Numerical Simulation on the Effect of Rock Joint Roughness on the Stress Field

In view of the influence of Joint Roughness Coefficient (JRC), which is for quantitative description of the joint surface roughness, on the stress field of the rock mass, compression test and shear-compression test were simulated on models with different joint roughness. The photoelasticity technique is applied to examine the feasibility of numerical simulation. The results show that numerical simulation results are in agreement with the results of photoelastic experiments. The stress concentration area is distributed near the joint plane. Thus, the joint plane controls the shear strength of the rock. In compression test, the maximum shear stress of the model is proportional to JRC and the normal pressure. In shear-compression test, when the ratio of the axial shear to the normal pressure is small, the maximum shear stress is nonlinearly positively correlated with JRC. When the ratio of the axial shear to the normal pressure is relatively large, the relationship curve between the maximum shear stress and JRC is parabolic. When the JRC is small, as the ratio of the axial shear force to the normal pressure increases, the maximum shear stress changes abruptly, and the maximum shear stress after the mutation decreases significantly. The reason is that the upper and lower parts of the model have slipped, resulting in a redistribution of stress. In addition, when the JRC is 6 to 12, it is more likely to cause stress concentration.

Numerical simulation of spatiotemporal red blood cell aggregation under sinusoidal pulsatile flow

Previous studies on red blood cell (RBC) aggregation have elucidated the inverse relationship between shear rate and RBC aggregation under Poiseuille flow. However, the local parabolic rouleaux pattern in the arterial flow observed in ultrasonic imaging cannot be explained by shear rate alone. A quantitative approach is required to analyze the spatiotemporal variation in arterial pulsatile flow and the resulting RBC aggregation. In this work, a 2D RBC model was used to simulate RBC motion driven by interactional and hydrodynamic forces based on the depletion theory of the RBC mechanism. We focused on the interaction between the spatial distribution of shear rate and the dynamic motion of RBC aggregation under sinusoidal pulsatile flow. We introduced two components of shear rate, namely, the radial and axial shear rates, to understand the effect of sinusoidal pulsatile flow on RBC aggregation. The simulation results demonstrated that specific ranges of the axial shear rate and its ratio with radial shear rate strongly affected local RBC aggregation and parabolic rouleaux formation. These findings are important, as they indicate that the spatiotemporal variation in shear rate has a crucial role in the aggregate formation and local parabolic rouleaux under pulsatile flow.

An Optoelectronics-Based Sensor For Measuring Multi-Axial Shear Stresses

Tactile shear force sensors are increasingly popular for medical applications especially for orthopedic rehabilitation such as measuring interfacial shear stresses between a residual limb and prosthetic socket to manage socket fit and residual limb tissue health, or measuring shearing between a foot sole and shoe for assessing performance in athletes. However, there are considerable challenges in implementing shear force sensors for orthopedic applications due to the requirements for noninvasiveness, light weight, low power, and robustness against motion artifacts, normal force, or electromagnetic fields. To address these challenges, this paper describes the design, fabrication, and characterization of a simple, low-cost, optoelectronic sensor that can measure multi-axial shear stresses. The sensor is based on a red, green, and blue (RGB) light-emitting diode (LED) cycling among red, green, and blue lights onto a color pattern surface. As shear strain causes a displacement between the LED and the color pattern, the relative intensities of reflected lights among the different colors change. A photodiode is used to capture the reflected light intensity at each color illumination, allowing the determination of the color pattern surface displacement, and in turn the shear, along two axes. In this paper, the efficacy of the sensor under benchtop testing conditions is reported, confirming the potential of this technology for shear monitoring at orthopedic devices such as protheses or shoes. Future efforts will focus on miniaturization and packaging of the sensors, and characterizing their performance for more medical and other types of applications.