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Natural frequencies describe the pre-stress in tensegrity structures
KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.ORCID iD: 0000-0001-8064-5463
KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.ORCID iD: 0000-0002-5819-4544
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.

Place, publisher, year, edition, pages
2014. Vol. 136, 162-171 p.
Keyword [en]
Buckling, Health monitoring, Pre-stress, Resonance spectrum, Tensegrity
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-145176DOI: 10.1016/j.compstruc.2014.01.020ISI: 000336703700013Scopus ID: 2-s2.0-84899953269OAI: oai:DiVA.org:kth-145176DiVA: diva2:717053
Note

QC 20140514

Available from: 2014-05-14 Created: 2014-05-14 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Vibration Frequencies as Status Indicators for Tensegrity Structures
Open this publication in new window or tab >>Vibration Frequencies as Status Indicators for Tensegrity Structures
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
Tensegrity, Pre-stress, Vibration, Health monitoring, Buckling, Temperature effect, Vibration health monitoring VHM, Tensegritet, Förspänning, Vibration, Hälsokontroll, Knäckning, Temperatureffekt, Vibrationsbaserad hälsokontroll
National Category
Applied Mechanics Other Mechanical Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-145164 (URN)978-91-7595-095-2 (ISBN)
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
2. Vibration-based Assessment of Tensegrity Structures
Open this publication in new window or tab >>Vibration-based Assessment of Tensegrity Structures
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Vibration structural health monitoring (VHM) uses the vibration properties to evaluate many civil structures during the design steps, building steps and service life.The whole function, expressed by 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.One of the aims of the thesis was to improve the current models for resonance frequency simulation of tensegrities. This has been achieved by introducing the bending behaviour of all components, and by a one-way coupling between the axial force and the stiffness.The environmental temperature effects on vibration properties of tensegrity structures have been also  investigated. 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 coming from temperature changes could of several magnitude as those from damage.Coinciding natural frequencies and low stiffness are known issues of tensegrity structures. The former can be an obstacle in VHM, while the later normally limits their uses in real engineering applications. It has been shown that the optimum self-stress vector of tensegrity structures can be chosen such that their lowest natural frequency is high, and separated from others.The environmental temperature effects on vibration properties of tensegrity structures were revisited to find a solution such that the natural frequencies of the tensegrity structures are not strongly affected by the changes in the environmental temperature. An asymmetric self-stress vector can be chosen so that the criterion is fulfilled as well as possible. The level of pre-stress can also be regulated to achieve the solution. The last part of this thesis, services as a summary of the work.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. 158 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2016:05
Keyword
Tensegrity, Pre-stress, Vibration, Health monitoring, Buckling, Temperature effect, Vibration health monitoring VHM, Optimization
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-185789 (URN)
Public defence
2016-05-17, F3, Lindstedtsvägen 26, Stockholm, 13:46 (English)
Opponent
Supervisors
Note

QC 20160429

Available from: 2016-04-29 Created: 2016-04-27 Last updated: 2016-11-16Bibliographically approved

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Ashwear, NasseradeenEriksson, Anders

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