Crack Paths 2012

860°

Maximumshear

stress criterion

40°

o n a ng l e

20°

40°

80°

60°

Modemixity )

t i

e c

D 20° e f l -20°

Experiments

-40°

-60°

Maximumtensile

-80°

stress criterion

Figure 2. Crack deflection angles over the range of mode-mixity for constant torsion

with superimposed cyclic tension after Highsmith [10].

Highsmith concludes there is no single formulation at hand to predict crack direction

for all cases. A best-practise approach is proposed which is based on the stress intensity

factors at an infinitesimally small kink crack’s tip, k1 and k2.

cos

(2)

2 -

3 2 2 -

-

k

§ ¨ ©

K

cos

K

sin

· ¸ ¹

1

I

2

II

2 1cos s i n 3 c o s 1 k 2 K2 K - - - I II

(3)

- is the kink angle. Since crack deflection angles fall between

In the Eqs. (2) and (3),

the angles of maximumkink tip stress intensity factor, ki,max, and the maximumkink tip

stress intensity factor range, 'ki, a crack driving force combining the influence of both

was suggested:

k

(4)

i k k ' ' ˜

1,max w w i i

A fitting parameter, w, appears in Eq. (4) which is allowed to take different values for

the two cases, i = 1 (tensile stress dominated), and i = 2 (shear stress dominated). A

transition criterion similar to what is shown in Fig. 1 completes the approach.

Highsmith formulated his concept against the background of his overview on

published results. The reference to two more summarypapers by Liu [14] and Bold [15]

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