S. Feih

Tensile Strength Modeling of Glass Fiber—Polymer Composites in Fire

A thermal-mechanical model is presented to calculate the tensile strength and time-to-failure of glass fiber reinforced polymer composites in fire. The model considers the main thermal processes and softening (mechanical) processes of fiberglass composites in fire that ensure an accurate calculation of tensile strength and failure time. The thermal component of the model considers the effects of heat conduction, matrix decomposition and volatile out-gassing on the temperature—time response of composites. The mechanical component of the model considers the tensile softening of the polymer matrix and glass fibers in fire, with softening of the fibers analyzed as a function of temperature and heating time. The model can calculate the tensile strength of a hot, decomposing composite exposed to fire up to the onset of flaming combustion. The thermal-mechanical model is confined to hot, smoldering fiberglass composites prior to ignition. Experimental fire tests are performed on dry fiberglass fabric and fiberglass/vinyl ester composite specimens to validate the model. It is shown that the model gives an approximate estimate of the tensile strength and time-to-failure of the materials when exposed to one-sided heating at a constant heat flux. It is envisaged the model can be used to calculate the tensile softening and time-to-failure of glass—polymer composite structures exposed to fire.

High-Value SLM Aerospace Components: From Design to Manufacture

Today additive manufacturing is shaping the future of global manufacturing and is influencing the design and manufacturability of tomorrows products. With selective laser melting (SLM), parts can be built directly from computer models or from measurements of existing components to be re-engineered, and therefore bypass traditional manufacturing processes such as cutting, milling and grinding. Benefits include: 1) new designs not possible using conventional subtractive technology, 2) dramatic savings in time, materials, wastage, energy and other costs in producing new components, 3) significant reductions in environmental impact, and 4) faster time to market. SLM builds up finished components from raw material powders layer by layer through laser melting. SLM removes many of the shape restrictions that limit design with traditional manufacturing methods, thereby allowing computationally optimised, high performance structures to be utilised. Functional engineering prototypes and actual components can then be built in their final shape with minimal material wastage. Samples and small product runs can be produced quickly at comparatively low cost to test and build market acceptance without major investment. In this chapter we present and discuss some of the concepts and findings involved in the design, manufacture and examination of high-value aerospace components from Ti-6Al-4V alloy produced at the RMITs Advanced Manufacturing Precinct.

Modeling composite high temperature behavior and fire response under load

This paper discusses the characterization and modeling of thermoplastic and thermosetting matrix composites under load in fire. Small-scale tests were found to provide a cost-effective means of characterizing load-bearing behavior of composites in fire and a useful framework for materials development. This paper demonstrates the modeling of thermal and decomposition behavior during the test and the extension of this modeling to include mechanical response and failure behavior. The work necessitated measurement of strength and stiffness over a wide temperature range, with interesting results up to the point of resin decomposition. The approach was applied to three 12 mm thick glass reinforced systems: vinyl ester, polyester, and polypropylene. The laminates were subjected to a one-sided 50 kW·m−2 heat flux, using a propane burner. Thermal behavior was modeled using a simplified version of the Henderson equation to predict the evolution of temperature and residual resin content through the thickness. These parameters were then used, along with a material model, to predict the mechanical response in fire.

Modeling Compressive Skin Failure of Sandwich Composites in Fire

A thermal-mechanical model is presented for calculating the residual compressive strength of flammable sandwich composite materials in fire. The model can also estimate the time-to-failure of the laminate face skin to sandwich composites exposed to fire. The model involves a two-stage analysis: thermal modeling and mechanical modeling. The thermal component of the model predicts the temperature profile and amount of decomposition through sandwich composites exposed to one-sided heating by fire. The mechanical component of the model estimates the residual compressive strength of the sandwich composite and the onset of skin failure. The model is tested for sandwich composite materials with combustible glass/ vinyl ester skins and balsa core. Experimental fire tests are performed on the sandwich composites under combined compressive loading and one-sided heating at constant heat flux levels between 10 kW/m 2 (Tmax · 250°C) and 50 kW/m2 (·600°C). The model predicts that the time-to-failure increases with the skin thickness and decreases with an increase to the applied compressive stress or heat flux. The predictions are supported by experimental data from fire-under-load tests. It is envisaged that the model can be used to design sandwich composite materials with improved compressive load capacity in fire.

Modification of the carbon fiber/matrix interface using gas plasma treatment with acetylene and oxygen

Acetylene and oxygen gas plasma treatment of PAN-based carbon fibers to increase the interfacial shear strength was investigated. The effects of different gas mixtures and exposure times were studied. Changes in the fiber surface chemistry were characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy, and wetting tests. The adhesion of epoxy to treated fibers was measured using the microbond technique and untreated fibers were tested to serve as controls. It was found that a significant improvement (α = 0.01) in the interfacial shear strength was obtained for fibers exposed to the plasma for 1 min at a ratio of 2 : 1 (acetylene/oxygen). However, the plasma treatments did not influence significantly (α = 0.01) the tensile strength of any of the fibers. FTIR analysis of the plasma film produced on glass slides under the treatment conditions showed that the expected chemical film composition for polyacetylene was achieved. Analysis of the fiber surface by wetting tests in...

Failure model for phenolic and polyester pultrusions under load in fire

Abstract The failure of polyester and phenolic pultrusions under tensile and compressive load and a one sided heat flux of 50 kW m−2 has been studied. A thermal/mechanical model, based on the Henderson equation and laminate theory, has been used to model their behaviour. In tension, significant load bearing capacity was retained over a period of 800 s, due to the residual strength of the glass fibres. However, pultruded composites are susceptible to compressive failure in fire, due to the loss of properties when the resin T g is reached. The fire reaction properties reported here showed the phenolic pultrusions to perform better than polyesters in all fire reaction properties (time to ignition, heat release, smoke and toxic product generation). The measurements under load in fire showed that the phenolic system decayed at a slower rate than the polyester, due mainly to the very shallow glass transition of the phenolic, but also the char forming characteristic of the phenolic. The behaviour described here for phenolic pultrusions is superior to that reported for some phenolic laminates, the main reason probably being their lower water content.

High-temperature mechanical properties and thermal recovery of balsa wood

This article presents an experimental study into thermal softening and thermal recovery of the compression strength properties of structural balsa wood (Ochroma pyramidale). Balsa is a core material used in sandwich composite structures for applications where fire is an ever-present risk, such as ships and buildings. This article investigates the thermal softening response of balsa with increasing temperature, and the thermal recovery behavior when softened balsa is cooled following heating. Exposure to elevated temperatures was limited to a short time (15 min), representative of a fire or postfire scenario. The compression strength of balsa decreased progressively with increasing temperature from 20° to 250°C. The degradation rates in the strength properties over this temperature range were similar in the axial and radial directions of the balsa grains. Thermogravimetric analysis revealed only small mass losses (<2%) in this temperature range. Environmental scanning electron microscopy showed minor physical changes to the wood grain structure from 190° to 250°C, with holes beginning to form in the cell wall at 250°C. The reduction in compression properties is attributed mostly to thermal viscous softening of the hemicellulose and lignin in the cell walls. Post-heating tests revealed that thermal softening up to 250°C is fully reversible when balsa is cooled to room temperature. When balsa is heated to 250°C or higher, the post-heating strength properties are reduced significantly by decomposition processes of all wood constituents, which irreversibly degrade the wood microstructure. This study revealed that the balsa core in sandwich composite structures must remain below 200°–250°C when exposed to fire to avoid permanent heat damage.

Compression Failure of Carbon Fiber-Epoxy Laminates in Fire

This paper investigates the compression failure of carbon fiber-epoxy laminates when exposed to fire. The investigation gives insights into the softening and failure of carbon-epoxy laminates supporting compression loads in the event of aircraft fire. A thermomechanical model is presented for calculating the compression properties and failure of polymer matrix laminates under combined loading and one-sided heating by fire. The accuracy of the model to predict the failure time of carbon-epoxy laminates at different compression load levels and fire temperatures is determined with structural fire tests performed on woven carbon-epoxy panels. The model predicts a gradual increase in the failure time of the laminate with decreasing compression stress (down to 10% of the room temperature buckling load) and decreasing heat flux (or temperature) of the fire. This was confirmed by the fire tests, which showed good agreement between the calculated and measured failure times. Compression failure of the laminate usually occurred within relatively short times (less than a few minutes) by viscous softening of the polymer matrix. For long failure times, matrix decomposition was shown to influence the failure process. Parametric analysis using the model reveals that raising the glass transition temperature of the polymer matrix increases the compression failure time of laminates in fire. However, only small improvements to the failure time are achieved by raising the glass transition temperature of laminates exposed to high-temperature fires typical of postcrash aircraft accidents.

Experimental impact damage study of a z-pinned foam core sandwich composite

This paper assesses the impact damage and post-impact compression properties of a foam core sandwich composite reinforced with through-thickness z-pins. Low-speed flat-wise compression tests performed on the sandwich composite revealed that z-pins improved the elastic modulus, crush strength and absorbed energy capacity (by 260–300%). However, these property improvements do not necessarily translate into a reduction in the amount of damage suffered by the z-pinned sandwich composite under localised (point) impact loading. There was no reduction to the impact damage area or an improvement to the post-impact compression properties of the z-pinned sandwich composite at low-impact energies (when damage was confined to the impacted face skin). Z-pins were only marginally effective at reducing the impact damage when the impact energy was high enough to cause core crushing. Z-pins absorbed high-impact energy via splitting, microbuckling and fragmentation during core crushing which reduced slightly the amount of impact damage to the sandwich composite. However, this did not cause a significant improvement to the post-impact compressive stiffness and strength for most energy levels.

Fire structural modeling of polymer composites with passive thermal barrier

A coupled thermo-mechanical model is presented for calculating the compressive strength and failure of polymer laminated composites with thermal barrier when exposed to fire. Thermal barriers are used to protect composite structures from fire, and this article presents a model for calculating the improved structural survivability under compression loading. The thermal component of the model predicts the through-thickness temperature profile of the composite when protected from fire using a passive thermal barrier insulation material. The thermal analysis is coupled to a mechanical model that calculates the loss in compressive strength with increasing temperature and heating time. The model predicts the strength loss and failure time of an insulated composite supporting a static compressive load when exposed to fire. The accuracy of the model is evaluated using failure times measured in fire-under-compression load tests on a woven E-glass/vinyl ester composite protected with a passive thermal barrier. The model predicts reductions to the failure time with increasing heat flux (temperature), applied compressive stress, and reduced insulation thickness, and this is confirmed by experimental testing. It is envisaged that the thermo-mechanical model is a useful analytical method to design thermal barrier material systems to protect composite structures exposed to high temperature or fire.