Variability Of Mechanical Parameters Research Articles

Effect of heterogeneity of particle properties on the mechanical properties of sandy soil materials

The random arrangement of particle composition or the heterogeneity of the particles' inherent properties is what causes the variability of mechanical parameters in sandy soils. Combining the discrete element method with random field theory, a random discrete coupling modeling method is refined, in which a rolling resistance contact model was used to take into account the rolling friction characteristics of sand soil particles. Four random discrete element models (RDEM) are developed for typical working situations taking into account the two random field factors, i.e., the coefficient of variation and fluctuation range. The Monte Carlo method is employed to simulate the triaxial RDEM, and the stress-strain relations, parameters of strength and deformation are analyzed. The effects of the coefficient of variation (Cov) and fluctuation range (δ) on the variability of mechanical characteristics are studied. The results show that the mean shear modulus G, peak friction angle φp, and critical friction angle φc of random heterogeneous samples decline while the mean Poisson's ratio μ increases as the Cov increases. The mean values of parameters of deformation and strength of random heterogeneous samples eventually approach those of homogeneous samples as the δ increases.

In vivo multifrequency magnetic resonance elastography of the human intervertebral disk.

To test in vivo magnetic resonance elastography (MRE) of the human intervertebral disk (IVD). The feasibility of MRE in IVD was demonstrated in ex vivo bovine disks. Sixteen asymptomatic volunteers underwent multifrequency MRE of the lumbar spine (IVD L3/4 and L4/5, n = 32) using a posterior plate transducer connected to a loudspeaker and operated at five frequencies from 50 to 70 Hz. Full wave field data were acquired in 10 transverse slices of 2 × 2 × 2 mm(3) resolution. High-resolution maps of magnitude |G*| and phase angle φ of complex shear modulus G* were generated by multifrequency dual elasto visco (MDEV) inversion. Disk morphology was assessed by the Pfirrmann score (Pf). Morphological Pf was 1 in 25, 2 in 3, and 3 in 4 disks. |G*| decreased with Pf by a Pearson's linear correlation coefficient of R = -0.592 (P = 0.0004), while φ remained unchanged. Group mean mechanical parameters for Pf = 1 to 3 were |G*| = 6.51 ± 1.27, 5.29 ± 0.95, 4.03 ± 0.99 kPa, and φ = 1.190 ± 0.181, 1.170 ± 0.156, 1.088 ± 0.084 rad, respectively (p[Pf1-Pf3] < 0.001). The variability of mechanical parameters in one volunteer including diurnal changes was approximately 11%. Multifrequency MRE with MDEV inversion allows measurement of in vivo mechanical properties of IVDs and may provide additional information in disc degeneration beyond standard morphological changes.

Analysis on Variability of Mechanical Parameters in the Process of Rock Damage

How the mechanical parameters of rock change with the development of joints and fractures is still a puzzle. It is hard to measure rock joints accurately at present. Resistivity is an important parameter of rock and its value can reflect the degree of fragmentation of rock. A new method combining measurement method of Young’s modulus with resistivity survey method is proposed in this paper. To determine the variation of mechanical parameters in the process of rock damage, the uniaxial, tri-axial compression tests and resistivity survey on rock specimens are done. The results showed that the values of resistivity were continuously increasing, but Young’s modulus and mechanical parameters were decreasing in the process of rock damage. The relationships between resistivity and mechanical parameters are established based on the experiments in the process of rock damage. Therefore, the damage state of rock and the change of mechanical parameters of rock can be determined according to the variance of resistivity. The study is helpful for design and real- time stability evaluation on rock engineering.