Transcript Day_4 - Rose

MATERIALS ENGINEERING – DAY 4
Finish Fracture including the example problem
 Discussion of Fatigue Failure

YOU NEED TO BE ABLE TO:
State what is measured in a fatigue test and list
two reasons fatigue is important to designers.
 Name two factors that increase fatigue life and
two that decrease fatigue life.
 Define the fatigue limit (endurance limit), state
which materials exhibit this limit, and describe
how a designer would use the information.

CLASSIC EXAMPLE – COMET (FIRST
COMMERCIAL JETLINER)
http://en.wikipedia.org/wiki/De_Havilland_Comet
RECENT CLASSIC EXAMPLE – ALOHA
AIRLINES FLIGHT 243
WHAT’S DRIVING FATIGUE? CYCLIC
STRESS.
The part is subject to stresses that vary
cyclically. Part of the cycle is tension.
 Cracks initiate and propagate.
 When the crack reaches the critical length, the
part fails in a way predicted by fracture
mechanics (KI>KIC).
 This failure stress can be below the yield
strength.

GROSS APPEARANCE OF FATIGUE FAILURE

Here is a conrod that failed due to fatigue. Note
the fatigued area and the fast fracture area.
HOW TO RECOGNIZE FATIGUE IN A BROKEN
PIECE

Beach marks on the fracture surface x120
Indications of loading
changes
Striations
X700 (Show indiv.
Cycles)

IMPORTANCE OF FATIGUE
Fatigue is a major killer. It is an automatic
suspect in almost any failure.
 Fatigue appears in subtle ways, eg. Thermal
cycling.
 The use of rational, accepted design procedures
against fatigue is absolutely essential. (Subject
of a later course.)

FATIGUE TESTING
We need something that does cycles. Here is the rotating beam
test. (There are other tests as well.)
We get cycles to failure (N) at a corresponding stress amplitude,
S. We plot these on a curve.
THE S-N CURVES
Curves are based on
widely scattered data!
Here is SN curve for a
typical steel. Note:
There is an endurance
limit.
To the right is SN curve
for typical aluminum
alloy. Sorry! No
endurance limit.
S-N CURVE FROM THE HOMEWORK
WHAT’S DONE WITH THE CURVES
The curves provide a starting point for rational
fatigue design. In particular they are useful
where “high-cycle” fatigue failure is possible.
 If the cyclical stress is superimposed on a mean
stress, this mean stress is also important and
should be accounted for (R ratio).
 You will be introduced to the methodology in a
later course.
 Be aware of the statistical scatter on these
curves. You CAN get failure at a stress below the
curve. There is always a probability of failure,
but there are ways of making that probability
acceptably small.

WAYS TO REDUCE DANGER OF FATIGUE
1.
2.
3.
Keep stresses below the endurance limit.
Avoid stress concentrations. They may not hurt
much in the static loading case, but are deadly
in the cyclic loading case.
Use surface treatments.
1.
2.
3.
Carburizing or Nitriding makes the steel harder
(i.e. stronger) on the surface plus it introduces
residual compressive surface stresses.
Another method: shot peening.
Another method: polishing