In this article, the Strain-Life Method of Simcenter Testlab Neo which is used to predict fatigue life is explained. The Strain-Life Method is used to calculate the fatigue life of an object subjected to cyclic loading over time.
The Simcenter Testlab Neo Strain-Life method contains multiple processing steps to account for the different aspects of elastic-plastic strain behavior when predicting the life. The steps are outlined in Figure 1.
Figure 1: Outline of the strain life fatigue calculation process. On the left side of the process tree, the input time histories are converted to a local stress-strain time history which contain elastic-plastic behavior. On the right side of the process tree, fatigue damage is calculated from the local stress-strain time history.
These steps, as well as the inputs and outputs of the method, will be covered in the article.
Index: 1. Getting Started with Simcenter Testlab Neo Process Designer 2. Ramberg-Osgood Relation 3. Manson-Coffin-Morrow Strain Life Curve 4. Mean Stress Influence 5. Running the Strain-Life Calculation 6. Strain Life Outputs 7. Life Curve Blockset 7.1 Design Point Block Repeats 7.2 Design Point Block No Repeats 7.3 Design Point Block Infinite Repeats
1. Getting Started with Simcenter Testlab Neo Process Designer
Figure 2: After opening Simcenter Testlab Neo Process Designer (1), data to be processed can be added to the input basket (2). The Strain-Life method can be added to the process area (3) and processing parameters selected (4).
To get the Strain-Life Method setup:
Start Simcenter Testlab Neo Process Designer by double clicking on the appropriate icon from the Testlab Neo General Processing folder.
Select some time data (load, stress, or strain) and Add to Input Basket
Drag the Strain-Life method from the Fatigue Life Analysis library and connect it to the input basket. Turn on the Fatigue Life analysis add-in on under File -> Add-ins if necessary.
Before executing the process, Strain-Life processing parameters can be set.
The Strain-Life method processing settings are shown in Figure 3.
Figure 3: Strain-Life method settings.
The strain life method contains the following:
Inputs: Strain or load input time histories.
Material Data Properties: Material and geometric information of the object.
Method Parameters: Mean stress handling, size corrections, surface corrections.
Outputs: Life curve, design points, input time history histogram, number of repeats of the given input to failure, and more.
2. Strain Life Inputs
Possible starting time history inputs for the Strain-Life method include load, stress, and strain as shown in Figure 4.
Figure 4: Possible input time history blocks for the strain life method include load (top - red), stress (middle green), and strain (bottom - blue).
When using the Strain Life method, the appropriate Input type needs to be selected in the method properties (Figure 5) to match the time histories being analyzed.
Figure 5: In the Strain Life method properties, the Input Type parameter (lower right) needs to match the time histories being processed. Elastic-plastic strain can be used with strain gauge measurements, while Load is used with stresses or forces.
Either Load or Elastic-plastic strain can be selected as input type:
Load: Selected if force or stress is the input time history.
Elastic-plastic strain: Selected if input time history is strain.
If Load is selected, the force or stress time history will need to be converted to an elastic-plastic strain in order to perform fatigue life calculations. This conversion is described in the knowledge article What is Neuber s Rule? . Neuber s Rule is used in the conversion of elastic stresses to elastic-plastic strains.
If Elastic-plastic strain is selected, then no conversion is needed.
The next step is to create a local stress-strain time history using the elastic-plastic strain.
3. Ramberg-Osgood Relation
The Ramberg-Osgood relation is used to derive the corresponding stress time history block from the elastic-plastic strain time history. This pair of time history blocks are referred to as local stress-strain time history. For any location that the local stress-strain time history is known, the fatigue life can be predicted.
The Ramberg-Osgood material properties can be entered in the Material Data entry of the Strain-Life method properties as shown in Figure 6.
Figure 6: The Material Data parameter is used to define the material properties like the Ramberg-Osgood relation.
The cyclic hardening coefficient, cyclic hardening exponent, and Young s Modulus of the material can be entered as shown in Figure 7.
Figure 7: The Ramberg-Osgood relation information can be entered in lower right area of the Material data property menu in the method properties. In the upper left, there are icons that can be used to create new materials.
If the user wants to enter their own material properties, the editing icons in the upper left of the menu can be used to enter new materials.
Strain cycles will be extracted from the stress-strain time history. Damage will be calculated from the strain cycles, and their corresponding mean stresses, using the Manson-Coffin-Morrow curve.
After determining the number of strain cycles and their respective amplitudes, they can be converted into damage by using the Manson-Coffin-Morrow Strain Life curve (also called a EN curve).
In the properties of the Simcenter Testlab Neo Strain-Life method, the Manson-Morrow-Coffin properties can be entered under Material Data as shown in Figure 8.
Figure 8: Click on the Material data entry in the Strain-Life method properties to enter Manson-Coffin-Morrow information.
Material properties for some steels, aluminum, and other alloys are included by default with Simcenter Testlab Neo. They can be selected on the right side of the Material Data menu (Figure 9).
Figure 9: The Manson-Coffin-Morrow information can be entered in on the upper left side of the Material Data menu.
In addition to the strength and ductility information, the static failure and endurance limits can also be entered. The Manson-Coffin-Morrow equation does not have an upper and lower limit itself, so these limits are needed to ensure that extremely high and extremely low strain cycles are handled appropriately.
Static failure value for a material is the stress level that results in failure with the application of one cycle for the selected material. It is defined for both tension and compression.
If loads are below the endurance limit (Figure 10) of the material, no failure occurs.
Figure 10: Endurance limit can be specified three different ways.
The endurance limit can be specified three different ways: as a number of cycles, as a stress amplitude, and as a strain amplitude:
For a given design, the number of cycles can represent a life target. For example, If our product can withstand 1,000,000 cycles, it is enough to last the anticipated lifetime of use .
If the loads result in amplitudes below the stress amplitude or strain amplitude limits, no damage will accumulate.
This parameter is used to indicate the measurement conditions used to determine the material properties when measuring the EN-Curve:
The stress ratio (R) indicates the mean stress conditions present in the measurement. See mean stress correction article for more information about stress ratios.
Torsion or Tension: Indicated how the material was tested (Figure 11).
Figure 11: Tension or torsion
Poisson s Ratio: An important material property that relates the decrease in the lateral measurement of a material due to the stretching it along its length.
Various Mean Stress Influence schemes and other correction methods can be selected from the menu shown in Figure 13.
Figure 13: Strain-Life Method Parameter menu from Simcenter Testlab Neo allows correction for mean stress influence, sizes, and surface finish.
Two different blockset results are calculated in Simcenter Testlab Neo: weighted and un-weighted:
The un-weighted results use the Manson-Coffin-Morrow relationship without any adjustments for the mean stress influence, surface corrections, etc. They are based on strain or stress cycle amplitudes only as taken from the original time inputs.
The weighted results do include the adjustments for mean stress, surface correction, etc. These are expressed in value of the damage parameter (P). If a Smith-Watson-Topper correction is used, the value is assigned a stress unit, if a Morrow correction is used, the value is assigned a strain unit. The weighted results also include a PN curve.
Weighted and un-weighted output of the Strain-Life method in Simcenter Testlab Neo are shown in Figure 14.
Figure 14: Two sets of results (called blocksets) are created when using a mean stress influence correction.
Notice that the curve with Mean Stress Influence is referred to as a PN Curve, rather than an EN Curve.
Select from different mean stress influence corrections, including Smith-Watson-Topper and Morrow. No correction for mean stress is the default setting.
Given the same material, the size of the test object affects the likelihood of a failure occurring. The larger the object, the more surface area. Simply having more exposed surface area increases the chances that a surface imperfection could develop into crack or other failure. The default value is one, and it can be increased up until three. Increasing this parameter higher than one means that the object is larger, and failure is more likely to occur.
The surface material finish also affects the likelihood of a failure. For example, a smoothly polished surface is less likely to develop a failure than a rough as forged finish. Decreasing the number indicates a rougher finish. The default value for the correction is one, and the upper limit is two.
6. Running the Strain-Life Calculation
After selecting time data to be analyzed, and choosing the appropriate settings, the strain life calculations can be run.
Click on the Run button in the lower right button to start the calculation (Figure 15).
Figure 15: In the Simcenter Testlab Neo Process Designer, click on the Run button in lower left to start the Strain-Life calculation.
Results will be placed in the Active Analysis (Figure 16). After highlighting Active Analysis in the navigation tree, click in the pivot table to display the results.
Figure 16: Display the Strain-Life results by clicking on the pivot table cells of the Active Analysis .
After viewing the results, press the Accept button to store the results in the active project.
7. Strain Life Outputs
There are two outputs of the Simcenter Testlab Neo Strain-Life method: Life Curve(s) and Life Statistics (Figure 17).
Figure 17: Pivot table (upper right) output of the Simcenter Testlab Neo Strain-Life method consists of a set of life curves (left) and life statistics (right).
The life curve output of Simcenter Testlab Neo is a blockset. The blockset consists of multiple functions that are treated as a single piece of data in the Simcenter Testlab Neo software.
7.1 Life Curve Blockset
The Life Curve blockset is shown in Figure 18.
Figure 18: The Life Curve blockset output of the strain life method: cyclic histogram of the input time history (red line), the sensitivity to scaling the time history (blue line), and the design point (green dot). The design point is the intersection of the highest amplitude cycle (dotted orange line) and the total number of cycles to failure (dotted blue line) for the given input.
The Life Curve blockset is comprised of three different functions:
Histogram: A cycle histogram (red curve in Figure X) calculated from a rainflow analysis on the input time history. It contains the cumulative number of cycles and their respective amplitudes present in the signal.
Design Point: A single point (green dot in Figure X) that indicates the number of cycles to failure (the X value position, dotted blue) and maximum cycle amplitude (the Y value position, dotted orange) based on the original input time history. The design point uses the input time history with the material property settings to determine the total number of cycles the part can survive.
Life Curve: By scaling the input time history both higher and lower, the sensitivity of the design point to changes in cycle amplitude can be determined. For example, to simulate a safety factor of three, the histogram amplitudes are multiplied by three and a new design point is calculated. By calculating design points for various amplitude scalings, both lower and higher, a full life curve (blue curve in Figure X) is created. This life curve will have a very similar slope to the material property curve.
From these life curves, a single number output called the Design Point Block is calculated, which is described in the next sections.
7.2 Design Point Block Repeats
The Design Point Block is determined from the life curves as shown in Figure 19.
Figure 19: The Design Point Block indicates the number of repeats that the input time history cycles (red dot) can be applied until failure occurs (blue dot). The Design Point Block is the ratio of these two quantities (red line length versus blue line length).
The Design Point Block is the number of times, or repeats, that the input time history can be applied before failure occurs. As shown in Figure 19, it is the ratio of the number of cycles to failure (blue line) versus the cycles in the input signal (red line).
For example, if the input time history was 60 seconds, and the design point block repeat was 100, then the part could survive 6000 seconds at the location of the local stress-strain. In other words, the original time history could be applied 100 times before the part fails. Of course, the calculation is only for this location, a failure could occur at less than 100 repeats at some other location.
The Life Statistics output of the Simcenter Testlab Neo Strain-Life method (Figure 20), shows the number of Design Point Blocks in numerical form.
Figure 20: The Design Point Blocks (highlighted in orange) is the number of times the input signal can be repeated before the part fails. In this case, it is 125.38 repeats of the input signal.
Other numerical outputs also include damage (when damage is equal to one failure occurs), the design point Y-axis value (maximum cycle strain), and number of total cycles at the design point.
It is also possible that the Design Point Block value is less than one as described in the next section.
7.3 Design Point Block No Repeats
It is possible that the part would not survive a single application of the input time history block as shown in Figure 21.
Figure 21: The Design Point Block is less than one (0.93 in this case), because failure occurs during the application of the original, non-repeated, input time history.
In this case, the part fails during the application of the original input time history. Usually this would already be known from the original test!
7.4 Design Point Block Infinite Repeats
If the amplitudes are low enough, then the part could live forever as shown in Figure 22.
Figure 22: If the amplitude of the histogram cycles of the measurement do not exceed a specific level, there is no Design Point. The input time history could be applied an infinite number of times without causing failure.
When the projection of the amplitudes from the histogram does not intersect the life curve, the design point is undefined. In this case, without a Design Point, an infinite number of repeats (Figure 22) of the original time history can be applied without the part failing.