This study investigates the thermal deformation behavior of fiberglass composite polymer material with a phenol-formaldehyde matrix under various heating regimes. Samples are exposed to both uniform and one-sided heating at rates up to 600 °C/min, with peak gas temperatures reaching 2500 °C. The research analyzes the kinetics of thermal deformation and evaluates the material's response to thermal loading by calculating the stressed-deformed state of reinforced plastic specimens. Under one-sided heating, thermal expansion is measured, revealing that the thermal deformation coefficient (αₜ) exhibits a linear relationship with both temperature and applied stress. This linearity is instrumental in preventing bending in unconstrained samples. However, in pre-bent samples, the absence of a stress gradient (Δσ = 0) at high heating rates leads to elevated stress gradients across the sample. These effects are further validated through dilatometry experiments performed over a temperature range of 20 to 1100 °C. The findings offer critical insight into the thermal and mechanical performance of fiberglass composites under extreme thermal conditions, aiding in the design of thermally resilient structural components