In order to carry out high-precision machining of aerospace structural components with large size, thin wall and complex surface, this paper proposes a novel parallel kinematic machine (PKM) and ...formulates its semi-analytical theoretical stiffness model considering gravitational effects that is verified by stiffness experiments. From the viewpoint of topology structure, the novel PKM consists of two substructures in terms of the redundant and overconstrained parallel mechanisms that are connected by two interlinked revolute joints. The theoretical stiffness model of the novel PKM is established based upon the virtual work principle and deformation superposition principle after mapping the stiffness models of substructures from joint space to operated space by Jacobian matrices and considering the deformation contributions of interlinked revolute joints to two substructures. Meanwhile, the component gravities are treated as external payloads exerting on the end reference point of the novel PKM resorting to static equivalence principle. This approach is proved by comparing the theoretical stiffness values with experimental stiffness values in the same configurations, which also indicates equivalent gravity can be employed to describe the actual distributed gravities in an acceptable accuracy manner. Finally, on the basis of the verified theoretical stiffness model, the stiffness distributions of the novel PKM are illustrated and the contributions of component gravities to the stiffness of the novel PKM are discussed.
•Parallel kinematic machine with redundant, overconstrained structures is proposed.•It is for precision machining of large scale, thin wall and complex components.•Semi-analytical stiffness model considering gravitational effects is formulated.•Stiffness experiment is carried out to verify the semi-analytical stiffness model.•Stiffness distribution is illustrated and effect of component gravity is analyzed.
Many analytical and numerical analysis and design methods for geosynthetic-reinforced soil structures require a single-value (constant) estimate of reinforcement stiffness. However, geosynthetic ...reinforcement products are rate-dependent polymeric materials meaning that they exhibit time and strain-dependent behaviour under load. Hence, the appropriate selection of a constant (elastic) stiffness value requires careful consideration. A simple hyperbolic stiffness model is shown to be a useful approximation to the constant-load isochronous creep-strain behaviour of these materials at low load levels applicable to operational (serviceability) conditions of geosynthetic-reinforced soil structures. A large database of 606 creep tests on 89 different geosynthetic reinforcement products falling within seven different product categories was collected. From these data, isochronous stiffness values were determined for different combinations of duration of loading and strain level. Data from products falling within the same category were collected together to provide approximations linking the isochronous load-strain (creep) stiffness to the ultimate tensile strength of the material. These approximations are useful for analytical and numerical modelling particularly when parametric studies are undertaken to identify the sensitivity of model outcomes to reinforcement stiffness. Finally, three different geosynthetic-reinforced soil application examples are provided to demonstrate the important role of tensile stiffness on analysis and design outcomes.
•A simple hyperbolic model is developed for the isochronous load-strain (creep) behaviour of geosynthetic reinforcement materials.•The model is used to estimate reinforcement stiffness at low load levels consistent with operational conditions of reinforced soil systems.•Model parameters are fitted from a large database of creep tests on geosynthetic reinforcement materials from different product categories.•Approximations linking the isochronous load-strain (creep) stiffness to the ultimate tensile strength of the material are provided.•Three different geosynthetic-reinforced soil examples demonstrate the important role of tensile stiffness on analysis and design outcomes.
•The idea of improving the progressive collapse behavior by post-yield stiffness is proposed.•The mechanism of enhancing the resistance at CAA stage by post-yield stiffness is revealed.•Optimal ...secondary stiffness ratio is suggested based on theoretical and parametric analyses.•Experiments and FEM simulations on the collapse resistance of SFCB specimens are conducted.
The progressive collapse behavior of conventional reinforced concrete frame structures caused by the loss of columns has been widely investigated over the past years. However, few papers on the improvement of the structural progressive collapse resistance accounting for the mechanical properties of the reinforcement material are reported. As a kind of reinforcement material of reinforced concrete frames, steel-FRP composite bar (SFCB) has the typical feature of controllable post-yield stiffness. This paper explored and compared the progressive collapse characteristics of concrete frames reinforced with SFCBs, steel bars and hybrid reinforcements including both steel bars and FRP bars. Scaled pushdown experiments based on the alternate load path method were conducted to study the effect of post-yield stiffness on the progressive collapse resistance of beam-column sub-assemblages. The experimental results demonstrated that the SFCB specimens experienced three mechanical actions: flexural action, post-yield stiffness plus compressive arch action (PYSCAA) and catenary action. The feature of the PYSCAA differed significantly from the second action of the specimen reinforced with steel bars. Due to the effect of the post-yield stiffness, not only was the load capacity of SFCB specimens significantly enhanced, but also the transition displacements from the PYSCAA to the catenary action were delayed. Meanwhile, the finite element method was employed to simulate the whole progressive collapse process of the specimens, and the simulation results agreed well with the experimental ones. Parametric analyses based on numerical simulations proved that the application of SFCBs was beneficial to reduce the vulnerability of progressive collapse for concrete frames. The ratio of post-yield modulus to initial elastic modulus of SFCBs was found to be the key factor affecting the characteristics of the progressive collapse resistance, and the optimal ratio of 0.30 is recommended.
The photoplethysmogram (PPG) signal is widely measured by clinical and consumer devices, and it is emerging as a potential tool for assessing vascular age. The shape and timing of the PPG pulse wave ...are both influenced by normal vascular aging, changes in arterial stiffness and blood pressure, and atherosclerosis. This review summarizes research into assessing vascular age from the PPG. Three categories of approaches are described:
) those which use a single PPG signal (based on pulse wave analysis),
) those which use multiple PPG signals (such as pulse transit time measurement), and
) those which use PPG and other signals (such as pulse arrival time measurement). Evidence is then presented on the performance, repeatability and reproducibility, and clinical utility of PPG-derived parameters of vascular age. Finally, the review outlines key directions for future research to realize the full potential of photoplethysmography for assessing vascular age.
Quasi-zero stiffness (QZS) nonlinear isolation systems have demonstrated better performance than their linear counterparts. However, their optimal performance is achieved only in a small displacement ...range around the static equilibrium position. Based on the QZS system with one pair of oblique springs, this paper proposes a new limb-like QZS system with two pairs of oblique springs to enlarge the QZS range and thus improve its isolation performance. Two pairs of oblique springs are configured to provide the dynamic stiffness opposite to the vertical spring for generating QZS characteristics. In comparison with the corresponding QZS system with one pair of oblique springs, the proposed QZS system with two pairs of oblique springs can achieve a lower dynamic stiffness in a much wider region around the static equilibrium position. Based on the theoretical analysis, a prototype is designed and fabricated to physically realize the QZS isolation system. Experimental results are found to be in good agreement with the theoretical predictions which also confirm the proposed QZS system has better isolation performance than the corresponding QZS system with one pair of oblique springs. The proposed model can be adopted for isolating low frequency vibrations in practical applications.
Blunt traumatic thoracic aortic injuries (BTAI) are associated with a high mortality rate. Thoracic endovascular aortic repair (TEVAR) is the most used surgical strategy in patients with BTAI, as it ...offers good short- and middle-term results. Previous studies have reported an abnormally-high prevalence of hypertension (HT) in these patients. This work aims to describe the long-term prevalence of HT and provide a comprehensive evaluation of the biomechanical, clinical and functional factors involved in HT development.
Twenty-six patients treated with TEVAR following BTAI with no history of HT at trauma were enrolled. They were matched with 37 healthy volunteers based on age, sex and body surface area, and underwent a comprehensive follow-up study, including cardiovascular magnetic resonance, 24-hour ambulatory blood pressure monitoring and an assessment of carotid-femoral pulse wave velocity (cfPWV, a measure of aortic stiffness) and flow-mediated vasodilation.
The mean age of patients was 43.5 ± 12.9 years and most of them were male (23/26, 88.5%). 120.2 ± 69.7 months after intervention 17 (65%) patients presented HT, 14 (54%) abnormal night-time blood pressure dipping and 6 (23%) high cfPWV. New-onset HT was related to a more proximal TEVAR landing zone and larger distal oversizing. Abnormal night-time blood pressure was related to high cfPWV, which was in turn associated with TEVAR length and premature arterial ageing.
In otherwise-healthy subjects undergoing TEVAR implantation after BTAI hypertension is frequent. TEVAR stiffness and length, the proximal landing zone and distal oversizing are potentially-modifiable surgical characteristics related to abnormal blood pressure.
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This paper presents an experimental insight into the performance of a mechanical oscillator consisting of an X-shaped-spring configuration. This configuration achieves an overall softening ...characteristic with quasi-zero stiffness behaviour far away from the static equilibrium point. Such a geometrical nonlinear configuration has attracted significant research attention in the last few years, particularly for its application as a vibration isolator with the possibility to extend the quasi-zero-stiffness region beyond that of the classical three-spring nonlinear isolator. However, previous experimental evidence has been limited to small amplitude vibration excitation only. Furthermore, it has been focused mainly on the isolation region, rather than on the large amplitude response, thus circumventing an insight on the damping effects and its modelling. To address this gap, in this paper, both frequency sweeps and random excitations are applied to a prototype device for experimental characterization. A nonlinear stiffness model is developed based on the geometry of the system and a nonlinear damping model is assumed based on experimental observation. The proposed model accurately describes the dynamic behaviour of the system as shown by comparison of theoretical and experimental data.
•Novel spring arrangements based on the X-shaped anti-vibration structure with tunable contact mechanism (NXSC) are explored.•An enhanced quasi-zero-stiffness (QZS) with large stroke can be obtained ...via specially-designed tuning mechanisms.•The resulting stiffness property is much more adjustable with significantly extended QZS in a wider displacement range.•The proposed NXSC structure is adjustable to different loading requirements and resonant frequencies.
A novel spring arrangement based on the X-shaped anti-vibration structure with tunable contact mechanism (NXSC) is proposed to explore much better low-frequency vibration isolation performance. An enhanced quasi-zero-stiffness (QZS) with large stroke is obtained by tuning contact position between the horizontal springs and vertical fixed rods. The loading capacity and working range of the NXSC structure can be extended by 2, 3 times compared with classical X-shaped structures. The proposed NXSC structure is adjustable to different loading requirements and resonant frequencies, and excellent vibration isolation performance can be achieved readily by tuning the contact parameter. Three types of the NXSC structure with different contact modes are designed. The effects of structural parameters on the loading capacity and QZS zone are analyzed. By tuning the contact parameter via special tuning mechanisms, the negative stiffness can be compensated completely to ensure the stability of isolation systems. Comparisons with a typical spring-mass-damper (SMD) isolator and an existing QZS isolator exhibit that the stiffness-displacement curve of the NXSC structure is much more adjustable with significantly extended QZS in a wider displacement range, and does not sacrifice loading capacity. Moreover, the NXSC structure is less sensitive to excitation amplitude than the traditional QZS isolator, and it does not produce frequency jump phenomenon and softening and hardening stiffness effects. Experiments are implemented to verify the theoretical result and demonstrate that the proposed NXSC structure possesses superior vibration isolation performance and can effectively attenuate vibration at low-frequency range. This NXSC structure presents a new and efficient way for manipulating beneficial nonlinearity based on the X-shaped structure for better engineering performance.
Force-displacement and stiffness curves of (a) a SMD isolator, (b) a typical QZS isolator and (c) the proposed isolator. Display omitted
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