Understanding Material Strength, Ductility and Toughness

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 it 0.2% to the
right on the stress-strain curve. 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, and so compression tests aren’t 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 GPa 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 to 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 will suited for applications where plastic deformation is to be avoided. T o 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. If you enjoyed the video please remember
to subscribe!

11 Replies to “Understanding Material Strength, Ductility and Toughness”

  1. I am so hyped, that someone as skilled as you is finally tackling engineering courses.
    Your channel will escalate (hopefully very soon) if you can keep this up 🙂

  2. Great overview of these important material properties. Simple and understandable animations along with a great verbal description.

  3. Great video! The only thing I would do. THE ONLY THING is drop the music. It's too reminiscent of instagram pizza popsicle recipe videos.


  5. a big thanks for explaining the concepts with animation. Keep making engineering videos which will help us for sure and im sure that u will get subscribers soon 🙂

  6. If YOU study MECHANICS;

    PARTS do "
    "wear and tear"

    And I also studied
    "PRESSURE plates" per

    And◇ " index springs◇ in arrays;

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