5
Structures
108
3.
Stress
To begin this section we can ask students a question about the
effects that forces produce inside objects. By doing this, we can
help them see that stress is related to what happens inside objects.
Their answers will reveal how close they are to the concept which
we are going to study.
The sidebar information box
Forces and deforming
can be used a
way of introducing the different types of stress. Simply draw on
the same bar the different combinations of forces (using arrows)
which correspond to a variety of stresses.
Another important point is to relate stress, not load, with the
breaking of a material or object. It is the stresses which deform
and break objects. Therefore, what concerns us is not so much the
force which acts on those objects but the effect on their insides.
Even though the quantification of stress is not a compulsory part
of this course, we can ask simple questions about the effects of
varying the section or load on a body and try to elicit spontaneous
answers from the students, for example: «The legs of this table
bear a certain stress. What would happen if we reduced the
length of the legs by half?»
3.1.
Types of stress
Before beginning to explain the different types of stress we
suggest getting students to look at some pictures where we have
applied different forces to a marshmallow. This will help explain
the deformation this object underwent and we can also ask them
to match each picture to the type of stress they believe is involved.
Once students have realised that there are different types of stress,
the next step is to name and explain them.
❚
Traction:
To test whether an object undergoes traction, it is
useful to get students to hold objects in their hands and to
apply forces to them in order to try to stretch them. In so doing,
the objects will undergo traction.
❚
Compression:
This stress occurs when forces are involved in
squashing or compressing an object. You can ask students to
identify objects in the classroom which are compressed.
❚
Bending:
This stress occurs when forces are involved in bending
an object. Ask students to identify objects in the classroom
which are doing this.
❚
Torsion:
This stress occurs when forces are involved in twisting
an object.
❚
Cutting or shearing:
This stress occurs when forces are
involved in cutting an object.
Each type of stress can be explained by demonstrating its effect by
pretending to deform a rigid bar and by providing real examples.
In general, types of stress are understood intuitively by students. It
helps a lot to identify them by using a gesture and a verb (stretch,
squash, twist, etc).
For each of the stresses presented, it is advisable to get the
students to carry out two types of mental operation. The first
one is to imagine the movement or situation required to recreate
the examples given. The second one, which can be set up as an
activity, is to identify the opposing forces which are involved in
each type of stress, locating them in specific situations.
104
105
5. Structures
5
+
www
3.
STRESS
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
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.
Understand
9.
What kind of stress do the legs of a chair support? When is the stress
greater: when you or someone heavier than you sits on a chair? Why?
10.
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?
11.
Listen and decide what kind of force is applied in each case and
what stress each object experiences.
Analyse
12.
Look at the photo. Analyse
the stress that the bridge will
support if a heavy load is in
themiddle. Illustratehow the
different parts of the bridge
would deform.
Bending
❚
This is when forces try to
bend
9
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
10
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.
Shearing or cutting
❚
When we apply this force, we are
using forces that try to
divide
11
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
12
on the wall, this stress is in action.
FORCES
AND DEFORMING
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.
❚
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.
Key concepts
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.
9
bend
:make something intoa curveor
angle
10
twist
:
turn the ends of something in
differentdirections
11
divide
:
to separate something
12
hook
:
apieceofmetalbent so that it
holds something
