WHY DID IT BREAK?
The Use of Microscopy in
Failure Analysis
David Taylor
Professor of Materials Engineering
Mechanical Engineering Dept.
Trinity College Dublin
How Things Break
Mechanism
Appearance
1) Ductile Fracture
Permanent deflection
and failure
2) Brittle Fracture
Mechanical
Property
Yield Strength (σy)
Cracking and
sudden failure
Toughness (KIC)
Ductile Failure
Bolts failing by ductile fracture (left) and brittle fracture (right)
SEM – Appearance of Ductile Failure
• Typical of good-quality metal
alloys (high energy failure)
• Dimples (craters) on fracture
surface.
Steel fracture surface
Craters on the moon
Brittle Fracture
aluminium ladder
ORIGIN OF BRITTLE FRACTURE
Brittle fractures start from defects or stressconcentrations in the material:
- Pre-existing cracks
- Manufacturing defects
- Notches and other stress concentrations
Brittle Fracture
Sometime you see marks (Chevron Marks) leading to
the origin, where there is a defect
(…but beware: this defect was
not the problem!)
Another example of brittle fracture starting
from a defect (internal pore)
• A good “rule of thumb”
is that, in most cases, if
the defect is dangerous,
then it will be big
enough to see by eye
(i.e. >1mm).
• There are some
exceptions to this rule.
Surface Roughness is a good indicator of
the amount of energy needed for fracture
Flat
surface
(low
energy)
Rough
surface
(high
energy)
SEM – Appearance of Brittle Fracture
• Confusing, because
sometimes cracks
can grow by the
same mechanism of
ductile fracture,
giving the same
dimpled surface…..
(I prefer to call this
“tearing”)
…but cracks can also grow by a brittle
mechanism: “cleavage”
• Fracture surface (brittle
fracture, steel)
• Transgranular cleavage
(through the grains).
• It also happens in brittle polymers (e.g. epoxy)
SEM – cleavage fractures,
intergranular
• Brittle fracture (inter- or
trans-granular) is often an
indication that something is
wrong…
Mechanisms of Failure - SLOW
Mechanism
Appearance
Mechanical
Property
3) Fatigue
Crack initiation
Fatigue limit (∆σo) and
and propagation
Fatigue crack
propagation threshold
(∆Kth)
Crack initiation
Limiting stress and
and propagation
Stress intensity
threshold KSCC
4) Stress-Corrosion
Mechanisms of Failure - SLOW
Mechanism
Appearance
Mechanical
Property
5) Creep
Gradual plastic
Strain rate,
deformation, internal
limiting stress,
damage
crack propagation
rates
Loss of material
Wear rate,
from surface
Surface hardness
6) Wear
FATIGUE
Fatigue failure occurs over a period of time, due to
cycles of stress.
Fatigue is the
most common
reason for
mechanical
failure
stress
time
FATIGUE CRACKS
Example: 737 aircraft landing gear.
Fast Fracture (brittle)
Beach Marks (clam
shell marks)
Fatigue (corroded)
Fatigue Cracks – more complicated
• Artificial hip joint
(Co-Cr alloy)
SEM - fatigue
Typical appearance of
fatigue fracture
surface.
Striations
STRESS-CORROSION FAILURE
Also called Stress Corrosion Cracking (SCC), this is
another type of failure that occurs by gradual crack
growth, like fatigue.
In this case you don’t need a cyclic stress; you need a
stress plus a corrosive atmosphere.
Examples: Aluminium alloys + NaCl solution
Copper alloys + ammonia
SCC shows much the same appearance as fatigue,
except at very high magnification (SEM – see below).
STRESS-CORROSION FAILURE
SCC is difficult to
diagnose.
Sometimes you see
corrosion on the
fracture surface and
not elsewhere
Sometimes you see
multiple cracking
and crackbranching.
SCC in Brass
Sometimes you see intergranular or
transgranular cleavage
CREEP
Creep is a gradual increase in plastic strain over time
which occurs at relatively high temperatures, usually
more than 0.3Tm (Tm is the melting point on the
absolute temperature scale (degrees Kelvin)).
Creep failures look like ductile failures (which is what
they are).
During creep, damage occurs in the material in the form
of small cracks and holes which form inside the
material, usually on the grain boundaries. These can
usually only be seen at high magnification (see SEM
below).
SEM - Creep
Voids on grain boundary
Cracks on grain boundary
Used to monitor creep in aging power plant, etc
WEAR
Wear occurs when two surfaces rub together (e.g. in
bearings and gears). Material is lost from one or both
surfaces.
This is usually obvious when looking at the surface.
Wear can also occur on fracture surfaces caused by
fatigue etc if they rub together.
“Fretting Fatigue” is a combination of wear and fatigue;
wear creates cracks near the surface which may then
grow into the rest of the component due to cyclic
stress.
Example: wear and fretting fatigue in an
artificial knee joint
Wear patch
Fatigue started
from here
Wear Debris
Failed O-ring
EDS in the SEM showed particles rich in Fe,Al, Si and
O with smaller amounts of other elements including Ti
and Zn. Typical wear debris.
Damage on O-ring due to
wear particle?
BASIC RESEARCH: Locust Wing
In-situ testing under the microscope
Basic Research
–
Damage and
Repair in Human
Bone
Microscopic
cracks form and
grow, and are
repaired
Bone cells near a crack
(LSCM, antibody stain)
In-situ SEM: fatigue tests on cell processes
Crack
opening