Fibre reinforced polymers (FRP) are finding increasing use in structures subjected to
high rate loading such as blast or impact. Proper design of such structures requires
thorough characterisation of the material behaviour over a range of loading rates from
quasi-static to impact. This thesis investigated the quasi-static and impact response
of Glass Fibre Polypropylene (GFPP) in compression, bending and delamination. The
bending and delamination response of Fibre Metal Laminates (FMLs) based on GFPP
and aluminium was also investigated at quasi-static and impact rates.
High strain rate (5x10^2 to 10^3 /s) compression tests were conducted on GFPP using
a compressive Split Hopkinson Pressure Bar (SHPB) and a Direct Impact Hopkinson
Pressure Bar (DIHPB), in the through-thickness and in-plane directions. In both loading
directions, the peak stress of GFPP increased linearly with the logarithm of strain
rate. For in-plane loading, the failure modes were dominated by localised fibre buckling
and kink bands, leading to delamination. The through thickness loading produced
macroscopic shear and spreading failure modes. However, both of these failure modes
are linked to in-ply fibre failures, due to through thickness compression causing transverse
tensile strain. Previous studies of similar materials have not explicitly stated the
link between through thickness compression and fibre failure associated with transverse
tensile strain.
A novel test rig was developed for Three Point bend testing at impact rates. The
specimen was supported at the outer points on a rigid impacter and accelerated towards
a single output Hopkinson Pressure Bar (HPB), which impacted the specimen
at its midspan. Previous impact bend test rigs based on HPBs were limited to testing
specimens with deflections to failure up to approximately 1mm, whereas the rig implemented
herein measured deflections up to approximately 10 mm. This configuration
permits the output HPB to be chosen purely on the magnitude of the expected impact
force, which resulted in superior force resolution to configurations used in other
studies. The HPB Impact Bend rig was used to test GFPP and aluminium-GFPP FML
specimens, at impact velocities ranging from 5 to 12 m/s. The flexural strength of GFPP
increased with strain rate, while the flexural response of the FML specimens was relatively
insensitive to strain rate.
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Several candidate delamination test geometries were investigated at quasi-static
displacement rates (1 mm/min), and the Single Leg Bend (SLB) test was identified as
suitable for adaptation to higher rate testing. Single Leg Bend delamination tests of
both GFPP and FML specimens were performed using the HPB Impact Bend rig, at
impact velocities of 6 to 8 m=s. The shape of the force displacement response for the
high rate testswas markedly different from the quasi-static tests, for both the GFPP and
FML specimens. Finite element (FE) simulation of the quasi-static and impact rate SLB
tests on GFPP indicated that the difference was probably due to the interaction of flexural
vibrations and stress waves in the specimen and the impacter cross member. The
experimental results and FE analysis suggest that the delamination fracture toughness
of GFPP decreases slightly as strain rate increases. High rate delamination tests on
FML specimens resulted in unstable crack growth.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/25150 |
| Date | 12 1900 |
| Creators | Govender, Reuben Ashley |
| Contributors | Langdon, Genevieive, Nurick, Gerald |
| Publisher | University of Cape Town, Faculty of Engineering and the Built Environment, Blast Impact and Survivability Research Unit |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, Doctoral, PhD |
| Format | application/pdf |
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