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/m 2 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 fireretarded 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.

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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 À2 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.

The influence of anisotropic strain on the valence band structure and related properties, including excitonic transition energies, transition polarization selection rules, band-edge hole effective masses, and exciton reduced effective masses, of polar and nonpolar plane GaN are systematically investigated using the well-known k⋅p Hamiltonian approach. We re-examine the band deformation potentials D3 and D4, and interband hydrostatic deformation potentials a1 and a2, and find that they take the values 9.4, −4.7, −3.0, and −12.4 eV, respectively. In order to correctly interpret the optical properties of GaN, the spin-orbit coupling effect cannot be neglected. Our numerical calculations show that pure linear polarization light emissions and absorptions can be obtained. In addition, the two topmost valence subbands can be effectively separated to reduce the band-edge density of state by manipulating the strain states in GaN epilayers, which is favorable for laser diode design. Furthermore, the band-edge hole effective masses exhibit significant in-plane anisotropy and are sensitive to the residual strain, while the influence of the residual strain on the exciton reduced effective masses is relatively weak.

The m-plane GaN films grown on LiAlO 2 ͑100͒ by metal-organic chemical vapor deposition exhibit anisotropic crystallographic properties. The Williamson-Hall plots point out they are due to the different tilts and lateral correlation lengths of mosaic blocks parallel and perpendicular to GaN͓0001͔ in the growth plane. The symmetric and asymmetric reciprocal space maps reveal the strain of m-plane GaN to be biaxial in-plane compress xx = −0.79% and zz = −0.14% with an out-of-plane dilatation yy = 0.38%. This anisotropic strain further separates the energy levels of top valence band at ⌫ point. The energy splitting as 37 meV as well as in-plane polarization anisotropy for transitions are found by the polarized photoluminescence spectra at room temperature. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2951618͔The wurtzite structure of III-nitrides leads to electrostatic fields along the ͓0001͔ direction due to spontaneous and piezoelectric polarization, when the film is grown on c-oriented substrates. 1 These built-in electric fields along the ͓0001͔ direction separate the electron and hole wave functions in a quantum well ͑QW͒ thereby reducing the recombination probability of electron-hole pairs. Consequently, it results in reduction of quantum efficiency of light-emitting diodes. 2 A way to overcome this deficiency is to grow GaNbased QWs along nonpolar orientations, for example, m plane 3,4 or a plane. 5,6 It has been shown that the electric field can be avoided in such nitride QWs. When ͓1100͔ or ͓1120͔ becomes the growth direction, the c axis of GaN lattice lies down in the growth plane. As a consequence, the C 6v hexagonal symmetry of the c growth plane is reduced to C 2v symmetry of the m growth plane. Firstly, it would show the anisotropic crystallographic characteristics. 7,8 Secondly, the nonpolar GaN films experience strong anisotropic deformation due to different in-plane lattice mismatches and thermalexpansion coefficients between GaN and underlying substrates. In the case of c-plane GaN films, isotropic strain in the c-plane preserves C 6v symmetry in the x-y plane so that no significant in-plane physical anisotropy occurs. The situation is quite different for nonpolar GaN films. Anisotropic in-plane strain components further lift the symmetry in the growth plane, which significantly modify the top three valence band ͑VB͒ states at ⌫ point. 9,10 Both the energy splitting and polarization selection of the transitions between conduction band ͑CB͒ and VB have been observed by absorption, reflectance, and photoreflectance spectroscopy. 11,12 Recently, transmission anisotropy spectroscopy has been utilized to obtain the energy splitting and polarization information of m-plane GaN films. 13 Although the in-plane strain components are important for the modifications in band structure of nonpolar GaN, it is still difficult to experimentally determine the full state of strain. In the case of m plane, GaN films are grown along GaN ͓1100͔ direction. The growth plane is the x-z plane, and the growth direc...