Fatigue Crack Paths 2003

fully reversed torsion tests

out on V-shaped

carried

a)

φ 12.5

φ 20

specimens, all characterised by a

R 40

notch tip radius equal to 0.5

60

80

60

mm.

90

Fatigue crack nucleation and

propagation phases are analysed

b)

φ 20

in detail by means of optical and

electronic microscopy. Despite

R 0.5

p = 0.5, 2, 4

R 0.5

the differences due to notch

shape and load level, all fatigue

c)

φ 28

φ 20

p = 4

2800

strength data are summarised in

terms of the Notch Stress

60

60

Figure 1. Geometry of the specimens.

Intensity Factor (N-SIF). Such

factors are calculated on the

basis of the stress distribution

related

to the uncracked

geometry, the notch being modelled simply as a re-entrant corner. N-SIF includes the

influence of notch dept and the size effect. Despite the complexity of the fatigue

phenomena, N-SIF are seen able to summarise the fatigue strength data in a single band

with a very limited scatter. This makes the N-SIF a useful and powerful tool in fatigue

life assessments in the presence of small value of the notch tip radius.

M A T E R I A LN DG E O M E TORFYS P E C I M E N S

The geometry of smooth and notched specimens is shown in Fig. 1. With reference to V

shaped notches (Fig. 1b), the V-notch depth ranges from 0.5 to 4 mm.The height of the

shoulder (Fig. 1c) is 4.0 mm. In all notched specimens the notch tip radius was kept

constant and equal to 0.5 mm. Such a value makes it difficult any correlation between

the theoretical stress concentration factor Kt and the fatigue trength reduction f, since he no ch s sitivi y index

exhibits a large scatter. Finite element analyses car i d out by using the

A N S Y S ®code gave a theoretical stress concentration factor (referr d to the net

area) equal to 1.94, 2.03 and 1.73 for the

symmetric V-notches with a depth equal to

4.0 mm, 2.0 m m and 0.50 mm, respectively. On the other hand, the

geometry with shoulders is characterised by a K net= 1.82.

Figure 2. Microstructure of the C40 steel

(normalised state).