Breakage Strength of Wood Sawdust Pellets: Measurements and Modelling

Wood pellets are an important source of renewable energy. Their mechanical strength is a crucial property. In this study, the tensile strength of pellets made from oak, pine, and birch sawdust with moisture contents of 8% and 20% compacted at 60 and 120 MPa was determined in a diametral compression test. The highest tensile strength was noted for oak and the lowest for birch pellets. For all materials, the tensile strength was the highest for a moisture content of 8% and 120 MPa. All pellets exhibited a ductile breakage mode characterised by a smooth and round stress–deformation relationship without any sudden drops. Discrete element method (DEM) simulations were performed to check for the possibility of numerical reproduction of pelletisation of the sawdust and then of the pellet deformation in the diametral compression test. The pellet breakage process was successfully simulated using the DEM implemented with the bonded particle model. The simulations reproduced the results of laboratory testing well and provided deeper insight into particle–particle bonding mechanisms. Cracks were initiated close to the centre of the pellet and, as the deformation progressed, they further developed in the direction of loading.

Stochastic finite element analysis of the diametral compression test of powder compacts and porous materials

This paper presents a stochastic finite element approach for modeling the mechanical behavior of powder compacts and porous materials under diametral compression test conditions. The main goal is assessing the validity of the diametral compression test as an indirect technique to estimate tensile strengths of brittle or quasi-brittle materials exhibiting porosity heterogeneity. Thus, the study seeks to predict the influence of porosity randomness on stress distributions and the spatial location of the highest tensile stress on thin disc-shaped specimens. The proposed formulation uses a stochastic framework that couples a random spatial field to the finite element analysis to include non-deterministic features. Two case studies consider comparable targets for the mean porosity but different coefficients of variations. For each case study, a total of 1000 realizations are conducted under identical loading and boundary conditions. The predicted stress distributions are compared to the ones from homogenous closed-form solutions from the literature. Then, the expected magnitude and location of the maximum tensile stress are evaluated by statistical means. Findings from the stochastic model show that porosity randomness induces stress concentration around less dense volumes and location deviation of the maximum tensile stress from the center of the discs. Likewise, porosity heterogeneity could affect the accuracy of experimental diametral compression tests even for small variance cases; and so, the reliability of the mechanical properties derived from models based exclusively on the classic assumption of material homogeneity.

Mechanical strength characterization and modeling of hydroxyapatite/tricalcium phosphate biocomposite using the diametral-compression test

This study reports the enhanced mechanical resistance of the composite bioceramics of hydroxyapatite (HAP) and tricalcium phosphate (β-TCP) used as bone substitute. HAP/β-TCP mixture was prepared by wet mixing of powders and characterized. Effects of powder manufacturing and sintering temperature on the densification, microstructure and mechanical properties of the composite were studied. The rupture strength (σr) was calculated using the Brazilian test. At 1250 °C, the relative density and mechanical strength of the HAP/β-TCP ceramics reached the maximum value of 89% and 43 MPa, respectively. Experimental results were modeled by the finite element method to determine the stress distribution in the compacted disc.