In composite materials, a low resistance to shear deformation, particularly in material planes dominated by matrix properties, is a severe weakness in fibre
138 composites. Relatively low shear stiffness and strength often compromise material performance. Interlaminar shear stress (ILSS) is dependent on the interfacial bonding at the fibre/matrix interface and thus sometimes used as an indicator of the interfacial bonding in composite. Composites with low ILSS are prone to delamination and have poor resistance to environmental degradation, which is detrimental to many applications. However, composites with too high ILSS may have a low toughness, as some of the toughening mechanisms such as fibre/matrix debonding, and crack deflection cannot be triggered.
Considerable experimental and analytical effort has been made in the development of shear testing methods. One of the major difficulties is the provision of a pure shear stress state in the specimen. A number of shear tests have been developed, which includes Iosipescu shear test (ASTM D5379), two-rail shear tests (ASTM D4255), [±45°] tensile shear tests (ASTM D3518) and short beam shear test (ASTM D2344) among others. However, there is no universal method suitable for the accurate evaluation of the shear properties for the extensive range of material geometries existing in composite technology. All the shear test methods, standardised or otherwise, have their own physical and geometrical limitations.
Short beam shear (SBS) test was chosen for this study, which is an interlaminar shear test method. A specimen with a low support span-to-depth ratio is subjected to three-point loading. Both bending and interlaminar shear stresses are induced, the axial bending stresses are compressive on the loading surface and tensile on the opposite surface. The neutral axis is where the bending stress passes through zero, and the interlaminar stress is at its maximum. For a shear test, by keeping the span-to-depth ratio low, the bending stresses can be kept low, promoting shear failures at the neutral plane. However, the concentrated loadings on the beam at the loading and support points create stress concentrations, complicating the stress state. No strain or displacement measurements are made because the span-to-depth ratio is too small. This means the shear modulus cannot be determined and a shear stress- strain curve is not obtained. Despite of the serious limitations, the test is in common
139 use owning to the ease of sample preparation. The sample can be very small, the text fixture can be relatively simple and a test can be performed very quickly. The SBS test is used extensively as a materials screening and quality control test in the composite field.
In this study SBS strength of the laminates was measured according to ASTM D2344 (Figure 4.4) [217] using an Instron universal testing machine. The width to thickness ratio of the specimen was 2, the span to depth ratio was 4 and the test rate was 1 mm/min. The average dimensions of the SBS specimens were 25 mm x 4.4 mm x 2.2 mm. The diameter of the loading roller was 6mm and diameter of the support rollers was 3 mm.
Expression for SBS strength:
A P 75 . 0 13 (4.2)
where P = maximum load
A = cross-sectional area
140 4.1.3.4 Compression after impact
It is thought that during service, composite structures are subjected to minor impacts and other damages, which are sometimes not visible on the surface. It is important to understand how a composite will perform after being damaged. The CAI test for composite laminates generally involves subjecting a plate to an out-of – plane low energy impact to introduce damage but not complete failure. The damaged plate was then loaded until failure by in-plane compression. The most common form of internal damage after impact is delamination. Delamination can lead to premature failure during compression loading caused by ply buckling and Mode-I dominated crack growth [218]. Tensile properties are fibre dominated and less affected by this type of damage unless the impact energy is high enough to cause fibre damage, hence compression test is carried out.
The test has been developed within the composite industry as a test method for damage tolerance assessment of composite laminated and is routinely carried out for materials developments, screening and quality control. There is no ISO or ASTM standard, although numerous studies have been done by companies and institution such as Boeing, National Aeronautics and Space Adminstration (NASA), Suppliers of Advanced Composite Materials Association (SACMA), and Royal Space Establishments (RAE) [218].
In this study CAI specimens were cut from composite laminates to size (89 mm x 55 mm) and impacted at 2 J, 4 J and 6 J, using a CEAST impact tester (Figure 4.5). Each specimen was clamped in a 40 mm diameter support and the dart diameter was 20 mm. Internal damage of the impacted specimens was examined using ultrasonic C-scan (Figure 4.6). The maximum width and length of the damage area was measured. The non-impacted and impacted specimens were then placed in a miniaturised Boeing CAI rig (Figure 4.7) [219] and loaded in compression at a rate of 0.5 mm/min. Five specimens were tested for each impact energy. The maximum compressive strength was recorded and the ratio of maximum compressive strength
141 for each impacted specimen to that of a non-impacted control specimen was presented as residual compressive strength.
Figure 4.5 Schematic diagram of an impact tester.
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Figure 4.7 Miniaturised CAI rig used in this study.