Behaviour of glass FRP composite tubes under repeated impact for piling application

Fibre composites have been a viable option in replacing traditional pile materials such as concrete, steel and timber in harsh environmental conditions. On the other
hand, the emergence of fibre reinforced polymer (FRP) composite tubes as a structural component and their corrosion-resistant characteristics made these materials potential in piling application. Driving these piles, however, requires more careful consideration due to their relatively low stiffness and thin walls. The possibility of damaging the fibre composite materials during the process of impact driving is always a concern. Research has therefore focused in understanding the impact behaviour of these materials in order for them to be safely and effectively driven into the ground.
This study investigated the behaviour of composite tubes subjected by repeated axial impact. The effects of impact event (incident energy and number of impact) on the instantaneous response and the residual properties of composite tubes were examined. Tubes made of glass/vinyl ester, glass/polyester, and glass/epoxy materials of different cross sections were considered. The impact behaviour of the tubes was experimentally and analytically investigated. An experimental study on the repeated impact behaviour of square composite tube was conducted. The result showed that the dominant failure mode of the tube
repeatedly impacted was characterised by progressive crushing at the upper end. This failure was manifested by inter and intra laminar cracking and glass fibre ruptures
with simultaneous development of axial splits along its corners. It was found that the drop mass and impact velocity (or drop height) have pronounced effects on the
collapse of the tubes at lower incident energies. Their effects, however, gradually decrease at relatively higher energies. The result also indicated that the incident
energy is the major damage factor in the failure of tubes for lower number of impacts. On the contrary, the number of impacts becomes the key reason as soon as the value of incident energy decreases.
The effects of the damage factors such as the level of impact energy, the impact repetitions, and the mass impactor on the residual (post-impact) properties
were also examined. The result of the investigation revealed that these factors significantly influenced the residual strength degradation of the impacted tubes. In
contrast, the residual modulus was found to be less affected by these factors since the damage brought by them is localised in most of the cases. The maximum reduction
on the residual moduli is roughly 5%. On the other hand, the residual strengths degraded by up to 10%. The flexural strength of the tube was the most severely affected by the impact damage than its compressive and tensile strengths. This result was due to the fact that the impact damage on matrix and fibre both contributed on the flexural strength degradation. Moreover, the presence of matrix cracks or
delamination lead to an increase in buckling instability during the flexural test, resulting to a much higher degradation compared to the other strengths. The comparison of the residual compressive strengths sourced at different locations along the height of the tube revealed that the strength reduction varied with its location. The degradation of the compressive strength of the impacted tube decreased when its location from the top of the tube increased. This result indicated that the influence of
impact damage on the degradation of residual compressive strength of the tube is concentrated only in region closer to the impact point.
Finally, theoretical prediction using the basic energy principle was performed to gain additional understanding on the damage evolution behaviour of composite tubes subjected by repeated axial impact. The damage evolution model was verified through experimental investigation on a 100 mm square pultruded tube. The model was applied to composite tubes of different cross sections and materials made from
vinyl ester/polyester/epoxy matrix reinforced with glass fibres. It was found that the experimental results on a 100 mm square pultruded tube and the proposed damage model agreed well with each other. The variation is less than 10% indicating that the model predicted reasonably the damage evolution of the tube subjected by repeated impact loading. It was also found that the energies describing the low cycle, high cycle, and endurance fatigue regions of the composite tubes are largely dependent on their corresponding critical energy Ec. The higher the Ec values, the higher the range of energies characterising these regions. The repeated impact curves (or Ec) of tubes
made from glass/epoxy is higher compared to the other matrix materials. Similarly, circular tubes have greater Ec values of comparable square and rectangular tubes. From this study, an improved understanding of the behaviour of glass fibre FRP composite tubes under repeated axial impact can be achieved. The information provided in this study will help in developing efficient techniques and guidelines in driving composites piles.