Page 256 - Failure Analysis Case Studies II

Failure Analysis Case Studies /I
D.R.H. Jones (Editor)
0 200 1 Elsevier Science Ltd. All rights reserved 24 1

FATJGUE FAILURE OF HOLD-DOWN BOLTS FOR A
HYDRAULIC CYLINDER GLAND

C. TAO,* N. XI, H. YAN and Y. ZHANG
AVlC Failure Analysis Center, PO Box 81-4, Beijing, 100095, P.R. China
(Receiced 22 September 1997)

Abstract-A hydraulic-cylinder gland system used in aircrah failed by leaking because the hold-down bolts
broke in the course of a trial run. The metallographic examination of the fracture surface and the stress
calculations for the bolts are described in this article. The investigation showed that the failure was caused by
fatigue and the reason for failure was considered in relation to the processing, surface condition and assembly
of the bolts. Measures to increase the fatigue strength of the bolts are proposed. CJ 1998 Elsevier Science Ltd.
All rights reserved.

1. INTRODUCTION
A hydraulic-cylinder gland system used in aircraft failed by leaking in the course of a trial run. The
gland was fixed with one hold-down and four hold-down bolts. Three of the four bolts broke in
service. The bolts were manufactured by turning and threading from 17-4PH steel. The nominal
composition of 17-4PH is OCr-17NiltCu-4Nb and typical mechanical properties are yield strength
= 1200 MPa, tensile strength ab = 1300 MPa after solution heat treatment at 1040"C, then
water quenching and tempering for 4 h at 495°C. This paper describes an analysis of the nature and
the causes of fracture as well as preventive measures for avoiding fatigue failure of the hold-down
bolts.

2. METALLOGRAPHIC EXAMINATION
A schematic drawing of the hold-down and bolts is shown in Fig. 1. The positions of the hold-
down bolts are indicated by 1#, 2#, 3# and 4#. Each bolt head was cross drilled with two assembly
holes at right angles to one another. The 3# bolt broke away in the middle of the threaded portion.
The 1# and 2# bolts broke away in the head between the assembly holes and the shoulder transition
radius. General views of the fracture surfaces taken in the scanning electron microscope are shown
in Fig. 2.
The fracture surface (Fig. 2(a)) of the 3# bolt was characteristic of a typical fatigue fracture, i.e.
there was a crack initiation zone, a fatigue crack propagation zone and a final ductile fracture zone.
The fatigue crack initiated at one position in the thread root at a machining mark. The crack
propagated towards the far edge of the thread. The origin mne was rough (Fig. 3) and had many
radial lines. The propagation zone was smooth and there were distinct fatigue striations (Fig. 4). In
comparison with the fatigue surface, the final ductile zone was smaller and was around 20% of the
total cross sectional area. According to the above-mentioned features, the fracture surface is
characteristic of fatigue. The final ductile fracture zone was typically dimpled. No material defects
were found in the fatigue origin zone.
The fracture surfaces of the 1# and 2# bolts initiated at the edges of the assembly holes, as shown
in Fig. 2(b) and Fig. 2(c). A schematic of the fatigue fracture sites is shown in Fig. 5. The cracks
obviously propagated towards the root of the bolt until the remaining cross section became unable
to support the load and failed by fast fracture. In comparison with the macro-fracture surface of 3#

*Author to whom correspondence should be addressed.
Reprinted from Engineering Failure Analysis 5 (3), 24 1-246 (I 998)