Fatigue behaviour of composite tubes under multiaxial loading

Damage mechanism and fatigue behaviour of uniaxially and
sequentially loaded wound tube specimens
Abstract
Frank Schmidt *, Peter Horst
Institute of aircraft design and lightweight structures, Technische Universität Braunschweig,
Hermann-Blenk Straße 35, 38108 Braunschweig, Germany
In mechanically loaded composites various damage types occur depending on different external load cases. One aspect
of this paper is to show the differences in damage and fatigue behaviour under pure tension/compression and pure shear
fatigue loading. For this purpose, fatigue tests with nominally defect-free wound tube specimens and non-destructive tests,
i.e. thermography, optical fracture analysis with high-speed camera and discrete damage monitoring are performed. Based
on the experimental data a comprehensive analysis of the successive failure mechanisms (matrix cracking, delamination
and final failure) is conducted. Thereby, the location of final failure can be found in an early stage of the fatigue life
observing high temperature areas (hot-spots). In the following, the effects of two different uniaxial load directions under
sequence loads, which mean a sequently change from pure shear load to pure tension/compression load after several life
cycles, are investigated.
Keywords: Polymer matrix composites; fatigue test methods; experimental data; non-destructive testing
1. Introduction
The increasing application of composite materials in several industries, e.g. automobile and aircraft
industry or wind-energy plants, demands a deeper understanding of the fatigue behaviour and its
modelling for different load cases. Thereby, the range of fatigue loadings is varied. Concerning the high
realistic complexity of load histories and arbitrary load ratios or intensities (e.g. aerodynamic loads of
aircraft wings or rotor blades of a wind-energy plant) investigations of multiaxial cyclic loading
conditions are required. Additionally, the sequence of different load directions and amplitudes is also
crucial for the occurrence and interaction of complex damage mechanisms and the fatigue behaviour of
composites.
Although constant-amplitude tests under uniaxial loading hardly represent realistic fatigue loading
conditions, mostly these simplified types of fatigue loading experiments are performed. However, these
investigations lead to a good understanding of the basic influences of fatigue damages and are essential
for the interpretation of further researches with multiaxial and sequence loadings.
The fatigue damage scenarios of 0/90 laminates show the initiation and progress of transverse cracks
with sharp tips (stage I), the development of local delaminations caused by the transverse cracks (stage
II) and the consolidation of local delamination surfaces with the final failure (stage III) [1-3]. Analysing
* Corresponding author. Tel: 0049-531-391-9921, Fax: 0049-531-391-9904
E-mail addresses: frank.schmidt@tu-bs.de, p.horst@tu-bs.de