Compression Testing of Thick-Section Composite Materials

Abstract

As composite materials become more attractive for use in large Navy structures, the need to understand the mechanical response of composites greater than 6.4 mm (0.25 in.) in thickness becomes a necessity. In this program, a compression test fixture that allows the testing of composites up to 25.4 mm (1 in.) in thickness and greater was designed and refined. This fixture was used to evaluate the effects of constituents, fiber orientation, and thickness on the compressive response of composite materials. In addition, the fixture was used to determine if the failure mechanisms observed for thick composites are similar to those that have been observed and reported for composite materials less than 6.4 mm (0.25 in.) thick. The in-plane moduli, in-plane and through-thickness Poisson's ratios, compression strength, and failure mechanisms of the thick composites were shown to be independent of material thickness. The predominant failure mechanisms for both materials were kink bands and delaminations, and were identical in geometry to those that have been reported for composite materials in the range of 2.54 mm (0.1 in.) thick. Although unchanged with thickness, the through-thickness Poisson's ratio for the carbon- and glass-reinforced laminates were found to be significantly nonlinear, resulting in changes in this property of up to 58% from the initial region of the strain-strain curve to the final region of the strain-strain curve.

A study on the compressive strength of thick carbon fibre–epoxy laminates

“…In the larger and thicker UD specimens the SCF influences the overall unidirectional behaviour by causing premature failures near the gripped region. Similar results and observations were reported in references [8,21]. Such effects are not as severe in the multidirectional laminates (small or large size) where the off-axis surface plies do reduce the SCF and allow the 0-degree plies (the load carrying layers) to fail closer to their intrinsic strength, see sections 4 and 5.…”

“…A possible explanation is that the off-axis plies protect the load carrying 0° layers from the stress concentrations at the tabs so managing to come closer or achieve their intrinsic strength. Compressive strength results of [0/0/90] ns laminates reported in reference [8] showed a decrease in strength of approximately 22% from 6.4 mm to 25.4 mm thick specimens. All failures occurred at the gauge length-tab termination interface due to high local stress as explained above for the IM7 system.…”

“…2. Even though the Weibull statistics could be used to explain the reduced strength in the UD specimens, the mode of failure is not a weakest link, but that affected by the stress concentration induced by the end tabs (see also [8,21]). As explained in the discussion and concluding remarks sections these premature failures are not observed in the large sublaminate-level scaled multidirectional specimens where size effects are not observed.…”

“…In terms of scaling of unidirectional laminates, the tendency of the thickness effects was revealed in the literature even though the thick specimens failed prematurely [8][9][10][11][12] . However, limited studies have been performed on 3-D scaling effects on the compressive strength, where all three dimensions are scaled up accordingly.…”

“…However, limited studies have been performed on 3-D scaling effects on the compressive strength, where all three dimensions are scaled up accordingly. In trying to explain the unexpected low failure strength of thick composites, possible explanations could fall into the categories of material or manufacturing related issues 8 and several questions can be raised. In terms of material aspects, the elastic constants or strength determined for thin laminates may not be applicable to thick composites.…”