The Young’s modulus – strength materials-selection map is used in conjunction with merit indices relating to elastic deformation and elastic energy storage. For simple tensile loading, to assess differences in maximum recoverable elastic deformation, the merit index 
is used. To compare maximum elastic strain energy per unit volume, the merit index is 
used, and for the maximum elastic strain energy per unit mass, the merit index 
is used.
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To derive the merit index 
, the cross-sectional shape of the tie in tension of the structure need not be considered, and engineering parameters are not involved in the derivation, making this a simple merit index to derive as follows:
From the definition of Young’s modulus, while the material is behaving elastically:
Therefore the maximum elastic strain (i.e. the deformation at the yield point) depends simply on the maximum elastic stress (i.e. stress before failure) which is the strength σf :
So to find the material with the greatest maximum recoverable deformation,
the merit index 
is maximised using the materials-selection chart shown. This involves moving
the merit index line towards the bottom and the right of the map. From doing
this it can be seen that cartilage, viscid silk, resilin
and skin have good values of this merit index.
Cartilage is found on the end of bone in joints and so it is important that
it is flexible while not being permanently deformed or breaking when the joints
bend. The same argument applies to skin, it would be rather gruesome if our
skin split open every time we bent our elbow! The merit index
is also used to find the best material for elastic hinges. This explains resilin’s
high value of the merit index as it is found in insect wing hinges. The insect
only has to pull the wing back and it will then be pulled forward by the elasticity
of the hinge (with resilin, this is particularly efficient as it has a very
low absorption of energy on elastic bending to and fro - i.e. resilin has a
high coefficient of restitution or resilience). This is discussed
further in the Elasticity
in Biological Materials teaching and learning package.
Spider's web
Viscid silk has a high value of this merit index, being able to stretch up
to three times its own length. Viscid silk makes up capture threads in a spider’s
web: it must entangle flies giving an advantage in a high 
value. Viscid silk also needs to absorb the kinetic energy of the fly, corresponding
to a high 
value. This is discussed further in the Elasticity
in Biological Materials teaching and learning package, where it is noted
that viscid silk has (in contrast to resilin) a low coefficient of restitution
- this reduces the elastic energy returned to the fly.
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