12 Juillet – Thesis defense - Cédric Perez

09 h In videoconferencing

Mixed mode I+II cracking of wood at different moisture contents: experiment and proposal for a R-curve model.

Cracking of wooden structural elements has been a challenge for many years. The risks
of cracking are closely tied to climatic variations and especially to their consequences, in
terms of moisture content variations of wood. Besides, cracks usually spread in mixed mode and intermittently as a function of the geometry and stresses applied.
The aim of this thesis is to characterize the fracture properties of wood and more specifically the maritime pine, for various moisture contents and crack mode propagation : in Mode I (opening mode), in Mode II (shear mode) and in mixed mode I+II. Fracture
properties of wood are characterized within the framework of equivalent Linear Elastic
Fracture Mechanics (eqLEFM) leading to the estimation of resistance to crack growth
curves (R-curves). Since specimens’ geometry affects fracture properties, a single geometry (Mixed Mode Bending) is used to characterize the properties of the different fracture modes. However, this choice leads to a high risk of unstable failure in many test configurations. Therefore, we had to use to an external displacement control technique to minimize these risks. This method allows to obtain a "complete" resistance curve for more than 60 % of opening tests (Double Cantilever Beam) and above 40 % for shear tests (End Notched Flexure), all moisture contents combined (5 % to 20 %).
At the same time, we developed a resistance to crack growth model expressed in mixed
mode I+II based on Mode I and Mode II’s R-curves, taking into account moisture content. This model, inspired by cohesive zone models, is based on two failure criteria expressed into energy : the first one is based on the resistance of elastic equivalent crack growth and relies on the notion of development rate of the process zone (FPZ), while the second one is based on the energy needed to develop the fracture process zone. The proposed model describes with a reasonable accuracy the experimental mixed mode I+II R-curve and this whatever the moisture content and the rate mixity are.

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