Fracture Behaviour of the Cross Laminated Timber

This four-year PhD project is focused on the investigation of the fracture behaviour associated with grain orientation and its effect on crack propagation of the cross laminated timber (CLT). Cross laminated timber is prepared by a new wood manufacturing technique. The CLT is glued from different timbers, and each timber has different percentage of latewood, grain orientation, and other natural defects. Wood is a kind of composite material found in nature. Due to international advocacy for energy efficiency and reduction, wood is a focus of attention again. In general, mechanical properties of wood vary with species, moisture content, temperature etc. Those individual characteristics always influence the mechanical properties and fracture behaviour of the wood. Therefore, how to precisely analyse the mechanical properties and fracture behaviour of the wood, to ensure quality, as well as in crease the safety of timber structures has become a critical issue.

In the past 5 years, Professor Wei-Chung Wang’s research group in the Photo mechanics Lab of NTHU has successfully applied the digital image analysis (DIA) technique to analyse the influence of the proportion of latewood and the grain orientation [1, 2].The behaviour during fracture of wood has been investigated using two-dimensional digital image correlation (2D-DIC).It was found that mode I fracture occurred at the very first appearance of a defect in the specimen. Then, the crack extended along the boundary between early wood and latewood. Later on, mixed mode I and II fracture occurred on passing into the latewood. Finally, mode I fracture occurred on the crack passing out of the latewood [3].
However, the fracture behaviour of the wood is difficult to observe and predict. Therefore, in this study, the challenge will be to develop appropriate methodologies to predict the stress concentration zone as well as the strain distribution when crack extension occurs.

In parallel, research at Liverpool has led to the development of quantitative comparison methodologies based on image decomposition [4] that allow data-rich strain fields, obtained from techniques such as DIC and ESPI, to be employed for the validation and updating of computational solid mechanics models [5] and in the analysis of flaws. With the help of the high-speed cameras at Liverpool, digital image correlation method can be used to analyse the full-field fracture behaviour of the wood in detail, and compared to mechanics models.

Applicants must have a good MSc or MEng degree in engineering, physics or an associated field. It is expected the successful student will spend equal periods of time at NTHU and UoL. The initial period at NTHU will focus on developing the experimental protocol for timber specimens and student will then take the specimens to UoL and perform analysis by using the high-speed digital image correlation method and image decomposition technique [4-6].