Thermo-mechanical fatigue (short TMF) is the overlay of a cyclical mechanical loading, that leads to fatigue of a material, with a cyclical thermal loading.
There are three mechanisms acting in thermo-mechanical fatigue Each factor has more or less of an effect depending on the parameters of loading.
The heated material flows more easily in tension, but cools and stiffens under compression.
Out of phase (OP) thermo-mechanical loading is dominated by the effects of oxidation and fatigue.
Oxidation weakens the surface of the material, creating flaws and seeds for crack propagation.
There are many different models to attempt to predict the behavior and life of materials undergoing TMF loading.
There are many different models that have been developed in an attempt to understand and explain TMF.
This page will address the two broadest approaches, constitutive and phenomenological models.
Constitutive models utilize the current understanding of the microstructure of materials and failure mechanisms.
These types of models are becoming more popular recently as improved imaging technology has allowed for a better understanding of failure mechanisms.
Phenomenological models are based purely on the observed behavior of materials.
They treat the exact mechanism of failure as a sort of "black box".
Temperature and loading conditions are input, and the result is the fatigue life.
These models try to fit some equation to match the trends found between different inputs and outputs.
It adds together the damage from the three failure mechanisms of fatigue, creep, and oxidation.
is the fatigue life of the material, that is, the number of loading cycles until failure.
[2][3] The life from fatigue is calculated for isothermal loading conditions.
The effects of temperature are treated in the oxidation and creep terms..
The life from oxidation is affected by temperature and cycle time.
Parameters are found by comparing fatigue tests done in air and in an environment with no oxygen (vacuum or argon).
Under these testing conditions, it has been found that the effects of oxidation can reduce the fatigue life of a specimen by a whole order of magnitude.
Higher temperatures greatly increase the amount of damage from environmental factors.
[5] Strain-rate partitioning is a phenomenological model of thermo-mechanical fatigue.
It accounts for different types of deformation and breaks strain into four possible scenarios:[6]
The damage and life for each partition is calculated and combined in the model
Because it fails to account for oxidation damage, it may overpredict specimen life in certain loading conditions.
The next area of research is attempting to understand TMF of composites.
The interaction between the different materials adds another layer of complexity.
Zhang and Wang are currently investigating the TMF of a unidirectional fiber reinforced matrix.
They have discovered that the large difference in the thermal expansion coefficient between the matrix and the fiber is the driving cause of failure, causing high internal stress.