Study On Fatigue Crack Growth And Crack Monitoring Based On Acoustic Emission Of Cast Steel And Butt Welds

Cast steel joints have been widely used in bridge structures,offshore platform structures and building structures with good mechanical properties,machining properties and weldabilities.Bearing the reciprocating loads such as trains,vehicles,waves and winds,cast steel joints face the initiation of fatigue cracks at the chamfers,butt welds which are employed to connect other components,casting defects or welding defects.The fatigue crack has the characteristic of concealment.The growth of fatigue cracks in components with great geometry size always lasts a quite long time.Moreover,the component or structure has no obvious change but local centration of stress and strain around the fatigue crack.The failure caused by fatigue crack has the characteristic of suddenness.The fatigue crack will lead to brittle fractures of components when it reaches the critical length.The sudden failure of cast joint will lead to great transformation of stress distribution in structures,and even lead to structural collapses.Therefore,detection of fatigue cracks,evaluation of fatigue crack damages and calculation of residual life are important to the safe application of cast joints.Based on the above background,this paper focuses on the fatigue crack monitoring and residual life calculation of G20Mn5QT cast steel and related butt welds.In this paper,the fatigue crack propagation behaviors are studied,the two-parameter driving force model is derived as the fatigue crack growth model,the fatigue crack growth mechanism is revealed and the fatigue crack monitoring method is established based on acoustic emission(AE)technology.(1)Study on fatigue crack growth behaviors of G20Mn5QT cast steel and related butt welds.The mechanical properties,chemical compositions and microstructure of G20Mn5QT cast steel and related butt welds have been investigated.The fatigue crack growth experiments under 3 stress ratios,various maximum loads and 2 specimen thicknesses have been conducted.And then the fatigue crack growth curves are obtained.The affecting parameters in fatigue crack growth rates(FCGR)are discussed.The stress ratio has a great effect on FCGR.The FCGR increases as the stress ratio increases.The specimen thickness has a small effect on FCGR.Under the same stress ratio,larger maximum load will lead to a slower FCGR.Comparing to the FCGR of G20Mn5QT,the FCGR of butt welds is slower at low stress intensity factor but faster at high stress intensity factor.Aiming to provide the basis of fatigue crack calculation and engineering design,the parameters of the Paris fatigue crack growth model with 50% confidence and 95% confiednce are calculated by reliability analysis.(2)Deduction of the two-parameter driving force model of fatigue crack growth.The model is based on the blunt crack tip assumption and material discretization assumption.The fatigue crack growth can be regarded as a series of failures of finite elements along the crack growth path.The modified SWT damage parameter is employed as the fatigue crack driving force to build the relationship with FCGR.The two-parameter driving force model can uniformly consider the effects of stress ratio,maximum load and load history on FCGR.The FCGR fomular is only determined by the sort of material.Based on the two-parameter driving force model,the finite element simulation of fatigue crack growth is achieved.The simulation has a good accuracy comparing to the experimental results.The FCGR formulars of cast steel and related butt welds are derived with 50%confidence and 95% confidence,aiming to improve the method of fatigue crack calculation and engineering design.(3)Investigation on fatigue crack growth mechanism of G20Mn5QT cast steel and related butt welds.The AE signals during fatigue crack growth of G20Mn5QT cast steel and related butt welds are collected.By k-means analysis,the AE signals during fatigue crack growth can be divided into three clusters.After analyzing the features in time domain and frequency domain,the three clusters are related to cyclic plastic deformations before crack tip,tensile fractures and shear fractures.The fatigue crack growth is a combination of cyclic plastic deformations before the crack tip,tensile fractures and shear fractures.The crack tip advancement can be regarded as a discontinuous process of the critical area brittlely ruptures as large number of tensile crack fractures and rare shear crack fractures when the fatigue damages due to cyclic plastic deformations reach critical status.The fatigue damage accumulation process releases around half of the total energies.The energy released by a single tensile fracture is quite small,but a large number of tensile fractures make the total accumulated energy slightly less than the energy released by fatigue damages.The energy released by a single shear fracture is larger than that released by a single tensile fracture,but the total accumulated energy is the least.The shear fractures are caused by casting or welding defects.(4)Establishment of fatigue crack monitoring method of G20Mn5QT cast steel and related butt welds based on AE technology.The method includes five steps,realizing the recognition of fatigue crack,location of fatigue crack,identification of fatigue crack damage and prediction of residual life.(1)The AE signals are simplified as characteristics.Energy proportions in different frequency bands are extracted by Fourier transform and wavelet packet analysis to evaluate the signal characteristics in frequency-energy domain.(2)The cracking material recognition to determine the location of fatigue crack whether on cast steel or butt welds is available with the help of artifical neural network.(3)The time difference arrival technique of AE signals is employed to local the position of fatigue crack.(4)Regarded as the fatigue crack damage,the two-parameter driving force is identified by AE signals with the application of artifical neural network.(5)The fatigue crack growth formulars of G20Mn5QT cast steel and related butt welds derived by the two-parameter driving force model is introduced to predict the residual life of fatigue crack.The fatigue crack monitoring method has a good accuracy on cracking material recognition and an acceptable accuracy on fatigue crack residual life prediction.However,the accuracy on fatigue crack location needs to be improved.This study has filled the research blank of fatigue crack growth of G20Mn5QT cast steel and related butt welds.Mover,the calculation method of fatigue crack growth as well as the monitoring method have been established to support the engineering application of G20Mn5QT cast steel.