3. Stress

Talking book

The soft sweet in the photos can be easily deformed because it is elastic. With objects made of more rigid material, the deformation is less obvious. When you are trying to find out what kind of force is acting on a structure, imagine that the material was more elastic and then think about what kind of deformation would be produced.

If we pull one of our fingers, bend it backwards, or push our palms together, we can feel a tension that grows as we pull, bend or push harder.
We also notice that the feeling is different in each case because we have applied force in a different way each time.

Stress is the internal tension which all bodies experience when one or more forces are applied to them.

3.1. Types of stress

Talking book

We classify stress by the deformation it produces: traction, compression, bending, torsion and shearing or cutting.

Traction

Traction is produced when forces try to stretch the body they act on.
These forces are opposing. That is, they go in the same direction but with different trajectories – away from each other.
Objects like the rubber band of a catapult, the cables on a suspension bridge or the chord on blinds.

Compression

We see this when forces try to crush or compress a body.
These forces are opposing, with the same trajectory – inwards on the object.
The legs of a table, the columns of a bridge or a person's legs while standing all experience this kind of force.

Bending

This is when forces try to bend the body they act on.
They are not opposing, in the same trajectory direction, but separate from each other.
Examples include the deck of a bridge between the supports or the bar in a wardrobe.

Torsion

In this case, the forces try to twist the body they act on.
The forces try to turn and they act in different directions.
Torsion happens when we sharpen a pencil or turn a key to open a door.

Stress is the internal tension that appears in a structure as a result of the action of a load.
We use:

- traction to stretch a body.

- compression to crush or compress a body.

- bending to bend a body.

- torsion to twist a body.

- shearing or cutting to cut a body.

Shearing or cutting

When we apply this force, we are using forces that try to divide something.
These forces act very near each other, but not opposing – one goes up and the other goes down.
When we cut a sheet of metal or when a picture hangs on a hook on the wall, this stress is in action.

What kind of stress do the legs of a chair bear? When is the stressgreater: when you or someone heavier than you sits on a chair?

The stress is greater when the force used is . The stress which the legs of the chair undergo is one of . The legs are compressed between our weight and the floor which does not let them sink into it.

What is the best way to break a piece of uncooked spaghetti: by stretching or bending it? What kind of force is acting on the piece of spaghetti in each case?

It is easier to break the spaghetti by it. This is a stress. This is due to the shape of the spaghetti and the fact that it’s so thin and hard that it can’t withstand being bent.

Listen and decide what kind of force is applied in each case and what stress each object bears.

a. There are a lot of books on that bookshelf.
b. The horse is pulling the car.
c. The man is putting up a flag.
d. I am making orange juice with this orange.
e. I'm cutting this paper in two pieces.
f. He is getting all the water out of the t-shirt.

Dynamic. Traction
Dynamic. Traction
Static. Compression / bending
Dynamic. Cutting or shearing
Static. Torsion
Static. Torsion

Look at the photo. Analyse the stress that the bridge will bear if a heavy load is in the middle. How would the different parts of the bridge deform?

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3. Stress

3.1. Types of stress

Traction

Compression

Bending

Torsion

Shearing or cutting

Activity 9

Activity 10

Activity 11

Activity 12