Study on Damage Mechanisms and Blast Mitigation of GFRPU and Coated Steel Plates

Conventional fabrication processes for fiber-reinforced composites are complex and stepwise, limiting their application in blast protection engineering. In this study, KH550 surface-modified glass fiber powder (1.5 wt%) was incorporated into a polyurea matrix, and a homogeneous glass fiber-reinforced polyurea (GFRPU) composite was successfully synthesized via an optimized one-step spray process. The strain-rate-dependent mechanical behavior (0.01–100 s−1), fracture mechanisms, molecular structure, and blast mitigation performance of GFRPU were systematically investigated through mechanical testing, SEM, FTIR, 75 g TNT contact explosion tests, and calibrated and partially verified finite element simulations. Results show that the surface-modified fibers form chemical crosslinks with the matrix, shortening the N–H hydrogen bond length from 3.023 Å to 2.983 Å and increasing the hydrogen bonding degree of the urea carbonyl region from 80.0% to 84.1%, thereby enhancing structural ordering and strength. At a strain rate of 100 s−1, the tensile strength of GFRPU reaches 50.27 MPa, 26% higher than that of the polyurea matrix. Under contact explosion, a 4 mm back-face GFRPU coating reduced the center-point deflection of the steel plate by 31.6% and suppressed brittle cracking, with interfacial debonding and coating tensile failure as the dominant failure modes. Considering protective efficiency and structural safety, 4 mm is recommended as the optimal coating thickness.