Effect of layer arrangement on post impact and open hole tensile properties of kenaf-glass hybrid composite laminates / Syarifah Yunus

Kenaf fibre reinforced polymer (KFRP) composites attract researchers, scientists,
academicians and also industry players because of their light weight properties,
abundantly available and low cost compared to those of synthetic fibre composites.
KFRP composites possess high potential to be used as primary and secondary structures
for many applications, especially automotive, aerospace and building construction.
Moreover, kenaf fibre plant is not harmful to human and has positive impact to
environment. However, there are several limitations of kenaf fibre composites such as
their low damage resistance and tolerance after subjected to low impact loading or
presence of defect, open hole or notch. Thus, in this study, kenaf fibre plies have been
hybridised with woven glass fibre mats in order to improve the post impact and open
hole tensile properties. These hybrid composite materials were fabricated by hand layup
and cold press techniques. Two different systems were fabricated based on specific
fibre layer arrangements of kenaf-glass fibre reinforced polymer (KGFRP) hybrid
composites, i.e. (i) Two layers of long kenaf fibre sandwiched two glass fibre mats,
[0°k/90°k/G]s and (ii) One layer of glass fibre mat sandwiched four layers of long kenaf
fibres, [G/0°k/90°k]s. In this study, the tensile strength, modulus and strain to failure at
maximum load for [G/0°k/90°k]s hybrid composites were 5%, 10% and 19% higher
than those of [0°k/90°k/G]s hybrid composites, respectively. The effect of layer
arrangement, for both KGFRP hybrid composites, on post impact and open hole
condition was evaluated. The results of post impact tensile test revealed that the
[G/0°k/90°k]s. i.e woven glass fibre at the outer layer better strength than [0°k/90°k/G]s
up to 10% higher as shown in Normalized Stress curve. The woven glass fibre at outer
layer helps to sustain more impact load than having kenaf at outer layer. As a
consequence, this hybrid composites exhibit more resistance to crack growth. However,
the result obtained from open hole tensile test the hybrid composite [0°k/90°k/G]s
having long kenaf fibre at the outer layer is less sensitive to holes size compared with
the [G/0°k/90°k]s . The tensile strength of the former laminate arrangements can be 15%
higher than the latter with respect to larger hole sizes. The structure of long kenaf at
outer layer influenced the strength of the hybrid composites as it holds higher loads
before the load is transferred to the glass fibre at the inner layer. The damage
mechanisms of KGFRP hybrid composite materials were then verified qualitatively
with the surface texture scanning machine, optical microscope, digital DSLR camera
and SEM. A closer examination on fracture surfaces for all specimens using SEM
showed that matrix cracking, fibre fracture, fibre pull-out and delamination were the
main failure mechanisms of KGFRP hybrid composite material. The damage area
obtained from the post impact and open hole of hybrid composites tests was then
correlated with the average stress criterion of Whitney-Nuismer equation with
modifying factor (MF) via fracture mechanics concepts. This correlation equation could
be used to predict the residual strength or strength degradation rate for both impact and
open hole cases of composite materials.