Ragnar Larsson

• Professor
• Head of Department at Chalmers University of Technology

A novel intralaminar model has, for the first time, been applied and validated for the rate-dependent failure of multidirectional carbon/epoxy laminates. Quasi-static compressive failure is evaluated by the growth of intralaminar rate-dependent damage combined with the interaction of cohesive zones for interlaminar delamination. A special feature o...

To increase the use of polymeric structural composites, a major issue is to properly account for intra-laminar failure mechanisms, such as fiber kinking induced under compression. We propose a new continuum damage model that can predict the fiber kinking response at the ply level. The model is based on a previous structure tensor-based model for th...

This study presents an efficient and robust inverse approach to obtain the heat flux distribution on the tool rake face in oblique cutting including the tool nose radius. In this approach, Machine Learning (ML) is used to establish the relation between the parameters associated with the heat flux distribution and the error functions expressing the...

Strain-rate effects in a unidirectional non-crimp fabric carbon/epoxy composite are addressed. To allow for kink-band formation including strain-rate effects and damage in such composites, the paper advances a recent model focused on compression loading at small off-axis angles. The model is based on computational homogenization with a subscale rep...

This article addresses the micromechanically motivated, quasistatic to dynamic, failure response of fibre reinforced unidirectional composites at finite deformation. The model draws from computational homogenization, with a subscale represented by matrix and fibre constituents. Undamaged matrix response assumes isotropic viscoelasticity-viscoplasti...

We present a gradient enhanced damage model for ductile fracture modeling, describing the degraded material response coupled to temperature. Continuum thermodynamics is used to represent components of the energy dissipation as induced by the effective material response, thermal effects and damage evolution. As prototype for the effective material s...

The present paper deals with preform deformation and resin flow coupled to cure kinetics and chemo-rheology for the VARTM process. By monitoring the coupled resin infusion and curing steps through temperature control, our primary aim is to reduce the cycle time of the process. The analysis is based on the two-phase porous media flow and the preform...

Fracture modeling and experimental validation of Compacted Graphite Iron (CGI) specimens loaded under quasi-static conditions at room temperature are considered. Continuum damage mechanics coupled to plasticity is adopted to describe the evolution of damage. The damage production is based on a recently developed rate dependent damage evolution law,...

The paper proposes a novel approach to model the in-plane resin flow in deformable thin-walled fiber preforms for liquid composite molding processes. By ignoring the through-thickness flow in large scale thin-walled components, the 3-D resin flow is simplified to an in-plane flow inside the preform by a specialized divergence theorem. Shell kinemat...

The paper presents a novel thermomechanically coupled distributed primary deformation zone model to assist the inverse identification of Johnson-Cook material parameters to be used for machining simulations. A special feature of the enhanced model is that the assumed stress field is temperature-dependent, where the thermomechanical coupling governs...

This article addresses dynamic behaviour of fibre reinforced polymer composites in terms of a transversely isotropic viscoelastic-viscoplastic constitutive model established at the unidirectional ply level. The model captures the prelocalized response of the ply in terms of rate dependent elasticity and strength without damage. A major novelty is t...

The present work expands the application of Puck and Schürmann Inter-Fiber Fracture criterion to fiber reinforced thermoplastic 3D-printed composite materials. The effect of the ratio between the transverse compressive strength and the in-plane shear strength is discussed and a new transition point between the fracture conditions under compressive...

In this paper, recent shell model is advanced towards the calibration and validation of the Vacuum-assisted Resin Transfer Molding (VARTM) process in a novel way. The model solves the nonlinear and strongly coupled resin flow and preform deformation when the 3-D flow and stress problem is simplified to a corresponding 2-D problem. In this way, the...

Graphene nanosheets and thicker graphite nanoplatelets are being used as reinforcement in polymeric materials to improve the material properties or induce new functional properties. By improving dispersion, de-agglomerating the particles, and ensuring the desired orientation of the nano-structures in the matrix, the microstructure can be tailored t...

The paper presents an approach to ductile failure modeling derived based on continuum thermodynamics and damage. A continuum damage enhanced formulation of the effective material is used to describe the degradation of the response. From the thermo-mechanically motivated dissipation rate, a novel damage driving energy that involves both stored energ...

Fibre-reinforced polymer composites are widely used in structural applications due to their high specific stiffness and strength. In some applications the response of dynamically loaded composite components must be analysed. For example, in crash analyses of structural components, where very high loading rates occurs, the composite behaviour is not...

To increase the use of polymeric structural composites, a major issue is to properly account for intra-laminar failure mechanisms, such as fiber kinking which is typically induced in compression. We propose a new set of continuum damage models that are able to predict fiber kinking response under compression. A structure tensor based formulation is...

Graphene flakes are used as additives in polymer matrices to improve the material properties. Critical aspects of obtaining graphene enhanced functional properties in polymer nanocomposites include the composition and morphological optimization. The concentrations of flakes must be optimized to create components’ material properties which achieve t...

A new approach for the analysis of the ductile fracture of thin-walled large scale structures is developed. The method proposes a subscale refinement of the elements containing the crack. It allows for smooth progression of the crack without furnishing required level of the mesh refinement, and a more detailed representation of the crack tip and cr...

The paper presents a novel porous media model for homogenized free surface flow, representing wet–out composites processing. The model is derived from concepts of homogenization applied to a compressible two–phase flow, accounting for capillary effects and the concept of relative permeability. Based on mass balance considerations, we obtain a non–l...

In the present paper, we address the delicate balance between computational efficiency and level of detailing at the modelling of ductile fracture in thin-walled structures. To represent the fine scale nature of the ductile process, we propose a new XFEM based enrichment of the displacement field to allow for cracks tips that end or kink within an...

We propose a new and computationally efficient continuum damage based model that is able to predict the compressive response in the fibre direction. Instead of invoking the geometric instability into the material model, a key feature is to consider the geometrical fibre kinking instability on the macro-level based on a finite strain formulation. To...

In the era of process modeling of composite materials, 3D simulation of manufacturing processes is desirable considering the manufacturing trend where parts became more complex leading to complex 3D stress-strain states. Moreover, coupling of sub-processes that are happening simultaneously such as macro-scale preform processes, flow advancement and...

We propose a new and computationally efficient continuum damage based model, able to predict fibre shear failure under longitudinal compression for a UD ply. A structure tensor based continuum damage formulation is placed in context with the UD ply, where the elastic material response is governed by transverse isotropy. To represent the proper ener...

In the present paper, we present a novel finite-element method capable of handling most of the physics arising in the resin wet-out step for any composite system and processing case. The method is based on a compressible two-phase continuum formulation where a key feature is to model the involved physics via innovative use of the compressibility of...

Local-continuum damage models, such as the JC-dynamic failure criterion are often combined with the constitutive JC-model to represent the material behavior during machining. The major drawback is that the failure criterion exhibit a pathological mesh dependence. In literature it has been argued that a viscous regularization of the continuum materi...

In this paper we are concerned with the assessment of sub-models within a two-phase continuum mechanical FE framework for process modeling of composites manufacturing. In particular, the framework considers the inclusion of two deformation dependent models describing resin flow related to: (1) meso-scale wetting and compaction of individual plies a...

During machining processes, the work piece material is subjected to high deformation rates, increased temperature, large plastic deformations, damage evolution and fracture. In this context the Johnson-Cook failure model is often used even though it exhibits pathological mesh size dependence. To remove the mesh size sensitivity, a set of mesh objec...

The experiences from orthogonal machining simulations show that the Johnson-Cook (JC) dynamic failure model exhibits significant element size dependence. Such mesh dependence is a direct consequence of the utilization of local damage models. The current contribution is an investigation of the extent of the possible pathological mesh dependence. A c...

In this contribution, we discuss the potential of using a fully coupled multiscale method based on computational homogenisation for simulating the mechanical response of fibre reinforced thin-walled laminates based on shell theory. Emphasis is placed on the effect of e.g. RVE size on the results as compared to both a fully resolved 3D analysis and...

In this paper, a method to analyse and predict crack propagation in thin‐walled structures subjected to large plastic deformations when loaded at high strain rates—such as impact and/or blast—has been proposed. To represent the crack propagation independently of the finite element discretisation, an extended finite element method based shell formul...

In this paper we propose a multiscale method based on computational homogenization for simulating the mechanical response of a thin-walled porous structure. Due to the inhomogeneous nature of the porous material in the thickness direction, the length scale of the deformation-field variations in the thickness direction is in the same order of magnit...

The present contribution is a part of the work towards a framework for holistic modeling of composites manufacturing. Here we focus our attention onto the particular problem of coupled dual-scale deformation–flow process such as the one arising in RTM, Vacuum Assisted Resin Infusion (VARI) and Vacuum Bag Only (VBO) prepregs. The formulation conside...

A method for quantifying distortions arising in engine blocks of aluminum with liners of gray iron has been developed based on virtual finite element-based simulation tools. The key processing steps comprise the convection cooling step, associated with shrinkage and residual stress build-up, and the machining step. The engine block has been modeled...

We recently developed a simulation tool to simulate a quite wide class of infusion processes based on a compressible porous media theory formulation involving three constituents, solid, fluid and pore gas embedded in the voids. The aim of this tool is: firstly to model the highly deformable preform and its interaction with external loading and the...

In the present contribution an infusion simulation tool is developed, applicable to a quite wide range of composites manufacturing technologies. The paper focusses on isothermal, infusion like manufacturing processes involving highly deformable preforms and a free surface resin flow. There are two major issues addressed at the modelling of the infu...

In the present contribution we address the modeling of graphene membranes using a hierarchical modeling strategy to bridge the scales required to describe and understand the material. Quantum Mechanical (QM) and optimized Molecular Mechanical (MM) models are used to describe details on the nanoscale, while a multiscale continuum mechanical method i...

Through-the-thickness crack propagation in thin-walled structures is dealt with in this paper. The formulation is based on the cohesive zone concept applied to a kinematically consistent shell model enhanced with an XFEM-based discontinuous kinematical representation. The resulting formulation comprises the representation of continuous deformation,...

The paper deals with the modeling of thin, monolayer graphene membranes, which have significant electrical and physical properties used for nano- or micro-devices, such as resonators and nanotransistors. The membrane is considered as a homogenized graphene monolayer on the macroscopic scale, and a continuum–atomistic multiscale approach is exploite...

A constitutive model for anisotropic and tension–compression asymmetric response of a fibrous preform is developed and solved using a FE software. Applicability of the method to complex geometries is demonstrated by analysis the consolidation of an axisymmetric filament wound pressure vessel made from commingled yarns. Three different winding patte...

This study concerns the constitutive modelling of dissipative open-cell structural cellular solids under primarily finite compressive deformations and the corresponding non-linear finite element implementation. A thermodynamically consistent, mechanistic approach presented in Hård af Segerstad et al. [Hård af Segerstad, P., Larsson, R., Toll, S., 2...

In the present paper the delamination mechanism of a typical internal structure of the anisotropic conductive adhesive (ACA) interconnect for electronic packaging is modeled on the basis of micropolar theory and computational homogenization. The interface is treated as a finite Representative Volume Element (RVE), across which the macroscopic defor...

In this paper, a finite element (FE) code for modelling of pressure driven axisymmetric consolidation of composite material from commingled yarns is presented. The proposed model is developed on the basis of a two phase continuum model incorporated in an FE framework. The applicability of the code to axisymmetric geometries is demonstrated by analy...

In this paper, a multiscale method for simulating the mechanical response of a thin porous structure subjected to loading is proposed. Shell kinematics is adopted for the structure on the macroscopic scale while the stress resultants pertaining to the shell model are attained from the underlying porous microstructure via computational homogenizatio...

A finite element model is developed to solve consolidation problems for composites manufacturing. The model is developed from a generic two-phase continuum theory allowing for coupling between the solid and fluid responses. The code is applied to a case study consisting of consolidation of a hat stringer to evaluate nonlinear effects.

A mechanistic model is presented for an open-cell cellular solid consisting of a three-dimensional network of elastic struts. By considering the bending and torsion as well as stretching and buckling of the struts, we allow for length-scale effect in the macroscopic response. Constitutive equations are developed for the force and couple stress tens...

The paper addresses the possibility to project both mechanical and thermal phenomena pertinent to the fracture process zone into a cohesive zone. A wider interpretation of the notion cohesive zone is thereby suggested to comprise not only stress degradation due to micro-cracking but also heat generation and energy transport. According to our experi...

A conductive adhesive is a promising interconnection material for microsystem packaging. The interconnect features are of great importance to system responses under various loading conditions. The flip-chip packaging system with anisotropic conductive film (ACF) joint under thermal loadings has been investigated both experimentally and theoreticall...

A plastic ball grid array component interconnect has been experimentally investigated and modeled on the basis of micropolar theory. The experimental results were analyzed, and the data also provided the verification for the micropolar interface model. Two different interconnect cross sections, namely, one near the component boundary and the other...

A thermodynamically consistent approach is developed for modelling the response of an open-cell cellular solid at finite compressive strains. The cellular solid is considered as a network of struts, where each strut connects two vertex points. A hypothesis is proposed that the vertex points move affinely in the finite strain regime, where the strut...

The present paper considers constitutive modeling of structural foams undergoing large deformations in combination with high deformation rates. In particular, large inelastic compactions is in focus, involving hardening of the material as the point of compaction of the foam is approached, and rate dependent evolution of the plastic deformation. A p...

The focus of the present paper is on the finite element modelling of dynamic fracture based on the concept of locally enriched element shape functions in the vicinity of the crack, in line with the eXtended Finite Element Method (X-FEM). For this purpose, the proper governing equations for the case of a propagating crack within a hyperelastic mater...

A conductive adhesive is a promising interconnection material for microsystem packaging. The interconnect features are of great importance to system responses under various loading conditions. The flip-chip packaging system with anisotropic conductive film (ACF) joint under thermal loadings has been investigated both experimentally and theoreticall...

In the present paper we investigate the significance of gas–solid interaction in foamed polymers. In particular, we consider the response of foams to large compressive deformations at high rates. A main feature of the paper concerns the representation of gas-filled foams as a two-phase porous material, where the interaction between the phases is mo...

In the present paper, the homogenized mechanical response of an interface in a microsystem interconnection is established on the basis of micropolar theory. The interface is treated as a finite RVE (representative volume element), across which macroscopic discontinuities occur as expressed in terms of the regularized discontinuous displacement and...

The paper presents a higher order homogenization scheme based on non-linear micropolar kinematics representing the macroscopic variation within a representative volume element (RVE) of the material. On the microstructural level the micro–macro kinematical coupling is introduced as a second-order Taylor series expansion of the macro displacement fie...

The process of press forming commingled yarn composites is studied and modelled based on the theory of porous media. Two sub processes are considered: (1) the wetting and compaction of individual bundles and (2) the overall preform deformation. An experimental method is introduced, to study these two sub processes separately. The experiment suppres...

Anisotropic conductive adhesives (ACAs) are widely used in microsystem packaging as interconnection materials, and many studies have been done about their electrical features, but limited works have been done on the mechanical properties with respect to the overall reliability of the packaging, and even less attention is paid to the micro-scale con...

The increase in microsystem packaging density sets the requirement for component sizes in the system to become smaller and smaller. The scale decrease makes the analysis more complicated as the corresponding resolution should be improved to a great extent. Interconnection in the system is a typical interface structure that widely appear in the pack...

A general finite element approach for the modelling of fracture is presented for the geometrically non-linear case. The kinematical representation is based on a strong discontinuity formulation in line with the concept of partition of unity for finite elements. Thus, the deformation map is defined in terms of one continuous and one discontinuous po...

The main object of the present paper is to investigate the significance of gas-solid interaction in foamed polymers subjected to large deformations, particularly in compression, in combination with high rates thereof. The present development thus represents an extension of previous work in [1], where we developed a phenomenological model representi...

In the present contribution, the development of a thermo-mechanical cohesive zone model is discussed in the context of a strong discontinuity formulation according to the concept of partitions of unity [1]. On the basis of our previous work, [2, 3], and also the contributions, [4, 5], the deformation map is thereby defined in terms of mutually inde...

In this paper, the micro-polar theory is used to develop a numerical model for the prediction of the behavior of the material in the vicinity of the microsystem interconnection interface as well within the interface. This model, as compared with the classical continuum theory, can offer the possibility to include the size-effect for the simulation...

A molten hybrid-yarn composite is modelled as a continuum. Constitutive equations governing hydrostatic consolidation are formulated, based on a two-phase continuum mechanical framework developed in a recent paper [Larsson R, Wysocki M, Toll S. Process-modelling of composites using two-phase porous media theory. European Journal of Mechanics-A 2004...

The contribution focuses on applying micropolar theory which, as compared to the classical continuum theory, can offer the possibility to include e.g. a microstructural size-effect for the simulation and prediction of the interfacial stresses of microsystem interconnections, such as anisotropically conductive adhesives. As the dimension of the adhe...

The present paper considers constitutive modeling of structural foam undergoing large deformations. In particular, we are concerned with the modeling of large, inelastic compressive deformations, and the associated hardening material response as the point of compaction is approached. A phenomenological approach is adopted, and the foam is regarded...

A theoretical and computational framework which covers both linear and non-linear fracture behaviour is presented. As a basis for the formulation, we use the material forces concept due to the close relation between on one hand the Eshelby energy–momentum tensor and on the other hand material defects like cracks and material inhomogeneities. By sep...

The contribution focuses on the application of micropolar theory to establish a model for prediction of the interface behavior in microsystem adhesive interconnections. As the dimension of the adhesive itself is usually much smaller than that of neighboring components, and still may include internal structures, the interconnection is treated as an...

In this paper a finite element code for modelling of pressure driven axisymmetric consolidation of composite material from commingled yarns is presented. The proposed model is developed on the basis of a two-phase continuum model incorporated in a FE-framework. The applicability of the code to axisymmetric geometries is demonstrated by analysis of...

This contribution focuses on the delamination phenomenon in a thin structural composite, which represents a major source of degradation during deformation in a composite material. The problem is resolved at a “meso-level”, where separate plies are explicitly modelled and connected via interfaces to form a laminate. A recently proposed formulation o...

A biphasic continuum model is proposed for the modeling of a family of forming processes for fiber composites. The processes considered involve deformation of a fiber bundle network, wetting by penetration of resin into fiber bundles, and resin flow through the fiber bundle network. The continuum model represents three (or more) actual micro consti...

In this paper, delamination progression in a composite laminate is modelled by interface elements within a geometrically non-linear FE analysis setting. With the assumption of small continuum deformations, a rotational interface formulation is obtained from a recently proposed formulation based on the concept of a regularised strong discontinuity,...