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The truth about thermomechanical fatigue: Why is it so important for engines?
Today's engine design experts often have to consider a critical factor: thermomechanical fatigue (TMF). TMF refers to the fatigue phenomenon of a material under the interaction of cyclic mechanical load and cyclic thermal load. When building turbine engines or gas turbines, the importance of TMF cannot be ignored.
Thermomechanical fatigue not only affects the life of the material, but also directly affects the efficiency and reliability of the engine.
Failure Mechanism
There are three main failure mechanisms for thermomechanical fatigue:
Creep: The flow phenomenon of materials at high temperatures.Fatigue: Crack growth and propagation due to repeated loading.Oxidation: Changes in the chemical composition of materials due to environmental factors make the oxidized material more fragile and prone to cracking.
The impact of these three mechanisms will vary depending on the load parameters.
Load Phase
In intra-phase (IP) thermomechanical loading, the effects of creep are most significant when both temperature and load increase simultaneously. The combination of high stress and high temperature is ideal for creep. This hot material flows more easily when stretched, but cools and becomes harder when compressed.
In out-of-phase (OP) thermomechanical loading, the effects of oxidation and fatigue dominate. Oxidation weakens the material surface, forming defects and acting as seeds for crack propagation. As the crack grows, the newly exposed crack surface oxidizes, further weakening the material and causing the crack to extend.
In some cases, when the stress difference is much greater than the temperature difference, fatigue may become the sole cause of failure, causing the material to fail before oxidation can take effect.
Model Analysis
Currently, research on thermomechanical fatigue is incomplete, and scientists have proposed a variety of models to predict the behavior and life of materials under TMF loads.
Two main types of models will be discussed here: constitutive models and phenomenological models.
Constitutive model
Constitutive models leverage existing understanding of material microstructure and failure mechanisms. These models are complex and are designed to incorporate all our knowledge about material failure. With the advancement of imaging technology, this type of model has become increasingly popular in recent studies.
Phenomenological ModelPhenomenological models are based on the observed behavior of the material and view the failure mechanism as a "black box". After inputting temperature and loading conditions, the output is fatigue life. This type of model attempts to fit the relationship between different inputs and outputs using certain equations.
Damage Accumulation Model
The damage accumulation model is a type of constitutive model that sums the damage from three failure mechanisms: fatigue, creep, and oxidation.
This model is considered one of the most thorough and accurate TMF models because it considers the effects of various failure mechanisms.
Fatigue life
Fatigue life is calculated under isothermal loading conditions and is mainly affected by the strain applied to the specimen. The model does not take into account temperature effects, which are dealt with by oxidation and creep terms.
Oxidation Effects
Oxidation life is affected by temperature and cycle time. Experimental results show that under high temperature conditions, the influence of environmental factors will significantly reduce the fatigue life of the material.
Creep Effects
The effects of creep are evaluated by strain and load conditions at different temperatures and the material life is summarized from this.
Challenges so far and future prospects
Although damage accumulation models play an important role in the understanding of TMF, their complexity also requires many material parameters to be obtained through extensive experiments. How to balance the accuracy and operability of the model remains a major challenge for academia and industry.In the future, as materials science advances, we will be able to gain a deeper understanding of the mechanisms of thermomechanical fatigue, which will help design more durable engines. However, how to effectively transform this new knowledge into practical applications is still a question worth exploring.
