"The nominal (engineering) stress reaches a maximum (ultimate tensile strength) value at point x , corresponding to the onset of incipient necking. Beyond this point, deformation localizes in a neck region and the stress-strain state is not uniform in the specimen. Therefore, after point x, the curve is no longer representative of the actual stress-strain state at any given location of the specimen. ... Point c in this curve identifies the material state at the onset of damage"
The above quote is from the Abaqus docs. Focusing on the left image, in step 1 of this series we calibrated an elastic-plastic material all the way to point "c". It was necessary to model the specimens with FE models to capture necking behaviors. Then, in step 2, we used the state of stress at point "c" to determine the damage initiation surface, thus determining the Hosford-Coulomb damage initiation parameters. As the simulation reaches point "c", the damage criterion, HCCRT, reaches the maximum value of 1.0 After that, the scalar stiffness degradation value, SDEG, will transition from 0.0 to 1.0 as an element accumulates damage. When any element reaches SDEG=1, it will carry no more load and is removed from the analysis.
Both step 1 and step 2 used the test data up to point "c" (truncated to point "c"). The simulation models used in those steps also needed to run up to the same point in time/applied displacement. In step 3 we will need to use the test data all the way to the end, point "d", or even a bit beyond that point. This also means that all of our specimen FE models need to be modified to run out to a longer time / larger applied displacement.
In this Step 3 post we focus on the evolution of damage, that is the physics happening between point "c" and "d". For many specimens this can be quite an abrupt, short process from the first element failure to complete rupture of the specimen into two separate parts. It turns out that the determination of the *damage evolution parameter is often the least important parameter in the entire characterization.
For :
*Damage Evolution, type=DISPLACEMENT
0.0,
The value given on the data line is (from the docs):
Effective total or plastic displacement at failure, measured from the time of damage initiation.
The general rule of thumb is that this is a small value, perhaps zero. The smaller the value, the more abrupt (short) the process is from "c" to "d".
The animation below shows the progression of PEEQ and HCCRT as the smooth round specimen is pulled. The *Damage Evolution value is 0.0 Three elements at the center of the specimen are removed in frame 564. Successive elements are removed and the part splits entirely into two pieces.
Back to: Continuum Damage Mechanics: Worked Example
Back to: Continuum Damage Mechanics: References
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