Temperature has a great influence on the mechanical properties of ductile cast iron or nodular cast iron. A thermomechanical treatment was carried out at various elevated temperatures of 450 °C, 750 °C and 850 °C using a universal testing machine with a tub furnace. Specimens were held at these temperatures for 20 min to ensure a homogeneous temperature distribution along the entire length of the specimen, before a tensile load was applied. Specimens were deformed to various levels of uniform strain (0%, 25%, 50%, 75%, and 100%). These degrees of deformation were measured with a dial gauge attached to a movable cross plate. Three strain rates were used for each specimen and temperature: $1.8\times {10}^{-4} {\mathrm{s}}^{-1}$, $9\times {10}^{-4} {\mathrm{s}}^{-1}$ and $4.5\times {10}^{-3} {\mathrm{s}}^{-1}$. A simple analytical model was extracted based on the CT tensile test geometry and yield stress and a 0.2% offset strain to measure the fracture toughness (J_IC). To validate the analytical model, an extended finite element method (XFEM) was implemented for specimens tested at different temperatures, with a strain rate of $1.8\times {10}^{-4} {\mathrm{s}}^{-1}$. The model was then extended to include the tested specimens at other strain rates. The results show that increasing strain rates and temperature, especially at 850 °C, increased the ductility of the cast iron and thus its formability. The largest percentage strains were 1 and 1.5 at a temperature of 750 °C and a strain rate of $1.8\times {10}^{-4} {\mathrm{s}}^{-1}$ and $9\times {10}^{-4} {\mathrm{s}}^{-1},$ respectively, and reached their maximum value of 1.7 and 2.2% at 850 °C and a strain rate of $9\times {10}^{-4} {\mathrm{s}}^{-1}$ and $4.5\times {10}^{-3} {\mathrm{s}}^{-1},$ respectively. In addition, the simple and fast analytical model is useful in selecting materials for determining the fracture toughness (J_IC) at various elevated temperatures and different strain rates.