Effects of geometry on the controlled failure of architected joints

Abstract

In the study of fracture mechanics, fracture is always treated as a non-desirable phenomenon. Through the presented studies, we aim to embrace fracture and treat it as a feature. We consider architected materials, aka mechanical metamaterials, as the platform for applying the proposed failure by design concepts. The mechanical behaviour of architected materials, independent of the base material, is tailored by the careful tuning of their geometry and microstructure. In that way, architected materials can exhibit extraordinary mechanical properties i.e., negative Poisson’s ratio as well as superior stiffness-to-weight and toughness-to-weight ratios. Hence, architected materials offer limitless possibilities for exploration with the goal of pre-programming fracture behaviour and exploiting geometrically driven fracture processes.

The studies we present, are mainly focused around the concepts and applications of adhesive bonding and confinement, where load transfer needs to be optimal. Adhesive joints eliminate the use of traditional fasteners, hence decreasing the amount of points prone to structural failure. However, the extensive use of adhesives in large scale applications (primarily wind energy and aerospace) gives rise to various challenges including ease of disassembling during maintenance, sustainability, recyclability as well as their dependence on fossil fuels to name a few. Therefore, through the presented studies, we aim to incorporate architected materials in confined systems, having thick adhesive bondlines in mind, and to understand the underlying physics driving the fracture processes in such systems. We then attempt to quantify the effects of the small-scale structure on fracture initiation and propagation.

Although the studies presented can be seen as independent problems and case studies, the underlying motivation, objectives and overall narrative is shared. The common question being: "How geometric manipulations can control crack growth in confined systems?", directs our focus around the areas of fracture mechanics, composite materials and architected materials. We then introduce all above areas, highlighting the followed research aspects and laying out the necessary developments that took place. That is achieved by reviewing the literature of the individual modules while simultaneously developing the needed methodology for each module.

In the third chapter, “Micropolar Elastic Foundations”, we formulate a complete theoretical framework for incorporating micropolar elasticity in an “augmented Winkler” elastic formulation. We focus on deriving closed-form solutions for the restoring linear and rotational stiffness based on stress potentials. We then present double cantilever beam solutions on the derived framework, and we quantify the effects of micropolar elasticity on the deflection distribution responses. Lastly, we lay the benefits and limitations of the method based on measured material parameters from existing literature.

In the fourth chapter, “Testing framework for Architected Joints”, we develop a testing framework for evaluating mode I fracture toughness that targets confined 3-dimensional architected materials. We focus on providing design guidelines for conducting such experiments. The testing setup is based on the classical double cantilever beam configuration, but the geometry of the pre-crack is designed specifically for avoiding any stress concentrations at corners, ensuring a cohesive crack propagation. The proposed configuration is benchmarked by numerical simulations incorporating micropolar elasticity and the phase field fracture model, and can successfully capture the crack paths. In the fifth chapter, “Gyroids: Can size control crack growth?”, we performed an experimental study based on the framework developed in the third chapter. The obtained experimental results are then complemented by the analytical and numerical methods presented at the second chapter, highlighting a dependence of micropolar size effects on the observed crack paths. We provide an estimation for the critical strain energy release rate of the gyroid lattice, based on micromechanical assumptions. Furthermore, we define a crack shape index for characterising crack growth in confined lattices, that enables the quantification of crack shapes without the need for precise coordinates of the crack shape.

In the sixth chapter, “The Toughness of Auxetic Foams”, we delve into the low stiffness limit by testing open-cell poly-urethane foams. Next, thermomechanical treatment is performed to convert the foams and give them auxetic properties. The converted foams are then tested in uniaxial tension/compression, and their mechanical response is modelled with the Ogden hyperelastic model, complemented, within the fracture mechanics framework, by a traction-separation law. We then formulate a 1-dimensional semi-analytical Euler-Bernoulli beam model integrating the measured material properties to predict fracture behaviour under the double cantilever beam configuration. The model demonstrates a very good agreement with experimental results, quantifying the fracture toughness of the auxetic foam, revealing the influence of thermomechanical conversion.

In the last chapter, we summarise the research undertaken in this thesis, and we explore possible further directions that can be taken.