Investigation On Fracture Toughness And Crack Growth Mechanism Of High-strength Steels

With excellent comprehensive mechanical properties,high-strength steels have become the most widely used structural metallic materials and play an important role in current and future industrial development.High-strength steels are subjected to cyclic loading during service,resulting in fatigue and fracture problems becoming the key research issues and directions.With the development of fracture mechanics,the damage tolerance design has been the control method for the fatigue and fracture of key components in national defense,such as aviation and aerospace.It is generally recognized that the fatigue crack growth(FCG)and fracture toughness of materials are the basis of the damage tolerance design.Although much knowledge has been obtained,it is still unclear about the FCG and fracture toughness in high-strength steels.Consequently,an AISI 4340 high-strength steel was adopted as the target material in our study.The estimated methods for fracture toughness and crack growth mechanism would be elaborated.An improved Paris' law was proposed through introducing strength and toughness.Accordingly,the quantitative relation between the strength-toughness and FCG rate of materials would be established.Meanwhile,the micro-mechanism and macro-mechanical model for FCG of materials were explored.Our understanding of FCG and fracture toughness on high-strength steels would be applied to the engineering field,and provide a reference for the optimization in material selection and design for component reliability.Three methods were proposed to evaluate the plane strain fracture toughness KIC of metallic materials.1)Based on the energy distribution from the initial stage to critical instability state during crack growth,the quantitative relationship between the specimen thickness B and fracture toughness KIC was established.The fracture toughness KIC could be estimated by small size specimen.This work would be applied to the fracture toughness evaluation of high-toughness metallic materials.2)The quantitative relationship between the fracture energy density WF and shear lip width s was derived from the energy consumption during the fracture process.The lowest WF would be achieved when the shear lip width increases to the maximum value,revealing the competitive balance between flat and shear fracture.Additionally,based on the minimum energy density criterion,the quantitative relationship between the shear lip width s and fracture toughness KIC was established.3)According to the manner of energy consumption and minimum energy density criterion of impact toughness and fracture toughness,the linear relation between shear lip widths in two types of toughness was proposed.Furthermore,the quantitative relationship between impact toughness and fracture toughness in high-strength steels was obtained.The transition of crack growth mechanisms in steels with different strength and toughness was discussed.It is found that the fracture surface morphologies of AISI 4340 steel range from quasi-cleavage to predominated dimple fracture with increasing the fracture toughness.The 3D-XRT images reveal the manner of crack growth evolves gradually from discontinuity to continuity.The main crack would join the micro-voids near the crack tip by cleavage fracture under the stress-dominated,promoting rapid crack propagation.However,the growth and coalescence of microvoids around the crack tip would be linked with the main crack by the strain-controlled,resulting in the ductile fracture.When the above two mechanisms exist simultaneously in material,which might be explained by the competition balance between strain and stress.The microscopic fracture surface morphology of specimen can be divided into three sorts,which are associated with the energy consumption.Based on the energy principle,we proposed a quantitative relationship between the microscope fracture surface morphology and fracture toughness of materials.The predictive FCG rate formula and optimization criterion for fatigue property were established.Based on the Paris' law,the improved FCG rate formula was derived through introducing the strength and toughness.The FCG behavior of materials is affected by two factors:the tensile strength determines the critical crack growth rate and fracture toughness controls the crack growth process.The FCG rate of materials could be evaluated by the static mechanical properties,which has been verified in high-strength steels.Accordingly,a quantitative criterion for FCG rate and strength-toughness was established.The optimal FCG property could be selected among materials with contradictory strength-toughness relationship.Furthermore,the essence about simultaneously enhanced strength and toughness can improve the fatigue property would be elaborated.The improved FCG rate formula and quantitative criterion have been accurately verified in steel,titanium alloy and aluminum alloy,which would provide a theoretical basis for the fatigue optimization of engineering materials.The micro-mechanism and macro-mechanical model for FCG were be explored.It is found in fracture surface morphologies that the mixed fatigue striation and dimple occur during stage-? FCG,and the number of dimples increases with enhancing the stress intensity factor.The 3D-XRT images reveal multiple microcracks exist near the crack tip,and the side face shows discontinuous microvoids occur around the crack tip.Based on the crack tip stress field,the FCG mechanism was proposed,i.e.,the FCG process is the transition of the blunting and re-sharpening(BRS)to microvoid coalescence(MVC)mechanism.According to the BRS-MVC mechanism,a macro-mechanical model was established to describe the three stages of FCG,which has been accurately verified in engineering alloys.The?Kv is the equilibrium position of the BRS-MVC mechanism and becomes the safe parameter of FCG process.Three methods were proposed to acquire the ?Kv value.