Thesis title: Industrial or Urban Recyclable Materials for Structural Applications: Green Composites with Natural Fillers and Metals
Industrialization in recent decades has significantly increased pollution, adversely affecting the environment. This has driven the need for efficient and sustainable use of natural resources, as well as the development of low-carbon strategies to balance industrial growth with environmental preservation, human health, and economic stability. Therefore, the concept of circular economy and its application in various fields are being widely explored by researchers and institutions. The concepts of reuse, recycling and repair are becoming crucial in many sectors. This economic model is being adopted by various industries and countries, making it a pivotal approach to sustainable development. Therefore, this model has emerged as an effective tool for triggering sustainable development. Material recycling can be an excellent solution to achieve sustainable development. Also, efficient and intelligent design can result in material savings and high material utilization. For this, a detailed understanding of material behavior at both the macro and micro levels is essential, with the latter achievable through crystal plasticity.
In line with this, this study investigates the properties and behaviour of composite polymers and steels under various loading conditions, chosen for their sustainability and recyclability within a circular economy framework. Experiments were conducted on polymer composites made with varying proportions of recycled and virgin polymers to evaluate their mechanical properties under bending and tension. Bending tests were performed on a composite consisting of 80% recycled plastic (approximately 50% recycled polypropylene (PP) and 50% recycled high-density polyethylene (HDPE), which contained a small amount of low-density polyethylene (LDPE)) and 20% virgin high-impact PP copolymer. Tension tests were performed on five different polymer composite types, virgin PP and composites made by replacing a percentage of virgin PP with either regrind PP (ground virgin PP produced from scrap) or Revet material (a recycled blend of PP and polyethylene (PE)), to assess how the properties of virgin polymers change when partially or completely replaced with recycled polymers. The Ramberg-Osgood equation and stress-strain approximation method were employed to extract polymer composite properties, which were then used to create a finite element (FE) model in Abaqus. The FE model results were compared with experimental data to validate the extraction method. Additionally, the homogenisation concept was applied to derive homogenised material properties for developing the FE model, and these properties were compared with experimental results.
The study found that the Ramberg-Osgood equation, combined with the stress-strain approximation method, effectively extracts the material properties of polymer composites. The type and proportion of recycled polymer significantly affect the mechanical properties of virgin PP, particularly the modulus of elasticity and yield strength. Virgin PP exhibited the highest modulus of elasticity but one of the lowest yield strengths, while composites made partially with virgin and recycled polymers showed higher yield strength but a lower modulus of elasticity compared to virgin PP. Composites made completely with recycled polymers shows the lowest yield strength and modulus of elasticity. Thus, the optimal composition of polymer composite involves a trade-off between yield strength and modulus of elasticity, which can be tailored to a specific application if required. Although homogenisation can estimate the homogenised material properties of the composite, its practical application is questionable due to a deviation of up to 16% from experimental results.
Furthering the investigation into sustainable materials, this study also focused on steel St37, a recyclable and sustainable material with excellent mechanical properties. The concept of crystal plasticity was applied to evaluate the behaviour of steel St37 under monotonic and cyclic loadings. Crystal plasticity finite element modelling (CPFEM) was performed to replicate the stress-strain curve of steel St37 under monotonic and cyclic loading, achieved with the Ramberg-Osgood equation. Also, an investigation was done to evaluate the effect of the number of grains in the representative volume element (RVE) on the stress-strain curve achieved by CPFEM. The crystal plasticity material model was implemented in Abaqus via a user-defined material subroutine (UMAT) to simulate crystallographic slip processes.
In the study, it was observed that the CPFEM can be used successfully to replicate the behaviour of steel St37. CPFEM results in a very good curve fit, and therefore, the obtained crystal plasticity parameters are reliable and can be used in future for CPFEM of steel St37. From the derived crystal plasticity material parameters, it was observed that the values of crystal plasticity parameters for cyclic loading are comparatively higher, ranging from 70% to over 150%, than that of monotonic loading. This demonstrates that the steel St37 under cyclic loading exhibits higher resistance to deformation and yielding than steel under monotonic loading. The maximum error of 15.3% was observed in the resulting CPFEM stress-strain curve with the variance in the number of grains in the RVE. The error between the CPFEM and numerical stress-strain curve is smaller for the RVE with square grains arranged in a square lattice. Also, the influence of the number of grains in RVE on the resulting stress-strain curve is smaller for RVE with square grains arranged in a square lattice. From the study, it can be concluded that while working with CPFEM of steel St37, the RVE with 100 square grains arranged in a square lattice should be considered since it results in an error below 5% for both monotonic and cyclic loading.