Modeling of wood surface ignition by wildland firebrands

The probability of structural ignition is dependent both on physical properties of materials and the fire exposure conditions. In this study, the effect of firebrand characteristics (i.e., firebrand size, number of firebrands) on wood ignition behavior was considered. Mathematical modeling and laboratory experiment were conducted to better understand the conditions of wood ignition by a single or group of firebrands with different geometry. This model considers the heat exchange between the firebrands, wood layer and the gas phase, moisture evaporation in the firebrands and the diffusion gases of water vapor in the pyrolysis zone. In order to test and verify the model, a series of experiments to determine probability and conditions for ignition of wood-based materials (plywood, oriented strand board, chipboard) caused by wildland firebrands (pine twigs with a diameter of 6–8 mm and a length of 40 ± 2 mm) were conducted. The experiments investigated the firebrand impact on the wood layer under different parameters, such as firebrand size and quantity, wind speed, and type of wood. The results of experiments showed that the increase in wind speed leads to the increase in probability of wood ignition. Based on the received results, it can be concluded that the ignition curve of wood samples by firebrands is nonlinear and depends on the wind speed and firebrand size as well as their quantity. At the same time, there is no ignition of wood samples in the range of wind speed of 0–1 m/s. The ignition of wood is possible with a decrease in the distance between the firebrands with a decrease in the firebrand length. This result agrees more closely with the model

Strain rate effect on dislocation structure in dispersion-hardened alloy with incoherent nanoparticles

The paper studies the influence of strain rate on the density of the dislocation system components in dispersionhardened materials having different scale parameters of the strengthening phase in a wide temperature range. According to mathematical simulation, the plastic shear rate of heterophased alloys with incoherent nanosized particles, affects the formation of dipole dislocation structures and, consequently, the material hardening. At high temperatures, the formation of dipoles does not occur in the dislocation structure of the material with the smallest particles at any strain rate. It is shown that during plastic strain, the density of shear dislocations is higher than that of dislocations in prismatic loops at all strain temperatures and rates in materials with strengthening particles of different size

Influence of nanosized incoherent particles on dislocation annihilation in heterophase aluminum-matrix crystalline alloys

Using mathematical modeling, patterns of changing the densities of dislocation subsystem components in dispersion hardened materials have been revealed depending on the volume fraction and scale characteristics of the hardening phase at different deformation temperatures. It has been shown that in all examined materials with hardening particles of different sizes, the dislocation annihilation significantly decreases with the volume fraction of nanosized incoherent particles. It has been found that the dislocation density in prismatic loops is largely determined by the particle sizes and the volume fraction of the hardening phase

Elastoplastic deformation of rotating disk made of aluminum dispersion-hardened alloys

This paper studies the plastic deformation of a rotating disk made of aluminum dispersion-hardened alloys using mechanical tensile tests and a structured study using optical microscopy methods. Alloys such as AA5056 and A356 with dispersed Al3Er and TiB2 particles are used as the initial materials. Tensile strength testing of the obtained alloys shows that the addition of Al3Er particles in the AA5056 alloy composition leads to an increase in its ultimate stress limit (USL) and plasticity from 170 to 204 MPa and from 14.7 to 21%, respectively, although the modifying effect is not observed during crystallization. The addition of TiB2 particles to the A356 alloy composition also leads to a simultaneous increase in the yield strength, USL, and plasticity from 102 to 145 MPa, from 204 to 263 MPa, and from 2.3 to 2.8%, respectively. The study of the stress-strain state of the disk was carried out in the framework of deformed solid mechanics. The equilibrium equations were integrated analytically, taking into account the hardening conditions obtained from the experimental investigations. This made it possible to write the analytical relations for the radial and circumferential stresses and to determine the conditions of plastic deformation and loss of strength. The plastic resistance of a disk depends on the ratio between its outer and inner radii. The plastic resistance decreases with increasing disk width at a constant inner radius, which is associated with a stronger effect from the centrifugal force field. At a higher rotational rate of narrow disks, the tangential stresses are high and can exceed the USL value. A356 and A356–TiB2 alloys are more brittle than the AA5056 and AA5056–Al3Er alloys. In the case of wide rotating disks, AA5056 and AA5056–Al3Er alloys are preferable

Investigation of stresses induced due to the mismatch of the coefficients of thermal expansion of the matrix and the strengthening particle in aluminum-based composites

An experimental and theoretical investigation of the strength properties of aluminum alloys strengthened by dispersed nanoparticles, as well as the determination of the significance of various mechanisms responsible for the strengthening of the material, was carried out. Results of experimental investigation demonstrate that the hardening of aluminum alloy A356 by Al2O3 and ScF3 nanoparticles leads to an increase in the yield strength, ultimate tensile strength, and plasticity. Despite the similar size of Al2O3 and ScF3 nanoparticles, the physicomechanical properties of nanoparticles significantly affect the possibility of increasing the mechanical properties of the A356 aluminum alloy. A physicomathematical model of the occurrence of thermal stresses was developed caused by the mismatch of the coefficients of thermal expansion (CTEs) of the matrix and strengthening particles on the basis of the fundamental principles of mechanics of a deformable solid and taking into account the elastic properties of not only the matrix, but also the particle. The forming of thermal stresses induced due to the mismatch of the coefficients of thermal expansion of the matrix and the strengthening particle in aluminum-based composites was investigated. In the case of thermal deformation of dispersion-hardened alloys, when the CTE of the matrix and particles noticeably differ, an additional stress field is created in the vicinity of the strengthening particle. Thermal stresses increase the effective particle size. This phenomenon can significantly affect the result of the assessment of the yield strength. The strengthening caused by thermal mismatch makes the largest contribution to the yield strength improvement. The yield strength increments due to NardonXPrewo and Orowan mechanisms are much lower