Explosive blast damage resistance of fibre-polymer composites
Creators: name: Gargano, Alexander
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401602
subject: Composite and hybrid materials
classificationCode: 401609
subject: Polymers and plastics

description: Fibre-reinforced polymer composites have been used in navy ship structures since the mid-1940s due to their ability to be strong, stiff, durable and easy to repair. More recently, composite materials have been increasingly more used due to the number of advantages they offer over conventional metallic materials. One major advantage of composite materials is that they have high stiffness and strength properties, whilst being incredibly lightweight. Low top-side weight increases sea-keeping and range and reduces fuel consumption. However, the use of composite materials in large ship structures is a concern for the Australian and US Navy due to their brittle-type failure when subjected to an explosive blast event. There is also an increased risk that the moisture rich environment that navy ships are continuously exposed to may reduce their explosive blast survivability. <br> <br>In light of these challenges, the Office of Naval Research (ONR) funded a program to investigate the explosive blast response of commonly used naval composite materials (laminate and sandwich) and how their resistance is affected by long-term seawater immersion, which was the scope of this PhD research. The materials considered are representative composite materials used in naval ship structures. Three blast loading conditions are considered: far-field, near-field and contact. Finite element models and experimental tests are used to determine the deformation, damage and residual mechanical properties of several types of glass fibre and carbon fibre laminates following blast loading. The laminates considered were glass/vinyl ester, glass/polyester, carbon/vinyl ester and carbon/polyester materials, and all types are used in naval ship structures. The project also evaluates the deformation and damage to sandwich composites exposed to explosive blasts of varying intensity. The sandwich composites considered have a core of polymer foam or balsa wood.  <br><br>The effects of the fibre reinforcement, the polymer matrix and the bonding between the two (fibre-matrix interface) on the explosive blast resistance of composite laminates are experimentally investigated. When exposed to the same shock wave impulse, the resistance to deformation and damage was higher for glass fibre laminates compared with carbon fibre composites, and laminates with a vinyl ester matrix were superior to the polyester matrix materials. Also, laminates with strong interfacial bonding had greater explosive blast resistance than those with a weak interfacial bond. This work also identified that the blast resistance correlated with the flexural strain energy density of the laminates. The capacity of the glass fibres and vinyl ester resin to deform to greater strains without causing damage to the laminate resulted in superior blast resistance. Also, the delamination damage resistance of the vinyl ester laminates being higher than that of the polyester composites correlated with their greater modes I and II interlaminar fracture toughness properties, which was particularly important for superior blast properties in contact explosive tests. Laminates with strong fibre-matrix interfacial bonding had higher mechanical properties and interlaminar fracture toughness properties, which resulted in superior blast resistance.  <br><br>The explosive blast resistance of several types of sandwich composites is experimentally investigated. For the blast conditions used in this work, the resistance of the sandwich panels with glass fibre laminate skins to out-of-plane deformation was higher than the carbon fibre sandwich panels due to their greater energy absorption capacity. Also, the balsa core composites were superior to the PVC core composites at low blast impulse, however the weak shear strength of the balsa wood resulted in premature failure at medium blast impulses. The low compressive modulus of the PVC foam resulted in poor stress transfer to the backface skin, causing extensive damage to the front facesheet. <br><br>The effect of high mode I and II interlaminar fracture toughness properties on the explosive blast damage resistance was further explored using three-dimensional (3D) woven composites. These composites contained different volume fractions of through-thickness woven fibrous tows. It was found that at low-medium blast impulses, the damage resistance of the 3D composites was similar to the 2D laminate. At the highest blast impulse, however, the amount of delamination damage to the 3D composites was much lower than the 2D laminate. The blast damage resistance of the 3D composites increased with their z-binder content under high impulse blast conditions. Also, the 3D composites remained intact whereas the 2D laminate completely shattered. The superior blast resistance of the 3D composites only occurred when the shock wave impulse was high enough to create a large-scale bridging process zone that opposes extensive delamination cracking. <br><br>The effect of seawater immersion on the explosive blast resistance of composite laminates is also explored. It was demonstrated that long term seawater immersion of laminates degraded the polymer matrix and fibre-matrix interface, which reduced the flexural, compressive and interlaminar shear properties. The reductions to these mechanical properties resulted in greater amounts of deformation and damage to laminates when subjected to an explosive blast. However, the reduction to the blast damage resistance caused by seawater absorption was dependent on the type of laminate.<br><br>This project also developed and validated a numerical (finite element) model to analyse the deformation and damage to metals and laminates subjected to airborne explosive blasts. FE modelling and experimental testing revealed that mild steel and aluminium (AA5083) plates outperformed the laminate plates at all blast impulses when the target plates were the same thickness. However, when the blast resistance is considered on the basis of specific areal weight of the target plate, then the glass fibre laminates were superior to the metals. The finite element (FE) model was able to predict the out-of-plane deformation and damage to the metal and carbon fibre laminate plates for all blast impulses with reasonable accuracy. <br><br>These studies aimed to investigate the explosive blast resistance of current and future naval composite materials, and how their response is affected by long-term immersion in seawater. The outcomes of this PhD thesis will assist in the management of current composite naval vessels and aid in the material selection for future naval vessels for improved explosive blast resistance.