Investigation of polyurethane toughened epoxy resins for composite cryotank applications. Part II: Theoretical analysis of toughening mechanisms

Based on the phase separation phenomenon, cryogenic mechanical behaviors and toughening effects for polyurethane toughened epoxy (PU/EP) systems demonstrated in our previous work (Part I), as a followed investigation, this study aims to conduct theoretical analysis of temperature-dependent toughening mechanisms. First, 3D finite element unit cells of PU/EP systems with random particle distributions are established to evaluate the maximum stress concentration factor for the theoretical fracture energy calculation. Then, the temperature-dependent fracture energies of PU/EP systems are investigated quantitatively for the first time, and the theoretical results are shown to be in good agreement with the experimental results. From 10 phr to 40 phr, the particle bridging contribution rises from 47.3% to 65.5%, which elucidates that the particle bridging plays a major role in determining the fracture energy at room temperature. As the temperature decreases from RT to −183°C, taking PU10/EP as an example, the untoughened matrix proportion goes up from 30.9% to 65.2%; the particle bridging contribution decreases from 51.8% to 27.8%; the shear banding contribution drops from 17.3% to 7%. This work explores temperature-dependent toughening mechanisms in depth and offers an accurate quantitative evaluation of fracture energies for toughened epoxy matrix for composites cryotank design. • The temperature-dependent fracture energies of PU/EP systems are investigated quantitatively. • The particle bridging plays a major role in room temperature. • From RT to −183°C, the untoughened matrix proportion goes up while the particle bridging contribution and the shear banding contribution decrease.