Page 62 - Handbook of Properties of Textile and Technical Fibres

Testing and characterization of fibers 43

zones as well as the crystallites, which are oriented out of the Bragg position are not
seen. By such a method, the sizes and shapes of crystallites and the mode of segrega-
tion between crystalline and amorphous zones can be determined.

2.4 Mechanical characterization

2.4.1 Mechanical tensile testing procedure
2.4.1.1 Mounting specimens for tensile testing
The mounting of single fibers in a testing machine should be done with great care. The
fiber should be placed, securely without crushing or misalignment, in the grips of the
testing machine as these can lead to bending stresses in the fiber at the grips and pre-
mature failure. In both cases, errors in measurement of the fiber properties are the
result.
Some types of fiber specimens can be mounted directly in the testing machine, with
a minimum of care, such as protecting them in the grips with tabs of adhesive paper or
tape. However, for brittle fibers it is the common practice to mount the individual fibers
on stiff paper or cardboard tabs in preparation for testing, as illustrated in Fig. 2.15.
The tab has a central cutout that matches the desired gauge length for the test. A gauge
length of 25 mm is commonly used. A drop of quick drying epoxy or similar adhesive
anchors the fiber in place. The ends of the frame can be cut away, along the dotted line,
before the test and the part of the fiber passing over the two holes can be kept for sub-
sequent examination. The tab is gripped in the jaws of the testing machine and, just
prior to testing, cuts are made from each side to the central cutout, ensuring that
only the fiber is loaded during the test.
Investigating the effect of the fiber gripping method on the mechanical properties is
essential to provide consistent and usable results (Kim et al., 2015). In particular, ef-
fects of grip materials must not affect the elastic properties measured. Tests with
several gauge lengths make it possible to verify this point. In the case of brittle fibers,
such as carbon or ceramic fibers, failure results in the fragmentation of the specimen,
and this can be a problem if the initial fracture surface needs to be observed. In this
case, tests specifically designed to identify the initial fracture surface are carried out.
The whole specimen is immersed in liquid paraffin so that the energy released at break
is dampened by the surrounding medium. Alternatively, carefully coating the fiber
with grease can also give more controlled fractures. In the latter cases, it is advised

Cut Cut Cut Cut
Fiber

Epoxy
Figure 2.15 The fiber is glued onto a card frame which is then placed in the grips before being
cut away in the center to allow the fiber to be tested.