MCW : Modelling of crack initiation and propagation in wood under varying ambient conditions

Technical Committee MCW

General Information

Activity starting in: 2001

Subject Matter

The structure and composition of wood material as a natural grown multilevel composite material has a strong effect on its mechanical properties. Moreover, wood material as a complex mixture of different chemical compounds (long polymer chains [cellulose] and amorphous substances [lignin, etc.]), all having different thermal and hygroexpansion properties, is subject to interaction phenomena that make its mechanical performance dependent on ambient conditions. The strongest effect has the ambient humidity and especially its change. The continuum mechanics type modelling and prediction of the performance of wooden structures requires homogenisation of the properties of the microscopic material structure. The homogenisation and development of continuum models is more complicated for multilevel materials like wood than for materials having a more homogeneous microscopic structure. One major consequence of the oriented microscopic composition of wood is the extremely high anisotropy of stiffness, strength and fracture toughness. Maybe the most difficult area of modelling the mechanical properties of wood is the prediction of failure. The strong anisotropy forces cracks to follow certain routes in the microscopic structure, but since the cracks must pass all levels of the constitutive structure, the assessment of failure susceptibility is difficult. Moreover, crack propagation is affected by ambient conditions. Recently, considerable progress has been obtained in material studies of wood fracture concerning fracture propagation in the microscopic structure considered as an inhomogeneous material. Furthermore, recent fracture mechanics experiments have shown that occurrence of drying or moistening periods is of crucial importance on crack development. Change of moisture can be a factor whose significance exceeds substantially the direct effect of (constant) moisture content level. The strong impact of changing conditions can be explained by the eigenstresses caused by uneven hygroexpansion in the different levels of the microscopic structure. The objective of this work is to convey the newly obtained results of material studies to a form in which they can be utilised on the macroscopic level of continuum mechanics for the prediction of wood failure and assessment of load bearing capacity. A precise goal of the work is to produce recommendations of how to take into account the effect of changing humidity conditions in design methods. Also, recommendations of how to test wood for crack propagation in changing conditions are produced. The scientific contribution of the work will be the evaluation and review of models and empirical results.

Terms of reference

The committee is assumed to have a membership of 10-15 experts. The committee work will consist of the (1) collection and evaluation of existing knowledge and modelling methods, (2) development of improved methods, and (3) producing recommendations. Provisional timetable: the work is scheduled for three years: 2002-2004 as follows: - 2002 Collection and review of existing empirical data and models about the effect of varying ambient conditions on the fracture behaviour of wood. - 2003 Evaluation of models. Development and application of improved models. - 2004 Preparation of recommendations and final report.

Detailed working programme

The working programs includes the following tasks: Task 0 Define a detailed working plan. Task 1 Collect existing knowledge of the crack development in wood microscopic structure and crack surfaces. Task 2 Collect existing experimental results of the effect of ambient conditions on fracture behaviour of wood. Prepare a synopsis of the state-of-the-art of fracture mechanics applied to timber from the perspective of modelling varying conditions. Task 3 Evaluate methods to predict the crack initiation and propagation in wood and their capability to take into account the effect of changing ambient conditions. Develop improved approaches to model crack initiation and propagation. Use new developments to consider effect of moisture changes in numerical modelling. Task 4 Consider revisions of testing methods in order to account for changing conditions. Task 5 Collect conclusions and recommendations. Prepare final report to be published in a suitable media. Organise a workshop to announce results. Provisional schedule of meetings: - 1st meeting: Dec01/Jan02 - 2nd meeting: Apr02 - following meetings: first and third quarter each year

Technical environment

In the 1990's there were two fruitful RILEM TC's on fracture mechanics application to timber. (TC 110 on application of fracture mechanics to timber, 1989-1990, and TC133 devoted to wood fracture mechanics in mode II, 1992-1995). These previous two TC's promoted to the establishment of a European research project "Duration-of-load effect on different sized timber beams", in which among other things the fracture toughness of notched timber beams was tested and surprising results of the effect of humidity changes encountered. This proposal is closely linked to these previous TC's and builds on the foundations created by them. It also utilises the results of the mentioned research project.

Expected achievements

The main result of the TC will be recommendations for design code developers and building designers of how to take into account the effect of changing conditions on the fracture mechanical behaviour and load bearing capacity of wood. The task is challenging due to the presumably limited level of knowledge available at present. Lack of empirical data sets a limit for the potential accuracy for this kind of modelling work. Therefore, the TC will also pursue to deliver recommendations of new research areas: what should be done to clarify sufficiently the effect of moisture changes on the cracking bahaviour and load carrying capacity, and try to give suggestions of testing procedures in such conditions. The work of the TC needs to evaluate the state-of-the-art, but it is not a main aim of the work, rather a tool. The TC will produce new suggestions of methods for treating moisture content changes in numerical analysis of wood fracture. The TC plans to organise one workshop at the end of its work. Presentations will be invited from both TC members and other interested experts.

Group of users

The target group include standard organisations, designers of wood constructions, wood product manufacturers and research institutes. The academics will benefit from the results as recommendations of new research topics and coordination of research work.

Specific use of the results

The results will be applied to improve the use of wood as a construction material by improving its performance and economical use. The new information will enable improved development of wood products and enhance their manufacturing processes. It can be used to reduce and possibly avoid cracking at different stages of the manufacture, especially drying, and at end-use.

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