Fire response of light-weight structural materials under complex loads

Loh, T 2018, Fire response of light-weight structural materials under complex loads, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Fire response of light-weight structural materials under complex loads
Author(s) Loh, T
Year 2018
Abstract Many engineering sectors, such as naval, aerospace, automotive and civil, are increasingly employing light-weight materials for structural applications. Using experimental and analytical techniques, this PhD project aims to assess the structural properties in fire of three important types of light-weight materials; namely aluminium alloys, glass fibre laminates and glass fibre-polymer foam core sandwich composites. The fire performance of these light-weight materials are compared to steel. The thermal, physical and mechanical responses of the materials in elevated temperature and fire-like conditions are assessed, and the mechanisms controlling softening, damage and failure identified. The investigation is conducted for the axial tension and compression load states and a variety of stress and heat flux (fire intensity) conditions. Engineering structures are used in a wide range of geometric conditions in real world applications, and as such the effect of parameters such as material thickness, fibre orientation angle of laminates, and core density of sandwich composites on the fire performance is also investigated.

A comprehensive review of published research on the elevated temperature and fire performance of aluminium alloy and composites materials is presented in this PhD thesis. The literature review assesses the current state of both experimental and numerical testing of metallic and composite materials subject to combined fire conditions and structural loading. Furthermore, the fire-induced damage mechanisms and failure of the composite materials are assessed at a constituent level. Based on the current state of research, gaps in the experimental and numerical research of metallic and composite materials subject to mechanical loading and one-sided fire conditions are identified. The research presented in this PhD thesis aims to address some of these important research gaps.

The effect of fibre orientation on the deformation, softening behaviour and failure of unidirectional glass fibre laminates subject to fire is assessed experimentally and numerically in this PhD project. Experimental fire testing revealed fibre orientation has a large influence on the thermal, mechanical and deformation response of the laminate. The survivability of the laminate in fire-like conditions decreased rapidly with increasing fibre orientation (relative to the tensile loading direction). Increasing the fibre orientation angle caused the failure mode to change from fibre dominate to resin dominate, resulting in a large reduction in structural performance in fire. The softening and failure of the laminate is analysed using a thermal-mechanical model, and good agreement exists for on-axis or obtuse angles, however prediction of acute angled laminates where mixed-mode failure occurs is poor.

An experimental and analytical investigation is also conducted into the structural performance of glass fibre laminates of different thicknesses using experimental fire-under-load tests. The investigation comprised of laminates with a wide range of thicknesses (1.25 mm - 18 mm). The tests are conducted for both the tension and compression load conditions. Increasing thickness affected the thermal response of the laminate which in turn affected the mechanical response of the laminates during fire exposure. Very thin laminates had a very small through-thickness thermal gradient and could be approximated as thermally thin. However, as the thickness increased the through-thickness thermal gradient also increased and this reduced the softening rate of the laminate resulting in an increase in time-to-failure. The softening and failure of the laminates with different thicknesses subject to fire conditions are assessed via a thermal-mechanical model under both tension and compression loading.

Significant scatter in the time-to-failure of composite materials can occur during combined mechanical loading and fire exposure. The investigation into the fire performance of composite materials was extended here to assess the cause of scatter for both tension and compression load conditions. Variability in many properties of the laminate and its constituents are identified, and their ability to influence the fire performance assessed. The key sources of scatter are identified and a new statistical-based thermal-mechanical model is developed to predict the fire performance.

Sandwich composite materials find common use in many engineering applications such as naval vessels and aircraft. Naval sandwich composites frequently employ rigid polymer foams at various densities for use as core materials due to their good resistance to salt water degradation, low water absorption, and buoyancy. One such polymer foam is rigid polyurethane. Here, the effect of core density on the structural integrity of sandwich composites during fire exposure is investigated. Through analytical and experimental analysis, core density was found to influence the thermal response of the sandwich composite with in turn affected the mechanical response, failure behaviour and ultimately time-to-failure.

The effect of thickness on the fire performance of steel and aluminium alloy is numerically assessed in the tension and compression load conditions via a finite element model. The thermal-mechanical model, which has previously been validated to accurately predict the fire performance of aluminium in fire, was used for both metals. The effect of increasing thickness was smaller for aluminium than for steel due to its much higher thermal conductivity and lower creep activation energy. The relative fire-under-load performance of aluminium and steel is also assessed. The steel shows significantly better fire performance in both the tension and compression load conditions due to its superior elevated temperature properties, lower thermal conductivity and higher creep activation energy.

An investigation into the relative structural integrity of steel, aluminium alloy, glass fibre laminate and glass fibre sandwich composites subject to one-sided fire conditions is undertaken. The thermal response, deformation behaviour and structural integrity for the materials are compared. The thermal response is dependent on the material type, with composite materials experiencing much greater temperatures than metals when exposed to the same heat flux. The difference in thermal response is due to the difference in material properties as well as the difference in damage behaviour. Composite materials often exhibit damage that retards conductivity of thermal energy from the heat exposed surface to the unexposed surface whereas metals do not. The fire-structural response of the materials is assessed for tension and compression load conditions. Steel performs best under tension load conditions due to its superior elevated temperature properties. The glass fibre laminate performed second best due to the thermal resistance of the glass fibres. The fire performance of the aluminium alloy and sandwich composite was similar. Under compression load conditions the steel and aluminium performed significantly better than the composite materials; and the sandwich composite performed the worst. The poor performance of the composite materials was due to the reliance on the polymer matrix properties under compression loading.

The research presented in this PhD thesis contributes to the understanding of the thermal and mechanical response of load-bearing metallic and composite structures and their survivability when exposed to fire. Furthermore, important scientific insights into the damage behaviour and mechanisms contributing to failure during fire are identified. This research contributes significantly to the development of validated models and experimental data for light-weight materials in fire, and can be used to help evaluate the fire safety in modern engineering applications where fire is an ever present risk.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Composite and Hybrid Materials
Keyword(s) Fire
High temperature
Mechanical properties
Glass fibre
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Created: Wed, 27 Mar 2019, 13:14:35 EST by Adam Rivett
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