| 1. |
- Al-Ramahi, Nawres Jabar, 1980-
(författare)
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Comprehensive numerical analysis of stress state in adhesive layer of joint including thermal residual stress and material non-linearity
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main objective of this work is to improve understanding of the stress state in the adhesive layer of bonded joints and identify key parameters which govern performance of adhesive joints. This information is crucial for the prediction of the failure initiation and propagation with the further estimation of the durability and strength of adhesively bonded structures.A systematic numerical analysis of stress state in the adhesive layer of a single-lap and double- lap joint under various loading conditions (thermal and mechanical loading) and an alternative methodology to predict the direction for crack propagation within adhesive layer are presented in this thesis. To identification of the most important parameters of joints is done based on the assessment of the peel and shear stress distributions in the adhesive layer. The thermal residual stresses arising after assembling of joints at elevated temperature are accounted for in the analysis.Initially, accurate, realistic 3D finite element model with novel boundary conditions (displacement coupling) was developed and validated. The employed boundary conditions allow to eliminate the edge effect and simulate the behavior of an infinite plate of composite laminate with off-axis layers (monoclinic materials). It is also possible to decouple the edge effects induced by the finite specimen width from the interaction with ends of the joint overlap region. Due to these advanced setting it is possible to eliminate influence of some of the parameters as well as to reduce geometry of the model without losing precision. Thus, the model is optimized with respect to the number of elements as well as element size distribution and does not require excessive computational power to obtain accurate stress distributions even near to the possible sites with stress perturbations (e.g. corners, cracks, etc). Additionally to the geometrical parameters, various material models have been employed in simulations of adhesive joints. A linear and non-linear material models (adherend and adhesive) was used for the single-lap joint, while a linear material behavior was considered for double-lap joint. The geometrical non-linearity was also included in the analysis whenever required. To make results more general and applicable to a wide range of different joints the normalized (with respect to the thickness of adhesive layer) dimensions of joints were used. Depending on the analyzed type of joint (single- or double- lap), combination of similar and dissimilar (hybrid) materials for adherends are considered: a) metal-metal; b) composite-composite; c) composite-metal. In case of the composite adherend (carbon and/or glass fibers) different laminate lay-ups were selected: uni-directional ([08]T and [908]T) and quasi-isotropic ([0/45/90/-45]S and [90/45/0/-45]S). In general, discussion and conclusions concerning the importance of various joint parameters are based on the magnitude of the peel and shear stress concentration at the ends of the overlap. In order to identify general trends with respect to the influence of mechanical properties of adherends the master curves for shear and peel stresses are constructed and analyzed. To simulate effect of the residual thermal stresses on the behavior of joints different methods for assembly of joints were considered (using dedicated adhesive or employing co-curing method). The results of this investigation lead to the conclusions that the one of the most important factors affecting the simulation results is the sequences of application of thermo-mechanical loading for different assembly methods. It is shown that simple superposition of thermal and mechanical stresses (most common approach) in the adhesive layer works properly only for linear material but it gives inaccurate results if non-linear material is considered. The thesis demonstrates the appropriate way to combine thermal and mechanical loads to obtain correct stress distributions for any material (linear and non-linear). The analysis of the influence of residual thermal stresses has shown that the peel and shear stress concentration at the ends of overlap joint and the shear stress within the over-lap region are reduced due to thermal effect. In case of composite adherend the co-curing assembly method is more favorable (in terms of reducing stress concentrations) than using adhesive for joining the materials.Finally, the simulation of the crack propagation within the adhesive layer for the bi-material (steel and composite) DCB sample with thick adhesive layer was carried out. The alternative to traditional fracture mechanics approach is proposed for the prediction of the crack path in the adhesive layer: a maximum hoop stress criterion. The hoop stress on the perimeters of a relatively large circle around the crack tip is evaluated to predict the direction of the crack extension with respect to the existing crack. The fracture mechanics is used to validate this approach and it is proved that if the Mode I is dominant for the crack propagation the hoop stress criterion be successfully used to predict crack path in the adhesive layer. This methodology is much more effective (in terms of required time and resources) than energy release based criterion or even X-FEM.The main result of this thesis is a tool to obtain accurate stress distributions in the adhesive layer of joints. This tool provided better understanding of the behavior of adhesive joints and allowed to develop new approach for prediction of crack propagation in the adhesive layer. This is definitely a development in the design of stronger, more durable adhesive joints for lighter structural components.
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| 2. |
- 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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| 3. |
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| 4. |
- 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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| 5. |
- Di Stasio, Luca, 1988-
(författare)
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Influence of microstructure on debonding at the fiber/matrix interface in fiber-reinforced polymers under tensile loading
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- At the end of the second decade of the XXI century, the transportation industry at large faces several challenges that will shape its evolution in the next decade and beyond. The first such challenge is the increasing public awareness and governmental action on climate change, which are increasing the pressure on the industrial sectors responsible for the greatest share of emissions, the transportation industry being one of them, to reduce their environmental footprint. The second big challenge lies instead in the renewed push towards price reduction, due to increased competition (as for example, in the market for low-Earth orbit launchers, the entry of private entities) and innovative business models (like ride-sharing and ride-hailing in the automotive sector or low-cost carriers in civil aviation). A viable and effective technical solution strategy to these challenges is the reduction of vehicles’ structural mass, while keeping the payload mass constant. By reducing consumption, a reduced weight leads to reduced emissions in fossil-fuels powered vehicles and to increased autonomy in electrical ones. By reducing the quantity of materials required in structures, a weight reduction strategy favors in general a reduction of production costs and thus lower prices. Transportation is however a sector where safety is a paramount concern, and structures must satisfy strict requirements and validation procedures to guarantee their integrity and reliability during service life. This represents a significant constraint which limits the scope of the weight reduction approach. In the last twenty years, the development of a novel type of Fiber-Reinforced Polymer Composite (FRPC) laminates, called thin-ply laminates, proposes a solution to these competing requirements (weight with respect to structural integrity) by providing at the same time weight reduction and increased strength. Several experimental investigations have shown, in fact, that thin-ply laminates are capable of delaying, and even suppress, the onset of transverse cracking. Transverse cracks are a kind of sub-critical damage in FRPC laminates and occur early in the failure process, causing the degradation of elastic properties and favoring other, often more critical, modes of damage (delaminations, fiber breaks). Delay and suppression of transverse cracks were already linked, at the end of the 1970’s, to the use of thinner plies inside a laminate. However, thin-plies available today on the market are at least 10 times thinner than those studied in the 1970’s. This characteristic changes the length scale of the problem, from millimeters to micrometers. At the microscale, transverse cracks are formed by several fiber/matrix interface cracks (or debonds) coalescing together. Understanding the mechanisms of transverse cracking delay and suppression in thin-ply laminates requires detailed knowledge regarding onset of transverse cracking at the microscale, and thus the study of the mechanisms that favor or prevent debond initiation and growth. The main objective of the present work is to investigate the influence of the microstructure on debond growth along the fiber arc direction. To this end, models of 2-dimensional Representative Volume Elements (RVEs) of Uni-Directional (UD) composites and crossply laminates are developed. The Representative Volume Elements are characterized by different configurations of fibers and different damage states. Debond initiation is studied through the analysis of the distribution of stresses at the fiber/matrix interface in the absence of damage. Debond growth on the other hand is characterized using the approach of Linear Elastic Fracture Mechanics (LEFM), specifically through the evaluation of the Mode I, Mode II and total Energy Release Rate (ERR). Displacement and stress fields are evaluated by means of the Finite Element Method (FEM) using the commercial solver Abaqus. The components of the Energy Release Rate are then evaluated using the Virtual Crack Closure Technique (VCCT), implemented in a custom Python routine. The elastic solution of the debonding problem presents two different regimes: the open crack and the closed crack behaviour. In the latter, debond faces are in contact in a region of finite size at the debond tip; in the latter, the debond is everywhere open and no contact exists between the faces. In the open crack regime, it is known that stress and displacement fields at the debond tip present an oscillating singularity. A convergence analysis of the VCCT in the context of the FEM solution is thus required to guarantee the validity of results and represents the first step of the work presented in this thesis. It is found that the total ERR does not depend on the size of elements at the debond tip, while the values of Mode I and Mode II ERR depend on element size in the open crack or mixed mode case. It is furthermore shown that Mode I and Mode II ERR do not converge, i.e. their asymptotic behavior for decreasing element size is not bounded. Thus, error reduction between successive iterations cannot be used to validate the solution and comparison with another method is required. Results obtained with the Boundary Element Method (BEM), available in the literature, are selected to this end. Debond growth under remote tensile loading is then studied in Representative Volume Elements of: UD composites of varying thickness, measured in terms of number of rows of fibers, from extremely thin (one fiber row) to thick ones; cross-ply laminates with a central 90◦ ply of varying thickness, measured as well in terms of number of rows of fibers, from extremely thin (one fiber row) to thick ones; thick UD composites (modelled as infinite along the through-the-thickness direction). Different damage configurations are also considered, corresponding to different stages of transverse crack onset: non-interacting isolated debonds; interacting debonds distributed along the loading direction; debonds on consecutive fibers along the through-the-thickness direction. Among the most relevant results, it is found that neither the 90◦ ply thickness nor the 0◦ ply thickness influences debond ERR in cross-ply laminates, differently from what is observed for transverse cracks with the so-called ply-thickness and ply-block effects. On the other hand, debond interaction along the loading direction is shown to influence significantly the Energy Release Rate, but this interaction possesses a characteristic distance (in terms of number of undamaged fibers) that defines the region of influence between debonds. Finally, an estimation of debond size at initiation and of debond maximum size is proposed based on arguments from stress analysis (for initiation) and on Griffith’s criterion from LEFM (for propagation). For a debond in a cross-ply laminate, its maximum size is estimated to lie in the range 40◦ − 60◦ , which is in strong agreement with previous results from microscopic observations available in the literature.
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| 7. |
- Gonçalves Nunes, Stephanie, et al.
(författare)
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The effect of short carbon fibers on viscoelastic behavior of UHMWPE
- 2022
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Ingår i: ECCM 2022 - Proceedings of the 20th European Conference on Composite Materials. - Lausanne : EPFL Lausanne, Composite Construction Laboratory. - 9782970161400 ; , s. 314-321
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- At service temperatures, ultra-high molecular weight polyethylene (UHMWPE) is a highly viscoelastic (VE) material due to its low glass transition temperature (≈-113 °C). Since the mechanical response changes over time, the ability to predict and improve its performance over lifetime is an engineering concern. Adding short carbon fibers (SCF) as reinforcement (10 wt%) is expected to improve the material instant and long-term properties. VE relaxation functions for UHMWPE and composite at different temperatures (25-100 °C) are obtained from experimental data used to find parameters in a Schapery's type linear VE model. Then, relaxation functions of the SCF (randomly distributed) composite are predicted using the quasi-elastic approach. The results show that fibers affect positively the VE properties of UHMWPE and that the temperature- and time-dependent matrix behavior affects the stress transfer to fibers However, due to uncertainty regarding the input parameters, limiting the applicability of the chosen quasi-elastic approach, the quantitative agreement is not perfect.
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