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Uncovering the three deadly mechanisms of material fatigue: How do they affect our technology?
With the advancement of science and technology, research on materials science has received more and more attention. Among them, thermo-mechanical fatigue (TMF) has become an important consideration in many high-tech applications, especially in the design of turbine engines or gas turbines. Increased acoustic butterfly noise or unstable turbine speed can be directly related to the fatigue behavior of the material.
TMF refers to the fatigue phenomenon caused by the material being subjected to periodic mechanical loads and periodic thermal loads at the same time. According to current research, there are three main failure mechanisms of thermomechanical fatigue: creep, fatigue and oxidation. Let’s explore how these mechanisms affect the properties of materials and, in turn, our technology.
Failure Mechanism
Creep is the deformation behavior of a material at elevated temperatures. Fatigue is the growth and propagation of cracks due to repeated loading. Oxidation is the change in the chemical composition of a material due to environmental factors. Oxidized materials are more brittle and more prone to cracking.
The impact of these three failure mechanisms will vary with the loading parameters. For example, under in-phase (IP) thermomechanical loading conditions, creep becomes the dominant factor as both temperature and load increase simultaneously. Here, the combination of temperature and high stress causes the material to flow to a greater extent, reducing its strength.
In contrast, under out-of-phase (OP) thermomechanical loading, the effects of oxidation and fatigue are more significant. Oxidation weakens the surface of the material, causing the crack to become the initial defect. As the crack expands, the newly exposed crack surface will be oxidized again, increasing the brittleness of the material.
In addition, in OP TMF loading, when the stress difference is greater than the temperature difference, fatigue can be the primary cause of failure and the material may be extremely sensitive, even failing before the effects of oxidation become noticeable.
Model
In order to better predict the behavior of materials under TMF loading, various models have been developed. Two basic models will be introduced here: constitutive models and phenomenological models.
Constitutive model
Constitutive models strive to exploit the current understanding of the material's microstructure and its failure mechanisms and are generally complex because they attempt to incorporate all knowledge about material failure. As imaging technology advances, this type of model is gaining more and more attention.
Phenomenological ModelPhenomenological models rely entirely on observations of material behavior and treat the failure mechanism as a “black box.” In this model, temperature and loading conditions are used as inputs, and the fatigue life of the material is ultimately derived. Its characteristic is that it attempts to use some kind of equation to describe the trend between different inputs and outputs.
Damage Accumulation Model
The damage accumulation model is a constitutive model that adds the damage from three failure mechanisms, fatigue, creep and oxidation, to calculate the total fatigue life of the material.
Although such a model is accurate, it also requires large-scale experiments to derive multiple material parameters, which undoubtedly increases development costs and time.
Benefits and Challenges
The damage accumulation model can comprehensively reflect the impact of various failure mechanisms on material properties, which is crucial for the design and selection of high-performance materials. However, the complexity of this type of model is also one of the biggest challenges in current design, which requires the accuracy and reliability of experimental data, otherwise it will lead to incorrect usage judgments.
Strain rate distribution model
The strain rate distribution model is a phenomenological model that focuses on the inelastic strain behavior of materials and evaluates fatigue life by dividing the strain into multiple cases.
The model takes into account the effects of plasticity and creep on the fatigue properties of materials under different loading conditions and is applicable to complex loading conditions.
The accuracy and usability of these models become even more important when faced with harsh operating environments, such as high temperature and pressure. As industry's requirements for material performance increase, more research will focus on the improvement and application of these models.
Technological advances have gradually deepened our understanding of material fatigue mechanisms, but there are still many unknown factors worth exploring in the future. While promoting scientific and technological progress, it also makes us think carefully about the durability of materials. . Do we fully understand these fatigue mechanisms and their profound implications for future technologies?
