[Show abstract][Hide abstract] ABSTRACT: The work presented here concerns the ultimate strength predictions of simply supported, square plates of laminated composite material subjected to uniaxial in-plane compressive load. Plates having a range of thicknesses and initial geometric imperfections have been investigated. Several models are established, each based on first order shear deformation theory and assumption of small deflections. The approaches give reasonable but somewhat conservative estimates for the thicker plates considered, while for the thinner plates, neglect of the post-buckling behaviour makes the results very conservative. It will be necessary to use a large deflection plate theory for some of the models to realise their full potential. (c) 2013 Elsevier Ltd. All rights reserved.
Composites Part B Engineering 11/2013; 54(1):343-352. DOI:10.1016/j.compositesb.2013.05.017 · 2.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Surface interaction properties between the backing steel and liner in lined pipes are important features in terms of understanding the mechanical behavior of lined pipes. Presently API 5LD recommends a gripping force test to classify the surface interaction properties between the liner and the backing steel. In this paper, several details involving the influence of axial and hoop stress interaction, friction behavior and the influence of free boundaries are investigated in testing contexts. Published results using different testing regimes have been re-investigated in order to show the effects of accounting for axial strain release close to free boundaries, axial and hoop stress interaction and different friction behaviors. It has clearly been demonstrated that these effects must be accounted for when trying to accurately quantify surface interaction properties between liners and backing steels in physical testing contexts.
[Show abstract][Hide abstract] ABSTRACT: Adhesively bonded joints have found important application areas in the marine and offshore industry during the last years. One particular application is the use of bonded patches to repair steel structures such as floating production storage and offloading units (FPSOs). Experience has shown that FPSOs develop corrosion and cracks during service. Welding, which is frequently applied when repairing such kinds of defects, is hot work that is not allowed for FPSOs during production. Closing down the production can be very expensive. Thus, composite patch repair using adhesives is an attractive alternative, which means that there is a need to investigate the strength of bonded steel-composite joints.In the present paper, the strength of adhesively bonded lap-shear joints has been studied. Failure loads obtained experimentally have been presented and compared with theoretical predictions. Capacity estimates provided by traditional strength of materials approaches do not agree with experiments. On the other hand, results obtained by a recent inelastic fracture-based analysis represent measured strength values well. Furthermore, finite element analysis using cohesive elements for the adhesive bondline is shown to be a powerful tool in strength predictions of adhesively bonded joints. In addition to provide accurate estimates of the ultimate failure loads, the fracture process can be modelled, and the analysis method is applicable to a wide range of joint geometries.
International Journal of Adhesion and Adhesives 09/2012; 37:102–111. DOI:10.1016/j.ijadhadh.2012.01.015 · 1.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nonphysical pressure oscillations are observed in finite element calculations of Biot's poroelastic equations in low-permeable media. These pressure oscillations may be understood as a failure of compatibility between the finite element spaces, rather than elastic locking. We present evidence to support this view by comparing and contrasting the pressure oscillations in low-permeable porous media with those in low-compressible porous media. As a consequence, it is possible to use established families of stable mixed elements as candidates for choosing finite element spaces for Biot's equations. Copyright (c) 2011 John Wiley & Sons, Ltd.
International Journal for Numerical and Analytical Methods in Geomechanics 08/2012; DOI:10.1002/nag.1062 · 1.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mechanical impact loading of injection-moulded components was simulated. The material was a talc-filled and elastomer-modified polypropylene used in automotive exterior parts. The material model was the linear-elastic–viscoplastic SAMP-1 model, which features pressure-dependent yield stress, plastic dilatation and a simple damage model. The model was calibrated with data from tests in uniaxial tension, shear and uniaxial compression, utilising 3D digital image correlation for full-field displacement measurements. With the calibrated model, two load cases were simulated; centrally loaded clamped plates and three-point bending of bars. The predictions of force vs. deflection were good to fair. The results are discussed in terms of deficiencies of the calibration data, heterogeneity and anisotropy of injection-moulded components, and shortcomings of the model. In particular, the hardening curves at high strain rates are uncertain, and tests in biaxial tension would be useful.
Materials and Design 08/2012; 42:450–458. DOI:10.1016/j.matdes.2012.06.020 · 3.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Distributions of shear strains and strain states (triaxiality) were analysed for two in-plane shear test fixtures (Iosipescu and V-notched rail), using digital image correlation and numerical simulations. Three different polypropylene-based materials (two talc-filled compounds and one unfilled homopolymer) were tested. The three materials behaved differently in the shear tests. Most notably, cracks developed in tension near the notches for the particle-filled materials, while the unfilled homopolymer did not fracture. There were also differences between the materials regarding strain localisation between the notches, strain rates vs. strain level (for a given cross-head speed), thickness change in the sheared section, and triaxiality. The yield stresses in shear, uniaxial tension and uniaxial compression showed pressure sensitivity. At least for equivalent strain rates below 1 s−1, the strain rate sensitivity of the yield stress was approximately the same in these three stress states. The stress–strain curves obtained with the two methods were quite similar for these materials. There were some differences between the methods regarding the ease of mounting and aligning specimens, the complexity of specimen deformation patterns, and the uniformity of the shear strain distribution between the notches.
[Show abstract][Hide abstract] ABSTRACT: This study addresses the nonlinear stress-strain response in glass fibre
reinforced polymer composite laminates. Loading and unloading of these
laminates indicate that the nonlinear response is caused by both damage
and plasticity. A user defined material model is implemented in the
finite element code LS-DYNA. The damage evolution is based on the Puck
failure criterion , and the plastic behaviour is based on the
quadratic Hill yield criterion for anisotropic materials .
The European Physical Journal Conferences 08/2012; 26:04028-. DOI:10.1051/epjconf/20122604028
[Show abstract][Hide abstract] ABSTRACT: Mathematical models of cardiac electro-mechanics typically consist of three tightly coupled parts: systems of ordinary differential equations describing electro-chemical reactions and cross-bridge dynamics in the muscle cells, a system of partial differential equations modelling the propagation of the electrical activation through the tissue and a nonlinear elasticity problem describing the mechanical deformations of the heart muscle. The complexity of the mathematical model motivates numerical methods based on operator splitting, but simple explicit splitting schemes have been shown to give severe stability problems for realistic models of cardiac electro-mechanical coupling. The stability may be improved by adopting semi-implicit schemes, but these give rise to challenges in updating and linearising the active tension. In this paper we present an operator splitting framework for strongly coupled electro-mechanical simulations and discuss alternative strategies for updating and linearising the active stress component. Numerical experiments demonstrate considerable performance increases from an update method based on a generalised Rush-Larsen scheme and a consistent linearisation of active stress based on the first elasticity tensor.
Computer Methods in Biomechanics and Biomedical Engineering 07/2012; 17(6). DOI:10.1080/10255842.2012.704368 · 1.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Large-scale simulations of coupled flow in deformable porous media require iterative methods for solving the systems of linear algebraic equations. Construction of efficient iterative methods is particularly challenging in problems with large jumps in material properties, which is often the case in realistic geological applications, such as basin evolution at regional scales. The success of iterative methods for such problems depends strongly on finding effective preconditioners with good parallel scaling properties, which is the topic of the present paper. We present a parallel preconditioner for Biot's equations of coupled elasticity and fluid flow in porous media. The preconditioner is based on an approximation of the exact inverse of the two-by-two block system arising from a finite element discretisation. The approximation relies on a highly scalable approximation of the global Schur complement of the coefficient matrix, combined with generally available state-of-the-art multilevel preconditioners for the individual blocks. This preconditioner is shown to be robust on problems with highly heterogeneous material parameters. We investigate the weak and strong parallel scaling of this preconditioner on up to 512 processors and demonstrate its ability on a realistic basin-scale problem in poroelasticity with over eight million tetrahedral elements.
[Show abstract][Hide abstract] ABSTRACT: Uncertainty and variability in material parameters are fundamental challenges in computational biomechanics. Analyzing and quantifying the resulting uncertainty in computed results with parameter sweeps or Monte Carlo methods has become very computationally demanding. In this paper, we consider a stochastic method named the probabilistic collocation method, and investigate its applicability for uncertainty analysis in computing the passive mechanical behavior of the left ventricle. Specifically, we study the effect of uncertainties in material input parameters upon response properties such as the increase in cavity volume, the elongation of the ventricle, the increase in inner radius, the decrease in wall thickness, and the rotation at apex. The numerical simulations conducted herein indicate that the method is well suited for the problem of consideration, and is far more efficient than the Monte Carlo simulation method for obtaining a detailed uncertainty quantification. The numerical experiments also give interesting indications on which material parameters are most critical for accurately determining various global responses.
[Show abstract][Hide abstract] ABSTRACT: Lined pipes are Carbon Manganese pipes (backing steel) with a thin liner of corrosion resistant alloy, mechanically bonded to the backing steel. Lined pipes are cheap to produce compared to clad pipes, where the liner is metallurgically bonded to the backing steel, but they are also more complex to design for. One particularly challenging aspect is to determine load/displacement levels for potential disbondment between the liner and the backing steel. In that context, the strength of the metallurgical bond between the backing steel and the liner in a lined pipe may have an important influence. The metallurgical bond may be characterized by residual stresses in the liner and the friction coefficient between the inner surface of the backing steel and the outer surface of the liner. Current industry testing practice to determine the magnitude of residual stresses is defined in API 5LD, but these tests fail to consider boundary effects and Poisson’s ratio effects which have a substantial impact on the measured stress levels. An analytical formulation for stress levels in the liner close to free boundaries, and interaction between axial and hoop stresses are presented in this paper and validated by detailed finite element analyses. This formulation provides excellent transparency in terms of understanding which physical parameters are important in the surface interaction between the liner and the backing steel, and, among several applications, they are a highly useful tool to reinterpret the test regimes suggested in API 5LD.
[Show abstract][Hide abstract] ABSTRACT: The low-velocity, low-energy impact response of a mineral and elastomer modified polypropylene was characterised by instrumented falling-weight impact testing of plates with annular clamping. Most of the impact tests were performed at −30°C with incident impact velocities in the range 1.0–4.4m/s, and with plate thicknesses in the range 2.0–3.9mm. The following factors were investigated: moulding conditions (mould temperature, melt temperature, holding pressure), striker geometry, clamping, plate surface texture, melt flow weld lines and paint. The occurrence of brittle fracture was affected by all these factors, except the moulding conditions. Reducing the striker hemisphere diameter or changing to a flat striker induced brittle fracture. Removing the annular clamping led to a more brittle response. Plates with a weld line were more brittle than standard plates. The surface texture caused brittle fracture when the textured side was in tension under the striker. The paint induced brittle fracture at −30°C, but no adverse effect of the paint was observed at 20°C.
[Show abstract][Hide abstract] ABSTRACT: The low-velocity, low-energy impact response of a mineral and elastomer modified polypropylene was characterised by instrumented falling-weight impact testing of plates with annular clamping. Different loading conditions were assessed by varying plate thickness (2–4 mm), incident impact velocity/energy (up to 4.4 ms−1/34 J) and temperature (−60 to 20 °C). Force-deflection curves and fracture patterns were categorised and analysed. The main trends can be explained in terms of 1) deformations spanning from small-strain bending to large-strain stretching, 2) fracture responses spanning from linear-elastic brittle to highly ductile, 3) process-induced anisotropy, and 4) friction effects. With the highest impact velocity used in this study, plates thinner than ∼2.5 mm fractured at both −30 and 20 °C, although with different mechanisms. A remarkable finding was that the central radial crack under the striker ran parallel to the (injection moulding) flow direction for the most brittle fractures (at low temperatures), while it ran perpendicular to the flow direction in other cases.
[Show abstract][Hide abstract] ABSTRACT: Large-scale simulations of flow in deformable porous media require efficient iterative methods for solving the involved systems of linear algebraic equations. Construction of efficient iterative methods is particularly challenging in problems with large jumps in material properties, which is often the case in geological applications, such as basin evolution at regional scales. The success of iterative methods for this type of problems depends strongly on finding effective preconditioners. This paper investigates how the block-structured matrix system arising from single-phase flow in elastic porous media should be preconditioned, in particular for highly discontinuous permeability and significant jumps in elastic properties. The most promising preconditioner combines algebraic multigrid with a Schur complement-based exact block decomposition. The paper compares numerous block preconditioners with the aim of providing guidelines on how to formulate efficient preconditioners. Copyright (C) 2010 John Wiley & Sons, Ltd.
International Journal for Numerical and Analytical Methods in Geomechanics 01/2010; 35(13):1466-1482. DOI:10.1002/nag.973 · 1.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Weight-critical marine structures, such as high-speed craft, are often made of high-strength aluminium, and the usual joining method is welding. To improve the performance of high-speed craft there is a tendency to seeking lighter weight materials, especially in parts of the structure where weight-saving is particularly beneficial, such as in the superstructure. Light-weight materials can be fibre-reinforced composites. When joining structures made of composites to components of aluminium or steel, welding is no longer an option. On the other hand, adhesive bonding becomes an attractive joining method. Thus, there is a need to investigate the strength of bonded steel-composite joints.In the present paper, the stress distribution and strength of bonded double-lap steel-composite joints are investigated through theoretical analysis and experimental testing. The strength of a set of joints, with a variety of overlap lengths and two different bonding techniques and environmental conditions, has been measured experimentally. Furthermore, a new elastic–plastic stress analysis, accounting for adhesive shear deformations as well as axial and shear deformations in the adherends, has been derived.Predictions using the new theory derived herein as well as linear models are compared with the reported results. It is clearly demonstrated that linear theories are completely inappropriate for modelling such joints when loaded to failure. On the other hand, the strength predictions obtained by the new nonlinear theory agree well with the experimental results.
Composites Part B Engineering 01/2009; 40(1):29-40. DOI:10.1016/j.compositesb.2008.07.002 · 2.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper we present a mixed finite element method for modeling the passive properties of the myocardium. The passive properties are described by a non-linear, transversely isotropic, hyperelastic material model, and the myocardium is assumed to be almost incompressible. Single-field, pure displacement-based formulations are known to cause numerical difficulties when applied to incompressible or slightly compressible material cases. This paper presents an alternative approach in the form of a mixed formulation, where a separately interpolated pressure field is introduced as a primary unknown in addition to the displacement field. Moreover, a constraint term is included in the formulation to enforce (almost) incompressibility. Numerical results presented in the paper demonstrate the difficulties related to employing a pure displacement-based method, applying a set of physically relevant material parameter values for the cardiac tissue. The same problems are not experienced for the proposed mixed method. We show that the mixed formulation provides reasonable numerical results for compressible as well as nearly incompressible cases, also in situations of large fiber stretches. There is good agreement between the numerical results and the underlying analytical models.
Computer Methods in Biomechanics and Biomedical Engineering 01/2006; 8(6):369-79. DOI:10.1080/10255840500448097 · 1.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Adhesively bonded joints are frequently used in aircraft and aerospace structures. A bonding technique called overlamination, where the parts to be joined are connected by adherends that are laminated directly upon the assembly, is more common in the marine industry, and there is a need to assess the stress distribution in such joints.In the present paper, the interfacial stress distribution in overlaminated double-lap joints is investigated. New analytical models of the shear and transverse (through-thickness) normal stresses are derived. The analytical solutions are validated through comparison with finite element calculations, and the accuracy is shown to be high for the unbalanced steel–composite joints as well as the (nearly) balanced composite–composite joints considered. In the former case, the load transfer between the inner and outer adherends is concentrated to a single joint end, while the transfer is distributed to both ends in the latter balanced case.The analysis of the transverse normal stresses demonstrates that the critical failure mode in overlaminated double-lap joints may be related to excessive peel stresses. However, this problem may be alleviated by, e.g. tapering the end of the outer adherends.
Composites Part B Engineering 01/2005; 36(6):544-558. DOI:10.1016/j.compositesb.2005.01.002 · 2.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It is well known that geometric nonlinear effects have to be taken into account when the ultimate strength of single lap composite joints are studied. In the present paper we investigate for which level of loads or prescribed end displacements nonlinear effects become significant and how they appear. These aspects are studied by comparing finite element results obtained from geometric nonlinear models with the results from the linear ones. The well-known software package ANSYS is applied in the numerical analysis together with a self-implemented module in the C++ library Diffpack. Some of the results are also compared with classical analytical theories of idealized joints showing significant differences.The joints examined are made of cross-ply laminates having 0 or 90° surface layers. A combined cross-ply/steel joint and an isotropic joint made of steel are also studied. All the models except the all-steel one are assembled with adhesives, while the latter is welded.Through the investigation a considerable departure from linear behavior has been detected for a large regime of prescribed end displacements or external loads. Geometric nonlinear effects begin to develop for external loads that produces stresses which are far below ultimate strength limits and for average longitudinal strains that are less than 0.5%. It has also been detected that the distribution of materials within the joint has some influence on the nonlinear behavior. Thus, geometric nonlinear methods should always be applied when single lap (or other non-symmetric) composite joints are analyzed.
Composites Part B Engineering 07/2003; 34(5-34):417-427. DOI:10.1016/S1359-8368(03)00023-4 · 2.98 Impact Factor