How can I explain what second Piola-Kirchhoff stress is? 1

[tugni925]

From "

https://www.brown.edu/Departments/Engineering/Courses/EN224/Kinetics.html"

"The components of the first Piola-Kirchhoff stress are the forces acting on the deformed configuration, per unit undeformed area. They are thought of as acting on the undeformed solid . The second Piola-Kirchhoff stress has no obvious physical interpretation."


Excerpt is from "Numerical investigation of stiffness and buckling response of simple and optimized infill structures". I want to explain how second Piola-Kirchhoff stress comes into play here. I am not sure how to explain it when it has no physical interpretation, is it all just mathematical equations?

 

[SwinnyGG]

The second PK tensor connects the loads in the initial, undeformed condition to the initial geometry.

The simplest explanation I can think of is that PK2 describes the relationship between initial loads and geometry at time zero. The value of PK2 is that it is symmetric - i.e. its utilization is less computationally difficult.