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The investigation of transient heat transfer in composite materials on the example of carbon/epoxy composites.


J. Terpiłowski1, B. Gawron1 and G. Woroniak2

1Military University of Technology, Poland
2Bialystok Technical University, Poland

Keywords: thermal properties, flash methods
property: thermal diffusivity
material: carbon epoxy composites, epoxy resin

This paper describes the flash method in investigation of transient heat transfer in composites containing k layers of carbon fiber plate or glass, which the end result is the temperature characteristic of replacement thermal diffusivity ae(T) in a perpendicular direction to the surface layers of the plate in the matrix material. The method is theoretical and experimental. In implementing this method the following assumptions were adopted:
- model of heat transfer is one-dimensional and temperature decomposition inside the composite material is defined by a solution of the equation of the Fourier including boundary conditions of heat transfer;
- layers of the plate, in a form of braided carbon fiber inside epoxy resin are located parallel to a both extreme surfaces of a flat-parallel, cylindrical sample and perpendicular to a heat flux, which flowing through a sample;
- in simulate tests it was assumed that a model is constructed of homogeneous material (resin), where each of the layers of carbon plate with an average thickness lcp and thermal diffusivity acp was replaced by equivalent layer of resin with thickness l= lcp(ar/acp) and thermal diffusivity ar.

Thus, a replacement thickness l of sample made of resin, simulating a sample of the composite, is l= lr + kl. To determine the replacement value of the thermal diffusivity of composite ae(T) a modified flash method was used [1], which gives good results in studies of homogeneous materials. The study was carried out:
- after a laser shot in a front surface of composite sample with thickness lc a change of temperature difference between extreme surfaces of sample ΔT’(t) was recorded;
- then, using the standard procedure, the starting value of the characteristic time tau=(lc)‘sup’/(p ar) was determined. If the test sample was homogeneous, the theoretical curve ΔT(t), would be the same as ΔT’(t) and for this value tau;
- because it is not, then we choose by the trial and error method the value of tau = taue for which the curves ΔT’(t) and ΔT(t) are practically the same. Known value of taue allows to calculate l=p(tauear);
- replacement thermal diffusivity of testing composite sample at temperature T is calculated from equation ae(T)=(lc/(p taue).

The study was carried out for sample made with epoxy resin and carbon/epoxy composite samples in the shape of a disc with a diameter Φ = 12 mm and a thickness l = 1,33 ± 0,01 mm. Composite material consisted of polymer matrix – epoxy resin type EP 53, reinforced from 1 to 4 layers of carbon plate type Porcher 3692/120. Replacement thermal diffusivity of all pre-tested samples are ae = (0,75 ÷ 2,42)•10 m/s at the temperature range from 20°C to 150°C.

References
  1. J. Terpiłowski, Archives of Thermodynamics Vol.24, No. 1, pp.59-80 (2003)

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