| Thermal
shock on interfacial adhesion of thermally conditioned
Glass fiber/epoxy composites |
Definition
The fiber/matrix adhesion
is most likely to control the overall mechanical behavior of fiber-reinforced
composites. An interfacial reaction may result in various morphological modifications
to polymer matrix microstructure in proximity to the fiber surface. The interactions
between fiber and polymer matrix during thermal conditioning and thermal shock
are important phenomena. Thermal
stresses were built-up in glass fiber reinforced epoxy composites by up-thermal
shock cycles (negative to positive temperature exposure) for different durations
and also by down-thermal shock cycles (positive to negative temperature exposure).
The concentration of thermal stresses often results in weaker fiber/matrix interface.
A degradative effect was observed in both modes for short shock cycles and thereafter,
an improvement in shear strength was measured. The effects were shown in two different
crosshead speeds during short-beam shear test.
Differential thermal expansion is a prime cause of thermal shock in composite
materials. Thermal expansion differences between fiber and matrix can contribute
to stresses at the interface [1-5]. A very large thermal expansion mismatch may
result in debonding at the fiber/matrix interface and/or a possible matrix cracking
due to thermal stress [6-8]. The fiber/matrix interface is likely to affect the
overall mechanical behavior of fiber-reinforced composites. The
performance of fiber reinforced composite is often controlled by the adhesion
chemistry at the fiber/matrix interface. Thermal expansion coefficients of polymers
are substantially greater compared to metals or ceramics. That is why failure
of the bond between fiber and resin occurs under the influence of temperature
gradient. The common reinforcement for polymer matrix is glass fiber. One of the
disadvantages of glass fiber is poor adhesion to matrix resin. The
short beam shear (SBS) test results may reflect the tendency of the bond strength
where only the bonding level is a variable [9]. A large number of techniques have
been reported for measuring interfacial adhesion in fiber reinforced polymer composites
[10-16]. A need probably exists for an assessment of mechanical performance of
such composite under the influence of thermal shock. Thermal
stresses caused by temperature gradient should be given special attention in many
application areas. A better understanding of interfacial properties and characterization
of interfacial adhesion strength can help in evaluating the mechanical behavior
of fiber reinforced composite materials.
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