Study On Low-cycle Fatigue Crack Growth Performance Of Steel After Cold Spaying

Under fatigue load,the steel structure often has fatigue cracks on the surface or inside of the steel with the accumulation of damage,resulting in the stress redistribution of components and the integral reduction of bearing and deformation capacity.For the steel structure that has produced fatigue cracks,a reasonable crack repair method can restore the bearing capacity of the cracked structure to its original level,which not only saves resources,but also simplifies the construction technology.Cold spray,as an emerging technology developed in recent years,has been mainly used for additive manufacturing,crack repair,anticorrosion and wear resistance in the fields of aerospace,machinery manufacturing and automotive industries.In this paper,with cold spray as the technical means and low-cycle fatigue crack growth performance of steel as the research object,a series of experimental studies about low-cycle fatigue crack growth performance of steel and cold sprayed steel were conducted.Specific research contents and the conclusions obtained are as follows:(1)Cold spray tests were carried out on the surfaces of Q235 B and Q355 B steel.A5052 Mg-Al alloy powder and Al powder,in addition to their cold spray parameters,were optimized from 6 kinds of powder,316 L et al.SEM analysis shows that the 2types of sprayed coatings are both uniform with closely integrated particles and low porosity of 2% approximately.(2)Adhesive strength of cold-sprayed A5052 Mg-Al alloy and Al coatings on the surfaces of Q235 B and Q355 B was tested respectively.In addition,the results show that the bonding strength of Al coatings are 20% higher than that of A5052 Mg-Al alloy coatings,and grid blasting is not conductive to the deposition of metal particles on the surface of substrate as bonding strength of coatings on the sandblasted substrate is lower than that on the unblasted one.(3)8 groups of low cycle fatigue crack growth tests were conducted and their a-N curves were obtained.The results illustrated that the higher strength steel has,the more loading cycles crack initiation and growth require.Further,gird blasting on the surface of steel can effectively delay the initiation and growth of fatigue crack,while cold-sprayed A5052 Mg-Al alloy and Al powder on steel surface are effective on suppressing the growth of fatigue crack.However,the deposition performance of cold-sprayed particles will get worse after grid blasting on the substrate.(4)The lg(da/d N)-lg(Δ K)scattering diagrams of 8-group specimens were obtained,in which the data points tend to straight line and conform to the Pairs formula.Therefore,through the linear fitting method,Pairs formulas for each group were acquired.(5)TheΔCMOD-a and lg(da/d N)-lg(ΔCMOD)scattering diagrams of 8-group specimens were obtained,and the data points of former ones tend to exponential curve while those of latter ones to straight line.Through the ononlinear and liner fitting method,the relation formulas of ΔCMOD-a and da/d N for each group were acquired.(6)Finite element models based on XFEM were proposed to analyze the low cycle fatigue crack of steel,grid blasting and cold sprtray were both applied into the FEM.In addition,the models were verified by FEA results compared with test results.