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Fracture and Fatigue

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Ductile Fracture

Fig 7.2

Fracture

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Ductile and Brittle Fractures

66

Brittle Fracture of Ceramics

• Surface of a 6-mm diameter fused silica rod.

• Characteristic fracture behavior in ceramics– Origin point

– Initial region (mirror) is flat and smooth

– After reaches critical velocity crack branches

• mist

• hackle

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Fracture of Polymers

Modes of Fracture

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Toughness and Impact Testing

Fracture Toughness

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Impact testing (Cont…)

Figure 7.10

Figure 7.11

Effect of carbon

Fatigue

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Fatigues Testing

(After H.W. Hayden, W.G. Moffatt, and J.Wulff, “The structure and Properties of Materials,” vol. III, Wiley, 1965, p.15.)

Figure 7.17

Figure 7.18

Figure 7.16

Endurance Limit modifications

Reversed Bending 1.0

Reversed Axial 0.71

Reversed Torsional 0.6

T < 450 C 1.0

450 < T < 550 C 1-5.8-3 (T – 450C)

D < 8mm 1

8 < D < 250 mm 1.189 D-0.097

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Fatigue Crack Propagation Rate

(After “Metals Handbook,” Vol 8, 9th ed., American Society of Metals, 1985, p.388.)

Fatigue Crack Growth rate Versus ΔK

)()(.

)(

ALogKLogm

KALogdN

daLog m

(After P.C. Paris et al. Stress analysis and growth of cracks, STP 513 ASTM, Philadelphia, 1972, PP. 141-176

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Fatigue Life based on Paris Equation

∆

Simple Math

1 2

Creep in Metals

Figure 7.25

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Creep Test

High temperatureor stress

Medium temperatureor stress

Low temperatureor stress

Figure 7.27

Figure 7.28

Copper Alloy

Larsen Miller Parameter

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How long will a steel connector rod last if used in service at 430 C and 180 Mpa? A test at 520 C at 180 MPa failed after 300 hours.

L.M. Diagram of several alloys

Figure 6.37

After N.R. Osborne et. al., SAMPE Quart, (4)22;26(1992)

Fig. 7.32

LM for 0.2% strain

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Viscous Creep

Creep in crystalline materials

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