| 1. |
- Bru, Thomas, 1990
(författare)
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Behaviour and material properties of composites for crash modelling
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The transport industry must find solutions to reduce its impact on climate change. A promising way to reduce the weight of vehicles and therefore to reduce the CO2 emissions is to introduce components made of lightweight composite materials, in particular laminated carbon fibre reinforced polymers (CFRPs). Aside from the new design possibilities for lighter vehicle structures, CFRPs can also potentially offer large improvements in terms of energy absorption capabilities in comparison to traditional crash components made of metals. During crushing of composites, a large amount of energy is absorbed through stable progressive failure of the material. The crushing process is a complex phenomenon because it is driven by the combination of many failure mechanisms and frictional effects. A limited amount of research has been performed on crushing of composites, mainly because crashworthiness is not a critical requirement in the aerospace industry (the predominant market for advanced composites today). As a result, there is today no reliable numerical tool to predict the crashworthiness of CFRP structures, which is a hindrance to the introduction of composite materials in mass-produced automobiles. Joint research efforts from both numerical and experimental perspectives are needed to fill this void and reach the goal of reliable crash predictions of composite vehicles. The focus of this thesis is on a material characterisation strategy for crash modelling of composites. An experimental methodology is developed to provide relevant and accurate input to a physically-based material model for crash, currently being developed in parallel to this thesis. The material selected for this research is a CFRP with non-crimp fabric (NCF) reinforcements. The first step in the material characterisation is to extract the different strengths and stiffnesses of the material, which requires dedicated tests because of the orthotropic nature of NCF composites. In a second step, more specific inputs to ply damage models for progressive failure are extracted from experiments. Those parameters are (1) damage evolution laws, identified from Iosipescu shear tests, and (2) the longitudinal and the transverse crushing behaviour of unidirectional laminates, extracted from relatively simple crush tests on flat specimens.
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| 2. |
- Bru, Thomas, 1990, et al.
(författare)
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Biaxial transverse compression testing for a fibre reinforced polymer material
- 2020
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Ingår i: ECCM 2018 - 18th European Conference on Composite Materials.
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Konferensbidrag (refereegranskat)abstract
- Unidirectional laminates have been tested under uniaxial transverse compression and under biaxial transverse compression. Failure occurred by shear in an inclined failure plane parallel to the fibres. The transverse shear response of the material on the failure plane was evaluated from the tests. In the biaxial tests, the failure load was considerably higher than in the uniaxial tests. For a given transverse shear strain the transverse shear stress was also higher in the case of biaxial compression. It is also shown that using waisted specimens instead of prismatic specimens does not seem to bring noticeable benefits for through-thickness uniaxial compression tests. The experimental results presented here are important input to the development and the validation of damage models of fibre reinforced polymer materials accounting for the matrix nonlinear response in shear and compression.
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| 3. |
- Bru, Thomas, 1990
(författare)
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Material characterisation for crash modelling of composites
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The transport industry must find solutions to reduce its impact on climate change. A promising way to reduce the weight of vehicles and therefore to reduce the CO2 emissions is to introduce components made of lightweight composite materials, in particular carbon fibre reinforced plastics (CFRPs). Aside from the new design possibilities for lighter vehicle structures, CFRPs can also potentially offer improvements in terms of crash performance in comparison to traditional metallic structures. During crushing of composite structures, energy is absorbed through the stable progressive failure of the structure. The crushing process is a complex phenomenon involving the interaction of different competing failure mechanisms and frictional interactions taking place at different scales in the material. Today there is no reliable numerical tool to predict the behaviour of composite structures in crash scenarios, which is a hindrance to the introduction of composite materials in mass-produced vehicles. Joint research efforts from both numerical and experimental perspectives are needed to fill this gap. In this doctoral thesis experiments are carried out to extract relevant material properties for crash modelling, and to assist in the development and the validation of numerical models as a first step of a building block approach with increasing structural complexity. The material selected for the study is a carbon fibre/epoxy uni-weave non-crimp fabric (NCF) composite. The first step in the material characterisation is to extract the different strengths and stiffnesses of the material, which requires dedicated tests because of the orthotropic nature of NCFs. Because several compressive failure mechanisms are driven by the shearing of the matrix polymer, a methodology is presented to extract the damage evolution laws from Iosipescu shear tests and indirect shear tests (uniaxial and biaxial compression tests). A quasi-static test method that uses crush coupons of simple geometry is proposed to measure the crush stress of composite plies for different fibre orientations and to characterise the associated crushing mechanisms. The experimental results of the crush coupons are then compared to blind predictions from finite element simulations to assess the predictive capabilities of a ply-based material model coupling damage and friction in a continuum damage approach. This material model is currently being developed in parallel to this thesis. Its aim is to pre-emptively simulate structural tests in order to optimise the design of crashworthy structures and to limit the number of physical tests.
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| 4. |
- Bru, Thomas, 1990, et al.
(författare)
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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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Ingår i: ICCM International Conferences on Composite Materials. ; 2017
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Konferensbidrag (refereegranskat)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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| 5. |
- Costa, Sergio, 1987, et al.
(författare)
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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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Ingår i: Journal of Composite Materials. - : SAGE Publications. - 1530-793X .- 0021-9983. ; 53:12, s. 1681-1696
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Tidskriftsartikel (refereegranskat)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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| 6. |
- Costa, Sergio, 1987, et al.
(författare)
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Validation of a novel model for the compressive response of FRP: Numerical simulation
- 2017
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Ingår i: ICCM International Conferences on Composite Materials. ; 2017-August
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Konferensbidrag (refereegranskat)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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