Composite Materials Research Progress

210
W. Van Paepegem, I. De Baere, E. Lamkanfi et al.
composites is still to establish the S-N curve for that particular composite. The efforts to
combine such fatigue tests with a variety of online and offline monitoring techniques and
detailed numerical simulations of the experimental boundary conditions and observed
material degradation, are much more limited.
This paper wants to give a general overview of the different types of fatigue tests, the
available online and offline monitoring techniques and the indispensable need of finite
element calculations to understand the outcomes of these tests. As such, it should become
clear that one single experimental fatigue test, if properly instrumented and simulated, can
provide a lot more information about the fatigue behaviour of the tested composite material.
The next paragraphs will discuss:
• the different fatigue test set-ups and related online monitoring techniques,
• the inspection of fatigue damage,
• the finite element simulation of experimental boundary conditions.
2. Fatigue Test Set-ups and Online Monitoring Techniques
In this paragraph, a general overview of the most relevant fatigue test set-ups is given:
(i) tension-tension fatigue, (ii) bending fatigue, and (iii) shear dominated fatigue. The related
online monitoring techniques are discussed and some examples of measurements are briefly
presented.
An elaborate discussion of all types of fatigue testing, including tension-compression
fatigue, biaxial fatigue and torsional fatigue, can be found elsewhere [1].
2.1. Tension-Tension Fatigue
The uni-axial tension-tension fatigue test is the most widely used fatigue test. The coupon
geometry is a parallel-sided specimen, instrumented with tabs. The choice of the tabbing
material differs among the testing laboratories. Some prefer steel or aluminium tabs, but most
of them use glass/epoxy tabs, where the glass reinforcement has a [+45°/-45°]ns stacking
sequence. In most cases, the tabs are straight-sided non-tapered tabs.
A fatigue test is usually conducted with a servo-hydraulic testing machine, equipped with
grips that clamp the specimen. The alignment of the specimen is very important. No bending
loads must be induced in the specimen due to misalignment.
In tension-tension fatigue tests, the stress ratio R (= σmin/σmax) is often chosen to be 0.1.
The test frequency is always chosen as high as possible to limit the duration of the test and
minimize the cost, but the fatigue response of some composites strongly depends on the
frequency (especially in case of fibre-reinforced thermoplastics).
In the international standards, the number of cycles to failure is considered as the main
outcome of the tension-tension fatigue test. Yet it is worth the effort to use online
instrumentation methods.
The most simple and effective online measurement is the axial stiffness evolution. The
axial stiffness can be directly calculated from the axial stress (loadcell) and the axial strain
(extensometer). The axial strain must never be calculated from the axial displacement and the