The strength under heating information of rubber-like materials is lacking in the literature. Standard equipment is not able to reach high stretches in a controlled temperature chamber. Furthermore, most industrial laboratories only perform the uniaxial stretch case for the calibration of the material. Given the fact that rubber materials in their industrial application are likely to be subjected to different ratios of biaxial loading at once rather than only uniaxial, there is a great potential for a mistake which can lead to an engineering error. We have applied a new methodology for calibration of hyper-elastic up to failure rubber-like materials subjected to high service temperatures. This generalizes the theory and may serve as a design consideration. The simultaneous calibration of uniaxial and biaxial tests are done using experimental data from a self-designed test chamber. Biaxial failure envelopes are built from the ultimate stretches obtained. The calibration is based on iterative finite element simulations using a new coupled thermo-elastic theory. Energy limiters, depending on the temperature were introduced in the Helmholtz free energy in order to describe material failure. It is found that strength might be significantly decreased by heating while the stiffness only slightly depends on the temperature alterations.