Jjc Joris Remmers

DELAMINATION BUCKLING OF FIBRE-METAL LAMINATES

A fibre–metal laminate is a composite of metal and fibre-reinforced prepreg layers. An example of such a material is Glare. It consists of alternating layers of aluminium and glass-fibre-reinforced prepreg. The material can be sensitive to delamination buckling, which occurs when a partially delaminated panel is subjected to a compressive force. The interaction of local buckling and extension of the delaminated zone typically results in a decrease of the residual strength and, eventually, in a collapse of the structure. This phenomenon can be observed in experimental tests, but numerical analyses are needed to obtain a better understanding of the mechanisms and the critical parameters. In this paper, some experimental observations are discussed regarding delamination buckling in Glare and, on the basis of these observations, a numerical model is constructed at a meso-mechanical level. In this approach, solid-like shell elements are used to model the individual layers. They are connected by interface elements, which are capable of modelling delamination between the layers.

Model for the Scaling of Stresses and Fluctuations in Flows near Jamming

We probe flows of soft, viscous spheres near the jamming point, which acts as a critical point for static soft spheres. Starting from energy considerations, we find nontrivial scaling of velocity fluctuations with strain rate. Combining this scaling with insights from jamming, we arrive at an analytical model that predicts four distinct regimes of flow, each characterized by rational-valued scaling exponents. Both the number of regimes and the values of the exponents depart from prior results. We validate predictions of the model with simulations.

The cohesive band model: a cohesive surface formulation with stress triaxiality

In the cohesive surface model cohesive tractions are transmitted across a two-dimensional surface, which is embedded in a three-dimensional continuum. The relevant kinematic quantities are the local crack opening displacement and the crack sliding displacement, but there is no kinematic quantity that represents the stretching of the fracture plane. As a consequence, in-plane stresses are absent, and fracture phenomena as splitting cracks in concrete and masonry, or crazing in polymers, which are governed by stress triaxiality, cannot be represented properly. In this paper we extend the cohesive surface model to include in-plane kinematic quantities. Since the full strain tensor is now available, a three-dimensional stress state can be computed in a straightforward manner. The cohesive band model is regarded as a subgrid scale fracture model, which has a small, yet finite thickness at the subgrid scale, but can be considered as having a zero thickness in the discretisation method that is used at the macroscopic scale. The standard cohesive surface formulation is obtained when the cohesive band width goes to zero. In principle, any discretisation method that can capture a discontinuity can be used, but partition-of-unity based finite element methods and isogeometric finite element analysis seem to have an advantage since they can naturally incorporate the continuum mechanics. When using interface finite elements, traction oscillations that can occur prior to the opening of a cohesive crack, persist for the cohesive band model. Example calculations show that Poisson contraction influences the results, since there is a coupling between the crack opening and the in-plane normal strain in the cohesive band. This coupling holds promise for capturing a variety of fracture phenomena, such as delamination buckling and splitting cracks, that are difficult, if not impossible, to describe within a conventional cohesive surface model.

Two-Dimensional Mode I Crack Propagation in Saturated Ionized Porous Media Using Partition of Unity Finite Elements

Shales, clays, hydrogels, and tissues swell and shrink under changing osmotic conditions, which may lead to failure. The relationship between the presence of cracks and fluid flow has had little attention. The relationship between failure and osmotic conditions has had even less attention. The aim of this research is to study the effect of osmotic conditions on propagating discontinuities under different types of loads for saturated ionized porous media using the finite element method (FEM). Discontinuous functions are introduced in the shape functions of the FEM by partition of unity method, independently of the underlying mesh. Damage ahead of the crack-tip is introduced by a cohesive zone model. Tensile loading of a crack in an osmoelastic medium results in opening of the crack and high pressure gradients between the crack and the formation. The fluid flow in the crack is approximated by Couette flow. Results show that failure behavior depends highly on the load, permeability, (osmotic) prestress and the stiffness of the material. In some cases it is seen that when the crack propagation initiates, fluid is attracted to the crack-tip from the crack rather than from the surrounding medium causing the crack to close. The results show reasonable mesh-independent crack propagation for materials with a high stiffness. Stepwise crack propagation through the medium is seen due to consolidation, i.e., crack propagation alternates with pauses in which the fluid redistributes. This physical phenomenon challenges the numerical scheme. Furthermore, propagation is shown to depend on the osmotic prestressing of the medium. This mechanism may explain the tears observed in intervertebral disks as degeneration progresses.

A Partition of Unity-Based Model for Crack Nucleation and Propagation in Porous Media, Including Orthotropic Materials

In this paper, we present a general partition of unity-based cohesive zone model for fracture propagation and nucleation in saturated porous materials. We consider both two-dimensional isotropic and orthotropic media based on the general Biot theory. Fluid flow from the bulk formation into the fracture is accounted for. The fracture propagation is based on an average stress approach. This approach is adjusted to be directionally depended for orthotropic materials. The accuracy of the continuous part of the model is addressed by performing Mandel’s problem for isotropic and orthotropic materials. The performance of the model is investigated with a propagating fracture in an orthotropic material and by considering fracture nucleation and propagation in an isotropic mixed-mode fracture problem. In the latter example we also investigated the influence of the bulk permeability on the numerical results.

A discrete dislocation–transformation model for austenitic single crystals

A discrete model for analyzing the interaction between plastic flow and martensitic phase transformations is developed. The model is intended for simulating the microstructure evolution in a single crystal of austenite that transforms non-homogeneously into martensite. The plastic flow in the untransformed austenite is simulated using a plane-strain discrete dislocation model. The phase transformation is modeled via the nucleation and growth of discrete martensitic regions embedded in the austenitic single crystal. At each instant during loading, the coupled elasto-plasto-transformation problem is solved using the superposition of analytical solutions for the discrete dislocations and discrete transformation regions embedded in an infinite homogeneous medium and the numerical solution of a complementary problem used to enforce the actual boundary conditions and the heterogeneities in the medium. In order to describe the nucleation and growth of martensitic regions, a nucleation criterion and a kinetic law suitable for discrete regions are specified. The constitutive rules used in discrete dislocation simulations are supplemented with additional evolution rules to account for the phase transformation. To illustrate the basic features of the model, simulations of specimens under plane-strain uniaxial extension and contraction are analyzed. The simulations indicate that plastic flow reduces the average stress at which transformation begins, but it also reduces the transformation rate when compared with benchmark simulations without plasticity. Furthermore, due to local stress fluctuations caused by dislocations, martensitic systems can be activated even though transformation would not appear to be favorable based on the average stress. Conversely, the simulations indicate that the plastic hardening behavior is influenced by the reduction in the effective austenitic grain size due to the evolution of transformation. During cyclic simulations, the coupled plasticity-transformation model predicts plastic deformations during unloading, with a significant increase in dislocation density. This information is relevant for the development of meso- and macroscopic elasto-plasto-transformation models.

The competition between adhesive and cohesive fracture at a micro-patterned polymer-metal interface

It is Common Practice for Polymer-Metal Interfaces, Frequently Encountered in Microelec-Tronic Devices, to Improve Adhesion by Surface Roughening or Micro-Patterning. the Competitionbetween Adhesive Fracture and Cohesive Fracture in the Vicinity of a Patterned Interface, i.e., Inter-Face Crack Deflection, is One of these Key Mechanisms that Contribute Significantly to the Macroscopicadhesion. in this Paper, these Fracture Phenomena are Described Simultaneously by Cohesive Zoneelements with an Exponential Traction-Separation Law (TSL) for the Adhesive Failure and an Initiallyrigid, Exponentially Decaying, TSL for the Cohesive Failure. it is Demonstrated that the Conditions Atwhich Crack Kinking Occurs are Dominated by Fracture Strength Values as Opposed to the Commonlyused Fracture Toughness Values. Experimental Verification is Performed by Means of Four Point Bend-Ing Tests on Specifically Designed Micro-Patterned Polymer-Metal Samples.

Numerical Modelling of Self Healing Mechanisms

A number of self healing mechanisms for composite materials have been presented in the previous chapters of this book. These methods vary from the classical concept of micro-encapsulating of healing agents in polymer systems to the autonomous healing of concrete. The key feature of these self healing mechanisms is the transport of material to the damaged zone in order to establish the healing process. Generally, this material is a fluid and its motion is driven by capillary action which enables transportation over relatively large distances requiring little or no work. In the microencapsulated polymers as developed by White et al. [1], this liquid material is a healing agent, which is enclosed in the material by micro-encapsulation. When the capsule is ruptured by a crack, the healing agent will flow into the crack, driven by capillary action. Polymerisation of this healing agent is triggered by contact with catalysts which are inserted in the material and whose position is fixed. The new polymerised material will rebond the crack surfaces.

A Large Strain Discontinuous Finite Element Approach to Laminated Composites

A numerical model is presented for the geometrically nonlinear analysis of laminated composite materials. The formulation is derived in a consistent fashion from nonlinear continuum mechanics. The finite element formulation is briefly described, and a range of examples elucidate the performance and potential of the model. It is shown that geometric instabilities due to delamination can be captured with unstructured finite element meshes.

Computational Methods for Debonding in Composites

This contribution starts with a discussion of various phenomena in laminated composite structures that can lead to failure: matrix cracking, delamination between plies, and debonding and subsequent pull-out between fibres and the matrix material. The different scales are discussed at which the effect of these nonlinearities can be analysed. From these scales – the macro, meso and micro-levels – the meso-level is normally used for the analysis of delamination, which is the focus of this contribution. At this level, the plies are modelled as continua and interface elements between them conventionally serve as the framework to model delamination and debonding. After a a derivation of interface elements and a brief discussion of the cohesive–zone concept and its importance for the analysis of delamination, a particular finite element model for the plies is elaborated: the solid–like shell. Next, a more recent method to numerically model delamination is discussed, which exploits the partition–of–unity property of finite element shape functions. This approach offers advantages over interface elements, as will be discussed in detail.