Thermal radiation is a clean, flexible, efficient and effective means to supply energy to process a composite on-line. Radiative transfer in high-fiberdensity, unidirectional composites is complex, if analyzed completely. A detailed thermal model for on-line processing of unidirectional fiber composites by surface or volumetric radiative heating is presented. The physical geometry and imposed thermal and radiative boundary conditions correspond to a unidirectional, hoop-wound cylinder. In-situ (or continuous) curing of thermosets or on-line consolidation of thermoplastics is represented by the inclusion or omission, respectively, of the exothermic, chemical energy release term in the energy equation. Numerical results are presented for the temperature and degree of cure of graphite/epoxy and glass/epoxy cylinders. The effects of: surface or volumetric radiative heat flux; radiant-source emissive power level and efficiency; radiative emission from the composite; exothermic, chemical energy release; radiation’s angle of incidence; and independent and dependent scattering in the composite’s interior are presented. Validation of the model is presented in Part II of this paper, as well as recommended manufacturing process windows for process parameters, such as radiant-source emissive power level and winding speed. Part II also includes surface and volumetric radiative properties of unidirectional graphite/epoxy and glass/epoxy composites that were used in the numerical simulations.
The volume fraction of the fibers present in commercial filament wound structures, formed from either epoxy-impregnated tapes (“prepreg”) or fiber strands pulled through an epoxy bath, approaches 60 percent. Such close-packed structures are near the region that may cause dependent scattering effects to be important; that is, the scattering characteristics of one fiber may be affected by the presence of nearby fibers. This dependent scattering may change the single-fiber extinction coefficient and phase function, and thus may change the radiative transfer in such materials. This effect is studied for unidirectional fibers dispersed in a matrix with nonunity refractive index, and with large size parameter (fiber diameter to wavelength ratio) typical of commercial fiber–matrix composites. Only the case of radiation incident normal to the cylinder axes is considered, as this maximizes the dependent effects. The dependent extinction efficiency is found by solving the dispersion relations for the complex effective propagation constant of the composites. An estimation of this dependent scattering effect on the infrared in-situ curing of thermoset-hoop-wound structures is also conducted. It is found that the wave interference effect is significant for S-glass/3501-6 composite, and neglect of this effect tends to overestimate the temperature and cure state within the materials during IR in-situ curing.
A curing process for unidirectional thermoset prepreg wound composite structures using infrared (IR) in-situ heating is investigated. In this method, the infrared energy is from all incident angles onto the composite structure to initiate the curing during processing. Due to the parallel geometry of filaments in wound composite structures, the radiative scattering coefficient and phase function within the structure depend strongly on both the wavelength and the angle of incidence of the IR incident radiation onto the fibers. A two-dimensional thermochemical and radiative heat transfer model for in-situ curing of thermoset, hoop-wound structures using IR heating is presented. The thermal transport properties that depend on the process state are also incorporated in the analysis. A nongray, anisotropic absorbing, emitting, and scattering unidirectional fibrous medium within a matrix of nonunity refractive index is considered. The temperatures and degrees of cure within the composite during processing are demonstrated numerically as a function of the configuration of IR heat source, nondimensional power input, mandrel winding speed, and size of wound composite. Comparison between the numerical result and experimental data is presented.
Measurements of the thermal conductivity of a commonly used epoxy resin are presented as a function of temperature and degree of cure. The temperature range of measurement varies with degree of cure, as the range must be below the temperature that will initiate further curing of the resin. Measurements at degrees of cure of 1l.5%, 5/.2%, and 100% are presented, with corresponding temperature rangesfrom 290 k to 360 k, 373 k, and 500 k, respectively. The line-source method for determining thermal conductivity is used for the measurements. The errors present in the application of this method to this material system are discussed.
Experimental validation is presented for a detailed thermal model (described in Paper I) for on-line processing of unidirectional fiber composites by surface or volumetric radiative heating. Surface and volumetric radiative properties of unidirectional graphite/epoxy and glass/epoxy are presented: measurements of the complex refractive index of an uncured and cured 3501-6 epoxy resin as a function of wavelength; semi-empirical extinction and scattering coefficients and phase functions for graphite/epoxy and glass/epoxy as a function of wavelength and incident angle, assuming independent scattering; model predictions of the effects of dependent scattering (i.e. electromagnetic wave interference) in graphite/epoxy and glass/epoxy; and measurements of the directional-hemispherical reflectance of AS4/3501-6 as a function of wavelength, incident angle, unidirectional composite orientation, and degree of cure. Experimental temperature histories for in-situ (or continuous) curing of graphite/epoxy, hoop-wound cylinders using infrared (IR) heating at power levels of 5 and 7 kW and mandrel winding speeds of 0.1 and 0.15 m/s are presented. Good qualitative agreement is found between the experimental results and model predictions for AS4/3501-6. Recommended manufacturing process windows for graphite/epoxy and glass/epoxy are presented for several process parameters, such as radiant-source emissive power level and winding speed. Due to their higher radiation absorptivity and lower heat capacity, graphite composites generally have narrower process windows.
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