Ductility is erratic in its behavior and is not always repeatableÐeven under

laboratory conditions. Ductility of a metal is affected by the grain size,

the specimen shape, and the techniques used for manufacturing. A fracture

that results from elongation can be of two types: brittle or ductile, 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 a result, 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 of

cycles. A very common type of failure, 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 107 cycles, assuming that the resonance

frequency for a given blade is 103 Hz. This would tend to mean that the

material would fail within 104 seconds, about 2.8 hours, if the blade was

subjected to an alternating force, which would excite the blade resonance

frequency. This type of failure would be depicted by a chevron

type of markings on the failed surface, near the trailing edge of the blade.

4.0

2.0

1.0

10500 600 700

1000 1200 1600

504846444240

Temp,100,000Hrs

Life

40

60

U-500

FSX-414

GTD-222IN-738

GTD-111

KS

I

Kg

/cm

10

3×

–3

Nozzles

Larson -MillerParameterP T (20+ log t)LM = 10

–3×

N-155

Blades

°F

°C

Stress

1400

800

3.0

Figure 11-4. Larson-Miller parameter for various types of blades.

416 Gas Turbine Engineering Handbook