Need help understanding eigenvalues and eigenvectors

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Discussion Overview

The discussion revolves around understanding eigenvalues and eigenvectors in the context of finding principal stresses and the orientation for the principal axis of stress in a plane stress scenario. Participants are exploring the mathematical framework and physical interpretation of stress tensors, particularly through the lens of Mohr's Circle and related equations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant presents a problem involving principal stresses with given values of σx, σy, and τxy, and expresses confusion about the steps in the solution, particularly regarding the determinant condition.
  • Another participant notes that the image link for the problem does not seem to load, indicating a potential barrier to understanding.
  • A later reply clarifies that the stress tensor can be expressed in terms of its components and unit vectors, explaining the symmetry of the tensor and the relationship between τyx and τxy.
  • The same participant elaborates on the Cauchy stress relationship, detailing how the traction on a plane can be derived from the stress tensor and unit normal vector, leading to the eigenvalue problem.
  • The definitions of eigenvalues and eigenvectors are discussed, with the principal stress being described as the eigenvalue and the components of the normal defining the eigenvector.

Areas of Agreement / Disagreement

Participants express confusion and seek clarification on various aspects of the problem, indicating that there is no consensus on the understanding of the steps involved or the interpretation of the equations presented.

Contextual Notes

Participants highlight missing assumptions and the need for clarity regarding the definitions and relationships within the stress tensor, as well as the implications of the eigenvalue problem in this context.

Cisneros778
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Homework Statement


Find the principal stresses and the orientation for the principal axis of stress for the following cases of plane stress.

σx = 4,000 psi
σy = 0 psi
τxy = 8,000 psi


Homework Equations


See picture.

The Attempt at a Solution


https://mail.google.com/mail/u/0/?ui=2&ik=bc68d58ae7&view=att&th=139dbef260c42514&attid=0.1&disp=inline&realattid=f_h79p3pz70&safe=1&zw

I solved this problem using Mohr's Circle. However, the solution to the problem is different and I would like to understand it.
I do not know what the steps mean. Why does the determinate of that function must equal zero?
And what is the n1 and n2 about?
Finally, the 2x2 matrices adds two shear stresses where I was only given one τxy where does this other value come from?
 
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Any help please?
 
I'm not sure if anyone else is having trouble here, but your picture doesn't seem to be loading?
 
Cisneros778 said:

Homework Statement


Find the principal stresses and the orientation for the principal axis of stress for the following cases of plane stress.

σx = 4,000 psi
σy = 0 psi
τxy = 8,000 psi


Homework Equations


See picture.

The Attempt at a Solution


https://mail.google.com/mail/u/0/?ui=2&ik=bc68d58ae7&view=att&th=139dbef260c42514&attid=0.1&disp=inline&realattid=f_h79p3pz70&safe=1&zw

I solved this problem using Mohr's Circle. However, the solution to the problem is different and I would like to understand it.
I do not know what the steps mean. Why does the determinate of that function must equal zero?
And what is the n1 and n2 about?
Finally, the 2x2 matrices adds two shear stresses where I was only given one τxy where does this other value come from?

In terms of components and unit vectors, the stress tensor can be written as a sum of terms (similar to a vector) as follows:

σ = σxx ixix + τxy ixiy+ τyx iyix+ σyy iyiy

Since the stress tensor is symmetric, τyx = τxy

Therefore, the stress tensor is given by:

σ = σxx ixix + τxy ixiy+ τxy iyix+ σyy iyiy

This is where the other τxy you were asking about comes from.


If n is a unit vector oriented in some arbitrary horizontal direction within your material, then, in terms of its components in the x and y directions, n can be written as the following sum of terms:

n = nx ix + ny iy

According to the so-called Cauchy stress relationship, the traction (force per unit area) acting on a plane perpendicular to the unit normal n is obtained by dotting the stress tensor σ with the unit normal n:

\Sigma=(σxx nx + τxy ny) ix + (τxy nx + σyy ny) iy

If n corresponds to one of the principal direction of stress, then the traction on the plane normal to n is perpendicular to the plane, and parallel to n:


\Sigma=(σxx nx + τxy ny) ix + (τxy nx + σyy ny) iy = λ (nx ix + ny iy)

where λ is the principal stress.

From the above equation, we get:

σxx nx + τxy ny = λ nx

τxy nx + σyy ny) = λ ny

This defines your eigenvalue problem. The components of the normal define the eigenvector, and the principal stress defines the eigenvalue.
 

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