RESEARCH
I. Rapid Fatigue Evaluation
Fatigue is one of the most predominant modes that cause failure in a diverse array of engineering materials and structures. Preventing fatigue failure through reliable lifetime prediction methodologies has a major societal impact in terms of both economics and safety. However, the traditional method of evaluating fatigue properties needs a long-time experimental period to do plenty of fatigue tests. It usually takes more than several weeks and even several months, which severely obstructs the fast development of the modern industry. The desire for accelerating the fatigue evaluation process has always been very strong.
As we all know, fatigue failure is an irreversible process of microstructure evolution toward degradation. This process is companied by energy dissipation, manifesting as a self-heating phenomenon. The temperature variation during the fatigue process should contain information on the fatigue damage accumulation. Therefore, analyzing the self-heating effect and relating it with the fatigue damage is a promising direction to achieve rapid fatigue evaluation.
Past & current achievements
Along this direction, I have performed a series of research, from the aspects of theory, experiment, and simulation, and obtained some achievements:
Established a calculation model for estimating the intrinsic dissipation of metallic materials under high-cycle fatigue. The calculation model is based on a double exponential regression of the one-dimensional temperature distribution on fatigue specimens. The model was further improved by incorporating the effect of natural convection and radiation. Besides, I established a unifying theoretical framework to rebuild the three dissipation estimation approaches, i.e., the zero-dimensional, the one-dimensional, and the two-dimensional approach. Combining finite element simulation and fatigue experiments, I performed in-depth analysis and made meaningful discussions.
Proposed an energy method for the rapid evaluation of high-cycle fatigue properties. The energy method takes intrinsic dissipation as the fatigue damage indicator, which has a more definite physical meaning than directly using temperature rise. Since eliminating the interference of the internal friction that causes no damage and considering only the dissipation part related to microplastic deformation, the energy method can achieve a relatively highly accurate prediction. Adopting the proposed experimental approach with the energy method, it is possible to accomplish the evaluation of high-cycle fatigue properties, including the fatigue limit and the S-N curve, within 24 hours and consuming only two fatigue specimens.
Conducted systemic fatigue experiments on a type of high-strength stainless steel, FV520B. The advanced infrared thermographic technique was utilized to detect the fatigue self-heating phenomenon. I found that the intrinsic dissipation increases with the applied stress amplitude and there exists an inflection point on the curve of intrinsic dissipation versus stress amplitude. The inflection point reveals a transition of the generation mechanism of intrinsic dissipation, and the corresponding stress amplitude is precisely the fatigue limit. Fatigue failure occurs once the part of the intrinsic dissipation due to microplastic deformation accumulates to a threshold value, which is a material constant independent of loading history.
Developed a dislocation-point defect interaction theory to reveal the mechanisms of energy dissipation and fatigue damage and formulated a relationship between them. The vibration of dislocation lines and the dislocation multiplication under fatigue loading were theoretically analyzed. I further proposed a constitutive model for fatigue life prediction, where two subsets of internal state variables are introduced to capture the two types of microstructure motions, i.e., the recoverable motions related to the non-damage anelastic behavior, and the unrecoverable motions related to the fatigue damage behavior.
Future research plans
Based on my previous achievements, my future work will be carried out from the following aspects:
To further improve the proposed models and methods and apply them to more materials and other fatigue regimes. In this step, it is known that the performance is good for many metallic materials under high-cycle fatigue. However, some studies have shown the potential implementation of other materials, such as composites, polymers, and even additive manufactured elements. It is also promising to apply to the study of very-high cycle fatigue.
To develop a multi-scale constitutive model by integrating the dislocation dynamics theory and the crystal plasticity finite element method. Using the model to quantitatively analyze the relationship between intrinsic dissipation and fatigue damage would contribute to the self-heating-based fatigue study and guide the anti-fatigue design of materials.
To develop an experimental platform to achieve the simultaneous observation of microstructure evolution and self-heating phenomenon. The platform will integrate the infrared thermographic technique and the digital image correlation technique, as well as some microscopes. The observations using the platform would provide direct evidence for the damage and dissipation mechanisms.
To work on the standardization of the rapid fatigue evaluation approach and promote it to industrial production, manufacturing systems, and other fields. The standardization will include equipment, procedure, criteria, and database.
