maxax that independent of the stress ratio R is taken as the control variable to study the effect of the stress ratio R on da/dN under the same K_maxax value.Firstly a new formula describing da/dN is obtained by data fitting.Then?da/dN?_P formula with probability and probability for each stress ratio is obtained by probabilistic statistical study of the parameters in the formula.Finally the relationship between the parameters and R in the formula is obtained by regression analysis.The mathematical model of R-P-da/dN-K_maxax probability is established,and its geometric graphics study and engineering application are given.The specific content and results are as follows:?1?The fatigue crack propagation test data of 20 ADB610 steel CT specimens were processed by the seven-point increasing polynomial method,and 20(K_max,da/dN)data points were obtained.The 20 samples were divided into four groups,corresponding to four different stress ratios,each with five samples.And the fatigue crack growth rate formula da/dN=CKⁿ _maxwith K_maxax as the control variable are obtained by data fitting.?2?The two important parameters lgC and n in the da/dN=CKⁿ _maxformula of the five samples under the same stress ratio R were probabilistically studied.It is found that the optimal distribution model of the parameters lgC and n is the three-parameter Weibull distribution model.The expressions?da/dN?_P=C_P(K_max)^npand?da/dN?_P-K_maxax curves of each stress ratio that with probability and statistic significance are obtained through the three parameters Weibull distribution model.Which reduces the error caused by the randomness of crack propagation.?3?The effect of stress ratio on da/dN is studied.It is found that da/dN decreases with the increase of stress ratio R at the same maximum stress intensity factor K_max.This is in contrast to many studies on the effect of stress ratio R on da/dN.Which provides a new idea for studying the effect of stress on da/dN.The effect of the stress ratio on da/dN is mainly manifested in the expression dadN=CKⁿ_maxin the parameters lgC and n have a significant correlation with the stress ratio R.The parameter lgC decreases as the stress ratio R increases,and the parameter n increases as the stress ratio R increases.?4?The regression analysis shows that the parameters lgC and n have a quadratic function relation with the stress ratio R,and the mathematical model of R-P-da/dN-K_maxax probability is established by this conclusion.The mathematical model of the R-P-da/dN-K_maxax probability can be expressed as a three-dimensional surface family consisting of da/dN-K_max-R surfaces with different degrees of reliability P.The relationship among stress ratio R,K_maxax and da/dN can be studied more intuitively,conveniently and quantitatively by the three-dimensional surface family.The P-da/dN-K_maxax curve at arbitrary stress ratio R and the P-da/dN-R curve at a given K_maxax can be obtained by cutting the curved family.The R-P-da/dN-K_maxax probability mathematical model takes full advantage of the limited experimental data and quantify,functionalize,graph and visualize the relationship between stress ratio R and da/dN.It has a good engineering application value.