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An Experimental Investigation of Shape Distortions in Aerospace Composites
KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Lightweight Structures.ORCID iD: 0000-0001-7976-9223
2020 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Composite materials are increasingly used in primary structure of modern commercial aircraft. Its excellent material characteristics enables reduction of structural weight compared to traditional metal solutions and thereby offers reduction of fuel consumption and carbon dioxide (CO2) emissions. In the aerospace industry, carbon fibre reinforced plastics or CFRP is the most commonly used composite material, where the reinforcement is held together by a thermoset resin, often epoxy, referred to as the matrix.

When manufacturing aircraft composite parts, the curing temperature is usually in-between 120°C to 180°C. As the constituents, i.e. fibre and matrix, have significantly different thermal expansion, the temperature difference from manufacturing of parts to assembly and in-service use results in shape distortions and/or development of residual stresses. With an increased size and complexity of structural parts used in modern aircraft, the development of efficient methods for shape distortion analysis are therefore becoming increasingly important. Shape distortions come from numerous sources and some of them like thermal expansion and chemical shrinkage during curing are fairly well studied and understood. The focus of this thesis is on less researched parameters such as the laminate bending stiffness and effects of moisture content.

The bending stiffness of a laminate can be controlled by varying the thickness of the laminate, or by changing the layup sequence of individual plies. Paper A presents an experimental study on shape distortion were the effect of laminate bending stiffness is separated from that of the laminate thickness. The results show that it is possible to tailor the laminate layup in a way that is beneficial for in-plane loads, while still reducing the built-in stresses that occur in a composite component due to shape distortions.

The second parameter investigated in this thesis is the laminate moisture content. Composite materials used in aircraft structures will be exposed to environmental effects such as varying temperatures and moisture. The exposure is seldom constant but varies over time, depending on seasonal change and geographical area of aircraft operation. In Paper B, the influence of laminate moisture content on shape distortions is experimentally investigated. It becomes clear that laminate moisture content has such a strong effect on shape distortions that it is important to control and predict for all composite structures.

The results presented in this thesis show that both laminate bending stiffness and laminate moisture content have a great influence on shape distortions, and that further research and development is needed to improve the simulation methodology used within the aerospace industry. This is key to future cost-efficient production and assembly of large composite parts.

Abstract [sv]

Kompositmaterial utgör viktiga konstruktionsmaterial i moderna flygplansstrukturer. Deras goda mekaniska egenskaper leder till minskad strukturell vikt och därmed minskad bränsleförbrukning och utsläpp av koldioxid (CO2). Ett vanligt använt kompositmaterial inom flygindustrin är kolfiberarmerad plast eller CFRP. CFRP består av lastbärande kolfiber sammanbundna av en härdplastmatris bestående av epoxi.

Vid tillverkning av kompositartiklar inom flygindustrin används härdningstemperaturer på vanligtvis mellan 120°C till 180°C. Då den termiska expansionen hos kompositens delar, fiber och matris, skiljer sig mycket, resulterar stora temperaturskillnader under tillverkningen i formförändringar hos kompositdetaljen och/eller uppbyggnad av restspänningar. Allt eftersom kompositdetaljernas storlek och komplexitet ökar i moderna flygplan så blir behovet av att förstå dessa formförändringar och kunna modellera dess effekter allt större. Det är många faktorer som påverkar uppkomsten av formförändringar, där de mest kända innefattar skillnader i termisk expansion och kemiskt krymp hos den härdande matrisen. I denna avhandling är dock fokus på två mindre undersökta faktorer; laminatböjstyvhet och påverkan från laminatets fukthalt.

Böjstyvheten hos ett laminat kan varieras genom att ändra laminattjockleken och/eller uppläggningssekvensen av enskilda lager. I artikel A presenteras en experimentell studie där inverkan av ett laminats böjstyvhet på formförändringen separeras från laminattjocklekens inverkan på formförändringen. Resultaten som presenteras i artikel A visar att det är möjligt att anpassa laminatupplägget på ett sätt som är fördelaktigt för i-planet laster samtidigt som det minskar de inbyggda spänningarna som uppstår i en komplex kompositartikel på grund av formförändringar.

I den andra delen av denna avhandling så har kopplingen mellan laminatets fukthalt och formförändring undersökts. Kompositmaterial som används i flygplanstrukturer kommer att utsättas för miljöeffekter såsom varierande temperaturer och fukt. Exponeringen är sällan konstant utan varierar över tid beroende på årstid och i vilket geografiskt område flygplanet befinner sig i. I artikel B sammanställs resultat från en experimentell studie där påverkan av laminatfukthalt på formförändringar har undersökts. Det är tydligt att fuktinnehållet har så stor inverkan på formförändringar att det måste beaktas vid analys av komplexa kompositstrukturer för att kunna nå tillförlitliga prediktioner från modeller och simulering.

Resultaten som presenteras i denna avhandling visar på att ytterligare forskning och utveckling behövs för att förbättra de beräkningsmodeller som används inom flygindustrin för att förutsäga formförändringar hos kompositartiklar. Detta är viktigt för att möjliggöra en mer kostnadseffektiv tillverkning och sammanbyggnad av stora kompositdelar i framtiden.

Place, publisher, year, edition, pages
Sockholm: KTH Royal Institute of Technology, 2020. , p. 41
Series
TRITA-SCI-FOU ; 2020:03
National Category
Composite Science and Engineering
Research subject
Aerospace Engineering
Identifiers
URN: urn:nbn:se:kth:diva-269076ISBN: 978-91-7873-443-6 (print)OAI: oai:DiVA.org:kth-269076DiVA, id: diva2:1410956
Presentation
2020-04-03, Live-streaming use https://kth-se.zoom.us/j/103973058, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Vinnova, 2015-06057
Note

QC20200302

Available from: 2020-03-17 Created: 2020-03-02 Last updated: 2020-03-27Bibliographically approved
List of papers
1. Thickness effect on spring-in of prepreg composite L-profiles – An experimental study
Open this publication in new window or tab >>Thickness effect on spring-in of prepreg composite L-profiles – An experimental study
2019 (English)In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 209, p. 499-507Article in journal (Refereed) Published
Abstract [en]

This paper presents the results and analysis of an experimental study of laminate thickness effects on the springin and shape distortion of thermoset composite L profiles. The primary objective is to achieve a broader understanding of how shape distortion is affected by laminate bending stiffness and part thickness of L-shaped laminates whose thickness varies between 1 and 12 mm. The larger thicknesses in particular have not received much attention in previous research. This work further aims at distinguishing the pure (geometrical) thickness effect from that of the coupled laminate bending stiffness by comparing laminates with different lay-ups. The work is performed on test specimens subjected to both a standard cure cycle and post-cure heat treatment at elevated temperatures. In parallel, finite element (FE) analysis is performed to evaluate if variation in the bending stiffness or the laminate thickness affects the predicted spring-in angle. The results clearly show springin dependence on laminate thickness and bending stiffness, whereas this dependence is not well predicted by the FE approaches. It is concluded that both effects exist and that shape distortions are more strongly related to bending stiffness than to laminate thickness.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Process simulation, Cure behaviour, Prepreg, Thermal properties
National Category
Composite Science and Engineering
Research subject
Aerospace Engineering; Fibre and Polymer Science
Identifiers
urn:nbn:se:kth:diva-268925 (URN)
Funder
Vinnova, 2015-06057
Available from: 2020-02-25 Created: 2020-02-25 Last updated: 2020-03-02
2. Moisture effect on shape distortions of curved prepreg laminates
Open this publication in new window or tab >>Moisture effect on shape distortions of curved prepreg laminates
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The influence from moisture content on shape distortion of curved thermoset composite laminates is investigated experimentally. The particular objects of study are L-shaped carbon/epoxy specimens with a quasi-isotropic layup and the thickness varying between 1 and 12 mm. The effect on the shape distortion is quantified by means of spring-in angle measurements vs. accelerated moisture uptake utilising a climate chamber at 90°C and 95% relative humidity. The results clearly show a strong dependence on spring-in angles from laminate moisture content – the effect is in fact in the same order of magnitude as the spring-in from thermal and chemical shrinkage during curing. Moisture does thus not only affect shape distortions but also has to be taken into consideration and be carefully controlled when assessing spring-in due to other parameters. Finally, a model based on the experimental results predicting the spring-in angle is presented.

Keywords
Carbon fibres, Cure behavior, Process simulation, Moisture
National Category
Composite Science and Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-268919 (URN)
Funder
Vinnova, 2015-06057
Note

QC 20200226

Available from: 2020-02-25 Created: 2020-02-25 Last updated: 2020-03-02Bibliographically approved

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