Investigation of the thermal anisotropy of unidirectional carbon fiber reinforced composite plates using optically gener
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Optically generated thermal waves have been used to measure the thermal diffusivity of a unidirectional carbon fiber reinforced composite plate (CFRC) both parallel and perpendicular to the fiber direction. The optically generated thermal waves have been used in combination with a noncontact optical detection technique. The diffusivity perpendicular to the fiber direction can also be determined by attaching a pyroelectric detector to the back of the sample. The value obtained this way agrees well with the results from the optical detection technique. An anisotropy factor of about 18 has been measured for a unidirectional CFRC, which agrees well with literature values obtained with completely different techniques.
I. INTRODUCTION Thermal waves can be generated in materials using an intensity modulated laser beam. Several noncontact methods exist to measure the thermal diffusivity of the material. A noncontact method (the mirage effect technique) and a technique requiring contact with the sample (pyroelectric detection) were chosen to measure the thermal diffusivity of a carbon fiber reinforced composite. Carbon fiber reinforced composites (CFRC's) are made of a number of layers of carbon fibers, impregnated with epoxy resin. The fiber direction can be varied from layer to layer to give the plate the desired mechanical properties. In unidirectional CFRC's, the fibers have the same orientation in each ply. As a consequence, the thermal properties of the plate are anisotropic, the thermal conductivity being different parallel and perpendicular to the fiber direction. Measuring the thermal conductivity of anisotropic materials is not straightforward, as can be seen in Ref. 1.
capacity of the sample, respectively. The term on the right-hand side of Eq. (1) is the thermal source. If a harmonically intensity modulated Gaussian laser beam is incident on the surface z = 0 of the sample (see Fig. 1), g is equal to:
g(r,z) =
Pa lira1
(2)
where P is the power of the incident beam, corrected for the reflection factor of the surface, a is the optical absorption coefficient of the sample, a is the beam width, co is the angular modulation frequency, and r the radial distance from the center of the beam. Ar+ laser
Position sensitive detector
II. THERMAL CONDUCTIVITY IN ISOTROPIC AND ANISOTROPIC MATERIALS A. Isotropic materials The thermal conductivity equation for isotropic materials can be written as 2 : ksV2
(1)
dt
In this equation, Ts is the temperature, ks the thermal conductivity, and p and C are the density and the heat ^Senior Research Assistant, N. F. W. O., Belgium. 3106 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 12, Dec 1993 Downloaded: 18 Mar 2015
He Ne laser FIG. 1. Measuring setup for mirage effect technique. © 1993 Materials Research Society IP address: 140.182.176.13
W. Lauriks et al.: Investigation of the thermal anisotropy of unidirectional carbon fiber reinforced composite plates
The boundary conditions that apply to Fig. 1 are continuity of temperature at the surfaces: Tg(z = 0) = Ts
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