Modelling Delaminations in Composite Panels: Including Novel Theories for Post-Buckling and Delamination Growth

Even though composite materials are being applied more and more often, these materials are often not used to their full potential. In particular, composite materials are very sensitive to Compression-After-Impact (CAI), typically leading to knockdown factors in the order of 0.65. Many researchers have tried to predict the effects of Compression-After-Impact, either using finite element or analytical methods. While non-linear finite element methods can capture most of the damage mechanisms accurately, large computational power is required rendering such methods unpractical for optimisations. On the other hand, many analytical methods are limited in accuracy, especially at higher impact energies, which are often critical for design. In particular, no analytical model has ever been developed which can capture the progressive post-buckling behaviour of a panel with elliptical delaminations, let alone in a variable stiffness laminate. In order to solve this problem, a novel general theory for post-buckling was proposed. With this theory, the non-linear effects of post-buckling phenomena can be captured using a quasi-linear system of equations. This theory was used, in combination with the Rayleigh-Ritz method, to model the progressive post-buckling behaviour of variable stiffness composite laminates with single and multiple delaminations under compression. In general, very good agreement was found in comparison with non-linear finite element methods. A novel theory was also proposed to predict growth of post-buckling elliptical delaminations by analysing the delaminating resin layer as elastic foundation. The methods developed in this thesis shows promising results for applications in models which determine the compressive strength of composite panels after impact or the compressive strength of composite panels with delaminations in general. In particular, models using these theories will be able to capture the combined effect of various damage phenomena for panels with larger delaminations, which is typically a limitation of existing models. Moreover, the developed theory for post-buckling is not only applicable to the Rayleigh-Ritz method but can be used for general post-buckling calculations of panels and beams. In particular, this theory could also be interesting for finite element calculations.Aerospace EngineeringAerospace Structures & MaterialsAerospace Structures & Computational Mechanic

The extreme mechanics of viscoelastic metamaterials

Mechanical metamaterials made of flexible building blocks can exhibit a plethora of extreme mechanical responses, such as negative elastic constants, shape-changes, programmability and memory. To date, dissipation has largely remained overlooked for such flexible metamaterials. As a matter of fact, extensive care has often been devoted in the constitutive materials' choice to avoid strong dissipative effects. However, in an increasing number of scenarios, where metamaterials are loaded dynamically, dissipation can not be ignored. In this review, we show that the interplay between mechanical instabilities and viscoelasticity can be crucial and can be harnessed to obtain new functionalities. We first show that this interplay is key to understanding the dynamical behaviour of flexible dissipative metamaterials that use buckling and snapping as functional mechanisms. We further discuss the new opportunities that spatial patterning of viscoelastic properties offer for the design of mechanical metamaterials with properties that depend on loading rate.Comment: 12 pages, 7 figure

Buckling Metamaterials for Extreme Vibration Damping

Damping mechanical resonances is a formidable challenge in an increasing number of applications. Many passive damping methods rely on using low stiffness, complex mechanical structures or electrical systems, which render them unfeasible in many of these applications. Herein, a new method for passive vibration damping, by allowing buckling of the primary load path in mechanical metamaterials and lattice structures, is introduced, which sets an upper limit for vibration transmission: the transmitted acceleration saturates at a maximum value in both tension and compression, no matter what the input acceleration is. This nonlinear mechanism leads to an extreme damping coefficient tanδ ≈ 0.23 in a metal metamaterial—orders of magnitude larger than the linear damping coefficient of traditional lightweight structural materials. This principle is demonstrated experimentally and numerically in free-standing rubber and metal mechanical metamaterials over a range of accelerations. It is also shown that damping nonlinearities even allow buckling-based vibration damping to work in tension, and that bidirectional buckling can further improve its performance. Buckling metamaterials pave the way toward extreme vibration damping without mass or stiffness penalty, and, as such, could be applicable in a multitude of high-tech applications, including aerospace, vehicles, and sensitive instruments.</p

Viscoelastic snapping metamaterials

Mechanical metamaterials are artificial composites with tunable advanced mechanical properties. Particularly interesting types of mechanical metamaterials are flexible metamaterials, which harness internal rotations and instabilities to exhibit programmable deformations. However, to date such materials have mostly been considered using nearly purely elastic constituents such as neo-Hookean rubbers. Here we explore experimentally the mechanical snap-through response of metamaterials that are made of constituents that exhibit large viscoelastic relaxation effects, encountered in the vast majority of rubbers, in particular in 3D printed rubbers. We show that they exhibit a very strong sensitivity to the loading rate. In particular, the mechanical instability is strongly affected beyond a certain loading rate. We rationalize our findings with a compliant mechanism model augmented with viscoelastic interactions, which captures qualitatively well the reported behavior, suggesting that the sensitivity to loading rate stems from the nonlinear and inhomogeneous deformation rate, provoked by internal rotations. Our findings bring a novel understanding of metamaterials in the dynamical regime and opens up avenues for the use of metamaterials for dynamical shape-changing as well as vibration and impact damping applications.Comment: 10 pages, 7 figure

Oligomodal metamaterials with multifunctional mechanics

Mechanical metamaterials are artificial composites that exhibit a wide range of advanced functionalities such as negative Poisson’s ratio, shape shifting, topological protection, multistability, extreme strength-to-density ratio, and enhanced energy dissipation. In particular, flexible metamaterials often harness zero-energy deformation modes. To date, such flexible metamaterials have a single property, for example, a single shape change, or are pluripotent, that is, they can have many different responses, but typically require complex actuation protocols. Here, we introduce a class of oligomodal metamaterials that encode a few distinct properties that can be selectively controlled under uniaxial compression. To demonstrate this concept, we introduce a combinatorial design space containing various families of metamaterials. These families include monomodal (i.e., with a single zero-energy deformation mode); oligomodal (i.e., with a constant number of zero-energy deformation modes); and plurimodal (i.e., with many zero-energy deformation modes), whose number increases with system size. We then confirm the multifunctional nature of oligomodal metamaterials using both boundary textures and viscoelasticity. In particular, we realize a metamaterial that has a negative (positive) Poisson’s ratio for low (high) compression rate over a finite range of strains. The ability of our oligomodal metamaterials to host multiple mechanical responses within a single structure paves the way toward multifunctional materials and devices