Fractography is well-known in the field of metals, but
Fractography of plastics is a relatively new field.
In fractography we study fracture surfaces
When a material fractures it produces characteristic features on the surface
The study of these characteristic features helps to determine the root cause of failure in that material.
Fracture analysis of composites, reinforced plastics and plastics containing high filler content is very important
as it can help to develop high quality product with better service life
In today's video we will discuss common fracture modes which includes ductile, brittle and fatigue fractures.
We will also discuss why and how the plastic material fail?
What are the features that help to identify the type of fracture and also we will discuss role of
microscopy in fracture analysis by case study.
So, fracture is basically separation of a solid into two or more parts under application of load or stress
Depending on the type of load, fracture may be defined by tensile fracture,
compressive fracture, shear fracture, fatigue fracture and fracture etc.
However, these fractures are mainly characterized by either ductile or brittle fracture.
The process of fracture mainly involves phenomenon of crack initiation and crack propagation
Due to stress concentration or void formation or inclusions
Crack initiation takes place and it propagates depending on the nature of the material and amount of the stress applied
leaving some characteristic features corresponding to the particular type of failure
The ductile fracture undergo an appreciable plastic deformation before actually breaking apart
Plastic deformation is permanent deformation.
Whereas brittle fracture occurs with elastic deformation, which is non-permanent or temporary deformation
So when force or stress is applied on a material its shape or size changes due to the force.
As we can see here in the schematic.
This change is known as deformation.
This force can be tensile, compressive,
torsional or bending force.
As deformation occurs internal intermolecular force arises that oppose the applied force.
If the applied force is large enough to resist the intermolecular force the material undergo plastic
deformation and if the applied force is not large enough to overcome the
intermolecular forces then the material undergo elastic deformation
In elastic deformation when force is applied the material deform but
deformation disappears when the external force is removed and
material returns to its original shape.
It depends mainly on the chemical bonding of the material
The best example for elastic deformation comes from elastomers such as elastic rubber
When we pull the lever it stretches if we stop pulling it, it's comes back to its original size.
This shows phenomenon of elastic deformation
But when pulling force is very high it breaks it cannot go back to its original shape
This is known as plastic deformation
plastic deformation is permanent deformation or
Change in shape of it solid material with or without fracture under action of force
Plastic deformation is irreversible and permanent
It occurs due to the breakage of limited number of chemical bonds between atoms that make up that material
If we look at stress strength curve
Elastic deformation occurs at very low strain value or at very low deformation
Whereas plastic deformation occurs at relatively high strain value
therefore brittle fracture that occurs with elastic deformation is characterized by low energy process and
ductile fracture that undergoes plastic deformation is
identified as high energy process
Ductile fracture requires much more energy compared to brittle fracture
So we can say that brittle and ductile failures are two extreme behavior of a material like black and white
But we should not forget that there are many shades of grey in between.
So many polymeric condition can make a plastic material
partially ductile and partially brittle
Therefore most of the plastics so intermediate ductility
Unless they are deliberately made to the very ductile or very brittle
So ductile material undergo plastic deformation
Which requires high energy therefore material undergo necking before breaking
Whereas moderately ductile material undergo little plastic deformation and then break in brittle manner
Brittle fracture is a low energy process therefore material breaks at low stress without any plastic deformation
This gives shiny and smooth fracture surface
Brittle fracture occurs in two stages first initiation of crack and second rapid propagation of the crack
leading to complete fracture in catastrophic manner
A brittle crack often starts at a pre-existing defect such as void or inclusion
A crack can also initiate in a defect free material in a region of high stress concentration
Such as at the edge of the drilled hole or notch
Brittle fracture involves the sudden failure of material by rapid crack growth immediately after crack initiation
Therefore brittle fracture is also called as fast fracture
Fully brittle fracture involves the rupture of interatomic bonds ahead to the crack
as illustrated here in the schematic
this produces a fracture surface that is called the cleavage fracture
A cleavage crack grows between the atomic planes along a specific
crystallographic direction that has the lowest atomic bond length
Brittle fracture is the worst type of failure
Because it is fast and show no visible sign of damage or prior warning that the material will break
Brittle fracture can normally be identified by a smoothness of the failed surface
The main reason of brittle fracture is stress concentration
that is present in the structure of molded or
manufactured product
Discontinuity that develop during molding or service can also initiate brittle fracture
The discontinuities can be in the form of notch, corner, holes, laps,
folds, large inclusions, cracks, etc. All these leads to brittle failure
A stress concentration or stress raiser is a location in a material where stress is concentrated
A material is stronger when force is evenly distributed over its area.
So a reduction in area for example caused by crack results in localized increase in stress
So stress concentration is the area in a material or sample where force is not evenly
distributed and accumulates there
When force in that area become much larger than what material can withstand it starts to crack
Stress concentration is the major factor that contributes to brittle fracture
Brittle fracture occurs due to the overloading
There is another type of brittle fracture that arises due to the cyclic loading or cyclic force
This is known as fatigue fracture.
Fatigue fracture occurs due to alternating or cyclic loading
these fractures occur only after a considerable period of service.
A fatigue fracture can be easily recognized from the appearance of a smooth and polished surface that corresponds to
slow growth of crack
During the crack propagation step
characteristic pattern of concentric rings spread over the smooth reason of fractured surface
These concentric rings are known as beach marks or striation
Beach marks or iterations are lines or curved radiating outward from the point of initiation of the failure
So Brittle fracture surface mainly so flat and shiny surface, Chevron and heckle pattern
fatigue brittle fracture show striation
These features are not visible from naked eye therefore requires optical and electron microscopy
When we look at a flat surface of a pipe or a plate that broke in a brittle manner
"V" shaped Chevron making pattern can be seen
V shaped Chevron patterns are basically zigzag pattern of crack growth
Cracks usually grow in direction opposite to the orientation of Chevron marks
So Chevron marks indicates about direction of crack initiation and direction of crack propagation
Similarly these feather-like features, which is also known as hackles is also characteristic of brittle fracture
Depending on the amount of force applied to the specimen the rate of crack propagation can be more or less elevated
In case of very high propagation rate, the fracture surface pattern is more complex and
a feather like hackle pattern form
similarly formation of river pattern also indicates about the brittle fracture
Fatigue type of brittle fracture can be recognized from the appearance of three distinct features first shiny flat
initiation area
second flat dull a smooth area
surrounding the shiny region that shows a slight change in surface texture
It is a transition zone from slow to fast crack growth and third is striation
This area is directly related to the type of loading and applied stress on the specimen
Due to cyclic load beachmarks appears radiating outward from the point of initiation of the failure
Like brittle fracture ductile fracture also appears with distinct features that can be identified easily
The classic example of ductile fracture is a tensile specimen
ductile fracture provides some warning before failing
Usually the material undergo necking before breaking
The sample starts to fracture due to formation of microvoids in the neck region
These microvoids grow in size and begin to join together to produce an internal crack
The final separation of cross-section occurs by shear rupture
The fracture has a typical characteristic of cup and cone or fibrous dull appearance
After the necking down the final rupture produces shear lip, which forms the wall of the cup
These cup-like depression at microscopic levels are known as dimples
The dimples form when the microvoids torn apart during the rupture process
Under tensile fracture dimples are either round or oval depending on the ductility of the material
So ductile fracture surface mainly produces dull fibrous morphology,
dimples on the surface, cup and cone shape
One such example of cup and cone ductile fracture can be seen in case of carbon fiber reinforced epoxy composite
Where high magnification SEM images of fractured surface of carbon fiber shows
formation of cup and cone
That indicates that the composite fractured in ductile manner
Dimple fracture is another feature of ductile fracture
A fracture surface of dimple rupture exhibits tiny cup like depressions
As we can see in SEM image of high density polyethylene fractured sample
Similarly
fibrous dull appearance of tensile fractured specimen of polyethylene also indicates ductile type of fracture
Polyethylene is a ductile material and requires high energy to break the specimen
therefore SEM image of polyethylene specimen source fibrous morphology due to stretching of material and
indicating that material failed in ductile manner
So we can see that using fractographing we can identify
root cause of material failure. Once we identify it we can prevent it and
improve the quality and service life of the product
That's all for today if you have any questions or suggestions, please comment below
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