This paper describes research concerning the resin impregnation and characterization of multidimensionally braided fiber-reinforced composite materials. These materials are an alternative to traditional laminated structures, having the potential for being more damage tolerant. Three graphite fiber systems were used in this investigation, and three processes were investigated for resin impregnation of the multidimensionally braided material using vacuum or pressure. Two were resin transfer techniques and the third was a resin film lamination technique. While all three methods are presented, the latter technique was chosen for impregnating the test specimens due to the consistently low void content and superior surface quality achieved by this method. Three variables having an important bearing on the performance of braided materials were investigated. These included the effect of braid pattern, tow size, and edge condition on the tensile, compressive, flexural, and interlaminar shear properties. The properties were obtained in the braid direction only. The cutting of the specimen edges substantially reduced both tensile and flexural strengths and moduli. Of the three braid patterns investigated (1 × 1, 1 × 1 × 1/2F and 3 × 1), the 3 × 1 braid showed superior tensile performance and the 1 × 1 × 1/2F pattern exhibited superior flexural properties. Variation in fiber tow size caused variations in tensile, flexural, and short-beam shear properties. The 12K tow size specimens exhibited the best performance. All braided composite materials in the uncut edge condition showed significant improvements in their short-beam shear strengths, being equal to or greater than unidirectional laminated composites. This latter characteristic may be one of several indicators that multidimensionally braided composites are inherently damage tolerant.

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.

No abstract

The design, analysis and testing of composite-to-metal joints for the ARPA Man-Rated Demonstration Article are discussed. The first is a composite-to-titanium sphere joint designed to react sphere membrane stress with block compression loading. A series of uniaxial compression tests to simulate this joint assembly was conducted. Test results successfully demonstrated the ability of the joint to withstand block compression loading even in the presence of assembly fitup gaps. A polymer shimming material (Hysol EA9394) was also evaluated as a means to fill potential fitup gaps and was found to effectively reduce stress concentrations resulting from the gaps. The second joint discussed is one between 120-degree segments that make up a stiffener ring for a ringstiffened composite cylinder. Joint design includes Hysol EA9394, boltedon titanium splice plates, and bondedon thermoset flange straps. 2D joint tests, single stiffener tests, and sub-scale ring-stiffened cylinder hydrostatic tests to validate the joint design are described.

A point stress criterion based on stress distribution, in conjunction with Hashin's interactive failure criteria for fiber composites, is applied to composite laminates containing throughthickness discontinuities. The point stress criterion assumes that failure occurs when the stress over some distance (characteristic distance) from the discontinuity is equal to or greater than the strength of the material without the discontinuity. The characteristic distance is determined from Vnotched beam specimens cut through the thickness of AS4/3501-6, S2 glass/3501-6, and AS4/PEEK unidirectional laminates. This characteristic distance is then used to estimate the strength of inplane Vnotched beam specimens, transverse flexure specimens, and an AS4/PEEK doublenotched cylinder loaded in axial compression. Good correlation between the predicted and measured strength was obtained for this case. Based on these results, additional investigations with other material systems and more interactive stress states are warranted.