M ATERIALS E NGINEERING – D AY 4 Finish Fracture including the example problem Discussion of Fatigue Failure

Y OU 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.

H ISTORICAL Problem with railway axles. Contemporary drawing from Wikipedia.

C LASSIC E XAMPLE – C OMET ( FIRST COMMERCIAL J ETLINER )

R ECENT C LASSIC E XAMPLE – A LOHA A IRLINES F LIGHT 243

W HAT ’ S D RIVING F ATIGUE ? C YCLIC S TRESS. 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 (K I >K IC ). This failure stress can be below the yield strength.

G ROSS A PPEARANCE OF F ATIGUE F AILURE Here is a conrod that failed due to fatigue. Note the fatigued area and the fast fracture area.

H OW TO RECOGNIZE FATIGUE IN A BROKEN PIECE Beach marks on the fracture surface x120 Indications of loading changes Striations X700 (Show indiv. Cycles)

I MPORTANCE OF F ATIGUE 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.)

F ATIGUE T ESTING 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.

T HE 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 H OMEWORK

W HAT ’ 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.

W AYS TO R EDUCE D ANGER OF F ATIGUE 1. Keep stresses below the endurance limit. 2. Avoid stress concentrations. They may not hurt much in the static loading case, but are deadly in the cyclic loading case. 3. Use surface treatments. 1. Carburizing or Nitriding makes the steel harder (i.e. stronger) on the surface plus it introduces residual compressive surface stresses. 2. Another method: shot peening. 3. Another method: polishing