Crack Paths 2006

to be used for predicting the material strength in terms of the proposed multiaxial P Mis

the one assuring the following trivial condition:

2 0 r b a 0 r L2 U ˜ Ÿ (4) U

According to the devised method, failure

220

in a notched brittle component occurs

200

when, at a distance from the notch tip

180

160

equal to the value given by Eq. (4), the

140

maximumprincipal stress, V1, equals the

V 0 =113.9 M P a

120

inherent material strength, V0.

100

r n =0.4 m m

80

60

=0.2 m m

r n

40

M E T H O D V A L I D A T I O N B Y

20

L/2=0.054 m m

0

E X P E R I M E N TDA LT A

0.0

0.1

0.2

0.3

0.4

r [mm]

In order to check accuracy and reliability

(a)

of the devised extension of the PM, the

128020

notched cylindrical specimens of P M M A

r n =0.2 m m

were tested under pure tension, under pure

torsion and under combined tension

torsion. In particular, for the biaxial tests

16802460 rn=0.4 m m

V0=111.7 M P a

three different ratios between nominal

and torsional

stress were

tensile

considered: Vnom/Wnom equal to 1.00, 0.55

240

and 0.23, respectively.

L/2=0.2 m m

Following the procedure sketched in Fig.

(b)

0.1

0.2

0.3 0 . 4 r [ m m ]

0 0.0

1, the critical distance value, L/2, and the

inherent material strength, V0, were

Figure 6. L/2 and V0 determined both determined by means of some calibrations

under plane stress ModeI loading (a) and results generated both under Mode I

loading (by testing the V-notched flat

under ModeIII loading (b).

specimens) and under ModeIII loading.

The stress-distance curves generated testing V-notches having notch root radius, rn,

equal to 0.2mmand 0.4mm, respectively, are plotted in Fig. 6. These diagrams show

that under plane stress Mode I loading, the applied procedure resulted in a critical

distance value, L/2, equal to 0.054mmand in an inherent material strength, V0, equal to

113.9 M P a (Fig. 6a), on the contrary, under ModeIII loading L/2 was equal to 0.2mm

and V0 equal to 111.7 M P a(Fig. 6b). It is interesting to observe that the above values

were in perfect agreement with hypothesis 1) and 3): the complexity of the stress field

damaging the material process zone influenced the value of L/2, but not the value of the

inherent material strength. According to the above experimental results, constants a and