KI

KII

∆Hcrest

a

CMOD

ksi .

√in

ksi√ .

in

ft

in.

in.

6.00

0.868

0.489

0.0111

-0.0806

7.50

0.743

0.483

0.0127

-0.0847

9.00

0.581

0.492

0.0140

-0.0881

10.50

0.387

0.521

0.0150

-0.0908

12.00

0.162

0.582

0.0155

-0.0924

13.02

0.000

0.643

0.0154

-0.0925

13.50

-0.083

0.678

0.0153

-0.0924

these analyses, *K*I decreases with increasing crack

length because the gravity forces acting to close

increased would constitute an unstable fracture

crack become more dominant. Figure A-8 shows

that the value of *K*II is relatively constant for crack

geometry. For a stable fracture geometry in equilib-

rium, the loading must be increased in order for the

lengths between 6.0 and 9.0 ft, but increases with

crack to propagate and propagation will cease when

crack length beyond 9.0 ft. If KII is of sufficient

a new equilibrium configuration is reached. An

magnitude, the crack direction may change. When

unstable fracture geometry leads to a catastrophic

significant shear stress (related to KII) is combined

failure unless the loading somehow changes to

with the normal stress, the direction of maximum

create a stable fracture geometry once a crack

tension stress changes. For a competent material

begins to propagate. Because this is a stable frac-

(homogeneous and defect free), the direction of the

ture geometry, the horizontal and vertical uplift

crack will propagate perpendicular to the direction

forces acting to open the crack and the vertical

of greatest tension. For materials that are not

forces acting to close the crack reach an equilibrium

homogeneous and defect free, the crack will propa-

configuration, and crack propagation ceases. For

gate where a much lower KIc value exists or where

A-10