•The error sources of inaccurate initial stiffness of self-centering braces are determined.•Parametric studies are conducted to study the effects of fabrication tolerances.•A prediction equation is ...proposed for estimating the reduced axial stiffness.•Stiffness prediction accuracy using the proposed equation is experimentally validated.
The self-centering energy dissipation (SCED) brace is an innovative structural component of seismic-resistant systems that provides a flag-shaped response with good energy dissipation and self-centering capacity. The initial stiffness of the SCED brace is a crucial parameter for brace design and a braced frame system. The current use of SCED braces is limited by the significant discrepancy between the actual and theoretically predicted initial stiffnesses. In this paper, the sources resulting in the inaccurate prediction are identified as the accuracy of the theoretical governing equations and the practical fabrication tolerances of the braces. The accuracies of the governing equations with different levels of complexity are analyzed and compared based on a typical SCED brace. To investigate the reduction effect of fabrication tolerances on the effective axial stiffness, a large number of numerical simulations are conducted on an axially loaded tube member with different tolerance characteristics, including the fundamental length tolerance, contact surface nonuniformity, and load level. According to the trends of the effective axial stiffness, a valid stiffness reduction factor is defined. It can provide good estimates of the reduced axial stiffness under different tolerance parameters. Additionally, the stiffness reduction factor is incorporated into a refined governing equation of an experimental SCED brace to verify the accuracy of the predicted initial stiffness when the effect of the fabrication tolerances is considered. It is shown that the modified governing equation estimates well the actual initial stiffness. Moreover, this prediction equation can be easily extended to other types of SCED braces once the tolerance parameters have been determined.
The negative stiffness exhibited by bi-stable mechanisms together with the tunable superelasticity offered by shape memory alloy (SMA) wires can enhance the dynamic resilience of a structure in the ...context of vibration isolation. The effects of negative stiffness and superelastic damping in base-isolated structures are here explored by carrying out an extensive study of the nonlinear dynamic response via pathfollowing, bifurcation analysis, and time integration. The frequency-response curves of the isolated structure, with and without the negative stiffness contribution, are numerically obtained for different excitation amplitudes to construct the acceleration and displacement transmissibility curves. The advantages of negative stiffness, such as damping augmentation and reduced acceleration/displacement transmissibility, as well as the existence of rich bifurcation scenarios toward quasi-periodicity and chaos, are discussed.
Perivascular adipose tissue (PVAT) is increasingly recognized as an essential layer of the functional vasculature, being responsible for producing vasoactive substances and assisting arterial stress ...relaxation. Here, we test the hypothesis that PVAT reduces aortic stiffness. Our model was the thoracic aorta of the male Sprague-Dawley rat. Uniaxial mechanical tests for three groups of tissue were performed: aorta with PVAT attached (+PVAT) or removed (-PVAT), and isolated PVAT (PVAT only). The output of the mechanical test is reported in the form of a Cauchy stress-stretch curve. This work presents a novel, physiologically relevant approach to measure mechanical stiffness ex vivo in isolated PVAT. Low-stress stiffness (
), high-stress stiffness (
), and the stress corresponding to a stretch of 1.2 (σ
) were measured as metrics of distensibility. The low-stress stiffness was largest in the -PVAT samples and smallest in PVAT only samples. Both the high-stress stiffness and the stress at 1.2 stretch were significantly higher in -PVAT samples when compared with +PVAT samples. Taken together, these results suggest that -PVAT samples are stiffer (less distensible) both at low stress (not significant) as well as at high stress (significant) when compared with +PVAT samples. These conclusions are supported by the results of the continuum mechanics material model that we also used to interpret the same experimental data. Thus, tissue stiffness is significantly lower when considering PVAT as part of the aortic wall. As such, PVAT should be considered as a target for improving vascular function in diseases with elevated aortic stiffness, including hypertension.
We introduce a novel and physiologically relevant way of measuring perivascular adipose tissue (PVAT) mechanical stiffness which shows that PVAT's low, yet measurable, stiffness is linearly correlated with the amount of collagen fibers present within the tissue. Including PVAT in the measurement of the aortic wall's mechanical behavior is important, and it significantly affects the resulting metrics by decreasing aortic stiffness.
•Effect of arbitrary stiffness profile on track geometry degradation is investigated.•Individual support stiffness harmonics induce extra resonance velocities.•Shorter wavelength and larger standard ...deviation increase degradation rate.•Varying stiffness influence converges for wavelengths around 16 sleeper bays.
This work addresses the contribution of the wavelength composition of the spectrum of the rail support stiffness profile to the expected long-term settlement. To that aim, purely harmonic stiffness variations of different wavelength are studied. The frequency-domain model with a double periodicity level previously developed by the first and last authors is adopted to embed the stiffness profile in one of the periodicity layers. Additional resonance velocities at which the resonance frequency of the track system coincides with the support-passing frequency or its multiples are found. The susceptibility to degradation is assessed both by quantifying the mechanical energy dissipated in the substructure under a moving train axle within one wavelength of the support stiffness variation, and the work performed by the wheel-rail contact force. It is shown that shorter wavelengths and larger standard deviations of varying ballast/subgrade stiffness result in an increasing energy dissipation in the substructure, and increase the work performed by the wheel-rail contact force, therefore leading to a reduced lifetime of the track. The energetic quantities increase for lower mean values of the stiffness profile, confirming the proneness of tracks on soft soils to degradation. The influence of varying stiffness vanishes for wavelengths of approximately 16 times the sleeper span, which is equivalent to a track length of about 10 m. High railpad stiffness values result in increased energy dissipation but the influence is limited. In general, an increasing train velocity amplifies the rate of track degradation, with no stabilizing trend in the high-speed regime (300 km/h).
Compliant mechanisms have the potential to be utilized in numerous applications where the use of conventional mechanisms is unfeasible. These mechanisms have inherent stiffness in their range of ...motion as they gain their mobility from elastic deformations of elements. In most systems, however, complete control over the elasticity is desired. Therefore, compliant mechanisms with variable, including zero, stiffness can have a great advantage. We present a novel concept based on the prestressing of open thin-walled multi-symmetric beams. It is demonstrated that by changing the prestress on the center-axis of these beams, a range of variable torsional stiffness can be achieved. For beams with a large warping constant, the stiffness changes from positive to zero and negative as the prestress increases, while for beams with a near-zero warping constant, the range of neutrally stable twisting motion increases. A planar equivalent is shown in this work to elucidate the notion, and numerical and experimental analyses are performed to validate the prestress-related behavior.
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•Variable torsional stiffness is achieved by axial preloading of thin-walled beams.•Tunable negative stiffness is achieved from beams with high warping constants.•On-off stiffness switch is achieved from beams with zero warping constants.•Tunable range of neutral stability is achieved for zero warping constant beams.
Liver cancer is the second leading cause of cancer mortality worldwide, causing more than 700,000 deaths annually. Because of the wide landscape of genomic alterations and limited therapeutic success ...of targeting tumor cells, a recent focus has been on better understanding and possibly targeting the microenvironment in which liver tumors develop. A unique feature of liver cancer is its close association with liver fibrosis. More than 80% of hepatocellular carcinomas (HCCs) develop in fibrotic or cirrhotic livers, suggesting an important role of liver fibrosis in the premalignant environment (PME) of the liver. Cholangiocarcinoma (CCA), in contrast, is characterized by a strong desmoplasia that typically occurs in response to the tumor, suggesting a key role of cancer-associated fibroblasts (CAFs) and fibrosis in its tumor microenvironment (TME). Here, we discuss the functional contributions of myofibroblasts, CAFs, and fibrosis to the development of HCC and CCA in the hepatic PME and TME, focusing on myofibroblast- and extracellular matrix-associated growth factors, fibrosis-associated immunosuppressive pathways, as well as mechanosensitive signaling cascades that are activated by increased tissue stiffness. Better understanding of the role of myofibroblasts in HCC and CCA development and progression may provide the basis to target these cells for tumor prevention or therapy.
The maintenance of appropriate arterial tone is critically important for normal physiological arterial function. However, the cellular and molecular mechanisms remain poorly defined. Here, we have ...shown that in the mouse aorta, resident macrophages prevented arterial stiffness and collagen deposition in the steady state. Using phenotyping, transcriptional profiling, and targeted deletion of Csf1r, we have demonstrated that these macrophages—which are a feature of blood vessels invested with smooth muscle cells (SMCs) in both mouse and human tissues—expressed the hyaluronan (HA) receptor LYVE-l. Furthermore, we have shown they possessed the unique ability to modulate collagen expression in SMCs by matrix metalloproteinase MMP-9-dependent proteolysis through engagement of LYVE-1 with the HA pericellular matrix of SMCs. Our study has unveiled a hitherto unknown homeostatic contribution of arterial LYVE-1+ macrophages through the control of collagen production by SMCs and has identified a function of LYVE-1 in leukocytes.
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•LYVE-1+ macrophages coat murine and human blood vessels harboring smooth muscle cells•Deficiency in LYVE-1+ macrophages induces arterial stiffness and collagen deposition•LYVE-1+ macrophages degrade collagen on smooth muscle cells via pericellular MMP-9•LYVE-1 on macrophage engages HA on smooth muscle for collagen degradation
Macrophages are essential to maintain tissue homeostasis. Lim and colleagues demonstrate that perivascular LYVE-1-expressing macrophages prevent arterial stiffness by controlling the expression of collagen in vascular smooth muscle cells, a process dependent on the engagement of LYVE-1 with hyaluronan on smooth muscle cells.
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