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Vibration Frequencies as Status Indicators for Tensegrity Structures
KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics. (Biomechanics, Computational and Structural Mechanics (BCSM))ORCID iD: 0000-0001-8064-5463
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

 Applications of vibration structural health monitoring (VHM) techniques are increasing rapidly. This is because of the advances in sensors and instrumentation during the last decades. VHM uses the vibration properties to evaluate many civil structures during the design steps, building steps and service life.

The stiffness and frequencies of tensegrity structures are primarily related to the level of pre-stress. The present work investigates the possibilities to use this relation in designing, constructing and evaluating the tensegrity structures.

The first part of the  present work studies the improvement of current models for resonance frequency simulation of tensegrities by introducing the bending behaviour of all components, and by a one-way coupling between the axial force and the stiffness. From this, both local and global vibration modes are obtained. The resonance frequencies are seen as non-linearly dependent on the pre-stress level in the structure, thereby giving a basis for diagnosis of structural conditions from measured frequencies. The new aspects of tensegrity simulations are shown for simple, plane structures but the basic methods are easily used also for more complex structures.

In the second part, the environmental temperature effects on vibration properties of tensegrity structures have been investigated, considering primarily seasonal temperature differences (uniform temperature differences). Changes in dynamic characteristics due to temperature variations were compared with the changes due to decreasing pre-tension in one of the cables. In general, it is shown that the change in structural frequencies made by temperature changes could be equivalent to the change made by damage (slacking). Different combinations of materials used and boundary conditions are also investigated. These are shown to have a significant impact on the pre-stress level and the natural frequencies of the tensegrity structures when the environment temperature is changed.

Abstract [sv]

Användandet av vibrationsbaserade hälsokontrollsmetoder (VHM) för strukturer ökar snabbt.Detta har möjliggjorts av utvecklingen inom  mätmetoder och mätutrus- tning under de senaste decennierna.Dessa metoder använder sig av de uppmätta eller simulerade vibrationsegenskaperna  underdesign-, uppbyggnads- och  nyttjandestadierna hos många slag av byggnadsverk.

Styvheten och resonansfrekvenserna hos tensegritets-strukturer är i hög grad beroendepå den aktuella förspänningsnivån. Föreliggande arbete undersöker möjlig- heterna attanvända detta beroende i konstruktion, byggande och utvärdering av sådana strukturer.

Den första delen av  föreliggande arbete studerar förbättringar av de vanligen användamodellerna för simulering av resonansfrekvenser hos tensegritetsstrukturergenom att införa de ingående komponenternas böjningsegenskaper, och genom att i enriktning koppla normalkraften till böjstyvheten. Genom detta kan såväl lokala som globalavibrationsmoder hittas. Resonansfrekvenserna ses därmed som icke-linjärt beroende avförspänningsnivån i strukturen. Detta ger därmed möjligheter att diagnosticera strukturenskondition från uppmätta frekvenser. De nya simuleringsmöjligheternademonstreras för enkla, plana strukturer, men de utvecklade metoderna kan också lättanpassas till mera komplexa fall.

Den andra delen av arbetet undersöker hur strukturernas vibrationsegenskaper ärberoende på temperatureffekter i omgivningen. I första hand beaktas säsongsvisa (likformiga)temperaturvariationer. Förändringar i de dynamiska egenskaperna beroende påtemperaturförändringar jämfördes med dem som beror på en minskande förspänning hos någonav de ingående kablarna. I allmänhet gäller att förändringarna i resonansfrekvenser kanvara av samma storleksordning som de som beror på skador (minskad förspänning).Olika kombinationer av material, och olika upplagsförhållanden undersöktes.Dessa egenskaper visades ha en betydande effekt på förspänningsnivån, och därmed ocksåpå resonansfrekvenserna, hos tensegritets-strukturerna som utsätts för temperaturvariationer.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , xii, 25 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014:10
Keyword [en]
Tensegrity, Pre-stress, Vibration, Health monitoring, Buckling, Temperature effect, Vibration health monitoring VHM
Keyword [sv]
Tensegritet, Förspänning, Vibration, Hälsokontroll, Knäckning, Temperatureffekt, Vibrationsbaserad hälsokontroll
National Category
Applied Mechanics Other Mechanical Engineering
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-145164ISBN: 978-91-7595-095-2 (print)OAI: oai:DiVA.org:kth-145164DiVA: diva2:716977
Presentation
2014-06-13, E31, Lindstedtsvägen 3, Kungliga Tekniska Högskola, Vallhallavägen 79, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20140514

Available from: 2014-05-14 Created: 2014-05-13 Last updated: 2014-05-14Bibliographically approved
List of papers
1. Natural frequencies describe the pre-stress in tensegrity structures
Open this publication in new window or tab >>Natural frequencies describe the pre-stress in tensegrity structures
2014 (English)In: Computers & structures, ISSN 0045-7949, E-ISSN 1879-2243, Vol. 136, 162-171 p.Article in journal (Refereed) Published
Abstract [en]

This paper investigates the effect of pre-stress level on the natural frequencies of tensegrity structures. This has been established by using Euler–Bernoulli beam elements which include the effect of the axial force on the transversal stiffness. The axial-bending coupling emphasizes the non-linear dependence of the natural frequencies on the pre-stress state. Pre-stress is seen as either synchronous, considering a variable final pre-stress design or as tuning, when increasing pre-stress is followed in a planned construction sequence. It is shown that for a certain tensegrity structure, increasing the level of pre-stress may cause the natural frequencies to rise or fall. This effect is related to whether the structural behavior can be seen as compression or tension dominant. Vanishing of the lowest natural frequency of the system is shown to be related to the critical buckling load of one or several compressed components. Modes of vibration show that when the force in the compressed components approaches any type of critical buckling load, this results in lower vibration frequencies. The methods in this study can be used to plan the tuning of the considered tensegrity structure towards the design level of pre-stress, and as health monitoring tools.

Keyword
Buckling, Health monitoring, Pre-stress, Resonance spectrum, Tensegrity
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-145176 (URN)10.1016/j.compstruc.2014.01.020 (DOI)000336703700013 ()2-s2.0-84899953269 (Scopus ID)
Note

QC 20140514

Available from: 2014-05-14 Created: 2014-05-14 Last updated: 2017-12-05Bibliographically approved
2. Influence of Temperature on the Vibration Properties of Tensegrity Structures
Open this publication in new window or tab >>Influence of Temperature on the Vibration Properties of Tensegrity Structures
2015 (English)In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 99, 237-250 p.Article in journal (Refereed) Published
Abstract [en]

Vibration health monitoring methods use the sensitivity of the natural frequencies to structural damage. Natural frequencies are sensitive to damage, but are also affected by environmental conditions like temperature changes. It is important to be able to distinguish between the effects of these different factors when using the vibration properties as a monitoring tool. This paper discusses the impact of damage and environment temperature changes on the natural frequencies of tensegrity ("tensile-integrity") structures, in particular noting that component bending is a prominent vibration mode, which motivates a use of non-linear beam elements with axial-bending coupling. The model considers not only thermal expansion effects, but also the change of the elastic modulus with temperature. Changes in natural frequencies produced by environment temperature changes are shown to be similar to the ones produced by damage. The geometry of tensegrity structures, the support conditions and the materials are found to be important factors. The sensitivity of the natural frequency to temperature changes is found to be dependent on pre-stress level.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-145177 (URN)10.1016/j.ijmecsci.2015.05.019 (DOI)000358625300023 ()2-s2.0-84937559648 (Scopus ID)
Note

Updated from "Manuscript" to "Article". QC 20150901

Available from: 2014-05-14 Created: 2014-05-14 Last updated: 2017-12-05Bibliographically approved

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