Page 40 - Handbook of Properties of Textile and Technical Fibres

P. 40

Testing and characterization 2

of ﬁbers

1
1
Anthony R. Bunsell , Sébastien Joann es , Alba Marcellan 2
1 2
MINES ParisTech, Centre des Matériaux, Evry Cedex, France; UMR CNRS 7615, ESPCI
Paris, Université Pierre et Marie Curie, Sorbonne-Universités, Paris, France

2.1 Introduction

As we have seen in Chapter 1, ﬁbers owe their ﬂexibility to their ﬁneness, as it is
related to the reciprocal of the “ﬁber diameter” to the fourth power. This means that
if the diameter of a right circular cylinder (a reasonable model of a ﬁber as a ﬁrst
approximation) is reduced by a half, it becomes 16 times more ﬂexible. Thus, despite
very high tensile moduli, ﬁbers are highly ﬂexible and can easily be organized into
complex structures such as threads or woven fabrics. This characteristic is obviously
very important in determining their use; after all sheet metal, or even chain mail, never
became fashion items except for medieval knights and they had to be careful not to fall
over in case they could not get up. Drapeability, coupled with lightness, therefore ex-
plains, to a great extent, the success of ﬁbers for clothing and these are often the rea-
sons why ﬁbers ﬁnd use in technical applications. However, this means that the nature
of structures made from ﬁbers is different from other structural materials. Cloth is a
two-dimensional structure, which can easily take complex forms, bending in more
than one plane to take the shape of a person or of a complex mold. Most other struc-
tural materials are three dimensional and have to be worked to achieve the ﬁnal desired
form. In that context, the single ﬁber undergoes multiaxial mechanical loadings and
complex ﬁbereﬁber contacts. From a fundamental point of view, some trivial ques-
tions arise: How morphology and mechanical properties (and possible the related
anisotropy) of the single ﬁber impacts on the response of the structure (i.e., the assem-
bly)? What are the microstructural parameters responsible for ﬁber stiffness and/or
tenacity? Because ﬁbereﬁber contacts are crucial for the interrelated characteristics
of the assembly, what is the contribution of the ﬁber surface morphology? Even as ﬁ-
ber reinforced composite materials the composites are usually in the form of thin
sheets. It also means that ﬁbers are mostly used in tension as they usually buckle under
compression and this makes characterizing ﬁbers particularly challenging (Bunsell and
Schwartz, 2000).
As we have already seen, the ﬁneness of ﬁbers had led the ﬁber industry to develop
its own units of measure, based on weight per unit length. Engineers working with bulk
materials usually normalize characteristics by the cross-sectional area of the material.
In this way, force to failure is converted into stress. The accurate measurement of the
cross-sectional area of a single ﬁber, however, remains difﬁcult. Through the years,

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