EXPERIMENTAL STUDY AND MULTISCALE NUMERICAL SIMULATION OF THE MECHANICAL BEHAVIOUR OF CRACKS IN VISCOELASTIC MATERIALS: APPLICATION TO BILINGA (NAUCLEADIDEERRICHI)

Abstract

The mechanical behaviour of local wood specie (Bilinga) in the Southwest region of Cameroon under the bending loading condition during the rainy and dry seasons are studied. The objective of this study was to determine the response of the wood (Bilinga) under mechanical stress in both weather conditions and also to study the crack initiation and its propagation of this wood of any of the species using the finite element analysis and compare its result using the stochastic model. To meet these objectives, the three-point flexural tests were used to determine the mechanical properties of the wood studied. A finite element software ANSYS workbench 2020 R1 is used for the numerical simulation on a macroscopic level by one of the most recent technologies called the Smart crack growth which was introduced from the 2019 version. The geometry was modelled in SolidWorks (2016) with initial cracks of 4mm and 8mm introduced in each sample and imported to ANSYS work which has all the tools to perform linear fracture and further analyses. The stress intensity factor (SIF) determines the fracture toughness of the material which is subjected to linear-elastic fracture mechanics (LEFM) where the stress intensify variable is represented as <em>K_IC</em>. Fatigue crack growth has been modelled using Paris’ law. The crack growth was simulated based on Mode I crack specimen with the initial crack length of 4mm and 8mm respectively. The stochastic quasi3D-modeling of crack growth on microscale used for comparing the crack growth rate using the heterogeneous material approach and taking into account the microstructure and fracture mechanism of the Bilinga wood. After analysing 10 samples each of Bilinga from the rainy and the dry season, results for Bilinga of the rainy season shows an elasticity modulus of 8.828 MPa as compared to that of the dry season which is 6.7415 MPa, for the stress intensity factor, Bilinga of the rainy season was 16,347 MPa√m as compared to that of the dry season which is 9,478 MPa√m. For the energy release rate, Bilinga of the rainy season was 134428 J/m² as compared to that of the dry season which is 145575 J/m²and for the rupture energy, Bilinga of the rainy season was 1875.12 kJ/m^2, as compared to that of the dry season which is 1276.25 J/m^2.For the moisture content, Bilinga of the rainy season was 14% as compared to that of the dry season which is 12% and for the density, Bilinga of the rainy season was 780kg/m³ as compared to that of the dry season which is 680 kg/m³. The result of stochastic modelling of the crack growth in the array of cracks and pores of characteristic size shows that the simulation is close to FE-modelling results. Therefore, stochastic simulation of crack growth in the wood at the mesoscale and microscale shows the lower local stress intensity factors and slower crack growth due to the existence of the scale-time hierarchy. The crack growth rate at the macroscale equals <strong><em>v_cr</em></strong> = 0.845-0.9·10^-3which corresponds to the macroscopic stress intensity variable <em>K_I</em>_C.