Theory Of Critical Distance-based Fatigue Strength Analysis And Life Evaluation Of Notch Parts

Different kinds of notches and geometrical discontinuities(such as grooves,notches,holes,etc.)are commonly introduced to satisfy various functional requirements,and it is often accompanied by complex stress and stress gradient state.With the development of major equipment towards complexity,large-scale and systematization,due to the cost and test conditions of full-size component fatigue tests,the fatigue life distribution of engineering components is usually derived from the fatigue testing data of small-scale and equal proportion samples,but the phenomenon that the fatigue limit and life are usually reduced with the structure and gap sizes is ignored in this process.Moreover,with the influence of load fluctuation and material properties and other factors,fatigue strength and life usually show great dispersion.Thus,it is necessary to analyze the influence of size effect,notch effect and uncertain factors on fatigue life assessment of notched components.Accordingly,this thesis summarizes the influence of size effect,notch effect and uncertainty on critical distance,fatigue life,and preforms the following work:(1)A modified critical distance model considering the influence of size effect on fatigue life and critical distance is proposed by combining the theories of critical distance and stress gradient,in which relative stress gradient at notch tip is applied to quantify the effect of size effect on fatigue life.Moreover,notch specimens of GH4169,Al 2024-T351 and En3B with different notches are utilized for model validation and comparison,and results indicate that the fatigue predicted life based on critical distance model modified by relative stress gradient is better.(2)Based on the theories of stress gradient and critical distance,an effective stress model considering the influence of notch effect on fatigue life is proposed.Moreover,the Weibull distribution is introduced to quantify the dispersion of fatigue life,and a fatigue life prediction framework considering the influence of size effect,notch effect and uncertainty on fatigue life is proposed.The fatigue life prediction method is verified and compared based on the fatigue test data of notch specimens of different materials.(3)Finite element model of a compressor disk under typical working conditions is established,and the stress distribution in the dangerous area is extracted.Then,the probabilistic fatigue strength analysis and life evaluation of compressor disk are carried out based on the modified critical distance model and effective stress model.The results show that the prediction results are more consistent with the actual values than the traditional critical distance method and the high stress volume method.