Abstract
Carbon fibres, owing to their highest tensile strength and stiffness amongst all fibres used in composite fabrication, are most popular in structural composites. However, out-of-plane impact forces are a major design concern when carbon fibre is used since its impact resistance is lower than that of glass fibre. There is considerable literature available on the impact and post impact behaviour of carbon fibre-reinforced polymer composites (CFRP). However, there is a gap in literature on the effect of heat/fire on impact damage to CFRPs, especially concerning the nature of particulates released during these conditions. Quite often nanoparticle additives are introduced in the matrix of the CFRP to increase impact resistance and other properties including flame resistance, which could also be released along with other debris, during impact and fire.
The aim of this research was to understand the behaviour of carbon fibre/epoxy composites (CFE) containing nano and micro particulate additives subjected to simultaneous impact and heat/fire conditions. This required novel laboratory scale testing equipment to be designed and developed with the ability to combine impact and heat/fire conditions testing whilst capturing any released debris/particulates and post-test characterisation of physical and chemical properties of the captured particles, from which the associated health environmental concerns could be established.
The design of such novel equipment incorporated a pendulum impactor to create impact whilst the sample was exposed to a radiant cone heater at a particular heat flux for a specified period of time.
Moreover, protocols using the bespoke impact and fire equipment for testing samples under different conditions and capturing particulates released, both from the front and back faces, along with those in the effluents were established. Parallel to the development of the set-up, high fibre ratio CFE composites, comparable to aero grade materials, were produced by resin infusion. CFE laminates with incorporated organomodified montmorillonite clays (nanoclays), layered double hydroxide (LDH), graphene oxide (GO) and carbon nanotubes (NT), were also produced.
These samples were then subjected to 19J impact by a hemispherical type impactor as: impact only, impact after 50kW/m² radiant heat exposure at 90s, 120s,180s and 240s and impact during 75kW/m² fire conditions, 20s post ignition. A loss of stiffness as a function of heat exposure time was found to affect the damage type observed for CFE samples. The introduction of nanoparticle additives LDH, NT and GO; reduced impact resistance during impact only and impact at 90s heat exposure, whilst at extended heat exposure duration, past 180s, improved damage resistance owing to increased charring and char reinforcement, especially in samples containing GO.
Particulates released during impact only testing were fibres and resin particles. Impacts after 50kW/m² radiant heat exposure at 90s, 120s, 180s and 240s, carbon fibres, decomposed resinous particles and resin coated fibres were released. During fire conditions additional soot particles were released. Quantitative and morphological analyses of captured particles from CFE samples demonstrated that the sizes of decomposed resin particles reduced with increasing time up to impact at 180 or 75kW/m² heat flux. The type, size and shape of particles released from samples containing nano particles were similar to those released from CFE without nanoparticles with the exceptions that there was a significant increase of resin particles released from LDH samples during impact only and particulates captured during fire conditions appeared less decomposed.
During impact only and impact during 50kW/m², fibres released regardless of sample composition, were in a size range (diameter 5-7.5µm and length 5-250µm) which can be deposited in the nose and throat region of the respiratory tract. On two separate occasions, fibres were sampled from effluents during impact after 50kW/m² radiant heat exposure at 90s, indicating released fibres could remain airborne. Fibres of reduced diameter and fibril bundles were observed from CFE samples without nanoparticles during fire conditions. Their dimensions (diameters less than 1μm and lengths 5-20µm) were within a range (diameters less than 3μm and lengths between 3-80μm) that could potentially enter the alveoli region of the respiratory tract and resist biological removal mechanisms, making them the greatest concern regarding potential health effects.
Many of the released resin particles observed throughout the research from all sample compositions and test conditions, had respirable diameters less than 4µm. Some resin particles were in or close to nanoscale in which they can deposit more effectively in the alveolar region of the respiratory tract. The introduction of heat and the decomposition of the resin particles potentially increased their deposition efficiency compared to impact only.
A specific study investigating sub-micron sized particles released in effluent during impact and fire conditions involved combining a Dekati Low Pressure Impactor (DLPI) and a Ecomesure Mini Particle Sampler (MPS) with the bespoke impact and fire equipment. The results from the DLPI showed that up to 75% of the soot and particles released could deposit in the lungs reaching the bronchi region at a minimum, whilst for the MPS 75% of soot agglomerates released from CFE samples could deposit deep in the lung in the bronchioles and alveoli.
Finally, an in vitro toxicological study using murine macrophage cell culture media was performed on particles released from CFE and CFE containing GO during impact and fire to finalise the research. The results indicated that GO induced pro-inflammatory and oxidative stress responses when diluted alone or as part of soot agglomerates and fibres of reduced diameter induced a cytotoxic response.