As can be seen from Tables 3 and 4 and Figs. 3 and 4, for both UTS and E, all four main parameters are statistically significant (Standard Effect is higher than the significance line for 95% significance) and can control the mechanical response of the material, although the number of layers is highly significant for both UTS and E, and is significantly more important than any other parameter. It is surprising that the type of reinforcement has such a small effect on the mechanical response, with the Standardised Effect on UTS for ‘reinforcement’ being only 27% of that for ‘layers’ (Fig. 3), although for E, the effect is more significant, being 70% of that for ‘layers’ (Fig. 4). The type of matrix material has been shown to have very little effect on the mechanical response, with the Standardised Effect for ‘matrix’ being only 12% of that for ‘layers’ for UTS (Fig. 3) and only 5% of that for ‘layers’ for E (Fig. 5). Also, for E, the ‘matrix’ parameter is only just significant (2.63) compared to the significance limit of 1.98. Thus, lower-cost, and more abundant Nylon matrix material offers similar performance to the more expensive Onyx. ‘Orientation’ has a standardized effect of only 3% for UTS and 19% for E compared to those for ‘layers’.
The selection of parameter levels for main parameters can be informed from the Main Effects Plots (Figs. 5 and 6 and Tables 5 and 6). These indicate the change in observed average (over all samples) mechanical response as a result in change in parameter level. From Figs. 5 and 6 and Tables 5 and 6 it is clearly evident that the results for UTS and E follow similar, but not identical trends. For ‘matrix’ parameter, both UTS and E, selecting Onxy over Nylon does provide a small increase in both UTS and E, but, this is only (UTS) from 5.58% below global mean to 11.66% above (Table 5) and (E) from − 1.55% below global mean to 1.55% above (Table 6). Thus, optimal properties are achieved by selecting Onyx over Nylon as matrix material, although, as discussed later, the improvements in mechanical response may bot outweigh the extra cost incurred.
For ‘reinforcement’ parameter, there is a large effect in moving from GF to CF, with large increases in both UTS (from 0.14% below global mean to 22.06% above – Fig. 4 and Table 5) and E (from 0.12% below global mean to 42.75% above – Fig. 5 and Table 6). There is a nearly equally sized decrease in properties for both UTS and E when selecting KF, shifting to 22.94% below global mean (UTS) and 16.98% below global mean (E). Thus, optimal properties are obtained by selecting CF as reinforcement material.
As discussed initially above, the ‘layers’ parameter has the largest influence over the mechanical properties. From the Main Effect plots for UTS and E (Figs. 5 and 6), moving from 4 layers to 12 layers provides a improvement in UTS from 46.52% below global mean to 46.56% above global mean; the same change results in raising E from 30.76% below global mean to 30.76% above global mean. Thus, optimal properties are obtained by using a higher number of reinforcement layers, and, as we see later, is an affordable choice.
Fibre orientation, within the bounds of these trials (0,45 and 0,90 bi-directional orientations) has very little impact on either UTS (2.87 at significance limit of 2.0) (Fig. 3) or E (9.84 at significance limit of 1.98). The Main Effect plots also demonstrate the marginal improvement in moving from a (0,45) pattern to a (0,90) pattern, with UTS increasing from 1.23% below global mean to 1.27% above global mean (Table 5), and E increasing from 5.78% below global mean to 5.80% above global mean (Table 6). This small effect may be due to the similarity between the two lay-up patterns. Higher performance was achieved by Klift et al , achieving 400 MPa (σ = 20.35) UTS for Nylon-Carbon samples, using a concentric ring lay-up, with our Nylon-CF only achieving 249 ± 6 MPa. Although the results are not directly comparable as Klift et al. used an increased number of reinforcement layers of 16, compared to 12 in our research. It is clear though from this research that a (0,90) pattern does help to optimise the UTS and E and has no significant effect on the cost (as we see later).
It can be deduced from the analysis of cost verses strength (Fig. 7) and stiffness (Fig. 8) that the lowest cost to strength ratio (most desirable) is achieved using a GF reinforced Nylon with 12 layers and (0,90) lay-up, at 0.0221 ± 0.0002 $MPa− 1, and achieving 220 ± 2 MPa UTS (79 ± 1% of highest UTS, achieved using CF reinforced Onyx with 12 layers and (0,90) lay-up. The equivalent material using (0,45) only achieves a UTS of 187 ± 4 MPa (67 ± 1% of highest UTS) at a cost to strength ratio of 0.0261 ± 0.0005 $MPa− 1. The highest performing composite, 278 ± 4 MPa (CF reinforced Onyx with 12 layers and (0,90) lay-up) has a cost to strength ratio of 0.0303 ± 0.0004 $MPa− 1, 37 ± 2% higher cost than the optimum cost to strength ratio material. Thus, for a 21 ± 1% increase in strength, a 37 ± 2% increase in cost is incurred, making the most cost-effective material option highly attractive for all but the most demanding applications.
The stiffest material is CF reinforced Onyx with 12 layers and (0,90) (15.5 ± 0.3 GPa), and is 2nd most cost-effective material (0.49 ± 0.06 $GPa− 1) (Fig. 8). The most cost-effective material is GF reinforced Nylon with 12 layers and (0,45) (0.40 ± 0.02 $GPa− 1), which is 20 ± 3% lower cost but only retains 39 ± 2% of the stiffness of the stiffest material. It is therefore only practical to use this most cost-effective material where high stiffness is not a design requirement. Only three materials have E > 10 GPa, and these are all CF reinforced with 12 layers. It is therefore practical to use the stiffest material (CF reinforced Onyx with 12 layers and (0,90)) for all but the most cost-sensitive applications. KF does not provide any technical advantage over CF and is also not competitive economically.