Research On Prediction Model For Disc LCF Life And Experiment Assessment Methodology

A disc failure in an aero engine can lead to a catastrophic result, so the disc is classified as the critical parts of both fracture and durability. Low-cycle-fatigue (LCF) is the most principal failure mode of engine discs. With the increasing of performance and thrust-to-weight ratio of aero engines, higher stress and temperature will be borne by discs, and higher reliability is required. All of these give a new challenge to the precision of the prediction model for disc LCF life and experiment assessment methodology. The advanced countries in aero engine, such as America, England, France and Russia, have set up the sophisticated system of disc LCF life prediction and experiment assessment. There are marked differences in this field between domestic and overseas. So researches on disc LCF life prediction models and experiment assessment methodologies are very important for the theory and practical engineering application.In this thesis, disc LCF life prediction models and experiment assessment methodologies were studied thoroughly and systematically in theories, experiments and engineering applications. The thesis is structured in three parts. The first part is the research and improvement of disc LCF life prediction models and methods, from chapter 2, chapter 4 to chapter 6. Firstly, the characteristics of disk LCF, some safe life prediction models and factors affecting life were reviewed. Secondly, an improved Walker strain life prediction model was put forward to predict the disc LCF life under unsymmetrical loads and the main life regions.Thirdly, a modified expression to account for the entire number of critical locations in each disc in life prediction was deduced. Finally, the LCF life of a turbine disc was predicted using the improved model and the modified expression. The result shows that the model and the expression developed in this thesis are simple, practical, and the accuracy of life prediction is higher compared to the other common models and methods.The second part is LCF experimental investigation of disc and its material, from chapter 3 to chapter 4. Firstly, an experimental rotor of turbine disc was designed and manufactured, and the high temperature LCF test for the full-scale turbine disc was successfully carried out at the vertical disc spin test facility. The crack initiation and propagation lives of the turbine disc were determined using fracture anti-derivation technique. Secondly, axial strain controlled LCF tests of disc alloy GH4133 were performed, at the strain ratios R=-1 and R=0.44, 250℃. All of these tests also have provided the verification platform of disc LCF life prediction models and methods.The third part deals with the reliability assessment methodologies of small sample disc life, namely chapter 7. Firstly, according to the sequence statistic theory, the mathematical formulae of small sample life scatter factors were systematically derived based on logarithm normal distribution and two-parameter Weibull distribution. The scatter factors calculated with the formulae fit well with both"general specification for aircraft gas turbine engines"(DEF STAN 00-971) and"joint services guide specification"(JSGS-87231). These indicate the formulae are reasonable. Secondly, fatigue life distribution for two home-made disc materials were investigated based on the goodness-of–fit test and failure rates function. The results show that the three-parameter Weibull distribution in which the shape parameter is greater than 1 is the best mathematical model of fatigue life for disc materials GH4133 and 1Cr11Ni2W2MoV in a wide range. Lognormal distribution is suitable for the lower life range of home-made disc materials. Hypothesis test must be made when two-parameter Weibull distribution is used to describe fatigue life distribution for home-made disc materials.