Moving down in stiffness a little bit more,
we see composite materials, which will be of interest later in this section.
We now revisit the implications of posture on stress in a simplified bone model.
In this figure,
we see how changes in posture influence loading in bones in a simplified model.
For a more upright posture, there is nearly pure compressive loading.
This results in a uniform stress distribution,
which in our example does not exceed the ultimate strength of the material.
In the more sprawled animal, we see a combination of compression and bending.
The principle of superposition let's us simply add the stress profiles to see
the resulting profile on the right.
This resulting profile has peak stress above the ultimate strength
of the material.
And so in this case, the bone in the sprawled animal would break.
As mentioned, all materials up to this point have been isotropic.
Which means when loaded in different conditions, they respond the same way.
We now consider anisotropic materials such as composites,
which have very different responses depending on loading conditions.
In this video, we see an example of an anisotropic material that
has a grain similar to something like wood.
When the material is loaded in the direction of the grain,
it is very compliant.
When it is rotated 90 degrees, however and loaded against the grain,
it becomes very rigid.
In this video,
we see the implications of combining two of these samples in different ways.
When the two samples are combines with that the grains are parallel, once again,
the combined samples are very compliant when loaded in the direction of the grain.
When the samples are combined, so that the grains are perpendicular.
We now have a more isotropic material, which is stiff in both directions.
