Experimental and theoretical study on the propagation characteristics of stress wave in filled jointed rock mass

This study is to theoretically and experimentally investigate the propagation of stress waves in the filled joint set. The time-domain recursive method is used to derive the propagation equations in the filled joint set, and the filled joints are further simplified into structural planes without joint thickness. The split-Hopkinson rock bar is modified to simulate P wave propagation normally across the parallel filled joints. The relationship among stress-closure curve, joint specific stiffness, transmission coefficient and loading rate is analyzed. The results show that, for the rock mass with a single joint, both the joint specific stiffness and transmission coefficient of different filling materials increase with loading rate. More serious particle breakage of the filling materials leads to lower joint specific stiffness and transmission coefficient. For the rock mass with two joints, the joint specific stiffness of each joint affects the transmission coefficient of the filled joint set. It is found that our theoretical calculations are basically consistent with the experimental ones, and the joint specific stiffness can well characterize the propagation behavior of stress wave in the filled parallel rock joints.

Model for in-vivo estimation of stiffness of tibiofemoral joint using MR imaging and FEM analysis

Background Appropriate structural and material properties are essential for finite-element-modeling (FEM). In knee FEM, structural information could extract through 3D-imaging, but the individual subject’s tissue material properties are inaccessible. Purpose The current study's purpose was to develop a methodology to estimate the subject-specific stiffness of the tibiofemoral joint using finite-element-analysis (FEA) and MRI data of knee joint with and without load. Methods In this study, six Magnetic Resonance Imaging (MRI) datasets were acquired from 3 healthy volunteers with axially loaded and unloaded knee joint. The strain was computed from the tibiofemoral bone gap difference (ΔmBGFT) using the knee MR images with and without load. The knee FEM study was conducted using a subject-specific knee joint 3D-model and various soft-tissue stiffness values (1 to 50 MPa) to develop subject-specific stiffness versus strain models. Results Less than 1.02% absolute convergence error was observed during the simulation. Subject-specific combined stiffness of weight-bearing tibiofemoral soft-tissue was estimated with mean values as 2.40 ± 0.17 MPa. Intra-subject variability has been observed during the repeat scan in 3 subjects as 0.27, 0.12, and 0.15 MPa, respectively. All subject-specific stiffness-strain relationship data was fitted well with power function (R2 = 0.997). Conclusion The current study proposed a generalized mathematical model and a methodology to estimate subject-specific stiffness of the tibiofemoral joint for FEM analysis. Such a method might enhance the efficacy of FEM in implant design optimization and biomechanics for subject-specific studies. Trial registration The institutional ethics committee (IEC), Indian Institute of Technology, Delhi, India, approved the study on 20th September 2017, with reference number P-019; it was a pilot study, no clinical trail registration was recommended.

Theoretical Analysis and Experimental Research into the Mechanical Properties of Al-Ti-Al Symmetrical Laminated Plate

In order to have both the surface corrosion resistance of aluminum alloy and the high specific strength characteristics of titanium alloy, titanium alloy TC4, and aluminum alloy 6061 can be used to make aluminum-titanium-aluminum (Al-Ti-Al) three-layer laminated plate by hot rolling. In this paper, the classical laminated plate theory was used to calculate the stiffness, specific stiffness, strength, and specific strength of the laminated plate. The results showed that when the coating rate of titanium alloy TC4 was 0.5, bending specific stiffness and bending specific strength were the minimum, but all other parameters increased with the increase of the coating rate of titanium alloy TC4. Therefore, in actual production, the coating rate of titanium alloy should be avoided being 0.5. Then, the rolling experiments of the Al-Ti-Al laminated plate were carried out with different temperatures, reduction rates, and thickness ratios. Finally, the tensile test and energy spectrum analysis of the laminated plate were carried out. The results showed that, with the increase of rolling temperature, the tensile strength, the extensibility, and the thickness of the diffusion layer increased; if the coating rate of titanium alloy TC4 was between 0.2 and 0.33, the mechanical properties, the bonding strength, and the thickness of the diffusion layer increased with the increase of the coating rate of titanium alloy TC4.

The effects of cables' strain and specific stiffness on the stiffness of cable-driven parallel manipulators

A cable-driven parallel manipulator is driven by a set of cables instead of rigid links. Since cables usually have more flexibility than rigid links, the stiffness of a cable-driven parallel manipulator has been a concern for many applications that require controllable system stiffness. This paper studies how cables' strain and specific stiffness affect the stiffness of a cable-driven parallel manipulator that has p degrees of freedom and [Formula: see text] cables. A decoupled stiffness model of a cable-driven parallel manipulator is derived mathematically. In the decoupled stiffness model, cables' specific stiffness is decoupled from the other factors that affect the stiffness of the cable-driven parallel manipulator, namely, cable strains, positions of anchor points on the end-effector, and extended lengths and orientations of cables. The concept of stiffness change ratio is proposed to reflect how significantly the stiffness of a cable-driven parallel manipulator can be regulated at a specific pose. The decoupled stiffness model shows that it is cable strains, rather than just cable tensions, that determine the stiffness change ratio of a cable-driven parallel manipulator at a specific pose. It is mathematically proved that, at a specific pose, the stiffness change ratio of a cable-driven parallel manipulator using cables with an extended strain range is larger than or equal to that of the cable-driven parallel manipulator using cables with the original strain range.