JEC COMPOSITES MAGAZINE - Issue #63 - March 2011 - 84

medical/biocomposite

degradation on the mechanical properties and to find the interfacial shear strength values of the composites.

Materials and methods
Woven carbon fibre fabrics were supplied by Hexcel, based upon Magnamite AS4 fibre. The weave style was a balanced 12K, 2x2 twill type giving a superficial density of 285 g/m2. The fibre diameter was approximately 7 microns and was confirmed via microscopy. The quoted elastic modulus is 228 GPa and the fibre density is 1.79 g/cm3. The PCL matrix was used in both an in-situ polymerised form (using distilled caprolactone monomer) and as poly(ε-caprolactone) pellets. PCL is a semi-crystalline, linear aliphatic polymer with a modulus ≈ 0.4 GPa, strength ≈ 16 MPa and melting point of 58-63°C [5]. Carbon fibre/PCL composites were prepared by two methods: film stacking (FS), which involves heat compression of alternated layers of fibre and polymer sheets, and in-situ polymerisation (ISP), which consists in reaction-injection moulding a polymer monomer into a mould cavity containing fibre layers [4]. The carbon fibres were heatcleaned to remove the manufacturer’s coating layer. Four- and eightply composites were produced to examine thickness and volume fraction effects. Long-term degradation tests were carried out to monitor changes in the mechanical properties and the interfacial bonding condition of the composites. But they are not discussed in this article. Single-fibre tensile testing In order to extract interfacial shear strength values, it is necessary to know the properties of the fibre being tested. This information was obtained via single-filament tensile testing according to ISO 11566_1996(E), using Method B to calculate the tensile modulus. Epoxy resin (X60450, Force UK) was used as an adhesive. The test was carried out at 20˚C with a gauge length of 25 mm and a crosshead speed of 1 mm/min. 30 samples were tested, with an individual diameter assumed to be constant at 7 μm. The Minitab software program was used to complete the regression which leads to the Weibull parameters. The system compliance was determined by testing 3 specimens at gauge lengths of 5-40 mm, at a crosshead speed of 1 mm min-1. This allowed a linear regression plot, and a value of 0.1371 mm N-1 was recorded.

To produce these dog-bone samples via the film stacking method, the fibres were fixed between two PCL strips. The resulting sandwich was then placed into a steel mould in a hot press for 5 minutes at 100°C. A 5 bar compression stroke was applied for 30 seconds and the mould was cooled to room temperature for 1 minute in a separate press (again, under 5 bar pressure). The thickness of the sample was controlled to 0.20 mm using steel shims. A fabrication method was also developed to produce in-situ polymerized SFFT specimens, although it was a challenge to prepare usable test samples due to strict handling requirements. Certain modifications were made to the process used for macroscopic composites. A 0.1 mm thick shim was made to fix the single filaments. The ends of each fibre were fastened to the shim with silicone rubber in order to keep the fibres straight. Eleven such fibres could be aligned in the mould at a pitch of 15 mm (see Figure 2).
Fig. 2: Mould diagram for the preparation of a single-fibre in-situ polymerised sample

jec composites magazine / No63 March 2011

Single-fibre fragmentation testing Single-fibre fragmentation testing (SFFT) was used to assess the relative interfacial bonding efficiencies [6,7] in the two types of fabrication methods. Single carbon fibres were taken from heat-cleaned bundles for these tests [8]. The test involves a single fibre which is totally encapsulated in a Fig. 1: SFFT sample (a) matrix tensile dog-bone coupon (see before testing and (b) after testing (fibre in blue) Figure 1).

Two shims were used to ensure symmetric fibre position within the sample thickness. After injection and curing, the resulting single composite sheet was hot-pressed (100°C) at 30 bar for 1 minute to a thickness of 0.2 mm, controlled by a shim. Finally, the individual samples were cut from this sheet into dog-bone shape. An optical microscope was used to determine the diameter of each fibre before preparing the SFFT specimens. The specimens were then tested in a tensile testing machine and loaded using a continuous method at 0.25 mm min-1 until load saturation was reached (i.e. the commencement of load shedding). The average fragment length in the samples was determined by transmitted-light optical microscopy. When the specimen is saturated with fractures, the maximum fibre fragment length is referred to as critical length lc. The shear stress at the interface is assumed constant along the fragment and, assuming a constant diameter, an average shear strength τ can be determined from a simple balance of force which results in:

JEC COMPOSITES MAGAZINE - Issue #63 - March 2011

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #63 - March 2011