Assessing latent initiator based multiple healing in epoxy matrix GFRP laminated composites
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Date
2020-07
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Department of Mechanical Engineering, NITS
Abstract
The matrix of any structural materials typically consists of brittle polymer, which is
susceptible to crack initiation and growth. To resist this flaw, the concept of self-healing is
introduced. Present research work demonstrates a latent initiator based healing agent is
used externally into the damage region to restore its original mechanical properties after
healing followed by a strong covalent bond at the interface. Also, healing can be possible
in the multi-step. In comparison to another healing approach, the healing efficiency of
healed samples was found higher compared to the virgin sample. In order to achieve this
objective, latent initiator based healing agent grafted silica nanoparticles were synthesized
using the one-pot/sol-gel method (IFSs) and multi-step process/post-synthesis grafting
method (IGSs). Further, the performance and healing ability IFSs and IGSs reinforced
epoxy composites have been investigated. After detail investigations, the results concluded
that IFSs/IGSs has no healing capability for the resistance of crack in the epoxy
nanocomposites. Looking opportunity towards maximum healing efficiency with unaltered
mechanical properties, imidazole as a suitable latent initiator based healing agent for
extrinsic healing capabilities of IFSs/epoxy composites using have been investigated. After
a successful demonstration towards enhancing the toughening and healing performance of
nanocomposites, multiple healing capabilities of GFRPs laminated composites using the
same healing agent have been investigated further.
Initially, the effect of varying content of imidazole on tensile, flexural and hardness
properties of epoxy nanocomposites was demonstrated. Composites (IENC-110) added
with 10 wt. % of imidazole and 1 wt. % IFS showed maximum enhancement in tensile
strength by 61 % (66 MPa) compare to NE (41 MPa) respectively. However, the tensile
modulus is remained unchanged as compared to NE. For flexural properties, the significant
enhancement in flexural strength and flexural modulus by ~96 % (102 MPa), and 15 %
(2.83 GPa), respectively for IENC-15 composites was found. Similarly, the maximum
enhancement in hardness for IENC-110 composites by 19 % (24.57 HV) was found as
compared to that of NE (20.71 HV). Further, multiple fracture test of SENB was performed
to evaluate fracture toughness and healing performance of epoxy nanocomposites. The
results showed the maximum recovery in fracture toughness for IENC-110 composites by
67 % (1.17 MPa.m0.5) as compared to the virgin sample (0.70 MPa.m0.5) was achieved over
3rd number of healing cycles. However, the maximum healing efficiency for IENC-110
composites of 140 % due to critical peak loads while 167 % due to fracture toughness at
the 3rd number of healing cycles was achieved. Finally, the appearances of hackel marks in
the fracture surface under fractography analysis also justify the significant enhancement in
surface roughness.
Further, multiple healing capabilities of GFRPs laminated composites using the same
healing agent have been investigated. The effect of varying content of imidazole on tensile,
flexural and hardness properties of GFRPs composites was demonstrated. The maximum
enhancement in tensile strength, tensile modulus, strain and modulus of the toughness of
GFI-10 composites of ~34 % (~464 MPa), ~13 % (~6.80 GPa), ~13 % (~8.16 %) and ~64
% (~22 MJ.m-3) respectively were found. This is mainly ascribed to the formation of strong
interface between the fiber and the matrix due to the addition of imidazole. Further, the
optical micrograph of the tensile fracture surface of the virgin sample showed poor
through-thickness delamination where the fibers are failed like a broom. In contrast, the
addition of imidazole in epoxy/GFRPs composites restricts the through-thickness
delamination and reduced the blooming effect with partial fiber breakages. For flexural
properties, the maximum enhancement in flexural strength, flexural modulus, flexural
strain and work of flexural of GFI-10 composites of ~34 % (~389 MPa), ~22 % (~19.70
GPa), ~100 % (~0.04 mm/mm) and ~72 % (~16.76 MJ.m-2) respectively were found. This
attributed the formation of strong bonding between glass fiber and matrix due to sustaining
the more flexural stress. Similarly, for hardness properties, the maximum enhancement in
the hardness of composite (IEGF-110) added with 10 wt. % of imidazole and 1 wt. % of
IFS of 21 % (74.44 HV) was found. This may be attributed to the high packing density of
the final network. Further, multiple fracture test of DCB was performed to evaluate
fracture energy and healing performance of GFRPs composites. The results demonstrate a
maximum recovery in fracture energy for GFI-10 composites by 83 % (1.87 kJ/m2) as
compare to the virgin (0.37 kJ/m2) over 12th healing cycles. It suggests that healing with
excess (10 wt. %) imidazole with respect to epoxy within the delamination surface
increases the number of bonds that bounds the delaminated surfaces strongly during the
repetitive healing. However, the maximum healing efficiency for GFI-10 composites of
154 % due to critical peak loads while 182 % due to fracture toughness at 12th number of
healing cycles was achieved. Finally, the optical micrograph of the mid fracture surface of
DCB samples showed the relatively rough fractures surface because of fiber bridging and
fiber pull-out for GFI-10. Thus, the capability of imidazole to restrict the damage in FRPs
composites is evident from the fractography investigation. This work successfully
demonstrates the capability of imidazole introduce healing capability in GFRPs.