Page 260 - Failure Analysis Case Studies II

245

I
I i
t, @ 4.134
iiiii

0th
Fig. 8. Stress acting on the fracture surface for I# and 2# bolts.

3.1. Stress (ath) acting on the thread root
First, the effect of stress concentration was not considered. The stress acting on the minimum
section of the bolt, i.e. the minor diameter, d, = 4.134 mm, was the maximum. The oil pressure
transmitted by the hold-down platform (d2 = 44.17 mm) was borne by four bolts. The maximum
oil pressure P,,, was 3.42 MPa. Take the static estimation as follows:

d:
44.172
d: 44.172
Pn- P71-
Pn-
P71-
4 4 4 4
28.5P,
-
-
Oth = 2 - 28.5P,
2
-
=
-
-
-
-
Oth
714.134'
714.134'
dl
dl
~
~
4K-
4K-
4 4
giving
ath.max 28.5Pm,, = 97.5 MPa. (2)
=
The fatigue strength of the alloy is much greater than and the bolts should not fail from
the thread root if there is a good surface finish, low stress concentration and absence of tensile
residual stress.
The result of tensile tests for the same group of bolts showed that ab was around 1260 MPa. This
showed that the maximum stress was about 1/13 of tensile strength of material even in the minimum
section.

Fig. 9. Comparison of fracture surfaces for (a) tensile fracture, and (b) 3# bolt fatigue fracture.