ESDU 10011:2010

$126.75

Endurance of Titanium and Titanium Alloy Structural Elements Subjected to Simulated Acoustic Loading

Published By	Publication Date	Number of Pages
ESDU	2010-03	42

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Category: ESDU

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Description

INTRODUCTION

This Item gives the results of fatigue tests on titanium and
titanium alloy specimens, excited by narrow-band loading of random
amplitude with zero mean load in order to simulate the stress
response to acoustic loading. The specimens, also called coupons,
are small structural elements which are typical of those used in
aircraft and space structures. The coupons are either simple
cantilevers or built-up T-pieces. The specimens were vibrated until
failure occurred. For a narrow band Gaussian process the number of
upward zero crossings per unit time is nearly equal to the centre
frequency of the process, thus the number of cycles to failure, or
the equivalent endurance, is equal to the product of the centre
frequency (in Hz) and the time to failure (in s) (Reference 17).
The equation relating S_rms , the
root mean-squared value of alternating stress, to N, the
number of cycles to failure, is assumed to have a form similar to
the Basquin equation.

where is the fatigue strength exponent or the Basquin exponent
and is a constant. The aim of the tests was to produce plots of
stress against number of cycles to failure in a logarithmic
coordinate system, and to find a line which is best fitted to the
data. The common logarithm of the stress is plotted against the
common logarithm of the endurance and a straight line is fitted to
data by the method of least squares. The slope of the line is a.
The constant k=S_rms(1) refers to
the value of S_rmsat 1 cycle. Although the S
-Nline is obtained from coupon tests, the results could be used to
predict the life of more realistic stiffened panel structures.
Extrapolation of the lines beyond 10⁸ cycles, where no
data is available, is expected to lead to an underestimate of
endurance. The Item was originally published as ESDU 73010 (see
also Reference 6).

Failure is characterised by a decrease in the resonance
frequency of the coupon and an increase in damping. Cracks are
sometimes visible at failure. The stress is measured by a reference
strain gauge or transducer, located approximately at a position
where a crack is expected.

The materials used are commercially pure titanium, a titanium
alloy with 2% copper and a titanium alloy with 6% aluminium and 4%
vanadium. Further details of the materials are given in Section 5.
In one series of tests superplastic forming and diffusion bonding
was used in the preparation of the coupons. Some tests were carried
out at elevated temperatures.

Additional information