In Abaqus/CAE Module Visualization or in Abaqus/Viewer, FIELD outputs, such as stresses and strains, are output in frames. Here, frames stand for time points or individual frequencies, for example. Now, there is a possibility to use the values of different frames to calculate new outputs.
An example to better estimate the fatigue strength of a component will explain the procedure. However, it is not a general procedure for fatigue strength verification.
Three independent load cases (PERTURBATION Steps) are considered:
Step 1 : Tension load -> 100 MPa tensile stress
Step 2: As 1 plus bending load in positive Y-direction, load case “Up”
Step 3: As 1 plus bending load in negative Y-direction, load case “Down”
Step 1: Tensile stress 100 MPa
Step 2: Tensile and bending stress superimposed
Step 3: Like Step 2 , but reversal of the bending load
In this simple example, you can manually determine some values to evaluate the fatigue strength.
This results in an additional bending stress of 150 MPa. The maximum upper stress Smax is 250 MPa. The minimum stress Smin is -50 MPa. Thus, the mean stress is: (Smax+Smin)/2= 100 MPa and the amplitude is ABS(Smax-Smin)/2= 150 MPa.
We will now show how to determine these values in Abaqus/CAE via the calculation with frames.
Now, we generate the calculation and field putput of the mean stress and amplitude from different frames.
Step 1: Generate output values via frames of the steps
Step 2: Selection of the frames for the evaluation, here the last increment of the load cases “Up” and “Down”, operation: maximum over all frames.
Any number of load cases can be selected from any number of steps.
Step 3: Select the output value, here S11, then select “Apply” to save without leaving the dialog.
All stress components are available.
Step 4: Now, change the operation to minimum value for S11 and exit the dialog with “OK”.
Step 5: Now, the generated frames can be used to determine amplitude and mean stress.
The previously generated output values now appear in the automatically generated “Session Step” as frames 1 and 2.
Step 6: Determining the mean stress
Step 7: Determining the amplitude
Mean stress and amplitude are now stored in frame 0
Mean stress and amplitude
Determination of the load via fatigue strength diagram (here: Smith diagram)
Red: material fatigue strength
Green: Reduced fatigue strength (material fatigue strength reduced by technological size factor and roughness factor).
Maximum stress 250 MPa at 100 MPa mean stress
Minimum stress -50 MPa at 100 MPa mean stress
The method shown here refers to uniaxial loading and does not claim to be a general procedure for fatigue strength verification.
Rather, it is intended to clarify the generation of output values via frames and fields and to demonstrate a pragmatic method for rapid estimation of component fatigue strength.