Most polymers fail into manners either in ductile or brittle manner.
There are many factors which influence if a polymer will fail in ductile or brittle manner.
In this video I am going to talk about these two failure modes and
main cause of leading these failures.
A good understanding of these failure mechanism is important for material selection and also for product design.
So failure is basically separation of a body into two or more parts or permanent distortion
under application of load or stress. As I already mentioned
there are two types of failure mechanism ductile and brittle failure.
The process of failure mainly involves phenomenon of crack initiation and crack propagation.
Due to a stress concentration or void formation or other factors
crack initiation takes place and it propagates depending on the nature of material and
amount of the stress applied.
In ductile failure, a crack moves slowly and is accompanied by an
appreciable plastic deformation before actually breaking apart.
Plastic deformation is irreversible or permanent deformation.
Whereas brittle failure occurs with elastic deformation where crack propagation occurs very fast.
This type of deformation is irreversible and temporary.
Now the question is what is deformation and how what do we mean by plastic and elastic deformation.
As we know when force or stress is applied on a plastic material
It's shape changes. This change in shape is known as deformation.
During deformation
the intermolecular forces of the material oppose the applied force.
If the intermolecular force is large enough to resist applied force the sample returns to its original state
when load is removed.
This is known as elastic deformation.
If the intermolecular force is not sufficient enough to overcome the applied force
then the sample undergo permanent deformation.
It cannot go back to its original shape even after the force is removed.
This type of deformation is known as plastic deformation.
At microscopic level elastic deformation occurs in the form of bond stretching and
rotation, which is temporary.
The best example for elastic deformation comes from elastomers such as elastic rubber.
When we pull the elastic rubber band on the ends it stretches
If we stop calling it, it comes back to its original shape.
This shows phenomenon of elastic deformation.
But when pulling force is very high and it breaks the material shows plastic deformation.
Plastic deformation is permanent and irreversible deformation.
It occurs due to breakage of limited number of chemical bonds between atoms that make up the material.
If we look at a stress-strain curve of a polymeric material
We can see that amount of stress required to deform a polymeric material changes with extent of deformation
As elastic deformation occurs at very low strain value which is less than 5 percent and
plastic deformation occurs at relatively high strain value.
In elastic reason a stress increases linearly with strain which indicates materials resistance to deformation
At certain point which is known as yield point
materials resistance to deformation
Decreases as a result material can be easily deformed.
In this phase
we can see that strain increases without increase in stress indicating stretching or elongation in material.
The yield point is the point that indicates the limit of elastic behavior and beginning of plastic behavior.
So those plastic materials that fail at very low strain are known as brittle material and
Those who requires high energy to break or fail at high strain are known as ductile material
The mechanism by which brittle and ductile material fail is known as brittle and ductile failure.
The main cause of brittle failure is stress concentration that is present in the structure of molded or
manufactured product.
Discontinuities that develop during molding or service life in shape crack formation
The discontinuities can be in the form of notch, corners, holes, splints, laps,
folds, large inclusions, cracks
Crystallographic planes, etc
Are stress concentration which is also known as stress raiser is a
location in an object where stress is concentrated.
An object is the stronger forces evenly distributed over its area.
So a reduction in area for example
caused by crack or wired results in localized increase in stress. So a stress concentration is
an area in a material or a sample where force is not evenly distributed and
force accumulates there
when force in
That area becomes larger than what the material is supposed to be stand or designed to withstand
it starts to crack from there.
As we can see here in case of a sample having a void the line of force are not straight
near the void.
The line of force has high density
near the void compared to list of the area.
Therefore material starts to crack from here
Brittle failure is worse type of failure because it involves the sudden failure of the material by rapid crack growth
Immediately after the crack initiation this type of failure is also known as fast failure.
A classic example of ductile failure is tensile specimen
Ductile failure provides some warning before failing. The material deform elongates
and decrease in cross-section before failure.
Elongation and decreasing cross-section refers to necking.
That means usually the material undergo necking before breaking.
Whereas brittle failure is quick and no prior warning.
The sample starts to fracture due to formation of microvoids in the neck region.
These micro voids grow in size and begin to join together or
coalesce to produce in internal crack.
The final separation of the cross-section occurs by shear rupture.
In ductile failure, there is absorption of massive amount of energy
therefore crack propagation occurs very slowly before the fracture
That is why
Plastic deformation occurs at a very high strain value because material undergo large deformation before failing.
So we learn about two extreme behaviors of a material which are brittle and ductile failures.
But many of the polymeric materials fails in ductile to brittle manner.
That means many polymeric conditions can make a plastic partially ductile and partially brittle.
Therefore most of the plastics so intermediate ductility
unless they are deliberately made to be very ductile or very brittle this ductile to brittle transition depends on
microstructure, molecular structure, time, temperature and percentage strain.
In summary we can say that
Brittle failure undergo with appreciable elastic deformation and ductile failure occurs with plastic deformation
elastic deformation is
Reversible and temporary whereas plastic deformation is permanent and a reversible.
Brittle failure occurs at low strain and ductile failure occurs at high strain.
Stress concentration is the main cause of brittle failure whereas ductile failure occurs due to void formation/
Ductile failure gives prior warning before failing whereas brittle failure is sudden and fast.
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