## 3 Simplification of Physical Property Parameters of Materials with Void Defects  
The void-defect affects the thermal properties of the glass fiber and the epoxy resin in composites laminate. According to the characteristics of the unidirectional composite laminate, the parameters are homogenized and the following hypotheses are Formulated [30]:  
1. Glass fibers are evenly distributed in the resin matrix, void defects only exist in the resin matrix, and the material is assumed to have no other manufacturing defects. In the calculations, the GFRP laminate with void defects is regarded as a structure having a mixed volume of air and the GFRP.
2. The glass fiber and the resin matrix are in firm contact, and the contact resistance is negligible.
3. The performance parameters of the material in a transient temperature field remain to be constant, which do not change as the temperature changes. In the x-axis direction, the phases of the fiber and the resin are all continuous during the heat conduction. In the y-axis direction, the resin is in the continuous phase, and the fiber is in the discrete phase.  
The properties of the experimental material with different void fractions were homogenized, and the thermal performance parameters \(\rho\), \(c\), \(\lambda_{h}\) are calculated as follows:  
\[\rho =\rho_{g}\cdot V_{g}+\rho_{a}\cdot(1-V_{g}), \tag{12}\] \[c =c_{g}\cdot V_{g}+c_{a}\cdot(1-V_{g}),\] (13) \[\lambda_{h} =\lambda_{gx}\cdot V_{g}+\lambda_{a}\cdot(1-V_{g}), \tag{14}\]  
where \(\rho_{g}\) and \(\rho_{a}\) represent the density of GFRP and the air respectively. \(c_{g}\) and \(c_{a}\) represent the specific heat capacity of GFRP and the air, respectively. \(\lambda_{gx}\) represents the thermal conductivity in the longitudinal direction of the fibers of GFRP. \(\lambda_{a}\) represents the thermal conductivity of the air. \(V_{g}\) represents the volume fractions of the experimental material except the voids. \(\rho\) represents the density of the experimental material. \(c\) represents the specific heat capacity of the experimental material. \(\lambda_{h}\) represents the thermal conductivity in the longitude direction of the fibers of the experimental material.  
The thermal conductivity in the transverse direction has a greatest influence on the surface temperature of the specimen, which can be acquired by an infrared camera. Therefore, the thermal  
Figure 1: Orthogonal coordinate system of laminate.  
conductivity in the transverse direction of the fibers is simplified, and the thermal conductivity \(\lambda_{l}\) in this direction can be represented as follows:  
\[\lambda_{l}=(\frac{V_{g}}{\lambda_{gy}}+\frac{1-V_{g}}{\lambda_{a}})^{-1}, \tag{15}\]  
where \(\lambda_{gy}\) is the thermal conductivity in the vertical direction of the fibers of GFRP.  
After using Equations (12)-(15) to calculate the performance parameters of \(\rho\), \(c\lambda_{h}\),\(\lambda_{l}\), we substituted these parameters into (11) to obtain the calculation model of the internal energy storage \(E\) in the process of defect evolution:  
\[E= \sigma\varepsilon V-\rho c\frac{\partial T}{\partial t}+\lambda_{h} \frac{\partial^{2}T}{\partial x^{2}}+\lambda_{l}\frac{\partial^{2}T}{\partial y ^{2}}, \tag{16}\]  
where \(\lambda_{h}\) is the thermal conductivity in the longitudinal direction of the fibers, \(\lambda_{l}\) is the thermal conductivity in the vertical direction of the fibers.  
According to Formula (16), if the \(\sigma\), \(\varepsilon\), \(V\), \(c\), \(\rho\), \(\lambda_{h}\) and \(\lambda_{l}\) in the process of defect evolution are already known, the internal energy storage value can be obtained.