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Author:Hämäläinen, Hilkka
Title:Three-point Bend Test of a Composite: Modeling and Validation
Komposiitti kolmipistetaivutuksessa; Mallinnus ja validointi
Publication type:Master's thesis
Publication year:2013
Pages:x + 55 s. + liitt. 9      Language:   eng
Department/School:Sovelletun mekaniikan laitos
Main subject:Lujuusoppi   (Kul-49)
Supervisor:Tuhkuri, Jukka
Instructor:Valjus, Petteri
OEVS:
Electronic archive copy is available via Aalto Thesis Database.
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Location:P1 Ark Aalto  4307   | Archive
Keywords:composites
three-point bending
komposiitti
FEM
kolmipistetaivutus
Abstract (eng): The goal of this thesis was to create an FEM model of a composite rope and to validate it using test data.
The composite consists of four continuous unidirectional Carbon Fiber Reinforced Plastic (CFRP) bars surrounded by a Thermoplastic Polyurethane (TPU) coating.

The literature review is concentrated on the basics of composite mechanics, the Classic Beam Theory and the Finite Element Method.

The model was validated using three-point bend tests, in which the deflection was controlled and the force was measured.
Two different failure modes were discovered; half of the samples failed through fiber micro buckling, while the others only suffered minor damage on the tensional side.
The force-deflection curves were linearly increasing in the beginning, and then the stiffness gradually dropped.
The behaviour is similar to the beam theory with large rotations.
The Classical Beam Theory predicts the slope of the linear section very accurately.

The FEM model was built using solid, parabolic 20-node elements for the rope volumes and rigid contacts for the pressing roll and the support.
Due to software licensing issues, the TPU had to be modelled as linear elastic despite that fact that it is a hyper elastic material.
This caused local error in areas close to the pressing roll and the support.
The support roll friction factor was not measured in the tests and proved to have a strong effect on the modelling results.

The modelled force-deflection curve had the same shape as the tested ones, and the slope of the linear part was 0.7 % smaller than the tested one.
However, since the friction factor in the support was not known, the model could not be validated.

In future work it is suggested that the test arrangements be redesigned in order to minimize the friction.
The friction also needs to be measured.
More samples should be tested for statistical reasons, and a proper nonlinear material model should be made for the TPU.
Abstract (fin): Diplomityön tavoitteena oli komposiittiköyden mallintaminen elementtimenetelmällä sekä mallin validointi kokeiden avulla.
Köysi koostuu neljästä termoplastisen polyuretaanin (TPU) ympäröimästä komposiittilatasta.
Latoissa on jatkuvia, yksisuuntaisia hiilikuituja epoksimatriisissa.

Kirjallisuuskatsauksessa esiteltiin komposiittien, klassisen palkkiteorian ja elementtimenetelmän perusteet.

Validointia varten tehtiin kolmipistetaivutuskokeita, joissa taipuma ohjattiin ja voima mitattiin.
Kahdestatoista testatusta näytteestä kuusi hajosi kuitujen nurjahdettua, lopuissa oli vain osittaista vauriota vedolla olleella pinnalla.
Voimataipumakäyrät olivat lineaariset alussa, kunnes taipuman kasvaessa jäykkyys alkoi pudota.
Käyrien muoto muistuttaa suurten taipumien palkkiteoriaa.
Yksinkertaistettu palkkimalli ennusti lineaarisen osan kulmakertoimen tarkasti.

FEM-mallissa käytettiin parabolisia 20-solmuisia tilavuuselementtejä köydelle sekä jäykkiä kontakteja simuloimaan paininta ja tukirullaa.
Ohjelmiston lisenssirajoitusten takia hyperelastise11e TPU:l1e jouduttiin käyttämään lineaarista materiaalimallia.
Tämä johti paikallisesti vääristyneihin muodonmuutoksiin painimen ja tukirullan läheisyydessä.
Tukirullan kitkaa ei mitattu kokeessa, mutta se vaikutti suuresti mallinnustuloksiin.

Mallinnuksen tuloksena saatu voima-taipumakäyrä oli muodoltaan kokeellisten käyrien kaltainen, ja lineaarisen osan kulmakerroin oli 0,7 % kokeellista pienempi.
Tukirullan kitkan takia mallia ei kuitenkaan voitu validoida.

Jatkotutkimuksia varten suositellaan koejärjestelyiden muokkaamista kitkan minimoimiseksi.
Kitka tulisi sen lisäksi myös mitata.
Kokeet tulisi toistaa useammalla näytteellä tilastollisuuden vuoksi, ja TPU:lle tulisi käyttää asiaankuuluvaa materiaalimallia.
ED:2013-12-10
INSSI record number: 48179
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