Infinite life is often used in designing critical components of products with demanding use. Examples include crankshafts of an engines, vehicles for public transportation, spacecraft, etc.
What is meant by infinite life? Ferrous materials have an "infinite life" region defined by an ‘endurance limit’. The endurance limit is a specific stress level for a material, where stress cycles below a certain amplitude and mean will not accumulate fatigue damage.
The Goodman-Haigh diagram is used to check if a cyclic stress time history is within the infinite life region for a product made of a given material (Figure 1).
Figure 1: Goodman-Haigh diagram
It is important that none of the stress cycles in a load history exceed the infinite life endurance limit. If they do, the material will behave as if the infinite life region does not exist, and failure will occur given enough additional cycles, even if they are below the endurance limit.
Goodman published his original diagram in 1899. Haigh added alternating and mean stress in 1917. The combination of these two is referred to as the ‘Goodman-Haigh Diagram’.
This article has the following sections: 1. Goodman-Haigh Diagram 1.1 Stress Cycles 1.2 Material Information 2. Using the Goodman-Haigh Diagram 3. Infinite Life in Simcenter Testlab Neo 4. Infinite Life in Simcenter Tecware
1. Goodman-Haigh Diagram
Two major pieces of information are needed to use a Goodman-Haigh diagram:
Stress cycles: A stress cycle time history of the expected loading that includes both alternating and mean stress information
Material Information: The yield strength, ultimate strength, and endurance limit of the part material
The material information is used to define an infinite life region. The stress cycles are plotted against this region to see if they are contained within it.
1.1 Stress Cycles
A stress time history can be broken down into individual cycles. A cycle has an alternating component as shown in Figure 2.
Figure 2: Alternating stress levels
A stress cycle can also have a mean stress. This mean stress puts the part in either net compression or tension as shown in Figure 3.
Figure 3: Mean stress: compression versus tension
The mean stress is very important factor in governing the fatigue life. Net tension on a part tries to pull it apart, which significantly reduces its life. Net compression pushes a part together, which is not as damaging.
In the Haigh diagram, the alternating and mean stress of the cycles will be plotted against each other as shown in Figure 4.
Figure 4: Alternating versus mean stress
The alternating stress level is plotted on the Y axis. The mean stress level is plotted on the X axis. Negative mean stress is compression, and positive mean stress is tension.
1.2 Material Information
Using a static stress-strain test on a material, the following material properties can be determined:
Yield Strength – Stress level at which there is a transition between the elastic region and plastic region of the material, where the relationship between stress and strain ceases to be linear
Ultimate Strength – Stress level where the material starts to fail
These material properties are determined via applying static loads to the material and plotting the relationship of stress and strain as shown in Figure 5.
Figure 5: Yield and Ultimate strength are determined from static stress-strain test
The Yield strength and Ultimate strength are plotted on the Goodman-Haigh diagram as shown in Figure 6.
Figure 6: Ultimate strength and yield strength are plotted on diagram
A yield envelope is created by connecting the yield strength points. However, this yield envelope is symmetric around the Y-axis, and does not distinguish between compression and tension.
Additional material information is needed from a dynamic/cyclic stress test. The result of a dynamic stress test can be found in a SN-curve as shown as shown in Figure 7.
Figure 7: SN-Curve with Infinite Life
The endurance limit is determined from the SN-Curve. The endurance limit is then plotted on the Goodman-Haigh diagram as shown in Figure 8.
Figure 8: Goodman-Haigh diagram with Endurance limit
An infinite life region can then be created by:
Connecting the endurance limit to the ultimate strength on the tension side (called the Modified Goodman line)
Project the endurance limit on the compression side
This infinite life region defined by these connections and projections are shown in Figure 9.
Figure 9: Infinite life region defined by Modified Goodman line
This infinite life region has a smaller region for tension versus compression, as would be expected. A stress time history can then be evaluated against the infinite life region.
2. Using the Goodman-Haigh diagram
The mean and alternating stress of a stress time history is plotted on the Goodman-Haigh diagram as shown in Figure 10.
Figure 10: Mean and alternating stress plotted against Infinite life region
This is done for each cycle in the time history. Each cycle is evaluated as to whether it falls in the infinite life region. In Figure 10, the stress cycles are contained entirely in the infinite life region.
Any stress time history, no matter how complicated, can be broken into individual cycles via the rainflow counting process. These cycles produced by the rainflow counting process include a mean and alternating stress.
Projecting from the origin to the cycle versus the region, a factor of safety can be calculated (Figure 11).
Figure 11: Factor of Safety
In this case, the factor of safety is approximately two: the ratio of the magenta and green lines. In many engineering applications, a factor of safety of three or higher is often desired. This would ensure that the part would survive with three times higher than expected loads.
Start Simcenter Testlab Neo Process Designer. Select the data to analyze and add it to the Input Basket. Load the "TecWare" add-in under File -> Add-ins (Figure 12).
Figure 12: In Simcenter Testlab Neo, load the "TecWare" add-in.
A new worksheet is added in the Processing Tab called "Tecware". After activating the tab, load a Tecware Batch File using the button in the upper right (Figure 13).
Figure 13: After activating the Tecware tab (lower left), select the Tecware Batch file in the upper right.
Load the Process called "TL_Goodman and Damage calculation.tbd"
In the new menu, select the Material Excel XML sheet as shown in Figure 14. Be sure to edit the file with the desired material data if needed.
Figure 14: Loading the Material Excel XML Sheet
Click the "Run" button in the lower left of Simcenter Testlab Neo. If desired, add a strain offset to any of the channels as needed (Figure 15). The offset could be users to account for an assembly strain. Press OK when done.
Figure 15: Add an offset to account for assembly strain if desired.
The process will execute. A Microsoft Word report is generated and should appear automatically.
4. Infinite Life in Simcenter Tecware
Using Simcenter Tecware, infinite life calculations can be made using the Tecware Processbuilder ( Figure 16) and the files attached to this article (upper right). They include a Installation and Instructions (*.docx), a ProcessBuilder file (*.pb), and other additional files.
Figure 16: Simcenter Tecware ProcessBuilder diagram for Goodman Infinite Life calculation.
After installing the files and running the process, reports are generated automatically in Microsoft Word as shown in Figure 17.
Figure 17: Goodman Infinite Fatigue Life report from Simcenter Tecware
If all cycles (indicated by triangles) fall within the Goodman triangle area, then infinite life is achieved. If the cycles are outside the triangle, as shown in Figure 13, infinite life is not possible.