Peter Myler

Thermo-mechanical Responses of Fiber-reinforced Epoxy Composites Exposed to High Temperature Environments. Part I: Experimental Data Acquisition

This first part of a series of papers on the thermo-mechanical responses of fiber-reinforced composites at elevated temperatures reports the experimental results required as input data in order to validate the kinetic, heat transfer, and thermo-mechanical models being developed and to be discussed in subsequent papers. Here the experimental techniques used for the determination of physical, thermal, and mechanical properties and their significance for particular models are discussed. The fire retardant system used to improve the fire performance of glass fiber-reinforced epoxy composites is a combination of a cellulosic charring agent and an interactive intumescent, melamine phosphate. Thermogravimetry is used to obtain kinetic parameters and to evaluate the temperature-dependent physical properties such as density, thermal conductivity, and specific heat capacity, determined using other techniques. During flammability evaluation under a cone calorimeter at 50 kW/m2 heat flux, thermocouples are used to measure temperatures through the thicknesses of samples. To investigate their thermo-mechanical behavior, the composites are exposed to different heating environments and their residual flexural modulus after cooling to ambient temperatures determined. At a low heating rate of 10°C/min and convective conditions, there was a minimal effect of fire retardant additives on mechanical property retention, indicating that fire retardants have no effect on the glass transition temperature of the resin. On the other hand, the fire-retarded coupons exposed to a radiant heat from cone calorimeter, where the heating rate is about 200°C/min, showed 60% retention of flexural modulus after a 40-s exposure, compared to 20% retention observed for the control sample after cooling specimens to ambient temperatures.

Towards the design of sandwich panel composites with enhanced mechanical and thermal properties by variation of the in-plane Poisson's ratios

The finite element (FE) method has been used to study the mechanical and thermal properties of both conventional and re-entrant (i.e. negative Poissons ratio) honeycombs, which may be used as the cores of sandwich panel composites. Failure of the honeycomb structures was simulated using a crack propagation method developed in-house. The cell-wall stress build up in the conventional honeycomb was calculated to be significantly reduced relative to the re-entrant honeycomb under (2D) hydrostatic loading, implying that the conventional core will undergo significantly less internal damage than the re-entrant core. Conversely, the re-entrant honeycomb performs better than the conventional honeycomb under thermal loading conditions. The size and pathway of the crack formed during the simulation is dependent on the failure stress distribution used in the crack propagation routine.

Crash performance of cellular foams with reduced relative density part 2: rib deletion

Abstract The finite element method has been used to investigate the crashworthiness of regular conventional and re-entrant cellular foams in pristine and defect forms. Defects were introduced by randomly deleting vertical ribs (cell walls), diagonal ribs and vertical-plus-diagonal ribs from the honeycomb models, which with suitable boundary conditions approximated to three-dimensional foams. Generally, the calculations show that deleting ribs leads to a reduction in magnitude of the Youngs moduli and Poissons ratios, although there are some notable exceptions. In particular, the Youngs modulus and Poissons ratio along the x-axis (perpendicular to the vertical ribs) actually increase in magnitude slightly when vertical ribs are deleted. In the case of the re-entrant honeycomb, a transition from auxetic (negative Poissons ratio) to non-auxetic (positive Poissons ratio) behaviour occurs at a critical threshold for diagonal rib deletion. For the conventional honeycomb, the Poissons ratio for loading in the y-direction remains approximately constant when diagonal ribs are deleted.

Fiber-reinforced epoxy composites exposed to high temperature environments. Part II: Modeling mechanical property degradation

This article is part of a series on the thermo-mechanical responses of fiber-reinforced composites at elevated temperatures and it follows the first part containing experimental results. A flame-retardant system consisting of a cellulosic charring agent and an interactive intumescent additive (melamine phosphate) has been used in order to improve the post-fire mechanical performance of glass fiber-reinforced epoxy composites. The effect of one-sided radiant heat on the residual flexural stiffness of laminate coupons exposed to incident heat fluxes of 25 and 35 kW m-2 was investigated. The flame-retarded coupons retained a higher percentage of their original room temperature flexural modulus after heat exposure while the control specimens showed inferior material property retention over the same exposure periods. A heat transfer (thermal) model based on Henderson’s equation is used to predict the through thickness temperature profiles and subsequently the mass loss due to the resin matrix decomposition at elevated temperatures. The theoretical results from the heat transfer model are validated against experimentally obtained data and then coupled with a mechanics model that describes material property-temperature dependence in order to predict the residual flexural stiffness, after heat damage. The accuracy of the thermo-mechanical model was validated against the experimental data and a good agreement was observed.

‘Egg-box’ panel for commercial vehicle front – compressive loading tests

The numbers of serious accidents and fatalities from accidents between commercial vehicles and vulnerable road users (VRUs) are still high and show no sign of declining. For pedestrians, this is traumatic due to high incompatibility of vehicle fronts. Hence, decisions have been made to implement countermeasures that have the potential for casualty reduction. This paper proposes an ‘egg-box’ energy-absorbing panel to be incorporated in a commercial vehicles cab front and sides. As a consequence it is expected to reduce the load between vehicle and pedestrians, while at the same time eliminating the risk of serious injuries and fatalities. The egg-box panel herein has been compressed under quasi-static conditions in order to study the collapse mechanism of the structure and quantify the corresponding load regimes for various stages of collapse. An attempt has been made to understand the structures energy-absorption capability from the resulting load-deflection curves. Also, a simple assessment of the structures crashworthiness is discussed and comparison made to existing energy absorbers. The maximum load that a 30 mm high panel structure can withstand is approximately 18 kN. Unlike in a concertina tube where a sudden drop in load is observed after the formation of the first plastic hinge, there is no drastic fall in this load after the collapse has started. A steady load is observed with the progressing deformation. With the height reduced to 20 mm, the maximum load capacity increases by 20% approximately along with increase in the stiffness.

Modelling flaming combustion in glass fibre-reinforced composite laminates:

A heat transfer model based on the well-known Henderson equation has been modified to allow for self-sustained ignition and the flaming combustion phenomena of E-glass fibre-reinforced epoxy composites to be predicted from first principles using known thermal-physical and thermodynamic data for their constituents. The modifications consider: (1) the assignment of thermodynamic conditions (e.g. ignition temperature and mass flux of volatiles) necessary and sufficient to trigger self-sustained ignition, and (2) the inclusion of an integrated loop allowing the heat energy generated from the flaming combustion process to be fed back into the burning laminate. The model compares moderately well with experimental results obtained from cone calorimetric measurements. The additional modelling capabilities considered in this study provide the basis for an analytical model that can more accurately predict the thermal response and flaming combustion of glass fibre-reinforced polymer composites exposed to a one-sided radiant heating environment in the presence of an ignition source.

Wheelchair occupant kinematics during a rail carriage crash

Over recent years, a number of legislative tools and codes of practice have been put in place to provide wheelchair users with greater access and freedom of using trains. However, much more is still needed to be done to improve the safety of wheelchair occupants (WOs) while on board. In the United Kingdom, under the Disability Discrimination Act, only approved dimensions are the minimum requirements for use of a wheelchair in a rail carriage. This paper seeks to provide an analysis of the experimental kinematics of a WO under frontal impact tests carried out using a 57 kg dummy subjected to a sudden stop generating a pulse of about 15 g, which is less than the ECE M1 vehicle category. However, this is more severe than the 5 g pulse that covers both the European Commissions TSI and prEN 15227 with regards to train crashes. Results of frontal impacts are presented where the resulting kinematic characteristics are used to assess the potential injury severity of the occupant. A loaded sled with a combined mass of about 200 kg was accelerated horizontally to 3.3 m/s and suddenly stopped using a rigid barrier to simulate the worst loading condition. Eight restraint scenarios are covered. These form the basis for making recommendations considering the safety of a WO during a crash of a rail carriage. The baseline scenario constitutes not securing the wheelchair and not restraining the occupant. This also represents the worst case for injury potential. The ideal scenario involves securing the wheelchair and restraining the occupant using a three-point occupant restraint system. The paper concludes by providing recommendations for seating configurations to improve the survivability of a WO during a crash in a railway vehicle.

Crash performance of cellular foams with reduced relative density part 1: rib thickness variation

Abstract Finite element modelling at the macroscopic level is used to investigate the effect of varying the rib thickness of open cell conventional and re-entrant periodic foams on the crash performance due to energy absorption. The data show that the mechanical properties and, hence, crashworthiness vary not only with foam-relative density, but also on how the relative density variation is achieved. Reducing the ‘diagonal’ rib thickness leads to a more marked decrease in E y as well as in E x , whereas ν xy and ν yx both increase in magnitude. The results have implications for applications in which the crashworthiness is to be maximised or minimised with respect to reduction in the relative density. The crashworthiness of orthotropic re-entrant foams is also shown to be considerably greater than conventional foams in one principal direction.

Flat nose low velocity drop-weight impact response of carbonfibre composites using non-destructive damage detectiontechniques

Abstract This work is mainly concerned with the nondestructive post-impact damage evaluation of carbon fibre reinforced laminated composite panels subject to low velocity drop-weight impact by flat and round nose impactors. Quasi-isotropic laminates consisting of eight-, sixteen-, and twenty-four plies were impacted by flat and round nose impactors at different velocity levels. Load-time history data were recorded and plotted to correlate loaddrop as damage level to the impactor nose profiles. Test produced data, non-destructive damage detection techniques: visual, ultrasonic, and eddy- current, and computer simulations were utilised to identify and quantify status of the impact induced damage. To evaluate damage in relatively thick laminates (consisting of 24-Ply), the damage ratios and deflection quantities were correlated to the corresponding impactor nose profiles. Damage induced by the flat nose impactor to thick laminates was compared against the data produced by the round nose impactor. Results show that relatively thin laminates were largely affected by the impactor nose. Reasonable difference was observed in damage caused by flat and round impactor nose profiles to thick laminates impacted at relatively higher velocity impacts. Resultswere compared and validated against simulation produced data.

The effect of fibrous reinforcement on optical and impact performance of fibre-reinforced transparent glass composites

Fire-resistant laminated glass composite containing intumescent silicate as an interlayer between two glass sheets is a widely used transparent building material. To improve the impact and other mechanical properties of this composite structure, a transparent silicate matrix has been reinforced with alkali- and UV-resistant synthetic (polypropylene, polyamide 66, glass) and metallic (steel) fibres as of nonwoven webs or woven meshes. The refractive indices (RIs) of the fibres and the matrices were measured and the transparency of the laminated composites depended upon fibre RI as well as reinforcement structure. All fibres were successful in significantly enhancing impact properties of laminated glass composites with alkali-resistant glass fibres showing the best performance.