Mechanical response and damage monitoring in hybrid composites under extreme loading conditions

The rising interest in composite materials within aerospace, defense, and automotive industries has prompted a thorough investigation of their material behavior and development of in-built sensing technique for continuous structural health monitoring under different extreme loading conditions. Therefore, the primary goal of this dissertation lies in understanding of materials behavior response and damage detection in hybrid composites under various dynamic loading conditions that are quite common in real-life applications. Firstly, an experimental study is performed to investigate the damage sensing in intra-ply glass/carbon hybrid composites at high strain rate shear loading conditions. Double-notch specimen geometry along with a four-circumferential probe measuring technique is used to measure piezo-resistance response. A split Hopkinson pressure bar (SHPB) apparatus in conjunction with high-speed imaging is used to conduct dynamic shear characterization. Orientation of glass and carbon fibers in intra-ply composites considerably influenced the dynamic shear strength and piezo-resistance sensitivity. Among the four intra-ply layups, the layup that has carbon fibers along the loading direction shows the highest shear strength (30 MPa) whereas alternating (+45°/-45°) layups of intra-ply composite shows peak piezo-resistance (35%) change. Moreover, machine learning techniques are employed to predict the piezo-resistance response of hybrid composites under dynamic shear loads. Secondly, an experimental investigation is performed to study dynamic mode-I fracture toughness and damage sensing capabilities of hybrid composites using a modified SHPB setup with a novel loading fixture. Six different hybrid glass/carbon composites (two inter-ply and four intra-ply) are considered in this study. A modified four probes measurement system and high-speed imaging are used to determine piezo-resistance response and dynamic fracture toughness. Results show that G-inter (G-C-G-G-C-G layup, G: Glass and C: Carbon) inter-ply composite shows the highest fracture initiation toughness (GIC) of 740 J/m² and 620 J/m² whereas alternating (+45°/-45°) intra-ply composite shows better damage sensing capability with a piezo-resistance change of 1170% and 590% under dynamic and static loading conditions, respectively. Again, machine learning techniques are employed to predict the piezo-resistance response of hybrid composites under dynamic fracture loads. Later, the damage mechanisms and piezo-resistance response are understood in glass/carbon intra-ply hybrid composites under blast loading conditions. Two intra-ply orientations, repeating ((G45C45)R) and alternating ((G45C45)A) glass/carbon layers, are tested under three boundary conditions configurations. A shock tube apparatus is employed to generate varying shock loads, while 3D digital image correlation technique is utilized to track shock wave interaction and deformation. Results highlighted that boundary conditions play a significant impact on blast mitigation. The (G45C45)A specimen dissipated 18% ~ 33% more energy and exhibited 100% ~ 300% higher piezo-resistance changes compared to (G45C45)R. There is a strong correlation among the center point out-of-plane displacement, energy loss, and the piezo-resistance response during the shock load under all three boundary conditions. Subsequently, high-velocity impact mechanics studies are explored in glass/carbon hybrid composites under projectile loading. The effect of different intra-ply orientations, inter-ply stacking sequences, liquid metal compositions (1 and 2 wt.%), and projectile shape on ballistic limit, energy absorption, and piezo-resistance response is investigated. The stepped conical end projectile absorbs up to 42% more energy and triggers a 60% maximum piezo-resistance change compared to the cone end projectile. The addition of liquid metal boosts ballistic limit by about 20% and energy absorption by roughly 50% but reduces damage-sensing sensitivity due to increased electrical conductivity. Finally, damage monitoring and fracture toughness characterization of additively manufactured carbon nanotubes (CNTs) embedded ABS composite (ABS-EC) are investigated. Four printing orientations (0°/90°, ±45°, 90°, and 0°) display varied fracture toughness under dynamic and static loading, with ±45° configurations showing the highest dynamic mode-I fracture toughness (2.54 MPa*m¹⸍²) and 90° orientations presenting weaker interfaces. Among them, 0°/90° exhibits the highest (500%) piezo-resistance response, while 90° shows the lowest (95%). Annealing enhances fracture toughness by 226% for ABS with no CNTs, however, annealing does not improve the fracture toughness of ABS-EC because embedded CNTs restrict polymer chain movement.