37 Fatigue testing

The most useful form of fatigue test is the stress-life S-Nvtest, performed in load control. Care is needed to define the fatigue life of a foam specimen. In tension-tension fatigue and in shear fatigue, there is an incubation period after which the specimen lengthens progressively with increasing fatigue cycles. The failure strain is somewhat less than the monotonic failure strain, and is small; a knife-edge clip gauge is recommended to measure strain in tension or in shear. The fatigue life is defined as the number of cycles up to separation. In compression-compression fatigue there is an incubation period after which the specimen progressively shortens, accumulating large plastic strains of the order of the monotonic lock-up strain. Axial strain is adequately measured by using the cross-head displacement of the test frame. The fatigue life is defined as the number of cycles up to the onset of progressive shortening. As noted in Section 3.2, it is important to perform fatigue tests on specimens of adequate size. As a rule of thumb, the gauge section of the specimen should measure at least seven cell dimensions in each direction, and preferably more.

Compression-compression fatigue is best explored by loading cuboid specimens between flat, lubricated platens. It is important to machine the top and bottom faces of the specimens flat and parallel (for example, by spark erosion) to prevent failure adjacent to the platens. Progressive axial shortening commences at a strain level about equal to the monotonic yield strain for the foam (e.g. 2% for Alporas of relative density 10%). The incubation period for the commencement of shortening defines the fatigue life Nf. The progressive shortening may be uniform throughout the foam or it may be associated with the sequential collapse of rows of cells.

Tension-tension fatigue requires special care in gripping. It is recommended that tests be performed on a dogbone geometry, with cross-sectional area of the waisted portion of about one half that of the gripped ends, to ensure failure remote from the grips. Slipping is prevented by using serrated grips or adhesives.

Shear fatigue utilizes the loading geometries shown in Figures 3.5 and 3.8:

1. The ASTM C273 lap-shear test applied to fatigue (Figure 3.5(a))

2. The double-lap shear test (Figure 3.5(b))

3. The sandwich panel test in the core-shear deformation regime (Figure 3.8)

Initial evidence suggests that the measured S-N^fcurve is insensitive to the particular type of shear test. The ASTM lap-shear test involves large specimens

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-Core

Figure 3.8 The sandwich-beam test, configured so that the core is loaded predominantly in shear which may be difficult to obtain in practice. The sandwich panel test has the virtue that it is closely related to the practical application of foams as the core of a sandwich panel.

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