Page 178 - Failure Analysis Case Studies II

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The applied torque and thrust during the reaming of the hole on which failure occurred was as
follows:

0 torque varied between 358,599 and 466,170Nm.
0 the original thrust was thought to be constant at approximately 4454 kN: however, on further
checking it was found that the actual thrust was 5033 kN, i.e. 13% above maximum.
By design, the drive torque is transmitted through the connection via splines, and the drive head
bolts are intended to carry only the applied thrust.
The bolts are 32 mm (1.25 in.) in diameter by 89 mm (3.50 in.) long SAE Grade 8 hexagonal head
cap screws having a torque specification of 1140 Nm (840 ft Ib). Prior to installation, all the bolts
are coated with an anti-seize compound.

4. SITE VISIT

A site visit was made in order to carry out an inspection of the raise boring machine, which had
been brought to the surface and had been dismantled in the company workshops. During “brcaking-
out”, it was noticed that the torque of the drive head cap screw or “centre bolt” was well below the
normal figure.
The fractured bolt sections in the locating holes had been extracted and clearly identified in
clockwise sequence from I to 32 (position 1 being at the 6 o’clock position for reference purposes).
It was not possible to extract the sections of bolts 21, 25 and 28 due to seizure in the holes. The
fractured bolt sections were subsequently “matched” to their corresponding bolt head sections by
fracture surface comparison. The original orientation of each bolt in the locating holes had been
marked on the bolt heads.
It was clear from the position of fracture of the bolt sections still situated in the body of the
machine and the positions of fracture of the other bolts that failure had occurred at or near the
joint between the cover and the body.
The underside surface of the cover, including the area containing the locating holes, showed
general rusting from the ingress of water.

5. EXAMINATION OF THE FRACTURED BOLTS
The fractured bolts were visually examined on-site, and then examined in the laboratory using a
binocular microscope after suitable cleaning. Apart from bolts 7 and 28, which had failed by 100%
tensile overload, the failure of the drive head bolts was associated with fatigue. A view of the
fracture surfaces of bolts 19 and 20, showing typical areas of fatigue, is shown in Fig. 6.
Each bolt was assessed in order to estimate the amount of fatigue crack propagation with respect
to the cross-sectional area. The results are presented in Table 1.
In order to try and understand the nature of the stressing which had produced the fatigue cracking,
the orientation of the fatigue crack origin(s) on each bolt, with respect to the original orientation
of the bolts in the locating holes, was determined. A diagram showing a plan view of the positions
of the fractured bolts and the corresponding fracture origins is shown in Fig. 7.
The general surface condition of the bolts was found to be poor, with extensive surface corrosion
and pitting corrosion in the threads (Fig. 8).

6. METALLURGICAL EXAMINATION
6.1. Chemical analysis
Three bolts were arbitrarily chosen for chemical analysis, the results of which are presented in
Table 2.