Research On The Impact Of Stress Ratio On Fatigue Of Rapid Prototyping Titanium Alloy

Rapid prototyping tianium alloy (TiAlRP) is original process in recent years,opens up a new field in forming process of titanium and recives a high degree ofconcern from aviation, because of many advantages, such as short cycle, low cost,high material utilization. Owing to high annealing strength, excellent hardenability,easy to manufacture complex components, TiAlRP material is very suitable for themanufacture of a variety of high load bearing aerostructures. Howerver, research onfatigue properties of material under the TiAlRP process is still in relatively scarce.TiAlRP components need to bear the alternating stress under different stress ratio inworking condition, which frequently leads failure, thus the study of stress ratio on thefatigue properties of TiAlRP sample is very important.Based on the integral relationship of the crack growth rate and fatigue life,considering both of mathematical models of fatigue life, a systematic investigation ofthe relationship between stress ratio (R) and fatigue life curve (S-N) was performed tobuild the fatigue life mathematical mode (R-S-N). Using randomized method ofquantitative equations, P-g-R-S-N mathematical model was build under differentreliability(P) and confidence(g).On the basis of theoretical studies, fatigue tests of TiAlRP samples (132specimens) were carried out under three kinds of stress ratio, to draw three fatiguelimits and6S-N curves of "stress amplitude life model " and " three-parametermodel ". According to the modified formula proposed in this paper, the establishmentof two R-S-N mathematical models, applicable for TiAlRP materials.And, twoP-g-R-S-N mathematical models were fitted under different reliability and confidence.Life assessment diagrams under typical stress ratio of two traditional fatigue lifemodels were given to facilitate the application in the project.The results showed that:(1) Fatigue limit: Stress ratio and the fatigue limit were in direct ratio. If thestress ratio increased from0.06to0.5, the fatigue limit increased20%. The Goodmantheory can accurately predicts the fatigue limit under different stress ratio, and theprediction value is lower than the test value, the error is less than6%.(2) Fatigue curve: Based on a large number of experimental data,6S-N curves of"stress amplitude"and"three-parameter fatigue"models, showed that "stressamplitude" model can accurately predict moderate fatigue life, and "three-parameterfatigue" model is more suitable for the prediction of long-life area.(3) Two proposed R-S-N mathematical models can accurately predict the fatiguecurve under any stress ratio. Based on experimental data under three stress ratio, theR-S-N model can be obtained, compared with the actual fatigue curve under the same stress ratio fatigue life, the maximum stress error were desprately7%and1%, whichproved the theories correction model has high engineering practical value.(4) Based on the randomized method of quantitative equations, confidence andreliability were considered to build P-g-R-S-N mathematical model of TiAlRPmaterial, which18mathematical equations under typical stress ratio can be obtainedfrom to give life assessment diagram. The life assessment diagram can help engineerto predict life of material and carry out fatigue design.