3.1. Solvent Wash Results
The results from the solvent wash procedure are shown below.
Figure 5 shows CFRP samples that have been stripped of the resin matrix, dried, and ready for determinations of its fiber volume.
Ten replicates were tested for each fiber type.
Table 5 displays the solvent wash results of the MSU produced pre-preg tows.
The prepreg IM7-GP had the closest fiber volume ratio to what is found in commercial settings. The first generation MSU SBCF showed a fiber volume about 17% lower than IM7-GP. Lower fiber volume could adversely affect the load bearing of the samples because the fiber carries most of the load when CFRPs are stressed.
Response values for Fmax and Δmax for all fiber types; IM7-GP continuous, Hexcel SBCF, and MSU SBCF are displayed in
Table 6. Results of analysis of variance (ANOVA) for average maximum force and maximum yield for three carbon fiber types are displayed in
Table 7. Main effect plots are shown in
Figure 6 and
Figure 7 to illustrate the relative adequate strength of each factor [
21].
It is clear from the contribution percentage that temperature has a dominant effect on the Fmax of both the Hexcel SBCF and MSU SBCF, showing 98.37% and 94.54%, respectively. Contribution dependency is the percentage that each factor contributes to the total sequential sums of squares. A higher percentage shows the factor accounts for more of the variation in the response variable. For the Δmax, this effect of temperature is reduced, though still a significant factor in the response. IM7-GP indicates that it is less dependent on temperature, showing a Fmax and Δmax contribution percentage of 1.26% and 11.94% for Fmax and Δmax, respectively. This response is expected since the IM7-GP fibers are continuous throughout the length of the sample despite temperature fluctuations, while the surrounding matrix continues to soften. On the contrary, SBCF are permitted to move freely when subjected to elevated temperatures. The forming tool and gap width effects on Fmax values of SBCF are not statistically valid enough to show dependency, while the yield displacement values do demonstrate a small contribution. Investigation into the combined contribution of the forming tool and gap widths is required to determine any further effects. The gap width effect on IM7-GP indicates relative high dependency on Fmax of 38.35%. The gap width effect on Δmax for IM7-GP shows a contribution dependency of 58.45%. Mechanically, the fiber is experiencing higher stress from the two directions as the gap width increases, (σx and σz). When the gap width increases, the force gauge registers lower forces because it records force only in the z-axis. Interaction plots in
Figure 6 illustrate the effect of temperature on the SBCF materials while displaying the generally insignificant effect of temperature on IM7-GP fibers.
p-values for forming tool diameter in IM7-GP are greater than 0.05, indicating its low significance, while gap width and temperature both show p-values less than 0.05, demonstrating that both sources are statistically significant. The average maximum force for Hexcel SBCF has low p-values (<0.05) for gap width and temperature, suggesting that both are significant. However, gap width has a minority contribution of 3.23%, while forming tool diameter has a p-value of 0.99, pointing toward low significance. All sources of average maximum yield displacement for Hexcel SBCF indicate statistical significance. In addition, MSU SBCF has low p-values (<0.05) for temperature and gap width, while forming tool’s nose diameter has p-values ≤ 0.5, though the contribution is hardly significant. From the ANOVA model it can be concluded that in general temperature has a significant effect on the Fmax of SBCF materials while it has lower influence on IM7-GP. Alternatively, gap width and forming tool’s nose diameter affect Fmax for IM7-GP, where the SBCF materials see little significance from the factors mentioned above.
Similar to Fmax, the ANOVA model indicates that temperature has little effect on the Δmax of IM7-GP but has a larger contribution to the Δmax for both Hexcel SBCF and MSU SBCF. Unlike Fmax though, there is gap width influence on the Δmax of IM7-GP but minor influence from the forming tool nose diameter. For SBCF materials, the contribution of forming tool diameter and gap width is moderately significant, yet still slightly lower than that of the temperature effect.
Lastly, with the individual factors considered, the combination of forming tool’s diameter and gap width (interaction) should be considered to determine if they affect the independent variables.
Table 8 displays the ANOVA results for all fiber types.
The results from
Table 8 demonstrate that the forming tool’s nose diameter and gap combinations do change the results of the ANOVA output. A return of near zero
p-values for all sources indicates that no individual factor can be completely dismissed. For IM7-GP continuous fiber, the ANOVA trend is similar, as shown in
Table 7, in which there is a greater contribution from the geometry as opposed to the temperature. This effect is even greater when considering Δmax, contributing of 62% from the forming tool diameter and gap width combined. The SBCF materials have large contribution percentages from the temperature effects on Fmax of 94.54% for Hexcel SBCF and 98.37% for MSU SBCF. The contribution of the forming tool gap combination on Δmax was much closer, leading to a conclusion that the geometry does affect the Δmax of the SBCF during testing.
Figure 8 displays the interaction plots for the ANOVA analysis of forming tool diameter and gap width combined for Fmax and Δmax, respectively. It follows that in general, for continuous fiber, the geometry has a dominant effect on the Fmax while for SBCF the temperature is the most significant contributing factor.