This PhD research project mainly examined the important yet incompletely understood field of the mechano-sorptive creep of softwood. It included the acquisition of accurate creep data during constant and cyclic moisture conditions. The study only concerns the loading of softwood in the longitudinal, parallel to the grain direction in bending mode. Mechano-sorptive creep deformation takes place when the wood is subjected to load in varying humidity conditions, where the creep deformation is substantially higher than the creep deformation under constant humidity conditions. Visco-elastic creep is defined as the creep strain that is primarily dependent on time. Further analysis on the pseudo-creep, which is reversible with moisture changes, was also investigated. The Young's Modulus (E) and density (p) of the samples used in this research were measured and the ratio of (E/p) was used to characterise the quality of the wood specimen. A program was written in Quick-Basic for the computer to take readings from the creep machines at all times during all creep tests for acquiring maximum creep data possible for obtaining better results. A 'steaming' technique was implemented to accelerate creep recovery, in order to allow the re-use of the same sample for repeated creep testing, so reducing the effects of variability. Measurements of visco-elastic creep, mechano-sorptive creep limit and pseudo-creep rate as functions of percentage relative humidity and E/p were investigated and analysed. The mechano-sorptive creep limit was successfully reached by the load-reduction method. The pseudo-creep rate was similar for samples with the same values of E/p at different relative humidity. The higher the relative humidity, the larger the magnitude of visco-elastic and the 'reference' creep at the mechano-sorptive creep limit. A higher value of E/p produced lower pseudocreep rate, visco-elastic creep and mechano-sorptive creep limit. Equations were derived from the one week visco-elastic creep experimental data in order to extrapolate visco-elastic creep to longer time durations, (medium, long and permanent periods, and compared to the new European code of practice for timber design, namely the Draft Eurocode 5), and at two critical relative humidity conditions, (63% and 90%rh), which correspond to the Service Classes 1 and 2 in the Draft Eurocode 5. Also, an equation to predict the mechano-sorptive creep limit at various values of E/p for the two critical relative humidity conditions, (63% and 90%rh), was acquired. The prediction of creep at various relative humidities along the pseudo creep line was also quantified. The extrapolated visco-elastic creep along with the total upper-bound creep values were compared to the creep allowances of the Draft Eurocode 5 for both Service Classes 1 and 2. Moreover, a design scheme for estimating the upper-bound creep, consisting of the addition of visco-elastic, mechano-sorptive and pseudo creep for Draft Eurocode 5 Service Classes 1 and 2 was achieved. A study of the degree of interaction between visco-elastic and mechanosorptive creep was further investigated. Two different experiments were performed to reach some conclusion. The results of the first experiment showed that mechanosorptive creep acceleration decreases the visco-elastic creep rate and there was some kind of interaction between the two creep mechanisms. Hence the two creep mechanisms should not be considered as a separate phenomenon. On the other hand, the results of the second experiment showed that the visco-elastic and mechanosorptive creep are not a single process but are related processes. Moreover, it was deduced that the mechano-sorptive creep does not depend on the accumulation of the visco-elastic creep rate.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:288110 |
| Date | January 2003 |
| Creators | Matar, Ali |
| Publisher | London South Bank University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.0112 seconds