CHE 333 Class 18 Fracture of Materials. Ductile or Brittle Failure Following elastic deformation,...
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Transcript of CHE 333 Class 18 Fracture of Materials. Ductile or Brittle Failure Following elastic deformation,...
CHE 333 Class 18
Fracture of Materials
Ductile or Brittle FailureFollowing elastic deformation, two different
processes can occur – plastic deformation
leading to ductile failure or movement to
brittle fracture with little or no plasticity.
The stress strain curve is shown for ductile
failure. After the UTS the ductility is shown
by the necking of the material. This would
only be for round bar.
The stress strain curve for brittle materials
is indicated, which would be for glass and
ceramics.
Brittle FailureSvS stops here
Fracture ProcessesFor ductile materials which have plasticityonce the neck starts, a sequence of structural changes occurs as the metal proceeds to failure. Internally voids areinitiated . Both the true stress and strainare both still increasing. With furtherincrease in strain the voids becomelarger, as they become circular orellipsoidal in shape. The small ligamentsof metal between the voids eventuallytears and an internal crack is initiated.With further true stress and strain increasethe last areas to fail are those connecting the internal crack to the surface. A “shear lip”at 45o to the surface characterizes this regionleading to “cup and cone” fracture in rod.
Ligament of material Stretches between voids
Micro Failure Processes.In single phase materials, small voids appear
at the cell walls as each dislocation effectively
carries a vacancy with it. These voids then
grow with further stressing
Micro Failure Processes
s
45o
Second Phase
In two phase material, several differentmicro processes can lead to void initiationgrowth and failure. If the second phase is either large or continuous,then a dislocation pile up in one phase will leadto a void formation. This will then grow in phase. The second phase will have strengthenedthe material but will also have eventuallyinitiated the failure process.Examples include pearlitic steels and other platelet structures such as those in titanium alloys Void from dislocation
pile up on slip plane
Micro Failure Example
Example of toughened material?
Micro Failure ProcessesWhen the second phase is small and
non discontinuous, other processes in
addition to dislocation pile up and a void
on the slip plane are possible. The controlling
factors are the strength of the second phase
particle and interface between the second
phase and the matrix material.
If the second phase is weaker or more
brittle than the matrix, then the second phase
particle may crack.
Age hardened aluminum alloys behave in
this manner
Micro Failure Example
Al 7079 Age Hardened alloy showing particle failure.
Micro Failure Examples
Macro View of Failure
Fracture Surface Features
Ductile failure in a titanium alloy which is two phase showing dimples from void formation
Final Failure in Shear Lip
Shear dimples in area of shear lip.
Brittle Failure Fracture Surface
A faceted type of fracture surface is often seen with a brittle failure as cracks are on a singlecrystal plane, no shear lip would be found..
Fracture Mechanics.Originally to explain why glass has low toughness where toughness is the ability to resist crack
growth in a material. Simple energy balance used – energy applied is the stress and it is used up by
creating fresh surface. Works well for brittle materials where no plastic deformation. For ductile
materials energy absorbed is complicated by plastic deformation. Fracture mechanics relates the size
of a defect in the material to the stress is can withstand before failure.
Kc = Ysc (pa)0.5
I II IIIMode
Plane Strain Fracture Toughness
Ductile to Brittle TransitionIn some materials, mainly steels, ductility can decrease very sharply with temperature, so a ductile
materials becomes brittle – know as the ductile brittle transition. The standard test is to use an impact
tester – a pendulum type hammer and the energy absorbed in failure is measured by how far the
hammer swings through after impact – the further the less energy.
Carbon content effect on DBTT for SteelsShear ratio to impact energy
Homework
1. For a material with a single crystal yield strength of 300 MPa, calculate the yield strength for a grain size of 50 micron and for a grain size of 2.5 microns. Assume k =1
2. Decide the crack initiation process in a tensile test for the following conditions:-
Matrix shear strength Matrix Normal Strength Interface Strength Particle Strength
a 25 60 35 55
b 55 50 60 60
c 55 60 35 45
d 45 55 45 35
Data units MPa.