| 1. |
- Bru, Thomas, 1990, et al.
(author)
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Validation of a novel model for the compressive response of FRP: experiments with different fibre orientations
- 2017
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In: ICCM International Conferences on Composite Materials. ; 2017
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Conference paper (peer-reviewed)abstract
- Crush tests have been performed on flat unidirectional non-crimp fabric (NCF) coupons with different fibre orientations as part of the validation of a ply-based damage model for crash. The fibre off-axis angle with respect to the crushing direction ranged from 0º to 90°. The results of the tests indicate that the crush stress remains unchanged for off-axis angles between 0° and 15°. The failure mode in these specimens was out-of-plane kinking. For 20° and 25° off-axis angles the crush stress dropped 20% and evidence of out-of-plane kinking were harder to find. For 45° off-axis angle a network of matrix cracks develops in the specimen and for 90° off-axis angle a brittle shear failure is observed. It is suggested that the out-of-plane kinking is promoted because of the natural waviness of NCF materials and that the high in-plane shear stress generated from 20-25° off-axis loading results in a transition from out-of-plane kinking to in-plane kinking. These hypotheses need, however, to be verified by an extended failure analysis of the crush specimens.
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| 2. |
- Costa, Sergio, 1987, et al.
(author)
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Development and validation of a finite deformation fibre kinking model for crushing of composites
- 2020
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In: Composites Science and Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 197
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Journal article (peer-reviewed)abstract
- A mesoscale model for fibre kinking onset and growth in a three-dimensional framework is developed and validated against experimental results obtained in-house as well as from the literature. The model formulation is based on fibre kinking theory i.e. the initially misaligned fibres rotate due to compressive loading and nonlinear shear behaviour. Furthermore, the physically-based response is computed in a novel and efficient way using finite deformation theory. The model validation starts by correlating the numerical results against compression tests of specimens with a known misalignment. The results show good agreement of stiffness and strength for two specimens with low and high misalignment. Fibre kinking growth is validated by simulating the crushing of a flat coupon with the fibres oriented to the load direction. The numerical results show very good agreement with experiments in terms of crash morphology and load response.
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| 3. |
- Costa, Sergio, 1987, et al.
(author)
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Finite element implementation of a model for longitudinal compressive damage growth with friction
- 2016
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In: ECCM 2016 - Proceeding of the 17th European Conference on Composite Materials. - : European Conference on Composite Materials, ECCM. - 9783000533877
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Conference paper (peer-reviewed)abstract
- A model for the longitudinal response of laminated fibre-reinforced composites during compressive damage growth is implemented in a Finite Element (FE) package and validated for mesh objectivity. The current work details the FE implementation of the fibre kinking model and in particular challenges associated with mesh objectivity. The numerical way to solve the stress equilibrium and stress compatibility equations simultaneously in an FE framework is also presented. The results show that the current model can be used to predict the kinking response and thus account for the correct energy absorption.
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| 4. |
- Costa, Sergio, 1987, et al.
(author)
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Improvement and validation of a physically based model for the shear and transverse crushing of orthotropic composites
- 2019
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In: Journal of Composite Materials. - : SAGE Publications. - 1530-793X .- 0021-9983. ; 53:12, s. 1681-1696
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Journal article (peer-reviewed)abstract
- This paper details a complete crush model for composite materials with focus on shear dominated crushing under a three-dimensional stress state. The damage evolution laws and final failure strain conditions are based on data extracted from shear experiments. The main advantages of the current model include the following: no need to measure the fracture toughness in shear and transverse compression, mesh objectivity without the need for a regular mesh and finite element characteristic length, a pressure dependency of the nonlinear shear response, accounting for load reversal and some orthotropic effects (making the model suitable for noncrimp fabric composites). The model is validated against a range of relevant experiments, namely a through-the-thickness compression specimen and a flat crush coupon with the fibres oriented at 45° and 90° to the load. Damage growth mechanisms, orientation of the fracture plane, nonlinear evolution of Poisson's ratio and energy absorption are accurately predicted.
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| 5. |
- Costa, Sergio, 1987, et al.
(author)
-
Mesh objective implementation of a fibre kinking model for damage growth with friction
- 2017
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In: Composite structures. - : Elsevier BV. - 0263-8223 .- 1879-1085. ; 168, s. 384-391
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Journal article (peer-reviewed)abstract
- A newly developed physically based model for the longitudinal response of laminated fibre-reinforced composites during compressive damage growth is implemented in a Finite Element (FE) software. It is a mesoscale model able to capture the physics of kink-band formation by shear instability, the influence of the matrix in supporting the fibres and the rotation of the fibres during compression, resulting in more abrupt failure for smaller misalignments. The fibre kinking response is obtained by solving simultaneously for stress equilibrium and strain compatibility in an FE framework. Strain softening creates pathological sensitivity when the mesh is refined. To make the model mesh objective, a methodology based on scaling the strain with the kink-band width is developed. The FE implementation of the current model is detailed with focus on mesh objectivity, and generalized to irregular meshes. The results show that the current model can be used to predict the whole kinking response in a 3D framework and thus account for the correct energy dissipation.
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| 6. |
- Costa, Sergio, 1987
(author)
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Physically based constitutive models for crash of composites
- 2019
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Doctoral thesis (other academic/artistic)abstract
- The transportation industry and passenger cars in particular are strong emitters of gases that contribute to the climate crisis. For this reason, the automotive industry investigates opportunities to reduce emissions, such as reducing the weight of the car. Composite materials, due to their high strength, stiffness and energy absorption to weight ratio, are a suitable material choice to reduce weight. The challenge here is that composites do not satisfy the fast development times and low costs required by the car industry. An efficient design phase, using more simulation and less physical testing, allows for time and cost-savings. However, there is a lack of efficient computational models to help the design with composite materials, which is fundamental for a widespread usage of composites in the automotive industry. This thesis presents the development, improvement and validation of constitutive models for composites in crash, focusing on compressive damage modes, matrix compression and fibre compression. The material being modelled is a carbon fibre/epoxy uni-weave Non-Crimp Fabric (NCF) composite. The properties of the composite constituents are homogenized to the ply level for a more efficient modelling. The matrix behaviour is modelled by combining damage and friction on the microcrack surfaces. The transverse mechanisms are modelled efficiently using a criterion for final failure, interaction of damage modes and a continuous response between compression and tension. The model is validated against 45- and 90-degree specimens. The fibre compression mode is fibre kinking growth, a very complex mechanism, responsible for high energy absorption. A homogenized 3D model based on Fibre Kinking Theory (FKT) is developed. It includes initial fibre misalignments and further rotations are governed by equilibrium with shear nonlinearity. The model is implemented in a commercial Finite Element (FE) software together with a mesh objective methodology. Furthermore, another formulation with similar physical principles but more suitable, efficient and robust for crash simulations is developed, implemented in an FE software and validated against experiments. The results show good qualitative and quantitative agreement. The proposed models allow for a reduction of physical testing required to develop crashworthy structures.
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| 7. |
- Costa, Sergio, 1987
(author)
-
Physically based fibre kinking model for crash of composites
- 2016
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Licentiate thesis (other academic/artistic)abstract
- Passenger cars are a major emitter of global warming gases which has led to tighter regulations being imposed on car manufacturers. An efficient way to reduce emissions is to reduce the weight of the cars. Composite materials, due to their high strength and energy absorption to weight ratio, are a suitable material choice to reduce the weight without affecting passenger safety. A major challenge today is the fast development times and low costs required by the automotive industry. An efficient design phase using more virtual tools and less physical testing allows time and cost-savings during the design phase. Fibres oriented longitudinally with the load and subjected to compression fail mainly by kinking, which is the damage mode responsible for most of the energy absorption. In this thesis the focus is on developing a physically based fibre kinking model for crash of composites. Fibre kinking is shear dominated, i.e. strongly influenced by the properties of the matrix as well as the alignment level of the fibres and the transverse loads. Modelling the complex physical mechanisms involved in crash at the microscale will result in prohibitively expensive simulations for the automotive industry. Therefore, in the present thesis, we homogenize the material while capturing the physical mechanisms involved, such as fibre rotation. The model parameters are physically meaningful and avoid cumbersome tests to obtain input for the model. Furthermore, the model is implemented in commercial Finite Element (FE) software together with a mesh objective methodology. The results show that the proposed model can be used to predict the whole kinking response in a 3D framework and thus account for the correct energy absorption.
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| 8. |
- Costa, Sergio, 1987, et al.
(author)
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Validation and improvements of a mesoscale finite element constitutive model for fibre kinking growth
- 2019
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In: ECCM 2018 - 18th European Conference on Composite Materials. - : European Society for Composite Materials. - 9781510896932
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Conference paper (peer-reviewed)abstract
- The present work is focused on the computational challenges and further verification and validation of an advanced fibre kinking model. This model was previously developed by the authors and implemented in a Finite Element (FE) code with a mesh objective formulation. The previous validation in terms of comparison with an analytical and a micromechanical model is herein extended to also encompass FE simulations of longitudinal compression in multiaxial stress states. In addition, numerical improvements have been added to the model targeting its computational efficiency and stability in order to handle multiaxial stress states and large structures.
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| 9. |
- Costa, Sergio, 1987, et al.
(author)
-
Validation of a novel model for the compressive response of FRP: Numerical simulation
- 2017
-
In: ICCM International Conferences on Composite Materials. ; 2017-August
-
Conference paper (peer-reviewed)abstract
- A progressive damage model for matrix compression is complemented with matrix tension in a physically based manner. The interaction of damage mechanisms undergoes a preliminary validation using single elements. The crushing response is validated with two different flat specimens with the fibres oriented transversely and at 45 degrees to the load. The model combines friction with damage to model the shear response accurately, which is necessary for reliable crush simulations. The behaviour in tension is history dependent, i.e. the model accounts for the stiffness reduction and strength to carry load in tension when previously damaged occurs in compression. The validation is performed against different tests showing the reliability of the model for different fibre orientation, specimen geometry and multiaxial loading scenarios. The crush response is well captured as well as the geometry and location of the different damage mechanisms.
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| 10. |
- Fagerström, Martin, 1979, et al.
(author)
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MODELLING AND TESTING THE CRASH BEHAVIOUR OF COMPOSITE VEHICLES COMPONENTS
- 2019
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Conference paper (other academic/artistic)abstract
- In the current contribution we will present the latest developments in the project “Modelling crash behaviour in future lightweight composite vehicles – Step 2”, involving 11 Swedish partners. On the material modelling side, a fully three-dimensional model to describe fibre kinking has recently been developed. The model is physically based and considers the fibre rotation during kink-band formation under large deformations. The FE implementation of the model is straightforward which allows for easy implementation. The validation of the model for stiffness and strength shows good correlation with the experiments. The influence of initial misalignments on the stiffness is well captured, the strength defined at the onset of unstable fibre rotation, is well predicted, and, in addition, the crushing response shows very good agreement with experimental results in terms of morphology in the crushing zone, as well as in the load response. To allow for computational efficiency, we have also developed and implemented (as a user element in LS-DYNA) an adaptive modelling strategy which allows for laminates to be initially modelled with only one element over the thickness.The user element kinematics can be adaptively enriched by introducing new degrees of freedom during the simulation to allow for more accurate stress predictions in critical regions by introducing discrete material interfaces, and for the modelling of delamination crack growth by introducing discrete crack surfaces interconnected with a cohesive zone law. In this work, special care has been taken to develop a robust method for explicit crash analysis. In the element, we also able to consider the correct intralaminar fracture toughness regularisation for various spatial discretisations. To assess and validate the models developed in the project, we have also conducted a series of bending and crushing experiments on component level. Three-point bending tests (in total 45 beams) have been conducted for three different carbon-epoxy material systems (pre-preg and vacuum infused), two different span lengths and two different lay-ups at several impact speeds. Similarly, crushing tests have been conducted for the same material systems by crushing tubes (in total 35 tubes) at various angles, with two different lay-ups and at two different loading speeds (quasi-static and dynamic). We believe that these tests serve as a very strong basis for any crash model validation.
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