| Although the traditional concept is that the main reason leads to fatigue failure of the crankshaft is bending moment, mounts of failure cases verify the fact that most of the failure fracture present multi-axial state. High explosive pressure and lightweight development trends of the diesel engine make the traditional infinite life design concept can’t be suitable for the new requirement. Based on the finite life design concept, the following studies were carried out: 1. An energy-based multi-axial fatigue life criterionUni-axial fatigue life criteria are relatively mature, but most of mechanical components are under multi-axial loading conditions and multi-axial fatigue criteria still have some drawbacks. Through a series of comparisons, a truth was found that when combined with the critical plane method, strain and stress fatigue criteria were doubted for lack of rigid continuous dynamics foundation, herein a new energy-based fatigue criterion was proposed in the thesis.The research process of the new criterion was divided into three parts:normal strain energy, shear strain energy and total strain energy part. Firstly, the SWT criterion overemphasizes on the maximum stress and ignores the decisive effect on fatigue life of stress/strain amplitude. Besides, the influences of mean stress on the elastic and plastic scopes are different from each other but the SWT model ignores the fact. In view of the fact mentioned above, normal strain energy part was proposed combining the SWT model with the Morrow criterion. Secondly, shear energy part was proposed based on the Glinka and Chen criterion. Considering the fact that the shear mean stress and normal stress were not taken into account in the Chen criterion and none concrete formula was given by the Glinka criterion, so not only the shear mean stress and the normal stress were all taken into consideration in the shear energy part of the new model but also concrete formula was proposed. Finally, the two energy parts were combined together. There were drawbacks of multi-axial additional factors of the FS and VF criterion, so a modified correction factor was proposed in the new criterion. Concrete expression of Nf was proposed and the different correction of mean stress was also taken into account in the formula.To verify the practicability of the new criterion, experimental data under different load conditions of materials SNCM630, LY12CZ, Pure Titanium, Titanium Alloy BT9,7075-T651 Aluminum Alloy and Titanium Alloy TC4 were employed. A series of comparisons were conducted based on the experimental data and predicted data with the new criterion. The results showed that although the load conditions are complicated, predicted and experimental data present well consistency with each other. In addition to the results of the material 7075-T651 Aluminum Alloy and two points of the material Titanium Alloy TC4, all the predicted data are located in the 2 scatter bands of the experimental data. In general, the new criterion presents better effect than the commonly used criteria such as Manson-Coffin, SWT, KBM, Von.Mises, Chen, FS, VF, and so on.2. Study of the crankshaft fatigue testThrough series of tests and comparisons based on the bending fatigue test, the following results can be obtained:if there are enough samples and time, the staircase method is the preferred method; if samples or the time is not so sufficient, the fatigue limit statistical method is recommended and the result maybe slightly conservative; if the fatigue limit statistical method is adopted, the 103 method is better than the QCI method; the three parameter weibul distribution is the prior choice to deal with the fatigue test data.Based on the study results above, one dimensional dynamics calculation of the crankshaft was conducted and the maximum dynamic torque was identified as the basic load of the torsional fatigue test of the crankshaft. After the torsional fatigue test data were obtained, comparisons between bending and torsional data were made. The results show that with 50% survival probability, the safety factor of the bending test is 2.0 and the torsional test is only 1.75. So the traditional concept that only bending test is necessary for the crankshaft is questionable.Considering the fact that torsional vibration leads to dynamic torque, to evaluate the influence of the dynamic torque on the fatigue, one dimensional dynamics calculations of the crankshaft with higher explosive pressure were conducted. The comparison results show that with 50% survival probability, the safety factors of the bending test are 1.9 and 1.64 with 19MPa and22MPa explosive pressure, separately, but the torsional results were only 1.6 and 1.37, correspondingly. Through the comparisons above, the opinion that shear stress is more harmful than normal stress is reasonable. The results also verify it is necessary to conduct torsional fatigue test and to make studies of multi-axial criterion.3. Study on the influence on crankshaft strength of quenching processSurface induction hardening is the most commonly method to elevate the fatigue strength for the steel crankshaft. How to embody the invigoration effect is important in the prediction of fatigue life of the crankshaft.Residual stress testing was carried out to get residual stress and fatigue tests with quenched and non-quenched crankshaft were done to obtain the hardening coefficient. Because stress state of the crankshaft presents 3 dimensional, the studies of calculating process of critical plane based on the 3 dimensional strain and stress tensor were conducted and a python program was written. Then the finite element simulation was done based on the testing loads. Fatigue life prediction was conducted with two ways, one used residual stress as mean stress to predict fatigue life and the other introduced the hardening coefficient directly. The comparison results between experimental data and the predicted data with the two proposed method show that using residual stress as the mean stress is a better way to embody the invigoration effect of the quenching process.4. Fatigue life prediction based on dynamic result of crankshaftTo calculate the fatigue life of the crankshaft under working condition, study of processing method of dynamic random load was done and the three-point rain-flow method was defined as the final method to deal with random loads.The stress and strain tensors present not only 3 dimensional state but also variable in service of the crankshaft, so calculating process of spatial dynamic stress was studied. Based on the python program mentioned above, a new program can be used to calculate the critical plane of the crankshaft under working condition was proposed.Conduct the dynamic calculation of the crankshaft and extract the strain tensor of critical nodes, and combine the new fatigue life criterion, critical plane method and rain-flow theory with together to predict the fatigue life of the crankshaft. During this process, residual stress is also taken into consideration through correcting the mean stress. The researchers not only obtain the fatigue life of 50% survival probability, the basic dispersion concept is also taken into consideration. The final results are that the result of 99.99% survival probability satisfied the 107 cycle requirement with all speeds but when the engine speed is equal to 2100r/min, the fatigue life of 99.997% survival probability is only 8.9×106 cycles. |
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