YEAR
materials deform elastically， then plastically， and are time independent.However， at higher temperatures， deformation is noted under constant loadconditions. This high-temperature， time-dependent behavior is called creep-rupture. Figure 11-3 shows a schematic of a creep curve with the various stages of creep. The initial or elastic strain is the first region that proceeds into a plastic strain region at a decreasing rate. Then a nominally constant plastic strain rate is followed by an increasing strain rate to fracture.
The nature of this creep depends on the material，stress， temperature， and environment. Limited creep (less than 1%) is desired for turbine blade application. Cast superalloys fail with only a minimum elongation. These alloys fail in brittle fracture-even at elevated operating temperatures.
Stress-rupture data are often presented in a Larson-Millercurve， which indicates the performance of an alloy in a complete and compact graphical style.认hile widely used to describe an alloy's stress-rupture characteristics over a widetemperature，life， and stressrange， it is also useful in comparing the elevated temperature capabilities of many alloys. The Larson-Miller parameter is
PLM二T(20 +log t)x10-3 (11-1)
where:
PLM二Larson-Miller parameter
T二temperature， oR
t二rupturetime， hr
Figure 11-.. Time dependent strain curve under constant load
The Larson-Miller parameters are plotted in Figure 11-4 for the specified turbine blade alloys. A comparison of A-286 and Udimet 700 alloy curves reveals the difference in capabilities. The operational life (hrs) of the alloys can be compared for similar stress and temperature conditions.
.uctilityand Fracture
Ductility is commonly measured by elongation and reduction in area. Inmanycases， all three stages of creep shown in Figure 11-3 are not present.At high temperatures or stresses， very little primary creep isseen， while inthe case of cast superalloys， failure occurs with just a small extension. This amount of extension is ductility. In a time-creep curve there are two elonga-tions of interest. One elongation is from the plastic strainrate， and the second elongation is the total elongation or the elongation at fracture.
Stress60 40
10
Larson -Miller Parameter PLM =T(20+logt) ×10 –3
Ductility is erratic in its behavior and is not always repeatable-even under laboratory conditions. Ductility of ametal is affected by the grainsize，the specimenshape， and the techniques used for manufacturing. A fracturethat results from elongation can be of two types: brittle orductile， depending on the alloy. A brittle fracture is intergranular with little or no elongation. A ductile fracture is trangranular and typical of normal ductile tensile fracture. Turbine blade alloys tend to indicate low ductility at operating temperatures.As aresult， surface notches are initiated by erosion or corrosion， and then cracks are propagated rapidly.
Cyclic Fatigue
All materials would fail at a certain load if cycled over a large amount ofcycles. A very common type offailure， which blades in turbines undergo is known as ""high cycle fatigue.'' This type of failure is caused when the blade is subjected repeatedly to an unsteady load. Most materials under these altern-ating loads would fail in about 107cycles， assuming that the resonance frequency for a given blade is 103 Hz. This would tend to mean that the material would fail within 104seconds， about 2.8hours， if the blade wassubjected to an alternatingforce， which would excite the blade resonance frequency. This type of failure would be depicted by a chevrontype of markings on the failedsurface， near the trailing edge of the blade.
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