Fiber flexural strength factor (FFSF)
The mROM (Eq. 5.1) could be adapted to analyze and predict the flexural strength of semialigned reinforced composites (Eq. 5.3)
where o/C, of, and Ojm* represent the flexural strength of the composite, the reinforcement, and the matrix, respectively, at the point of maximum flexural strength. The efficiency factor // includes the interphase, orientation, and length effects.
Parallel to the case of the tensile strength, the contribution of the fibers to the flexural strength of the composite material is represented by the term: // • o/F • V (Lopez et al., 2012c; Serrano et al., 2013), and a fiber flexural strength factor (FFSF) was defined isolating the contribution of the fibers to the flexural strength of the composite in the modified rule of mixtures (Eq. 5.4). The FFSF was defined as:
The value of // • o/F shows the influence of the reinforcement on the flexural strength of the composite, which is determined by the slope of the line in Fig. 5.7.
Figure 5.7 Fiber Flexural Strength Factor.
The proposed FFSF can be directly evaluated from the data obtained in the stress- strain test (Table 5.2) without further manipulation of the composite.
The value of the FFSF is 184.35 MPa, and if the 35% data is omitted 187.46 MPa. The value is 50% higher than the FTSF. The difference is lower than the 78% obtained for the SGW-reinforced composites (109 vs 194 MPa) (Lopez et al., 2013). There is research that suggests that, under some circumstances, the tensile and flexural intrinsic strengths have the same value (Hashemi, 2008). The differences between the values of the FTSF and the FFSF could indicate that in the case of the BSKP-reinforced PLA composites both intrinsic strength should be different. In fact, in a recent work, some of the authors exposed the possibility of computing the flexural intrinsic strength. The method assumes that the flexural and tensile intrinsic strengths ratio somehow is equivalent to the FFST-FTSF ratio (Lopez et al., 2013; Girones et al., 2011). While the authors admitted that the calculation is based on many unproven assumptions, the results rendered agree with the literature. In the case of the BSKP, the resulting intrinsic flexural strength amounted to between 990 and 1350 MPa, assuming 668-900 MPa as the possible intrinsic tensile strengths. The result is also in line with the flexural strength of SGW, with flexural strength between 1000 and 1200 MPa.