How do you compare the creep-fatigue behavior of different materials using life fraction rules?

2 How are life fraction rules derived?

Life fraction rules are derived from experimental data of creep-fatigue tests on different materials. In these tests, the material is subjected to a cyclic loading with a constant stress amplitude and a variable hold time at the peak or valley of the cycle. The hold time creates a creep deformation in the material, which affects its fatigue resistance. By varying the hold time and the stress amplitude, the fractions of creep and fatigue cycles can be controlled and measured. The number of cycles to failure is then plotted against the fractions of creep and fatigue cycles, and a curve or a surface is fitted to the data. The equation of the curve or the surface represents the life fraction rule for that material.

3 What are the advantages and limitations of life fraction rules?

The main advantage of life fraction rules is that they are simple and easy to use for comparing the creep-fatigue behavior of different materials. They can provide a quick estimate of the life of a component under a given loading condition, and help to select the most suitable material for a specific application. They can also be used to design experiments and optimize the loading parameters.

The main limitation of life fraction rules is that they are empirical and not mechanistic. They do not account for the physical mechanisms of creep and fatigue damage, such as crack initiation, propagation, and coalescence. They also do not consider the effects of other factors, such as temperature, strain rate, stress ratio, frequency, environment, microstructure, and history. Therefore, they may not be accurate or valid for all materials and loading conditions, and they may require calibration and validation with experimental data.

4 How to apply life fraction rules to different materials?

To apply life fraction rules to different materials, you need to know the equations of the life fraction rules for each material, and the fractions of creep and fatigue cycles for the loading condition of interest. You can obtain the equations of the life fraction rules from literature or from your own experiments. You can calculate the fractions of creep and fatigue cycles from the loading parameters, such as the stress amplitude, the hold time, the cycle time, and the creep and fatigue properties of the material. Alternatively, you can use software tools or charts that can perform these calculations for you.

Once you have the equations and the fractions of creep and fatigue cycles, you can plug them into the equations and solve for the number of cycles to failure for each material. Then, you can compare the results and see which material has the highest or the lowest creep-fatigue resistance. You can also use graphical methods, such as damage contour plots or interaction diagrams, to visualize the relationship between the fractions of creep and fatigue cycles and the number of cycles to failure for different materials.

5 What are some examples of life fraction rules for different materials?

The literature contains various life fraction rules for different materials, many of which are more complex than the linear damage rule. For example, austenitic stainless steels such as 316L and 304L use a modified linear damage rule, which incorporates a threshold stress for creep damage. This rule is expressed as D = fc + ff = 1 - R, where R is a constant that depends on the material and temperature. Nickel-based superalloys such as IN718 and IN100 use a nonlinear damage rule to account for the nonlinear accumulation of creep damage. This is expressed as D = (fc)^n + ff = 1, where n is a constant that depends on the material and temperature. Titanium alloys such as Ti-6Al-4V and Ti-6242 use a power law damage rule to reflect the power law relationship between stress amplitude and fatigue life. This is expressed as D = (fc)^m + (ff)^p = 1, where m and p are constants that depend on the material and temperature.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?