SAMPE Journal - January/February 2025 - 23

LRS approach to non-Raman active glass fiber (GF)
composites by coating GFs with a Raman active
coating[25]
ride[26]
and doping GFs with samarium fluo.
Since the first approach changes the fundamental
nature of the interface of interest and
the second approach yields considerable scatter
in the strain-profile data, these approaches have
not been widely adopted. However, the published
data does support Galiotis' conclusion that the K-T
model is not appropriate for calculating the F-M
IFSS in SFFT specimen prior to the occurrence of
extensive damage that may be caused by the fiber
fragmentation process. Additionally, carbon nanotubes
(which are Raman active) have been added
to several matrices in an attempt to determine the
matrix stresses around fiber breaks[27]
.
1.3 The Basis for the Current Approach on
non-Raman Active (Glass) Fiber Composites
GF composites are known to exhibit minimal
debonding during the fiber fragmentation process
when the GFs are bonded covalently to the matrix
using silane coupling agents or by hydroxyl-hydroxyl
bonding between the matrix and the fiber[28]
.
Additionally, to better understand the fiber fracture
dynamics, research has been done on GF-SFFT
specimens by integrating a novel molecular probe
into the epoxy matrix and analyzing the effects of
the fiber fracture process on the fluorescence lifetime
of the surrounding matrix[29]
revealed
that
the
stress-redistribution
. These results
process
associated with fiber fracture results in localized
changes in the matrix lifetime 5 to 10 fiber diameters
away from the fiber break. These lifetime
changes have been associated with matrix yielding
and/or damage due to breakage of chemical bonds
in the network.
In addition, no apparent yielding
of the matrix at the fiber ends, as is often inferred
from the quasi-static shear-lag models, was observed.
However, the birefringence pattens under
stress suggests that this localized matrix yielding/
damage has only minimal impact on the observed
fiber stress profile and its correlation with Cox
shear-lag models.
Since LRS cannot be easily adapted to measure
the fiber strain along its length in GF composites,
this paper proposes to determine a non-saturation
F-M ISS value by using a variant of Equation 7 to fit
the Cox-type equation to the average strain in the
fiber (〈eƒ
{zi,e,t}〉). To distinguish this non-saturation
value from the Galiotis approach, it will be termed
F-M (t〈ISS〉
). As will be subsequently discussed,
this approach only requires that the matrix be ductile
enough to induce at least several breaks in the
fiber prior to specimen failure and deform in a
nonlinear anelastic behavior during the SFFT.
Figure 5. Polycarbonate stress-strain curves
showing intrinsic yield point in compression,
tension and shear along with the extrinsic yield
point in tension. (adapted from[31]
).
www. sampe.org
JANUARY/FEBRUARY 2025
|
SAMPE JOURNAL |
23
2. PROPOSED NON-SATURATION METHODOLOGY
FOR ASSESSING F-M IFSS IN GLASS/
FIBER COMPOSITES
2.1 Nonlinear Anelastic Deformation Behavior
in Thermoset Resins
The yield point in a material is the point beyond
which the deformation ceases to be entirely elastic.
[30]
An amorphous glassy polymer such as polycarbonate
(PC,) deformed in tension can exhibit two
yield points (extrinsic and intrinsic) (Figure 5). Although,
the distinction between elastic (recoverable)
and plastic (permanent) deformation is difficult
to apply precisely to polymers, the intrinsic
yield point denotes the onset of a mixture of elastic
and plastic deformation. Thus, prior to the intrinsic
yield point the strain is completely recoverable,
hence the polymer deforms in a nonlinear anelastic
manner in the limited region that is between the
end of elastic deformation and the intrinsic yield
point.
The yield behavior of thermoset polymers (e.g.,
epoxies) are typically investigated in compression,
since at temperatures well below the glass
transition temperature, the specimens often fail
in tension by brittle fracture before global yield
occurs[32,33]
. Because of em
ƒ requirements for the
matrix in the SFFT, only high extension-to-failure
thermosets are investigated. The most common
is the diglycidyl ether of bisphenol-A (DGEBA)
cured with meta-phenylenediamine (m-PDA)[2]
DGEBA cured with Jeffamines (e.g., D-230)[24]
4,4'-methylenedianiline (MDA),[34,35]
.
and
have also been
investigated. Because flexibilizers and reactive diluents
are commonly used in composite manufacture,
DGEBA blended with diglycidyl ether of
1,4-butanediol (DGEBD, aliphatic reactive diluent)
or polyglycol diepoxide (DER-732, aliphatic flexibilizer),
have also been reported. These network
blends were cured with m-PDA,[24]
eutectic blends of m-PDA/MDA[24]
MDA,[34,35]
.
and
http://www.sampe.org
SAMPE Journal - January/February 2025

Table of Contents for the Digital Edition of SAMPE Journal - January/February 2025
