Tensile strength of wood in high temperatures before charring

Abstract

The area under the examination in this thesis is the behaviour of wood in exposure to high temperatures, before charring. The main focus is temperature's effects on tensile strength. This is first done by examining relevant literature and studies, after which the design aspects are analyzed. Next part of the thesis are the tensile strength experiments conducted in high temperatures.The testing method used in this thesis was steady-state test, where the temperature of the sample is kept constant, and the tension is increased until yielding. Samples were manufactured out of spruce.Based on the connection between temperature and tensile strength presented throughout this thesis, an example of a practical application is derived through a small software built for the purpose of this work. This software can be used as a tool in analysing the results of a fire simulation.The main objective of this thesis was to gather knowledge on the effects of the temperature on tensile strength, and evaluate the adequacy of different design approaches on the fire behaviour of wood, explained in the Eurocode 5. This analysis and complementation of the underlying patterns of the fire design of timber was mainly achieved via tensile tests in elevated temperatures. The results of these tensile strength experiments showed that the correlation between temperature and tensile strength presented in the Eurocode can be used as a good approximation in design applications.

Elastic Modulus, Thermal Expansion, and Pyrolysis Shrinkage of Norway Spruce Under High Temperature

“…However, the L direction from one measurement shows significantly lower elastic modulus value compared to e.g. [1,17], while scatter is within 14.6%. Several reasons may contribute to the less reliable measurement of L direction.…”

“…A sample which has not reached a constant height at the temperature of measurement, meaning, did not undergo the pyrolysis at the temperature may be resulting in lower moduli reading than the sample which underwent the pyrolysis at the set temperature. This can explain why the tests reported in the literature [4][5][6][7][8][9], and especially the tests performed on the larger specimen of the wood used in hereby research project (20 9 7 mm) [17], result in larger decrease of moduli with temperature and why the significant standard deviation is observed. The effect may be coming from the combination of shrinkage induced forces due to pyrolysis with tensile stress applied to sample (Fig.…”

“…The experiment uses Norway Spruce (Picea abies) samples sourced in the same way as those in other related FISAMA (Fire-safe materials by pyrolysis modelling) project studies (see e.g. [17,18]). The data related to the mass loss during pyrolysis of the samples from the same batch is provided by Rinta-Paavola and Hostikka [18].…”

“…The data related to the mass loss during pyrolysis of the samples from the same batch is provided by Rinta-Paavola and Hostikka [18]. Tensile strength and elastic modulus of moisture-conditioned at 45%RH and dry specimens were investigated previously by Kuronen [17] up to 250°C.…”

“…First of all, the presence of pyrolysis shrinkage necessitates additional data processing method to correctly extract elastic moduli values from the raw measurements (Figs. 16,17), which results are the main goal of the moduli program and presented in Figs. 18, 19, 20. Additionally, close observation at the equipment's measurement history and the processed data revealed a creep-like behaviour and increase of elastic moduli over time at a given temperature.…”