| 1. |
- Al-Ramahi, Nawres, 1980-, et al.
(författare)
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FEM analysis of stresses in adhesive single-lap joints with non-linear materials under thermo-mechanical loading
- 2018
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Ingår i: ECCM 2018 - 18th European Conference on Composite Materials. - : Applied Mechanics Laboratory. - 9781510896932
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Konferensbidrag (refereegranskat)abstract
- This study presents comprehensive numerical stress analysis in the adhesive layer of a single-lap joint subjected to various loading scenarios (mechanical and thermal loading). For this purpose numerical model (finite element method) with novel displacement coupling conditions able to correctly represent monoclinic materials (off-axis layers of composite laminates) has been developed. This model includes nonlinear material model and geometrical nonlinearity is also accounted for. The effect of thermal residual stresses (in adhesive) is analysed for various methods of manufacturing of single lap joint. The sequences of application of thermal and mechanical loads for the analysis of the thermal residual stresses in joints are proposed. It is shown that the most common approach used in many studies of linear superposition of thermal and mechanical stresses works well only for linear materials and produces wrong results if material is non-linear. The present study demonstrates suitable method to apply combined thermal and mechanical loads to get accurate stress distributions. Based on the analysis of these stress distributions the conclusions concerning the effect of the thermal residual stresses on peel and shear stress concentrations are made. The comparison between effect of thermal stresses in case of the one-step and two-step joint manufacturing techniques is made
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| 2. |
- Al-Ramahi, Nawres, 1980-, et al.
(författare)
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Investigation of end and edge effects on results of numerical simulation of single lap adhesive joint with non-linear materials
- 2018
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Ingår i: International Journal of Adhesion and Adhesives. - : Elsevier BV. - 0143-7496 .- 1879-0127. ; 87, s. 191-204
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents systematic numerical study of stresses in the adhesive of a single-lap joint with the objective to improve understanding of the main material and geometrical parameters determining performance of adhesive joints. For this purpose a 3D model as well as 2D model, optimized with respect to the computational efficiency by use of novel displacement coupling conditions able to correctly represent monoclinic materials (off-axis layers of composite laminates), are employed. The model accounts for non-linearity of materials (adherend and adhesive) as well as geometrical non-linearity. The parameters of geometry of the joint are normalized with respect to the dimensions of adhesive (e.g. thickness) thus making analysis of results more general and applicable to wide range of different joints. Optimal geometry of the single-lap joint allowing to separate edge effect from end effects is selected based on results of the parametric analysis by using peel and shear stress distributions in the adhesive layer as a criterion. Three different types of single lap joint with similar and dissimilar (hybrid) materials are considered in this study: a) metal-metal; b) composite-composite; c) composite-metal. In case of composite laminates, four lay-ups are evaluated: uni-directional ([08]T and [908]T) and quasi-isotropic laminates ([0/45/90/-45]S and [90/45/0/-45]S). The influence of the abovementioned parameters on peel and shear stress distributions in the adhesive layer is examined carefully and mechanical parameters governing the stress concentrations in the joint have been identified, this dependence can be described by simple but accurate fitting function. The effect of the used material model (linear vs non-linear) on results is also demonstrated.
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| 3. |
- Al-Ramahi, Nawres, 1980-, et al.
(författare)
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Model for numerical simulation and parametric analysis of composite adhesive joints under thermo-mechanical loading
- 2017
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Ingår i: ICCS20. - Paris : Società Editrice Esculapio, 2017. - 9788893850414 ; , s. 234-
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Konferensbidrag (refereegranskat)abstract
- Abstract: The current investigation focuses on development and verification of a modelfor numerical simulation of performance of adhesive joints under tensile loading. Differentcombination of materials in joints is considered: metal-metal, composite-composite andcomposite-metal. The objective of this paper is to present simulation results of joints usingan accurate finite element model including non-linear behaviour and large deformation.Moreover, several loading scenarios are analysed, including simultaneous application oftemperature and mechanical load. Not only the effect of temperature on mechanicalperformance of materials (adhesive as well as adherents) is analysed but also built up ofresidual thermal stresses during the manufacturing of joints are taken into account. Thisapproach is demonstrated by simulation of tensile tests of joints at several temperatures.Two scenarios of application of temperature and mechanical load using large deformationtheory are considered: 1) the thermal and mechanical loads are applied simultaneously (theproperties of the materials are adjusted accordingly to their performance at differenttemperatures); 2) temperature is applied on specimen which is not macroscopicallyconstrained and the obtained stress distribution is used as initial state for the nextsimulation of mechanical loaded joint. The influence of edge effects (due to limited widthof the joint) on the stress distribution within the joint are studied. In order to eliminatethese effects the periodic boundary conditions (BC) are used in the numerical model.These BC are adjusted to optimize numerical model and obtain efficient calculation routinefor analysis of stresses within interior part of the structure. The validity of these BCs isevaluated and verified by analysing number of case studies. The comparison between full3D FEM model and simplified 2D model is carried out. The resulting stress distributions inthe overlap region of joints are presented for different joints (the parameters are: materialcombinations, material models, geometry of adhesive layer, constraints and BCs) withcomprehensive analysis and recommendations for optimal numerical model that can beused in joint design.
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| 4. |
- Al-Ramahi, Nawres, 1980-, et al.
(författare)
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Numerical stress analysis in adhesive joints under thermo-mechanical load using model with special boundary conditions
- 2019
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Ingår i: IOP Conference Series: Materials Science and Engineering, Volume 518, Mechanical and Materials Engineering. - : Institute of Physics Publishing.
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Konferensbidrag (refereegranskat)abstract
- A numerical study of the adhesivejoint made of similar and dissimilar adherends subjected to thermo-mechanical loading is presented. A comprehensive numerical model was used for this purpose with the novel displacement coupling conditions which are able to correctly represent monoclinic materials (off-axis layers of composite laminates). The geometrical nonlinearity as well as nonlinear material model are also taken into account. Three different types of single-lap and double-lap adhesive joints are considered in this study: a) metal-metal; b) composite-composite; c) composite-metal. In case of composite laminates, four lay-ups are evaluated: uni-directional ([08]T and [908]T) and quasi-isotropic laminates ([0/45/90/-45]S and [90/45/0/-45]S). This paper focuses on the parameters which have the majoreffect on the peel and shear stress distribution within adhesive layer at the overlap ends. The comparison of behaviour of single-and double-lap joints in relation to these parameters is made. The master curves for maximum stress (peel and shear) at the ends of the overlap with respect to the bending stiffness and axial modulus of the adherends are constructed by analysing stress distributions in the middle of the adhesive.The main conclusions of this paper are: the maximum peel stress value for SLJ is reduced with increase of the adherend bending stiffness and for DLJ,similar behaviour was observed at the end next to the inner plate corner, while, at the end next to the outer plate corner peel stress is reduced with increase of adherend axial modulus.
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| 5. |
- Al-Ramahi, Nawres, 1980-
(författare)
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Numerical stress analysis in hybrid adhesive joint with non-linear materials
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents systematic numerical study of stresses in the adhesive of a single-lap joint subjected to various loading scenarios (mechanical and thermal loading). The main objective of this work is to improve understanding of the main material and geometrical parameters determining performance of adhesive joint for the future analysis of failure initiation and development in these structures.The first part of the thesis deals with development of a 3D model as well as 2D model, optimized with respect to the computational efficiency by use of novel displacement coupling conditions able to correctly represent monoclinic materials (off-axis layers of composite laminates). The model takes into account the nonlinearity of materials (adherend and adhesive) with geometrical nonlinearity also accounted for. The parameters of geometry of the joint are normalized with respect to the dimensions of adhesive (e.g. thickness) thus making analysis of results more general and applicable to wide range of different joints. Optimal geometry of the single-lap joint is selected based on results of the parametric analysis by using peel and shear stress distributions in the adhesive layer as a criteria and it allows separation of edge and end effects. Three different types of single lap joint with similar and dissimilar (hybrid) materials are considered: a) metal-metal; b) composite-composite; c) composite-metal. In case of composite laminates, four lay-ups are evaluated: uni-directional ([08]T and [908]T) and quasi-isotropic laminates ([0/45/90/-45]S and [90/45/0/-45]S). The influence of the abovementioned parameters is carefully examined by analyzing peel and shear stress distributions in the adhesive layer. Discussion and conclusions with respect to the magnitude of the stress concentration at the ends of the joint overlap as well as overall level of stresses within overlap are presented. Recommendations concerning use of nonlinear material model are given.The rest of the work is related to the various methods of manufacturing of joint (curing) and application of thermo-mechanical loading suitable to these scenarios. The appropriate sequences of application of thermal and mechanical loads for the analysis of the residual thermal stresses developed due to manufacturing of joints at elevated temperature required to cure polymer (adhesive/composite) are proposed. It is shown that the most common approach used in many studies of simple superposition of thermal and mechanical stresses works well only for linear materials and produces wrong results if material is non-linear. The model and simulation technique presented in the current thesis rectifies this issue and accurate stress distributions are obtained. Based on the analysis of these stress distributions the following conclusions can be made: joint processing at elevated temperature causes high stresses inside the adhesive layer; the residual thermal stresses will reduce the peel stress concentration at the ends of overlap joint and the shear stress within the overlap, moreover, this effect is more pronounced for the case of the one-step joint manufacturing in comparison with two-step processing technique.This study has generated a lot of results for better understand of behavior of adhesive joints and it will help in design of stronger, more durable adhesive single-lap joints in the future.
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| 6. |
- Almgren, Karin M., et al.
(författare)
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Contribution of wood fiber hygroexpansion to moisture induced thickness swelling of composite plates
- 2010
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Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 31:5, s. 762-771
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Tidskriftsartikel (refereegranskat)abstract
- One of the main drawbacks of wood fiber-based composite materials is their propensity to swell due to moisture uptake. Because the wood fibers are usually the main contributor to hygroexpansion, it is of interest to quantify the hygroexpansion coefficient of wood fibers, to compare and rank different types of fibers. This investigation outlines an inverse method to estimate the transverse hygroexpansion coefficient of wood fibers based on measurements of moisture induced thickness swelling of composite plates. The model is based on composite micromechanics and laminate theory. Thickness swelling has been measured on polylactide matrix composites with either bleached reference fibers or crosslinked fibers. The crosslinking modification reduced the transverse hygroexpansion of the composites and the transverse coefficient of hygroexpansion of the fibers was reduced from 0.28 strain per relative humidity for reference fibers to 0.12 for cross-linked fibers
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| 7. |
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| 8. |
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| 9. |
- Asp, Leif, et al.
(författare)
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Multiscale modelling of non-crimp fabric composites
- 2012
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Ingår i: Proceedings of the ASME International Mechanical Engineering Congress and Exposition--2012. - New York : American Society of Mechanical Engineers. - 9780791845196 ; , s. 581-590
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Konferensbidrag (refereegranskat)abstract
- Damage initiation and evolution in NCF composites leading to final failure includes a multitude of mechanisms and phenomena on several length scales. From an engineering point-of-view a computational scheme where all mechanisms would be explicitly addressed is too complex and time consuming. Hence, methods for macroscopic performance prediction of NCF composites, with limited input regarding micro- And mesoscale details, are requested. In this paper, multi-scale modelling approaches for in-plane transverse strength of NCF composites are outlined and discussed. In addition a simplistic method to predict transverse tensile and compressive strength for textile composites featuring low or no fibre waviness is presented.
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| 10. |
- Ben Kahla, Hiba, 1990-
(författare)
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Micro-cracking and delaminations of composite laminates under tensile quasi-static and cyclic loading
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aerospace industry is devoted to improving the aircraft performance while reducing its weight and limiting the emissions. Part of this objective can be accomplished with the use of high-performance long fibre reinforced polymer laminated composites. Being the first mode of damage under loading, intralaminar cracks initiate at the free edge of the off-axis plies and propagate along the respective fibre orientation. While these cracks grow as tunnels and increase in number, at some point two close cracks in plies of different off-axis orientation could intersect forming an envelope with the free edge. As loading continues, local delamination is expected within this envelope. The evolution and interactions of the different damage modes and the accumulation of damage under a specific loading are crucial in order to have a good understanding of the mechanisms and hence an accurate prediction of the mechanical properties´ degradation. This thesis is devoted to initiation and evolution of intralaminar cracking in plies and interlayer delamination in composite laminates.In the first part, quasi-isotropic Carbon Fibre/ Epoxy non-crimp fabric (NCF) laminates were studied under both quasi-static and cyclic loadings. The objective was to develop an efficient testing methodology for statistical damage evolution determination in Fatigue. The sequence of damage occurrences (intralaminar cracks in the different layers, delaminations at the different interfaces) loaded under quasi-static and tension-tension fatigue is first captured. To save characterisation time and costs, a simple model for predicting intralaminar cracking in laminates under cyclic loads was proposed and validated under low stress cyclic loads and low crack density. The model is based on Weibull distribution for the probability of cracking where part of parameters is obtained in quasi-static tests and part in a limited number of cyclic tests. The predictions of dependency of the cracking on the stress and number of cycles are validated against experimental observations of cracking in the 90-plies of quasi-isotropic NCF laminates as well as in tape based cross-ply laminates. In position where intralaminar cracks meet the specimen edge, local delaminations initiate due to the high 3D stress state. The delamination is further assisted by cracks in other off-axis plies, usually linking them. The average delamination length dependence on loading parameters is characterized and linked with the extent of the laminate stiffness reduction, showing using a simple ply-discount analysis that delaminations are the main reason for very large axial modulus reduction.In the second part, local delaminations and their effect on laminate stiffness are analysed using FEM. Expressions for the crack opening displacement (COD) determined using FEM are obtained and a modelling approach based on GLOB-LOC is performed for intralaminar crack case with local delaminations starting from the intralaminar crack. The delamination length is used as a parameter and studies are performed for different materials. Strong effect of delaminations on COD and on the axial modulus of the laminate is found. Finally, the last findings are used to simulate the damaged composite laminate behaviour in 4-point bending test. The bending stiffness of the laminate is significantly reduced by intralaminar cracks with delaminations. An approach, using the concept of the effective stiffness of the damaged ply is used. The so obtained effective stiffness matrix is a function of intralaminar crack density in the ply and the delamination length. The effective stiffness is used to calculate the bending stiffness of the damaged laminate. The laminate curvature calculated in this way is in a very good agreement with the curvature obtained in 3-D FEM simulations of the test with explicitly including cracks and delaminations in the model.
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