Single edge crack effect on mechanical property of polyimide flexible composite envelope

The existence of cracks has a great influence on the life span of the envelope material of aerostats, especially for stratospheric airships that operate in harsh environments. Understanding the relationship between fracture mechanism and composite structure of envelope materials is of great significance to the design and development of envelope materials. To explore the fracture mechanism of envelope material, a novel polyimide (PI) envelope was developed by lamination method. Then, the fracture behavior and mechanism of PI envelope with single edge crack was studied and analyzed in detail by means of experimental method and theoretical model analysis. It was found that the crack orientation has no significant effect on the fracture strength of PI envelope, while the increase of crack length will increase the strength loss rate and decrease the crack sensitivity. These phenomena suggest that the fracture strength loss of envelope is mainly determined by the number of cut-off longitudinal yarns. And the stress transfer effect of coating and transverse yarns which causes stress concentration in the crack tip region is another important factor affecting the fracture strength of PI envelope. Besides, the interfacial properties between coating and fibers as well as the size effect of coating also play an important role in the fracture property of PI envelope. Finally, stress intensity factor theory and critical stress field theory were chosen to study the fracture behavior of PI envelope. The calculation results indicated that critical stress field theory can better predict the fracture strength of PI envelope.