Dynamic response of a polymer and polymer composite systems: Experimental studies

Recently, research on the impact resistance of transparent hybrid glass-polymer systems (layered and particle-matrix systems) has been conducted at the Netherlands Organisation for Applied Scientific Research (TNO), showing the potential of these materials for the application in protection concepts. A soft, transparent polyurethane elastomeric polymer, Clear Flex 75 (CF 75 in short) was selected as matrix material. Knowledge about the impact resistance of particle(s)-polymer systems is also relevant for propellants, consisting of a polymer binder and solid energetic particles. The impact-induced damage will change the combustion properties and the performances of the propellant. Against this background, a combined research programme of experimental tests and computational modelling was defined on the mechanical response of hybrid particle(s)-polymer systems under dynamic loading. In this programme, CF 75 polymer is the matrix material, which is embedded with single and multiple particles. In the computational part, multi-scale modelling techniques for dynamic conditions are developed. In the experimental part, a step-wise approach with increasing complexity is conducted for studying the dynamic response in compression and tension of (i) a monolithic polymer matrix, (ii) a single-particle-polymer system and (iii) a multiple-particles-polymer system. The used apparatus is a small scale split Hopkinson bar (SHB) device developed at TU Delft, which can conduct the dynamic compression (SHPB) and tension (SHTB) tests at various strain rates. The experimental results are important for (i) the evaluation of a transparent impact-resistant device of a soft polymer material backed by rigid glasses, (ii) the development of a particle(s)-polymer system for propellants and (iii) the calibration of computational models for designing a rigid-particle-modified composite material system for protective applications. Firstly, the experimental setups were built for high-strain-rate tension (SHTB) and compression (SHPB) tests. These two setups were carefully evaluated and proved to be reliable for dynamic tests on a soft polymer material at different strain rate levels. The details on the technology for casting the specimens of the monolithic CF 75 polymer material, the single-glass-polymer system and the multiple-particles-polymer system were given, as well as the basic physical properties of CF 75 polymer material. Afterwards, the dynamic tension and compression experiments were conducted at various strain rate levels. During these tests, a high-speed camera was employed to record the dynamic loading process and a scanning electron microscopy (SEM) was used to carry out the post-test analysis. From the dynamic tension tests, the damage, fracture and toughness mechanisms of the monolithic CF 75 polymer material were revealed. The rate dependency of the mechanical properties and dynamic deformation behaviour were characterized. Then, the bonding of the glass-polymer interface was also characterized by dynamic tension loading on a single-glass-polymer system of the CF 75 polymer matrix with an embedded single 3mm-diameter glass particle. Debonding, cavitation and necking were illustrated, as well as the effects on the stress-strain response at various strain rates. Dynamic compression tests were carried out to characterize the impact-resistant performance of the monolithic CF 75 polymer material. They revealed the deformation behaviour, damage mechanisms, fracture characteristics, rate dependency of mechanical properties and temperature rise. Then, the single-glass-polymer system was tested by dynamic compression tests. The results clarify the particle effect on the dynamic compressive response. The debonding-induced decrease of yield stress and the contribution of glass particle failure to fracture energy are quantitatively characterized at various strain rates. Finally, the impact-resistant performance of the multiple-particles-polymer systems of CF 75 polymer matrix with embedded 25wt.% and 50wt.% 0.5mm-diameter PMMA particles was characterized by dynamic compression tests. For the multiple-particles-polymer systems, an improved static stiffness is explained. Multiple cracking from the particle-polymer interface is observed, which induces the decrease of yield stress. A high particle density results in the decrease of maximum stress and strain energy. However, crushing of the particles causes a faster increase of maximum stress and strain energy with increasing strain rate (indicated by a higher strain rate sensitivity index). In this thesis, a systematical investigation about the dynamic mechanical response of soft polymers was carried out. It includes the monolithic polymer material (as matrix material in the particle(s)-polymer system), the single-glass-polymer system (for studying the interfacial bonding and single particle effect), and the multiple-particles-polymer system (as a representative of the real particle(s)-polymer system for impact-resistant applications)