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^{1}Military University of Technology, Poland

^{2}Bialystok 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 a_{e}(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 l_{cp} and thermal diffusivity a_{cp} was replaced by equivalent layer of resin with thickness l= l_{cp}(a_{r}/a_{cp}) and thermal diffusivity a_{r}.

Thus, a replacement thickness l of sample made of resin, simulating a sample of the composite, is l= l_{r} + k_{l}.
To determine the replacement value of the thermal diffusivity of composite a_{e}(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 l_{c} 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=(l_{c})‘sup’/(p a_{r}) 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 = tau_{e} for which the curves ΔT’(t) and ΔT(t) are practically the same. Known value of tau_{e} allows to calculate l=p(tau_{e}a_{r});

- replacement thermal diffusivity of testing composite sample at temperature T is calculated from equation a_{e}(T)=(l_{c}/(p tau_{e}).

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
a_{e} = (0,75 ÷ 2,42)•10 m/s at the temperature range from 20°C to 150°C.

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

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