Most of the time, results of uniaxial tensile tests are used to explain plastic strain. That usually leads to an assumption that greater total strain leads to greater plastic strain (and vice versa). This statement only applies to uniaxial tensile test results and not to crash simulations generally.
Figure 1: Uniaxial Stress-Strain diagram
Strain is the physical quantity that expresses how much a material has changed from its original shape, with the use of a plus or minus sign.
- Strain does not consider whether change is elastic or plastic (if removes or not after springback).
- Whatever the content of the deformation, it comes out positive if object size increases and negative if it decreases.
Plastic Strain is the cumulative amount of permanent deformation.
- The amount of deformation expressed as positive value always, no matter if the object becomes smaller or larger in size.
- By default, is a positive value.
The following example showcase a steel plate pulled by external force and then crash back to original size by external force:
Figure 2: Plastic (up) vs Total (down) Strain contour
Figure 3: Plastic (up) vs Total (down) Strain values
From the above contours and diagrams, we can see that plastic strain grows from the beginning to the end of the simulation, while the total strain reaches a maximum value and returns to zero.
For crash applications we can conclude that:
- (Total) Strain is not generally the same with Plastic Strain
- Invisible and violent vibrations occur during crash that influence these values.
- Direction of deformation sometimes changes with object shape.
It strongly recommended to insert the proper Engine File Cards (or Keywords) to visualize the Plastic and Total Strain in Post Processing. The exact keywords for each case are given in Table 1.
|
SHELL ELEMENT |
/ANIM FILE |
/H3D FILE |
|
Plastic Strain through thickness |
/ANIM/SHELL/EPSP/ALL |
/H3D/SHELL/EPSP/NPT=ALL |
|
Total Strain tensor |
/ANIM/SHELL/TENS/STRAIN/ALL |
/H3D/SHELL/TENS/STRAIN/NPT=ALL |
|
SOLID ELEMENT |
/ANIM FILE |
/H3D FILE |
|
Plastic Strain through thickness |
/ANIM/BRICK/EPSP/ALL |
/H3D/SOLID/TENS/EPSP (scalar value) |
|
Total Strain tensor |
/ANIM/BRICK/TENS/STRAIN/ALL |
/H3D/SOLID/TENS/STRAIN/IR=ALL/IS=ALL/IT=ALL |
Table 1: Recommendations for visualizing Strain