Manipulating 2nd Order Tensor Expression: Isolating Q

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Discussion Overview

The discussion revolves around manipulating the expression involving second order tensors, specifically isolating the tensor Q from the equation A = Q : B, where A and B are second order tensors and Q is a fourth order tensor. The context includes theoretical exploration of tensor operations and their implications in material properties.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks to isolate Q in the equation A = Q : B, expressing uncertainty about the manipulation of tensor expressions.
  • Another participant questions the notation used for the double dot product, suggesting a need for clarification or reference to standard texts.
  • A participant explains the concept of double dot product, noting that it results in a scalar when applied to two second order tensors, and provides external references for further understanding.
  • One participant mentions the complexity of manipulating a fourth order tensor with a second order tensor and expresses discomfort with the rules of tensor operations, particularly regarding inversion and contraction.
  • A different perspective simplifies the problem by considering A and B as vectors and Q as a second order tensor, discussing the challenges of finding coefficients from the resulting equations.
  • Another participant reflects on the physical motivation behind the problem, highlighting the complexity of determining unique material properties from stress and strain states in anisotropic materials.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and approaches to the problem, with no consensus on the method to isolate Q or the implications of the tensor operations involved. The discussion remains unresolved regarding the best approach to manipulate the tensor expression.

Contextual Notes

The discussion reveals limitations in the clarity of notation and the complexity of tensor operations, particularly in the context of isolating a fourth order tensor from a given expression. There are unresolved assumptions regarding the nature of the tensors involved and the mathematical steps required for manipulation.

afallingbomb
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Given the expression

[tex] \textbf{A} = \textbf{Q} : \textbf{B}[/tex]

where A and B are second order tensors of rank 2 and Q is a second order tensor of rank 4.

How can I manipulate this expression to calculate for Q given A and B? In other words, isolate Q on one side of the equation. Thank you very much!
 
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The notation you are using

A=Q:B

is not one of the many standard textbooks. Perhaps you can explain it or provide a reference to some easily available text.
 
The double dot product between a fourth order tensor and a second order tensor is a second order tensor (now I'm referring to order as the number of subscripts... rank/order are used interchangeably in the literature). I can work it out using dyadics, but I'm not sure how to move around terms in the equation to isolate Q. There are many products to choose from and I'm not very comfortable with the rules, especially using an inverted second order tensor and perhaps (single or double?) contracting it on the left.
 
Alright, make it simpler. Let A,B be vectors and Q a second order tensor. So what you have then is an equation of the form

a = Qb

where a and b are vectors. Can you calculate nxn coefficients form n equations? You can't. But you can try to find a general solution for Q, with arbitrary coefficients. Is that what you mean? If so, try to do it for matrices and vectors - you will see what kind of algebra is needed.

The idea is: you have an inhomogeneous linear equation (for Q). Therefore a general solution is a sum of a particular solution of this equation and a general solution of the homegeneous one.
 
arkajad, thank you very much. You made me see that my problem was not well posed.

The physical motivation of this problem stemmed from my curiosity on determining a unique material property (stiffness or compliance) from a specified stress and strain state at a material point. In the most general, anisotropic case, there is no unique answer to this problem.
 
You are welcome!
 

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