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Compressive Failure of NCF Composites
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. KTH, Superseded Departments (pre-2005), Vehicle Engineering. (Lightweight Structures)ORCID iD: 0000-0002-3675-1354
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The necessity to reduce environmental impact promotes transportation industry to reduce energy consumption of vehicles. One possible way to improve vehicles' structural efficiency is to utilize modern composite materials that offer excellent mechanical performance-to-weight ratio. Mass production of composite parts requires cost effective manufacturing methods. One potential rational method is to use dry textile preforms and liquid moulding methods, e.g vacuum infusion or resin transfer moulding. Among different types of textile preforms, non-crimp fabrics (NCFs) are most attractive for load bearing applications as they offer considerably higher in-plane mechanical properties compare to other textiles such as wovens or random mats. Composites manufactured with NCF fabrics are characterised by distinct fibre bundles separated by resin rich areas. These bundles are not perfectly straight but have a small yet significant waviness, both in-plane and out-of-plane. The waviness will influence the performance of NCF composites and especially the compressive properties. Design of structural parts made of NCF composites requires both a thorough understanding of the compressive failure process and effective failure prediction models. This is particularly relevant for the critical compressive loaded parts, such as bolted joints.

The present work is concerned with the compressive failure of NCF composites and focuses on two major goals. First is to experimentally characterise the compressive failure process of various NCF composites and identify relevant damage modes and mechanisms. Secondly is to develop and propose suitable failure prediction models for reliable design of NCF composite parts with special emphasis on cost-effective methods relevant for industrial design processes.

In the present work, a combination of experimental studies, modelling methods development and implementation of advanced state-of-the-art failure criteria have been performed. Optical methods were used to characterise the damage mechanisms in the material at different stress levels. This allowed both identification of the critical damage mechanisms and the whole damage progression sequence. Engineering models were developed to predict the compressive failure of NCF composites. In the models, the fibre bundles' waviness was dealt with in a cost-effective way. The models utilise a state-of-the-art failure criteria that predict both intra-laminar and inter-laminar damage. The proposed models demonstrated good accuracy in the predictions of both compressive and bearing failures. In addition, a cost-effective high-fidelity meso-scale modelling methodology was developed for bearing failure prediction of NCF composites. The methodology demonstrated a potential for cost-effective and highly detailed analysis of the bearing failure process and possible method for parameter studies of mechanical properties and their relation to the reinforcement architecture.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. , p. 58
Series
TRITA-SCI-FOU ; 2019:56
National Category
Composite Science and Engineering
Research subject
Vehicle and Maritime Engineering
Identifiers
URN: urn:nbn:se:kth:diva-263886ISBN: 978-91-7873-388-0 (print)OAI: oai:DiVA.org:kth-263886DiVA, id: diva2:1370984
Public defence
2019-12-11, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20191120

Available from: 2019-11-20 Created: 2019-11-18 Last updated: 2019-11-21Bibliographically approved
List of papers
1. Failure of cross-ply NCF composites under off-axis compressive loads - An experimental study and a new strength prediction model including fibre bundle waviness
Open this publication in new window or tab >>Failure of cross-ply NCF composites under off-axis compressive loads - An experimental study and a new strength prediction model including fibre bundle waviness
2018 (English)In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 153, p. 49-56Article in journal (Refereed) Published
Abstract [en]

The design of reliable and efficient mechanical joints with non-crimp fabric (NCF) composites depends on several factors but knowledge on actual loading direction and an accurate compressive strength prediction is essential. Motivated by this, the current study is focused on the compressive strength of cross-ply NCF composites and the influence of fibre orientation in relation to the loading direction. Possible influence of stacking sequence on the compressive strength is also studied. Compression tests of cross-ply NCF composite laminates that are loaded at various off-axis angles are performed and the failure mechanisms are identified. An analytical semi-laminar based model for strength prediction of NCF composite laminates loaded in compression is then suggested. The model take in- and out-of-plane bundle waviness into account. Good agreement between the proposed model and experiments is observed.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Fabrics/textiles, Strength, Analytical modelling, Mechanical testing
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-239075 (URN)10.1016/j.compositesb.2018.06.022 (DOI)000448494100048 ()2-s2.0-85050100432 (Scopus ID)
Note

QC 20181121

Available from: 2018-11-21 Created: 2018-11-21 Last updated: 2019-11-18Bibliographically approved
2. Effect of stacking sequence and bundle waviness in quasi-isotropic NCF composites subjected to compression
Open this publication in new window or tab >>Effect of stacking sequence and bundle waviness in quasi-isotropic NCF composites subjected to compression
2019 (English)In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 178, article id 107423Article in journal (Refereed) Published
Abstract [en]

The current study is focused on the compressive strength of composite materials containing non-crimp fabric (NCF) reinforcement, and how ply stacking sequence and fibre waviness influence onset and growth of damage in such materials. Experiments reveal significant effects from stacking sequence, both on the compressive strength as such, and on the underlying failure mechanisms. The fibre waviness also has a strong influence on the strength. Fibre kinking is seen before ultimate failure for all configurations but some of them also show local delamination prior to kinking. A finite element simulation methodology is developed and used for the studied cases. It handles local variations of fibre orientations by corresponding re-orientation of stiffness matrices at element level. The simulations provide good predictions of intra- and inter-laminar failure considering both in- plane and out-of-plane fibre bundle waviness. The model is further used in a parametric study of the influence from bundle waviness on the compressive strength.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Fabrics/textiles, Strength, Finite element analysis (FEA), Mechanical testing
National Category
Composite Science and Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-262037 (URN)10.1016/j.compositesb.2019.107423 (DOI)000498274700006 ()2-s2.0-85072574900 (Scopus ID)
Note

QC 20191016

Available from: 2019-10-14 Created: 2019-10-14 Last updated: 2019-12-12Bibliographically approved
3. Failure mechanisms in NCF composite bolted joints. Experiments and FE model
Open this publication in new window or tab >>Failure mechanisms in NCF composite bolted joints. Experiments and FE model
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The current study focuses on the bearing failure process of NCF composites and associated damage mechanisms. A set of experiments on bolted joints between NCF composite and steel plates have been performed. The bearing damage onset and failure progression in the composite was monitored at different load levels by microscopic image analysis. Fibre kinking in 0° layers was found as the key damage mechanism that initiate and drive the bearing failure. Matrix cracking and delaminations were found present as well. A cost-effective FE model that predicts bolt bearing failure of NCF composites was proposed.The model utilises state-of-the-art failure criteria and predicts both intra- and inter-laminar progressive damage. A good correlation between the predicted damage development process and experiments was observed both in terms of failure modes and load levels.

Keywords
Fabrics/textiles; Strength; Finite element analysis (FEA); Mechanical testing; Non-crimp fabric (NCF); Bolted joints)
National Category
Composite Science and Engineering
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-263879 (URN)
Note

QC 20191126

Available from: 2019-11-18 Created: 2019-11-18 Last updated: 2019-11-26Bibliographically approved
4. Meso-scale modelling of the bearing failure in NCF composites
Open this publication in new window or tab >>Meso-scale modelling of the bearing failure in NCF composites
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The study is focused on the bearing failure of NCF composites. The study presents a cost-effective methodology for high-fidelity meso-scale modelling of NCF composites and bearing failure prediction. The proposed methodology does not require expensive measurements of the meso-structure, nor access to specimen of the material. Instead, the complex meso-structural geometry in the model is achieved by the included compaction analysis of a dry multi-layer NCF preform. The modelling process is assisted by the developed Python-based automated framework. The methodology was applied to predict bearing failure of the experimentally tested pin-loaded quasi-isotropic NCF composites. A good agreement of the predicted and measured meso-structural geometry was found. The proposed method could accurately predicted the meso-structural geometry of the manufactured NCF composite in terms of both tow paths and their cross sectional shapes. The predicted bearingfailure response was found to be in general agreement with experiments. However, predictions of damage initiation and progression were too conservative. This is explained by the conservatism of the material properties used and material model.

National Category
Composite Science and Engineering
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-263882 (URN)
Note

QC 20191126

Available from: 2019-11-18 Created: 2019-11-18 Last updated: 2019-11-26Bibliographically approved

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