Escola Superior de Conservació i Restauració de Bés Culturals de Catalunya
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A nano to macroscale study on structure-mechanics relationships of archaeological oak

Contributor(s): Thomas K. Bader, Karin de Borst | Thomas Ters, Susan BraovacMaterial type: ArticleArticleDescription: 12 pISBN: 1296-2074Subject(s): Archaeological wood | fusca arqueològica | Madera arqueológica | Mechanical properties | Microestructura | Microscopi | Microscopio | Microscopy | Microstructure | Nanoindentació | Nanoindentación | Nanoindentation | Propiedades mecánicas | Propietats mecàniques | Ultrasò | Ultrasound In: Journal of Cultural Heritage 5 14 5, 377-388Abstract: Mechanical properties of wood at different lenght scales of its hierarchical structure are governed by structural and compositional properties on smaller length scales. This opens up the possibility to use microsructural data for estimating mechamical properties, whitch are difficult to assess by conventional, destructive testing but are nevertheless of high relevance for conservation practice. Herein, we investigate such microsructue-mechanics relationship for a particular exemple, namely the load bearing archaeological oak of the Oseberg Viking ship, displayed at the Viking Ship Museum in Oslo, Norway. In order to identify the effects of degradation on the mechanical behavior and their relations to the microstructure, recent oak specimens of different geographical origin (Norway and Austria) are investigated as well. Wood exhibits a cellular structure. Its cell walls are composed of an amorphous polymer matrix consisting of lignin and hemicelluloses and embedded, stiff cellulose fibers. At the cell level, experimental studies comprised microscopic investigations of the cellular structure, chemical analyses of the composition of the cell walls, as well as nanoindentation tests on single cell walls. The same samples were also analyzed on the macroscopic level, where additionally mass density and annual ring data were measured together with ultrasonic stiffnesses. The chemical data clearly indicate deterioration in the archaeological oak, affecting mainly hemicelluloses and amorphous cellulose. At the cell wall scale, however, this does not necessarily lead to a weaker material behavior. The nanoindentation modulus, as a measure of the cell wall stiffness, was found to even increase. This is counterintuitive to our understanding of the effects of chemical degradation. It might be due to possible modification of lignin in the Oseberg oak, and thus have a stronger effect on the indentation modulus that the concurrent weakening of the interfaces between the load-carrying cellulose fibers and the connecting cell wall matrix when analyzing wood at the microscopic level. A similar effect is also observed for the transversal stiffness of macroscopic samples, which increases. In tension-dominated loading modes, however, the degradation of the degradation of the interfaces is the dominant effect, resulting for example in a considerable reduction of the macroscopic stiffness in longitudinal direction. This underlines the utmost relevance of the loading condition on the remaining load-carrying capacity of degraded wood. On the macroscale, effects of the geographical origin (i.e. growth conditions) on ring characteristics of the oak tissues override the effects of degradation on the mechanical behavior. They have to be carefully extracted in order to come up with conclusions on the effect of degradation from macroscopic test results. The identified microstructure-mechanics relationships provide the basis for-in further research steps-building mathematical models describing the relations between microstructural characteristics and macroscopic mechanical properties and, thereon, for structural analyses of historical wooden objects.
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Article de revista Article de revista Biblioteca de l' Escola Superior Conservació i Restauració de Bens Culturals de Catalunya
Journal of Cultural Heritage 5 (Browse shelf(Opens below)) Available Art-7

Mechanical properties of wood at different lenght scales of its hierarchical structure are governed by structural and compositional properties on smaller length scales. This opens up the possibility to use microsructural data for estimating mechamical properties, whitch are difficult to assess by conventional, destructive testing but are nevertheless of high relevance for conservation practice. Herein, we investigate such microsructue-mechanics relationship for a particular exemple, namely the load bearing archaeological oak of the Oseberg Viking ship, displayed at the Viking Ship Museum in Oslo, Norway. In order to identify the effects of degradation on the mechanical behavior and their relations to the microstructure, recent oak specimens of different geographical origin (Norway and Austria) are investigated as well. Wood exhibits a cellular structure. Its cell walls are composed of an amorphous polymer matrix consisting of lignin and hemicelluloses and embedded, stiff cellulose fibers. At the cell level, experimental studies comprised microscopic investigations of the cellular structure, chemical analyses of the composition of the cell walls, as well as nanoindentation tests on single cell walls. The same samples were also analyzed on the macroscopic level, where additionally mass density and annual ring data were measured together with ultrasonic stiffnesses. The chemical data clearly indicate deterioration in the archaeological oak, affecting mainly hemicelluloses and amorphous cellulose. At the cell wall scale, however, this does not necessarily lead to a weaker material behavior. The nanoindentation modulus, as a measure of the cell wall stiffness, was found to even increase. This is counterintuitive to our understanding of the effects of chemical degradation. It might be due to possible modification of lignin in the Oseberg oak, and thus have a stronger effect on the indentation modulus that the concurrent weakening of the interfaces between the load-carrying cellulose fibers and the connecting cell wall matrix when analyzing wood at the microscopic level. A similar effect is also observed for the transversal stiffness of macroscopic samples, which increases. In tension-dominated loading modes, however, the degradation of the degradation of the interfaces is the dominant effect, resulting for example in a considerable reduction of the macroscopic stiffness in longitudinal direction. This underlines the utmost relevance of the loading condition on the remaining load-carrying capacity of degraded wood. On the macroscale, effects of the geographical origin (i.e. growth conditions) on ring characteristics of the oak tissues override the effects of degradation on the mechanical behavior. They have to be carefully extracted in order to come up with conclusions on the effect of degradation from macroscopic test results. The identified microstructure-mechanics relationships provide the basis for-in further research steps-building mathematical models describing the relations between microstructural characteristics and macroscopic mechanical properties and, thereon, for structural analyses of historical wooden objects.

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