JEC COMPOSITES MAGAZINE - Issue #63 - March 2011 - 85

where d is the fibre diameter for a circular fibre cross-section. Since the fibre-matrix interface is placed under shear, the calculated value of τ is often used as an estimator of the composite interfacial shear strength. Because the fibre strength is length-dependent, it is more accurate to replace σf with the term σ(lc), which reflects the strength of the fibre at the prevailing critical length as follows:

Results and discussion
Single-fibre tensile tests (SFTT) The tensile test data were analysed using a Weibull plot via Minitab software, as shown in Figure 3, revealing shape and scale values of 5.59 and 4055 respectively. Table 1 shows the tensile properties obtained from the same data using the system compliance value to calculate the tensile elastic modulus. The lower strain to failure of fibres compared with the supplier’s data is attributed to the effects of heat cleaning [8].
Fig. 3: Weibull analysis of AS4 carbon fibres pre-treated for 15 min. at 400˚C
Tab.1: Tensile properties of AS4 carbon fibres pre-treated for 15 min. at 400˚C

Thus, by measuring lc, d and σ(lc), the IFSS could be estimated, calculating critical length from:

where is the average fragment length [7] and is obtained conveniently by dividing the gauge length of a specimen by the number of fragments. This calculation assumes that one takes values between lc/2 and lc following a normal distribution. The value of d is taken by a simple measurement. The determination of σ(lc) is less straightforward and the value is not generally measured directly since lc may be very small (less than 0.25 mm). The conventional approach relies on linear extrapolation of fibre strength measurements at higher gauge lengths. Another indirect method for determining σ(lc) involves a Weibull function. This relies upon strength being determined by a weakestlink principle, using scale and shape parameters for fibre strength σ0 and m. A correlation factor α also takes account of gauge length effects:

Specifications Average

Supplier data

The scaling parameter α/m can be estimated by testing the fibre at different gauge lengths and constructing a log-log plot of tensile strength versus gauge length. This remains a first-order approximation of strength in respect of an embedded fibre since, here, the applied load cannot provoke failure along the entire fragment length because some of this falls within the stress transfer regions or exclusion zones at the ends. As the average fragment length reduces so these effects become large. Various pre-treatments were introduced before film stacking in order to isolate the important influences on the interfacial shear strength (IFSS) of carbon fibre-PCL specimens. Cleaned fibres were immersed in (a) 99% caprolactone monomer at 120˚C for 24 hours (with and without a chloroform wash) or (b) a PCL-chloroform solution for 5 minutes.

Single-fibre fragmentation test results SFFT specimens with pre-treated fibres were tested to compare the effects of various wetting processes and pre-treatments on the apparent interfacial shear strength. Specimens were fabricated by both in-situ polymerisation and film stacking, after pre-treatment with either chloroform, caprolactone monomer or both. Details of the treatments and IFSS results are provided in Table 2.

The in-situ polymerisation samples provided the highest value of apparent IFSS of those combinations tested. This is consistent with the results of flexural testing for macroscopic composites [4]. While this is a reasonable outcome that follows from wetting by a low-viscosity monomer rather than a viscous melt, it seems apparent from the pre-treatments that there is a solvent action of the hot caprolactone monomer contributing to the effect. As the fibre pre-treatment becomes increasingly aggressive, the film-stacked specimen results approach

No63 March 2011 / jec composites magazine
1.69 ± 0.33 1.87 3,752.16 ± 760.97 4,278.00 208.10 ± 10.55 228.00 5.59 4055 -

Max tensile strain (%) strength (MPa)

Ef (GPa)

Shape value

Scale value

Research & development

JEC COMPOSITES MAGAZINE - Issue #63 - March 2011

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