| The fracture-splitting process of the connecting rod is a new processing technology that obtains the big-end joint surface of the connecting rod by a broken mode.By artificially prefabricating the incision in the position of the theoretical joint surface in the big hole,and then applying the tensile stress load perpendicular to the notch,stress concentration is generated at the root of the incision,and the crack is initiated and expanded to realize the rapid fracture separation of the connecting rod body and the connecting rod cover.Thereby a three-dimensional fracture joint is obtained.The precise positioning and perfect meshing of the three-dimensional uneven joint surface can greatly improve the assembly quality during the subsequent assembly of the connecting rod and carrying capacity of the connecting rod.Compared with traditional processing technology,the connecting rod fracture-splitting processing has the advantages of less processing steps,small equipment investment,low manufacturing cost,good product quality and high assembly precision.The bulge and fracture is the core process of the fracture-splitting process of the connecting rod.It is required that no large plastic deformation occurs during the fracture-splitting process,and that the fracture surface is in good shape in order to meet the requirements of subsequent processing process after fracture,the positioning accuracy of connecting rod assembly and the bearing capacity of connecting rod products.However,the connecting rod fracture-splitting is a process that gets"breaking"by"cracking".The crack expands immediately as soon as it initiates.The whole process is extremely rapid,and the influencing factors are complex,and it is easy to produce all kinds of bulge and fracture defects,including quality problems of fracture surface(including defects,slag inclusion,steps,etc.),fracture line deviation and and big head hole out of round defects,and so on.36MnVS4 is a new generation material used connecting rod fracture-splitting.Compared with the high-carbon microalloyed non-quenched and tempered steel C70S6which is mainly used at present,36MnVS4 has high tensile strength,yield strength and fatigue strength,and has broad application prospects.However,there are more quality problems in the fracture-splitting process of 36MnVS4 connecting rod.The qualification rate of bulge and fracture process of the 36MnVS4 connecting rod is lower than that of the C70S6 connecting rod.Analysis and control of fracture-splitting defects become the focus of research and problems that need to be solved in production.Fracture-splitting defects are difficult to avoid completely,the key is how to reduce and control.With the popularization and application of 36MnVS4,it is necessary to study the fracture-splitting characteristics and laws of the 36MnVS4 connecting rod,reveal the causes and mechanisms of the defects occurred and propose corresponding solutions.So far,there is no industry technical standard for the fracture-splitting quality.The description of the fracture-splitting defects,the setting of the defect parameters,and the test items,etc.have no rules to follow.Both the parts manufacturing company and the engine main engine factory adhere to technical claims that is beneficial to oneself.Some technical requirements are determined by empirically or reference peer data,and the technical standards of fracture-splitting need to be reasonably formulated.The fracture-splitting defects are introduced into the operation condition of the connecting rod.Based on the analysis of the influence of fracture-splitting defects on the service performance of connecting rod under engine operating conditions,the fracture-splitting machining defects are evaluated,which are important to scientifically formulate the quality specifications and standards for connecting rod fracture-splitting processing.This is also a problem that has not been involved in the research of connecting rod fracture-splitting technology.In this paper,the bulge and fracture process of the 36MnVS4 connecting rod was numerically simulated.The fracture-splitting characteristics and regularity of the36MnVS4 material were investigated.The mechanism of the fracture defects were revealled.And the corresponding prevention and control measures were proposed.Based on the reverse reconstruction,the structure analysis model of the fracture-splitting connecting rod was established.The working performance of fracture-splitting connecting rod under engine working condition was analyzed.The structural of the fracture surface and the other parts of the 36MnVS4 connecting rod were optimized to reduce the quality defects of the fracture splitting from the perspective of product design and realize the lightweight of the connecting rod.For unavoidable defects,the defect is introduced into the connecting rod working condition.Based on the performance analysis of the fracture-splitting connecting rod with the fracture-splitting defects under the engine condition,the defects were evaluated and the defect control standards were established.The main research contents and achievements are as follows:(1)Based on the fracture mode of the connecting rod and the fracture surface morphology,it was determined that the fracture nature of the connecting rod is the type I quasi-brittle fracture under the small range yield of the notch component.The stress and strain distribution at the crack tip,the cause of crack initiation at the notch root and the law of crack propagation were analyzed.The maximum tensile(main)stress fracture criterion was chosen as the unified criterion for the crack initiation and propagation of the connecting rod.(2)Composition and microstructure of 36MnVS4 and C70S6 were analyzed,and tensile,fatigue test and fracture performance test were carried out.Compared with C70S6,36MnVS4 had lower carbon content,more ferrite in the structure and higher grain size.The sectional shrinkage of 36MnVS4 and the elongation after fracture increased,so 36MnVS4 had higher plasticity and poor brittleness,which was not conducive to brittle fracture of the 36MnVS4 connecting rod.The fracture toughness of36MnVS4 is less than that of C70S6,so its notch sensitivity is slightly higher.(3)The fracture-splitting process of 36MnVS4 and C70S6 connecting rod were simulated by finite element simulation.The fracture-splitting characteristics and laws of36MnVS4 were investigated,and the stress-strain field,displacement field and load characteristics of the crack initiation and propagation of the two connecting rods were compared and analyzed.The 36MnVS4 connecting rod had a high notch sensitivity and was easy to crack,the location of crack initiation is scattered,random and uncertain compared with the C70S6 connecting rod,but the initiation time was earlier and the required cracking force was smaller.However,the crack propagation time of the36MnVS4 connecting rod was about 20%higher than that of the C70S6 connecting rod,which increased the possibility of defects production during the fracture-splitting process of the 36MnVS4 connecting rod.The plastic zone of fracture surface of the 36MnVS4connecting rod was larger than that of the C70S6 connecting rod.And there was a large plastic deformation at the boundary such as bolt hole,end face,and outer edge of joint surface,which made 36MnVS4 connecting rod have a poor quality fracture surface.Therefore,36MnVS4 connecting rod more likely to produce the quality defects of fracture surface such as tearing,slag inclusion and off-slag.(4)Combined with ABAQUS and FRANC3D software,the morphology of crack propagation and the change of crack structure in the process of connecting rod fracture were described.Furthermore,the mechanism of the quality defect of fracture surface,the defect of incomplete fracture,the defect of fracture line migration and the defect of big hole out-of-round were revealed.It was concluded that the quality defects of fracture surface such as slag dropping,slag inclusion,step,crack bifurcation,etc.,mainly occurred at the boundary of bolt hole and end face.The boundary of the fracture surface was in a state of plane stress,and it was shear deformation,which leaded to large plastic deformation.As a result,the torsion angle of the crack tip fluctuated significantly and the crack structure was unstable.Incomplete fracture defects were mainly caused by one side of the incision leakage processing or processing too shallow or obvious passivation of the root.Different incisions on both sides caused the stress concentration of the roots of the two sides to be different.Even a number of micro-cracks were generated,which resulted in the energy release rate during the crack propagation process being lower than the expansion resistance,and leaded to crack arrest.The first type of fracture line deviation defect mainly occurred in the connecting rod fracture-splitting,which was mainly caused by eccentric load.The big head hole out of round occurred after connecting rod fracyure-splitting processing.The head hole out of round was mainly generated before the crack starts.36MnVS4 started to crack early,and the plastic deformation before cracking was small,so the loss of round of the36MnVS4 connecting rod was lower than that of the C70S6 connecting rod.The outer edge of the connecting rod joint surface adopted arc transition,forming equidistant curve with the bolt hole,and the two end faces adopted wedge or arc transition,which could accelerate the speed of crack growth,facilitate the crack aggregation,and help to reduce the occurrence rate of fracture defects such as slag-dropping and slag inclusion.Therefore,the size and the shape of the fracture joint surface could be improved.(5)Combined with fracture-splitting processing experiment and reverse reconstruction technology,the three-dimensional reverse reconstruction of the fracture surface of the connecting rod was realized,and the structural analysis model of the fracture-splitting connecting rod was established to simulate the running condition of the connecting rod.The work performance including strength,stiffness and fatigue life of the 36MnVS4 fracture-splitting connecting rod were analyzed.The results showed that the fracture joint surface of the fracture-splitting connecting rod had three-dimensional concavity and convavity,which could make the connecting rod body and the connecting rod cover close contact and lock each other,reduce the relative movement between them,and improve bearing capacity of fracture-splitting connecting rod,especially the shear resistance.The fatigue life of the C70S6 and 36MnVS4 connecting rod were 1.4×10~7and 2.1×10~7 respectively,and the safety coefficient were 1.6858 and 2.9765,respectively.The 36MnVS4 connecting rod had a large weight reduction space.After lightening and improving design,the connecting rod met the requirements of strength,stiffness and service life,with the mass reduced by 21%.And after optimization,the size and shape of the joint surface were changed.The occurrence of fracture processing defects could be reduced.(6)The fracture-splitting connecting rod model with fracture surface defect and fault line offset defect were established respectively,and the quantitative description of the defects were realized.Based on the performance analysis of the fracture-splitting connecting rod with defects under engine condition,the defects were evaluated and the control criteria for fracture surface defects and fracture line offset defects were established.It was concluded that:(1)the strength of the connecting rod decreased with the increase of defect size;in the same size,the internal defect was more harmful than the marginal defect.Under the condition of ensuring the strength of the fracture-splitting connecting rod,the size of the defect of the connecting rod should be:defect on edge,when 1.5mm<b<2.5mm,a?-1.125b+5.0875;in-plane defect,when 1.4mm<b<2.1mm,a?-1.143b+4.8.(2)When the fracture line was offset,the force of the connecting rod and bolt changed compared with that of traditional connecting rod.With the angle increase of fracture line offset,the equivalent stress of the connecting rod increased gradually.The equivalent stress and shear stress of the fracture surface and bolt were higher.The tendency of the bolt to break was increased,resulting in a decrease in the load carrying capacity of the connecting rod during operatio.From a safety point of view,the allowable value of the deflection line offset angle was 6°. |
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