Report of 3-months stay in RISH (4/1-3/4/2023)

Prof. Joseph GRIL, Centre national de la recherche scientifique(CNRS), senior researcher, France

The principal activity was the study of wood ageing, resuming a research subject initiated 14 years ago during a previous invitation period. The inner core of structural elements from historical structures, unaffected by surface degradation, provides the material needed to investigate the effect of natural ageing on wood properties. It was observed that similar modifications, although not completely identical, were obtained through thermal treatment of modern equivalent material. Oak blocks were shipped from France to complete the database of previous research conducted in RISH. They will serve as modern reference to material to be extracted from remains of Notre-Dame-de-Paris (NDP) cathedral, presumably dating back to the 13^th or 19^th century, and tested during Summer 2023 in RISH by a master course student co-supervised with M. Matsuo-Ueda (JSPS Summer program). The blocks were cut into small specimens oriented in the longitudinal (L) or radial (R) direction in RISH wood workshop. Some of them, rapidly prepared while still in wet state, served to calibrate and verify experimental procedures: measurements of elastic properties by static 3-point bending were compared to free-free vibration methods; as an alternative to the forced bending commonly used to evaluate the musical properties of wood, the analysis of free bending using the BING© software (CIRAD, France) provides with a quick estimate of shear rigidity on top of specific modulus and damping. The other L and R specimens, cut after slow air drying, will be organized in matched groups and subjected to thermal treatments of varying temperatures and durations, attempting to simulate the effect of combined natural and thermal ageing expected in NDP wood. 

A number of parallel activities allowed reinforcing or extending scientific relationships with research groups in Kyoto or elsewhere in Japan. The regular participation to the seminar of the Laboratory of Sustainable Materials was a nice introduction to the development of ecofriendly wood-based materials, a potentially attractive subject for staff and students of Clermont Auvergne University interested by an international research experience. The same could apply to the research on the seismic behavior of wooden structure performed in the neighboring Laboratory of Timber Science and Engineering. Revisiting the analysis of mechanical data involving the effect of time and temperature with colleagues of Nagoya university or FFPRI was a nice challenge that spiced up the stay. Measuring the growth stress distribution in large keyaki logs, as a joint research between RISH, Nagoya University, and a traditional construction company in Gifu, was a great experience. In addition to an online presentation given at RISH open seminar in February and an invited talk at a technical session of the Annual meeting of the Wood Research Society in Fukuoka in March, several talks were given to a more restricted audience in RISH, Tokyo university…, on subjects ranging from conservation of cultural heritage to modelling of time-dependent behavior of wood and biomechanics of trees.

Figure: Using BING© device to measure vibrational properties of wood: the sound of a ball hitting the specimen is processed by a FFT analyzer and a dedicated program (https://ur-biowooeb.cirad.fr/en/expertises-products/products/bing-software)

Figure: measuring the internal distribution of longitudinal growth stress using hundreds of strain gages glued on diametral planks of Keyaki logs