Effects of climatic and mechanical variations on the durability of timber structures – CLIMBOIS

Task 1 : FRACTURE AND VISCOELASTIC OF WOOD MATERIAL
Analytical development in fracture mechanic under variable environment integrating both viscoelastic behavior and numerical modeling. The experimental tests (material scale) aim at identifying fracture parameters under variable environment conditions. Additional experimental tests (wood structure scale) aim at investigating spatial variability of both mechanical properties and wood nodes. The following task are planned :
Task 1.1. Analytical formalism of fracture mechanic
Task 1.2. Numerical modeling of fracture coupled with viscoelasticity
Task 1.3. Experimental validation (decoupling of modes, digital image analysis, creep tests, etc.)

Task 2. OPTIMISATION, UNCERTAINTIES, RELIABILITIES OF STRUCTURES
The main goal is to incorporate both uncertainties and spatial variability in the reliable formula. The following tasks are planned:
Task 2.1. Probabilistic modeling of monitoring results
Task 2.2. Simplified modeling of climate
Task 2.3. Spatial variability of wood material properties
Task 2.4. Specific formalism of optimization based on fracture mechanic

Task 3. Non Destructive Testing (NDT)
It focuses on optimizing the identification of «variables of interest« for both the design and the maintenance of wood structures based on monitoring results provided by NDT methods. The following tasks are planned:
Task 3.1. Contribution of acoustic emission technique in identifying both failure mechanisms and damage of wood material
Task 3.2. Experimental tests to propose ROC curves
Task 3.3. Optimizing the assessment of both «measured properties« and «variables of interest« using NDT methods.

The Tasks 1 proposed analytical models of non-dependent integral coupling both the effects of temperature and lifetime on wood material. These models were successfully implemented in the finite element program CAST3M. In addition, laboratory creep tests were conducted as planned in Task 1.3. Mechanical tests were performed on DCB specimen (variable inertia geometry) to evaluate the fracture behavior of Douglas FIR and Picea abies under variable humidity in climatic chamber.
The Task 2 allowed incorporating the uncertainties as well as spatial variability in general concept of reliable formula. This task was achieved by analyzing the NDT tests results (Task 3 and Task 1.3). The Task 2.3 proposed the applicability of stochastic calculus methods to quantify the effect of hazard on the variability of fracture parameters of wood material.
Finally, Task 3 mainly uses two NDT methods : acoustic emission (AE) and Digital Image analysis. AE method allowed identifying failure mechanisms of wood material based on the localization and monitoring the crack tip propagation. The preliminary tests (Pencil Lead Breaking tests) (Task 3.1 and Task 3.2) on DCB specimens provided input data for Task 2.1. Then, the preliminary results based on statistic analysis provided recommandations to better localise the crack tip.

In the future works, the developed methods will be generalized to crack propagation integrating process zone in mixte mode in orthotropic viscoelastic media. Finally, humidity variations will be taken into account in order to model effects of humidity increase during creep tests (Task 1.3). In addition, fracture tests in mixte mode will be conducted in a climatic chamber to reproduce climatic loading (Task 1), as well as to compare numerical to experimental results.
To incorporate the variability of defects (nodes), tests on girder as well as tests on structures (Douglas Fir and Picea Abies) will be conducted in real environment, in order to provide input data for Task 1.2 and Task 2.
The future tests (Task 1.3 and Task 1.4) as well as Digital Image analysis will provide input data for Task 2 in terms of spatial variability and reliability optimization.

Riahi H., Moutou Pitti R., Dubois F., Fournely E. On numerical evaluation of mixed mode crack propagation coupling mechanical and thermal loads in wood material. Vol. 5, 21-26, Springer, 2015.

Riahi H., Moutou Pitti R., Chateauneuf A. Dubois F. Stochastic analysis of mixed mode fracture in timber material using polynomial chaos expansion. Final Cost Action FP0904 Conference, Skellefteå, Sweden, 19-21 May, 2014.

Moutou Pitti R., Diakhaté M., Bastidas Arteaga E., Aoues Y., Angellier N., Riahi H., Chateauneuf A., Dubois F. ANR JCJC-2013 Project CLIMBOIS: effects of climatic and mechanical variations on the durability of timber structures: use of acoustic emission tool to evaluate wood mechanical behavior. COST Action FP1302 Wood Music. Paris, France, 26-28 February, 2014

Angellier N., Moutou Pitti R., Dubois F. Crack analysis of wood under climate variations.SEM 2015. Costa Mesa, USA, 8-11 June 2015.

Moutou Pitti R., Hamdi S.E., Dubois F. Riahi H., Angellier N. Numerical fracture analysis under temperature variations by energetic method. SEM 2015. Costa Mesa, USA, 8-11 June 2015.

Aoues Y. Bastidas-Arteaga E., Moutou Pitti R., Chateauneuf A. Reliability-Based Design Optimisation of timber trusses subjected to decay and climate variations. COST Action FP1303. Tallinn, Estonia, 4-5 March 2015.

Hamdi S.E., Diakhate M., Moutou Pitti R., Dubois F., Chateauneuf A. In situ damage monitoring in timber materials: Acoustic emission pattern recognition approach based on Hilbert–Huang transform. ICBBM 2015. Clermont Ferrand, 21-24 June2015.

Moutou Pitti R., Diakhate M., Bastidas Arteaga E., Aounes Y., Angellier N. ANR JCJC N° ANR-13-JS09-0003-01– Projet CLIMBOIS: Impact des variations climatiques et mécaniques sur la durabilité des constructions bois. 2èmes Journées du GDR CNRS 3544 Sciences du Bois, Champs-Sur-Marne, 19 – 21, Novembre, 2013.

Both environmental matters and use of natural materials with low energy impact lead to an increase in the use of wood material in the field of construction. This increase is governed by the understanding of both the mechanical behaviour and the durability of wood structures. Notched beam are usually subjected to both climatic and complex mechanical loadings. These last loadings are generally due to the heterogeneous behaviour of wood material. In order to increase its use in the field of construction, it is important to better understand the behaviour of in-service of wood structures under climatic loading (humidity, temperature, etc.) as well as mechanical loading. This project aims at investigating the durability of wood structures under variable climatic environment as well as several mechanical loadings. During these last years, two French laboratories LAMI (University Blaise Pascal) and GEMH (University of Limoges) conducted several studies to investigate material durability under multi-loadings (due to moisture diffusion and mecanosorptif law). The mechanical behaviour of wood becomes more complicated to understand when one would take into account various mechanical properties (anisotropy, viscoelasticity, orthotropic behavior). The first aim of this project is to propose efficient methods combining both viscoelastic constitutive laws and uncoupling of fracture mixed-modes for viscoelastic materials subjected to the loads listed above. Modelling of these methods will be also investigated. The second aim of this project is focused on both conducting creep tests, and investigating the randomly spatial distribution of properties/flaw (on notched specimen, undamaged beam, and beam with flaws). These tests will be conducted both outdoor and in a climatic chamber. The third aim of this project consists in conducting non destructive tests (optical methods, acoustic emission, ultrasonic, etc.) on cracking specimens. The expected results, non linear viscoelastic constitutive laws, will allow reducing time-computation, analyzing field deformations during cracking process, and determining the effects of several parameters on the lifetime of wood structure elements.