The present study investigates the failure process of Ethylene tetrafluoroethylene (ETFE) foil on 250 µm thick Novoflon ET 6235Z a material that is extensively employed in the building industry. ETFE structures tend to fail due to a rupture in the foil rather than reaching its ultimate stress or strain level in extended area. Determining the fracture toughness and the level at which unstable crack growth occurs is imperative when considering in-situ material repair. Despite the extensive research conducted on the viscoelastic mechanical behaviour of ETFE foils [Li and Wu (2015); Comitti and Federico (2024)], relatively few research found in connection to the fracture mechanical investigation of the foil. [Rigotti et al. (2019)] ETFE is a ductile polymer film, resulting in a large plastic zone around a crack tip, which highlights the necessity to the use of methods in elastic-plastic fracture mechanics. Here the energy release rate, termed J-Integral [Cherepanov (1967); Rice(1968)] connecting to the crack propagation (∆a) is investigated [Hegyi (2015)]. Unidirectional tensile tests were conducted on 50mm wide, 300mm long strips with a 6 mm slit position on the centre. Five different strain rates (10mm/min to 40mm/min) and three different temperature levels (16°C, 24°C and 32°C) were investigated. The J-integral calculated around the tip of a crack from the strain field measured by digital image correlation. The critical level of J-integral determined when the crack starts to propagate. The results show that the value of critical level of J-integral is independent of the measurement’s strain rate, but it is proportional to the temperature change. Furthermore, the investigation of the resistance curve of ETFE foils (J- ∆a) suggest that the strain rate has a major influence on the unstable crack propagation. The findings of this study offer significant potential for future research and development in the field of environmentally sustainable structural design, to prolong the service life of the material even in the event of a defect.