JEC COMPOSITES MAGAZINE - Issue #102 - January/February 2016 - 50

quality
Focus
This paper is written by Prof. Edith
Mäder, a world-renowned expert in the
field of interphases from the LeibnizInstitut für Polymerforschung Dresden
e.V., and by Textechno, a leading designer and manufacturer of precision test
equipment/systems for textile and manmade fibres. It describes how such test
equipment can be designed and used
for routine and ongoing quality assurance, both for glass and carbon fibre
composites. Textechno's CEO Dr. Ulrich
Mörschel and Dr. Michael Effing from
AMAC GmbH recently announced, in a
press release dated September 2015,
that they are cooperating in the field of
composites for business development
and market introduction of Textechno's
testing systems.

proach includes three closely interrelated
tasks: (1) comparison of micromechanical tests for interface characterization;
(2) review of existing practice, advantages of the pull-out test and adequate
data treatment, and (3) development of
semi-automatic equipment.
Comparative study of micromechanical
tests for interface characterization
A large number of micromechanical
tests have been developed to determine
the parameters of interfacial interaction
between fibres and matrices. These tests
can be divided into the two groups [2]
shown in Figure 1.
It is obvious that in an adequate test
configuration, the stress distribution
must be similar to that in a real composite. For composites with ductile matrices
and brittle fibres (the matrix elongationto-break is several times greater than the
fibre elongation-to-break, e.g. carbon
fibre-reinforced polymers), the fragmentation test is the most appropriate. On
the contrary, for composites with brittle
matrices, which fail through multiple
transverse cracking (with reinforcing
fibres bridging the cracks), the pull-out
test appears to be closer to reality. However, as a tool for investigating interfacial
adhesion, it can also be successfully used

for matrices with large elongation-tobreak, under the condition that interfacial debonding occurs at moderate relative matrix deformation near the fibre.
Tests based on single fibre pull-out,
which are regarded as especially
interesting and important because
they make it possible to relate the load
transfer ability of the interface to adhesion parameters at molecular level, are
considered below. The results obtained
and the applicability of different models
to the data will be discussed taking into
account specific mechanical properties
of the components.
Review of existing practice
and advantages of the pull-out test
Since their invention more than 50
years ago, the pull-out and microbond
tests have probably remained the most
popular micromechanical techniques for
determining the bond strength between
fibres and matrices. This is due to their
experimental simplicity, well-defined
test geometry and high reproducibility
of experimental results. In these tests,
an adhesion contact is formed between
the fibre and a matrix (Figure 1). After
matrix curing/consolidation, the fibre
is pulled out of the matrix. The applied
force is recorded as a function of the
displacement of the loaded fibre end.
A typical force-displacement curve is
presented in Figure 2.

Fig. 2: Typical force-displacement curve

50 jec composites magazine / No102 January - February 2016

An interfacial crack is initiated at some
point of the interface (close to the fibre
entry) when the applied force reaches a
critical value ("debond" force, Fd) and
then propagates along the embedded
length towards the opposite fibre end.
Interfacial friction plays an important
role in this process. The frictional force
arising in the debonded regions is added
to the adhesion force contribution in
still intact interfacial areas, and the force
applied to the loaded fibre end continues to grow (segment AB). Only when
the intact embedded fibre part becomes
too short, the force begins to decrease.
The maximum force, Fmax, recorded
during the test can be much greater than
the debond force, Fd. Then the whole
embedded length fully debonds and the
measured force drops from Fmax to a
smaller value, Fb. From this moment and
until complete pull-out, the "tail" force
(segment CD) is due to friction between
the fibre and matrix.
The interface strength is characterized
using two main approaches: fracture mechanics (energy-based approach), which
considers the critical energy release rate,
Gic, as a debonding criterion and the
main interfacial parameter, and shear-lag
analysis (stress-based approach), where
interfacial debonding is governed by the
local interfacial shear strength (IFSS),
τd. The adequacy of Gic and τd has been

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #102 - January/February 2016