PSI - Issue 13

Odounga Bernard et al. / Procedia Structural Integrity 13 (2018) 347–352 Odounga Bernard / Structural Integrity Procedia 00 (2018) 000–000

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can alter their structural e ffi ciency and thus lead to sudden failure during their service life. Studying the cracking of tropical timber is crucial to know their behavior and guide local people in their choice of building materials other than concrete and steel. This would also contribute to sustainable development discussed at the COP23 (2017) meeting. This paper is devoted to the study of the mixed mode fracture at room temperature of the following tropical species: iroko (Malicia Excelsa) , okume (Aucoumea Klaineana) and padouk (Pterocarpus soyauxii) . A Mixed Mode Crack Growth wooden sample (MMCG) and an Arcan system were manufactured and mounted in an electromechanical press. The specimens tested have a thickness of b = 20 mm , and the orientation of the Arcan device was α = 15 0 . The images were recorded by a camera during the test and were then processed with the grid method Gre´diac et al. (2016). The displacement and strain maps of the grid were then processed and analyzed. They enabled us to follow the crack opening and the crack growth along the test. The results are given in terms of displacement and strain maps, but also in terms of curves showing the force as a function of the opening of the crack. The decoupling of the modes made it possible to obtain the energy release rate for modes 1 and 2. The di ff erent energy release rates were calculated by the compliance method with displacement imposed. They are given here as a function of the length of the crack.

Nomenclature

a

crack length initial crack

a i

b thickness of the specimen F c critical force that induces a crack advanced of length a G energy release rate G c critical energy release rate G I energy release rate (part of mode 1) G II energy release rate (part of mode 2) G ic initial average values of the energy release rate G ic 1 initial average values of the energy release rate from mode 1 G ic 2 initial average values of the energy release rate from mode 2

MC internal moisture ∆ a crack advanced ∆ C compliance α

solicitaion angle of the specimen

2. Materials and method

2.1. Wood specimens

The wood was cut in Gabon and kept in a local environment for one month before being brought to France. It was kept in a room with controlled temperature conditions, see Fig.1 (a). These conditions are as follows: room temperature T = 22 0 ± 1 0 , relative room humidity HR = 60% ± 1%, see Fig.1 (b). The three species studied here, iroko, okume and padouk, have a relative densitie of 0 . 64, 0 . 44 and 0 . 79 respectively. The percentage of internal moisture content (MC) of the species is 7.94% for iroko, 9.12% for okume and 7.29% for padouk. These values of the internal humidity of the di ff erent species were obtained in two ways, by calculation and with a moisture meter, which is a direct measuring device. The dimensions of the MMCG wood samples are shown in Fig.1 (c). All specimens were machined in the RL plane, with the orientation of the crack following L. The specimens tested had an initial crack of a i = 24 mm , a thickness b = 20 mm , and were tested at 15 0 . All the test specimens were reinforced with washers and metal plates around the fillets and load holes.