in this video we're going to take a
closer look at strength ductility and
toughness three very important material
properties let's get started
strength is a measure of the stress a
material can withstand two different
values are typically used to define the
strength of a material the ultimate
strength and the yield strength let's
define them by taking a look at the
stress-strain curve obtained from a
tensile test the ultimate tensile
strength of material is defined as the
maximum stress which is reached during
the tensile test corresponding to this
point on the stress-strain curve it is
the maximum stress the material can
withstand during a tensile test the
yield strength is defined as the stress
at which a material begins to deform
plastically stressing a material beyond
its yield strength will result in
permanent deformation after the load is
removed many structures and components
are designed to ensure that they only
deform elastically
this makes the yield strength a commonly
used criterion for defining failure in
engineering design codes it is possible
for failure to occur at stress levels
well below the yield strength if the
applied load varies with time this
failure mode is called fatigue and I
will cover it in a separate video for
some materials the transition from
elastic to plastic deformation is not
clearly defined in these cases the yield
strength can be defined using the 0.2%
offset method this involves drawing a
line with the slope equal to Young's
modulus and shifting at 0.2% to the
right on the stress-strain curve
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so far we have only discussed strengths
in the tensile direction but what about
the compressive direction for ductile
materials like mild steel the yield and
ultimate strengths are very similar in
the tensile and the compressive
directions tensile tests are usually
easier to carry out than compression
tests in so compression tests are often
performed for these materials for
brittle materials like concrete and
ceramics however the material strength
is much larger in compression than in
tension I will talk about why this is
the case later on in the video first
let's look at some examples of strength
values for a few different materials
tensile yield strengths are shown for
ductile materials and ultimate
compressive strengths are shown for
brittle materials graphene is the
strongest material to have ever been
tested it is a two dimensional material
made up of a single layer of carbon
atoms arranged in a hexagonal lattice
and linked by strong covalent bonds
defect free graphene has an ultimate
tensile strength of up to 130 Giga
Pascal's or 19,000 ksi I've mentioned
material ductility a few times in this
video so now is probably a good time to
define it properly ductility is a
measure of the ability of a material to
deform plastically before fracture
let's return to our stress-strain curve
materials which undergo large amounts of
plastic deformation before they break
are called ductile materials these
materials fracture at very large strains
mild steel and gold are examples of very
ductile materials materials which
fracture at very low strains with little
to no plastic deformation are said to be
brittle glass and ceramics like
porcelain are examples of brittle
materials because they don't deform
plastically the concept of yield
strength is irrelevant for brittle
materials there is no defined transition
point between ductile and brittle
behavior typically a material which has
a strain at fracture of less than 5% is
considered to be brittle now that we
have covered what ductile and brittle
materials are let's return to the
question of why brittle materials are
much stronger in compression than in
tension
there are two factors at play the first
factor is that tensile loads tend to
encourage the formation and propagation
of cracks
whereas compressive loads do not as
illustrated here the second factor is
that in brittle materials very little or
no plastic deformation occurs to
redistribute stresses at existing flaws
this means that large stress
concentrations build up at the crack-tip
resulting in fracture
in ductile materials plastic deformation
occurs which relieves these localized
stresses
ductility can be dependent on
temperature a lot of different types of
steel for example are ductile at room
temperature but become brittle when the
temperature drops to below the ductile
to brittle transition temperature this
transition temperature is an important
design consideration because ductile
failure is normally preferred to brittle
failure one very famous example of
brittle failure is the Titanic the icy
waters of the North Atlantic are thought
to have caused the steel of the ship's
hull to drop below its ductile to
brittle transition temperature resulting
in catastrophic brittle fracture
toughness is the ability of a material
to absorb energy up the fracture it can
be calculated as the area under the
stress-strain curve if the area under
the stress-strain curve is large the
material will have high toughness and so
will be able to absorb a large amount of
energy before fracturing for a material
to have high toughness is should have a
good balance of both ductility and
strength low strength materials and
brittle materials tend to have low
toughness a related property is
resilience which is the ability of a
material to absorb energy when deforming
elastically it corresponds to the area
under the stress-strain curve but only
within the elastic region materials with
high resilience are well suited for
applications where plastic deformation
is to be avoided do summarize the yield
strength of a material defines the
stress at which it begins to deform
plastically the ultimate tensile
strength defines the maximum stress
which is reached during the tensile test
ductility is a measure of the ability of
a material to deform plastically
toughness is a measure of the ability of
a material to absorb energy up until
fracture and resilience is a measure of
the ability of a material to absorb
energy while deforming elastically
that's it for now
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