Civil Engineering Reference
In-Depth Information
h e SEM and AFM observations also showed that the twist present in the i bers can
be right- or let -handed and that thicker i bers exhibit a longer pitch. Measurements
of the pitch and diameter of the i bers allowed the authors to i nd a relation pitch-to-
diameter of approximately 7.1. In search of an explanation for the twist the authors
ruled out the chirality of the cellulosic chain as the origin because, if this was the case,
only right- or let -handed twist would be found and not both at the same time.
Following this work, another publication [103] by the same authors, reported the
collection of electrospun APC i bers, suspended between two electrodes, in order to
obtain oriented mats.
Tensile tests performed on these mats showed a linear behavior up to a strain value of
ε
y
= 0.103, which corresponds to a yield stress of σ
y
= 3.5 MPa (with a Young's Modulus
of 40 MPa). For strains higher than 0.206 the curve exhibits a plateau corresponding
to constant-stress deformation. A typical stress-strain curve obtained for these i bers is
shown in Figure 8.15.
It is important to mention that the values determined for the mechanical properties
of these i ber mats are inl uenced by the dii culty in measuring the real cross-section of
the i ber mat; the authors opted to make an estimation of this area by considering that
the mat was dense. h is approximation means that the considered area is several times
higher than the real cross-section area and, consequently, the real values for the Young's
modulus and yield stress should be several times higher than those determined by the
analysis of the tensile test results.
Further observations by means of scanning electron and polarized optical microsco-
pies performed at er the tensile test revealed that some of the i bers did not break and
split into two, but instead they opened i ssures. Figure 8.16 shows pictures taken by
scanning electron and polarized optical microscopies of some of those i bers.
Observation of Figure 8.16 shows that the electrospun i bers are in fact composite
i bers with an outer shell that represents 10% to 20% of the total diameter; this layer is
rigid and should be responsible for the elastic behavior found in the mechanical test.
h e inner part is sot and l uid and its presence provides one explanation for the exis-
tence of the plateau of deformation at constant stress, for higher strain values, at er the
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0
0
0,2
0,4
0,6
0,8
1
Strain
Figure 8.15
Stress-strain curve of a uniaxially align nonwoven mat of APC i bers. Up to ε
y
= 0.10 the
behavior is linear with E = 40 MPa. Above a yield stress of σ
y
= 3.5 MPa a plateau of deformation at
constant stress is observed [103]. Reproduced by permission of the Royal Society of Chemistry.
