# What is the relationship between force lines and the stress tensor field?

• em3ry
In summary, the force lines method is used in Solid Mechanics to visualize internal forces in a deformed body. Force lines represent the maximal internal forces and their directions across imaginary internal surfaces. However, stress in a solid body is a tensor, not a single vector. The relationship between force lines and the stress tensor field is that the stress tensor can be expressed as the sum of a mean hydrostatic stress tensor and a deviatoric stress tensor. The deviatoric stress tensor determines the principal stresses and their directions, which can be traced out as the force lines. The maximum principal stress is considered special as it is the primary contributor to the deformation of a solid body. Removing the maximum principal stress would leave the remaining deviatoric stress to cause
em3ry
Gold Member
Force lines method is used in Solid Mechanics for visualization of internal forces in a deformed body. A force line represents graphically the internal force acting within a body across imaginary internal surfaces. The force lines show the maximal internal forces and their directions.

But stress in a solid body is a tensor not a single vector. What is the relationship between force lines and the stress tensor field? If I know the force lines can I calculate the stress tensor field? If I know the stress tensor field can I calculate the force lines?

em3ry said:
Force lines method is used in Solid Mechanics for visualization of internal forces in a deformed body. A force line represents graphically the internal force acting within a body across imaginary internal surfaces. The force lines show the maximal internal forces and their directions.

But stress in a solid body is a tensor not a single vector. What is the relationship between force lines and the stress tensor field? If I know the force lines can I calculate the stress tensor field? If I know the stress tensor field can I calculate the force lines?

View attachment 274095
If you know the stress tensor, you can determine everything there is to know about the internal force distribution. Google Cauchy stress relationship.

em3ry
So are the force lines just the maximum principal stress at each point?

And I notice that the force lines never go in circles. I am guessing that when you remove the maximum principle stress then what's left over just goes in circles. And I guess that stress at a point is actually a vector plus a bivector.

Last edited:
em3ry said:
So are the force lines just the maximum principal stress at each point?

And I notice that the force lines never go in circles. I am guessing that when you remove the maximum principle stress then what's left over just goes in circles. And I guess that stress at a point is actually a vector plus a bivector.
I really don’t know what force lines are. With regard to stress, it is a second order tensor.

I am still not sure what the answer to the original question is but I think it probably has to do with this:

The stress tensor
can be expressed as the sum of two other stress tensors:

1. a mean hydrostatic stress tensor or volumetric stress tensor or mean normal stress tensor,
, which tends to change the volume of the stressed body; and
2. a deviatoric component called the stress deviator tensor,
, which tends to distort it.
So:

where
is the mean stress given by

Last edited:
em3ry said:
I am still not sure what the answer to the original question is but I think it probably has to do with this:

The stress tensor
can be expressed as the sum of two other stress tensors:

1. a mean hydrostatic stress tensor or volumetric stress tensor or mean normal stress tensor,
, which tends to change the volume of the stressed body; and
2. a deviatoric component called the stress deviator tensor,
, which tends to distort it.
So:

where
is the mean stress given by

This is all correct, but I don't see how it relates to your original question.

Unless the body is in hydrostatic equilibrium or is experiencing a purely volumetric change, the principal stresses and their directions can always be determined at each spatial location (i.e., the principal stresses are determined solely by the deviatoric stress tensor). Tracing out the directions of the principal stresses is what you might mean by the force lines.

Chestermiller said:
This is all correct, but I don't see how it relates to your original question.

Unless the body is in hydrostatic equilibrium or is experiencing a purely volumetric change, the principal stresses and their directions can always be determined at each spatial location (i.e., the principal stresses are determined solely by the deviatoric stress tensor).
OK. I see what you mean. Thanks

Chestermiller said:
Tracing out the directions of the principal stresses is what you might mean by the force lines.

All I know about force lines is what I read at wikipedia

https://en.wikipedia.org/wiki/Force_lines

But there are 3 principal stress lines and only one force line. I am trying to figure out why there is this discrepancy. The article seems to imply that the force line is just the maximum principle stress vector. But I can't be sure.

Whats so special about the maximum principle stress? If you remove the maximum principle stress then what is left?

Last edited:
em3ry said:
OK. I see what you mean. Thanks
All I know about force lines is what I read at wikipedia

https://en.wikipedia.org/wiki/Force_lines

But there are 3 principal stress lines and only one force line. I am trying to figure out why there is this discrepancy. The article seems to imply that the force line is just the maximum principle stress vector. But I can't be sure.

Whats so special about the maximum principle stress? If you remove the maximum principle stress then what is left?
What do you mean by removing the maximum principal stress?

Chestermiller said:
What do you mean by removing the maximum principal stress?

If the force lines are the maximum principle stress then what's the rest?

em3ry said:
If the force lines are the maximum principle stress then what's the rest?
Still don't understand. As best I can tell from the article, somehow they trace out the maximum principal stress directions. You do understand how the principal stresses and their directions are determined, right?

I don't know how to be any clearer

Assuming that the force lines are just the maximum principle stress then:

principle stresses = maximum principle stress vector + ?
principle stresses = force lines + ?

I am trying to understand what the force lines represent and what the components of the stress tensor represent

Are you familiar with the Cauchy stress relationship, which uses the stress tensor to determine the traction vector on an area of arbitrary orientation at a location within a body? This is key to understanding how the stress tensor works and how the principal directions and stresses are determined.

Yes

The you know that the stress tensor dotted with a unit normal to an internal area gives the traction vector on that area. This traction vector (force per unit area) has components normal to the area (normal component) and tangent to the area (shear component). There are exactly 3 directions of orientation for the area in which the shear component will be zero, and only the normal component will exist. These three directions are the three principal directions of stress at that location. The largest of these three normal tractions is called the maximum principal stress.

To get the principal stresses and principal directions, you set the dot product of the stress tensor and unit normal equal to a multiplier times the unit normal, and solve for the directions of the unit normal that satisfy this constraint.

I know the math. I am trying to understand what the math is saying. I am trying to understand the ideas.

I am trying to understand what force lines are and how they differ from the principal stresses.

em3ry said:
I know the math. I am trying to understand what the math is saying. I am trying to understand the ideas.

I am trying to understand what force lines are and how they differ from the principal stresses.
As best as I can understand from your article, the "force lines" are loci created by identifying the maximum principal stress direction at given location, extending a small segment of it to a neighboring location, and then determining the maximum principal stress at that location. In that way a continuous locus of maximum principal stress directions can be traced out.

em3ry
Whats so special about the maximum principal stress?

em3ry said:
Whats so special about the maximum principal stress?
The maximum principal stress determines the tensile failure criterion of the material and differences between the principal stresses determine the yield and shear failure criteria of the material.

You have been extremely helpful. I appreciate you taking the time to listen to me and to give me your extremely helpful feedback.

## 1. What are force lines?

Force lines, also known as lines of force or field lines, are imaginary lines that represent the direction and strength of a force field. They are used to visualize and analyze the effects of forces on objects in a given space.

## 2. What is a stress tensor field?

A stress tensor field is a mathematical representation of the distribution of stresses (forces per unit area) within a material or object. It includes both the magnitude and direction of the stresses at every point in the object.

## 3. How are force lines related to the stress tensor field?

Force lines and the stress tensor field are closely related, as force lines represent the direction and magnitude of forces, while the stress tensor field represents the distribution of stresses within an object. The direction of the force lines at a point is parallel to the direction of the stress tensor at that same point.

## 4. How does the stress tensor field affect the behavior of materials?

The stress tensor field plays a crucial role in determining the behavior of materials under different forces. It can help predict how a material will deform, break, or withstand external forces. Understanding the stress tensor field is essential in fields such as engineering, materials science, and physics.

## 5. Can force lines and stress tensor fields be used to solve real-world problems?

Yes, force lines and stress tensor fields are valuable tools for solving real-world problems in various fields. They can be used to analyze the structural integrity of buildings, design new materials, and predict the behavior of objects under different forces. They are also essential in understanding the mechanics of natural phenomena such as earthquakes and weather patterns.

• Mechanical Engineering
Replies
35
Views
4K
• Mechanical Engineering
Replies
1
Views
3K
• Mechanical Engineering
Replies
4
Views
1K
• Electromagnetism
Replies
6
Views
1K
• Atomic and Condensed Matter
Replies
3
Views
2K
Replies
3
Views
3K
• Mechanical Engineering
Replies
8
Views
1K
• Mechanics
Replies
5
Views
1K
• Special and General Relativity
Replies
21
Views
2K
• Introductory Physics Homework Help
Replies
4
Views
384