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The work describes experiments and models related to auxetic (negative Poisson’s ratio) foams subjected to low-frequency and variable amplitude 3-point bending loading. A custom 3-point bending vibration test rig is designed and used to perform the dynamic test of auxetic PU foam beams within low-frequency range (1–20 Hz) and 5 different displacement amplitudes. The auxetic foams tested in this work are manufactured using a simplified and relatively low-cost uniaxially thermoforming compression technique, which leads to the production of foams with transverse isotropic characteristics. Auxetic foam beam samples with two different cutting orientations and different thermoforming compression ratios rc (20–80%) are tested and compared, also with the use of theoretical Euler–Bernoulli-based and finite element models. The dynamic modulus of the foams increases with rc, ranging between 0.5 and 5 MPa, while the dynamic loss factor is marginally affected by the compression ratio, with overall values between 0.2 and 0.3. The auxetic PU foam has a noticeable amplitude-dependent stiffness and loss factors, while the dynamic modulus increases but slightly decreases with the frequency. The dynamic modulus is also 20–40% larger than the quasi-static one, while the dynamic and static loss factors are quite close. A modified Bouc–Wen model is also further developed to capture the amplitude and frequency-dependent properties of the conventional and auxetic foams with different volumetric compression ratios. The model shows a good agreement with the experimental results.

Background Dual antiplatelet therapy (DAPT) is recommended in secondary prevention after coronary artery bypass grafting (CABG), but it is inevitably associated with the risk of bleeding, of which gastrointestinal bleeding accounts for more than half. Proton pump inhibitors (PPIs) may increase the risk of major cardiovascular adverse events when reducing the risk of upper gastrointestinal bleeding. Therefore, the optimal duration of a PPI in combination with DAPT is unclear. Methods The “Proton Pump Inhibitor Preventing Upper Gastrointestinal Injury in Patients on Dual Antiplatelet Therapy after CABG” (DACAB-GI-2) study is a prospective, single-center, open-label, parallel, randomized controlled trial. A total of 232 eligible subjects who are scheduled or initiated on DAPT (clopidogrel plus aspirin or ticagrelor plus aspirin) for 12 months immediately after CABG will be enrolled and be randomized in a 1:1 ratio to either a 12-month pantoprazole treatment arm or a 1-month treatment arm. The primary outcome is to assess the rate of gastroduodenal erosions and ulcers evaluated by esophagogastroduodenoscopy (EGD) within 12 months after randomization, based on the modified Lanza score. Secondary outcomes include reflux esophagitis and upper gastrointestinal bleeding. Other pre-specified outcomes include major adverse cardiovascular events, graft failure, and all-cause death. Discussion This study aims to compare the efficacy and safety of 12 months and 1 month of pantoprazole treatment in preventing DAPT-related upper gastrointestinal mucosal injury after CABG. Trial registration ClinicalTrials.gov NCT03908593.

In engineering practice, a nonlinear system stable about several equilibria is often studied by linearizing the system over a small range of operation around each of these equilibria, and allowing the study of the system using linear system methods. Theoretically, for operations beyond a small range but still within the stable regime of an equilibrium, the system behaves nonlinearly, and can be described and investigated using the Volterra series approach. However, there is still no available approach that can systematically transform the model of a nonlinear system into a form that can be studied over the whole stable regime about an equilibrium so as to facilitate the system study using the Volterra series approach. This transformation is, in the present study, referred to as nonlinear model standardization, which is the extension of the well-known concept of linearization to the nonlinear case. In this paper, a novel approach to nonlinear model standardization is proposed for nonlinear systems that can be described by a Nonlinear AutoRegressive model with eXogeneous input (NARX) or a nonlinear differential equation (NDE) model. The proposed approach is then used in three case studies covering the applications in nonlinear system analysis, nonlinear system design, and nonlinearity compensation, respectively, demonstrating the significance of the proposed nonlinear model standardization in a wide range of engineering practices.

In this study, a novel criterion known as the comprehensive transmissibility is proposed to estimate the isolation performance of nonlinear isolation systems under complex loading excitations. A combined quasi-zero-stiffness (QZS) isolator is enumerated, and the comprehensive performance of other common isolator systems and the existing QZS system is analyzed. Based on the normalized differential equations of the isolation system, the isolation performances of different isolation systems are studied by using the comprehensive transmissibility criterion, showing that the combined QZS isolator has better isolation performance than others. In addition, the output frequency response function (OFRF) representation of the comprehensive transmissibility is derived, and the optimal design of the QZS isolation system is developed by using the OFRF. The simulation results demonstrate that the design can meet the requirements of the vibration isolator with both the force and displacement excitations and is promised to be applied to the design of nonlinear isolation systems in engineering practice. To solve the issue of the analysis and design of nonlinear vibration isolation systems under multi-excitation, a new criterion known as comprehensive transmissibility is proposed. Based on the comprehensive transmissibility, the isolation performance of the combined vibration isolation system is analyzed, and the output frequency response function (OFRF) method is applied to conduct the design of the isolation system.

In response to the identification problem of nonlinear systems with harmonic input, this paper studies the identification effect of the different splicing way of the multi-harmonic input based model identification approach. In this method, the relationship between the input and output signals of the nonlinear system is represented by the Nonlinear Auto-Regressive model with exogenous inputs (NARX) model. Firstly, the modeling framework of the NARX model based on the multi-harmonic input method is introduced. Then the effect of the different splicing way contained sequence and period of splicing data on modeling results is researched and proved in theoretically. Finally, a case study is discussed to simultaneously validate that the different multi-harmonic signal splicing forms have no effect on the identification results.

This paper proposes an improved transfer-matrix method (TMM) for investigating the steady-state response of complex rotor-bearing systems. The internal damping of the shafts is considered to enhance the accuracy of the method. A transfer matrix of ball bearings with clearance and Hertzian contact is established. In addition, the incremental harmonic balance method is combined with the TMM to obtain the steady-state response of the rotor systems and simplify the investigation processes. The simulation results verify the superiority of this method in programming, reducing the order of system matrix, and determining stability. Thus, this method is efficient in investigating the nonlinear dynamic characteristics of large-scale rotor-bearing systems.

REBCO-coated conductor is widely used in various HTS power applications. The HTS coils for power applications are generally impregnated with epoxy resin to enhance their mechanical stability, and the coil conductors would be subjected to transverse compressive loading in these applications. Although the effect of various operating conditions on the quench characteristics of REBCO-coated conductor have been investigated, the effects of compressive stress on quench characteristics of epoxy-impregnated REBCO-coated conductor has rarely been reported. This study examined the effects of transverse compressive loading on the minimum quench energy (MQE) and normal zone propagation velocities (NZPV) of epoxy-impregnated REBCO-coated conductor. The critical current of the conductor shows little degradation when transverse compressive stress is applied up to 80 MPa. Under the same compressive stress, the MQE decreases but the NZPV increases with the transport current. For fixed transport current, compressive stress increases the thermal contact conductance between conductor and epoxy which allows the heater energy of the conductor dissipating faster towards the surrounding epoxy, resulting in the increase of MQE and decrease of NZPV when increasing transverse compressive stress. The results indicate that higher compressive stress is helpful to decrease the quench damage of the conductor.

A novel vibration isolation device called the nonlinear energy sink (NES) with NiTiNOL-steel wire ropes (NiTi-ST) is applied to a whole-spacecraft system. The NiTi-ST is used to describe the damping of the NES, which is coupled with the modified Bouc-Wen model of hysteresis. The NES with NiTi-ST vibration reduction principle uses the irreversibility of targeted energy transfer (TET) to concentrate the energy locally on the nonlinear oscillator, and then dissipates it through damping in the NES with NiTi-ST. The generalized vibration transmissibility, obtained by the root mean square treatment of the harmonic response of the nonlinear output frequency response functions (NOFRFs), is first used as the evaluation index to analyze the whole-spacecraft system in the future. An optimization analysis of the impact of system responses is performed using different parameters of NES with NiTi-ST based on the transmissibility of NOFRFs. Finally, the effects of vibration suppression by varying the parameters of NiTi-ST are analyzed from the perspective of energy absorption. The results indicate that NES with NiTi-ST can reduce excessive vibration of the whole-spacecraft system, without changing its natural frequency. Moreover, the NES with NiTi-ST can be directly used in practical engineering applications.

Wetlands in Mu Us Desert have severely been threatened by grasslandification over the past decades. Therefore, we studied the impacts of grasslandification on soil carbon (C):nitrogen (N):phosphorus (P) stoichiometry, soil organic carbon (SOC) stock, and release in wetland-grassland transitional zone in Mu Us Desert. From wetland to grassland, the transition zone was divided into five different successional stages according to plant communities and soil water conditions. At every stage, soil physical and chemical properties were determined and C:N:P ratios were calculated. SOC stock and soil respirations were also determined to assess soil carbon storage and release. After grasslandification, SOC contents of top soils (0–10 cm) decreased from 100.2 to 31.79 g kg−1 in June and from 103.7 to 32.5 g kg−1 in October; total nitrogen (TN) contents of top soils (0–10 cm) decreased from 3.65 to 1.85 g kg−1 in June and from 6.43 to 3.36 g kg−1 in October; and total phosphorus (TP) contents of top soils (0–10 cm) decreased from 179.4 to 117.4 mg kg−1 in June and from 368.6 to 227.8 mg kg−1 in October. From stages Typha angustifolia wetland (TAW) to Phalaris arundinacea L. (PAL), in the top soil (0–10 cm), C:N ratios decreased from 32.2 to 16.9 in June and from 19.0 to 11.8 in October; C:P ratios decreased from 1519.2 to 580.5 in June and from 19.0 to 11.8 in October; and N:P ratios decreased from 46.9 to 34.8 in June and changed from 34.9 to 34.0 in October. SOC stock decreased and soil respiration increased with grasslandification. The decrease of SOC, TN, and TP contents was attributed to the reduction of aboveground biomass and mineralization of SOM, and the decrease of soil C:N, C:P, and N:P ratios was mainly attributed to the faster decreasing speeds of SOC than TN and TP. The reduction of aboveground biomass and increased SOC release led by enhanced soil respiration were the main reasons of SOC stock decrease. Grasslandification led to lowers levels of SOC, TN, TP, and soil C:N, C:P, and N:P ratios. Grasslandification also led to higher SOC loss, and increased soil respiration was the main reason. Since it is difficult to restore grassland to original wetland, efficient practices should be conducted to reduce water drainage from wetland to prevent grasslandification.

In order to improve the harsh dynamic environment experienced by heavy rockets during different external excitations, this study presents a novel active variable stiffness vibration isolator (AVS-VI) used as the vibration isolation device to reduce excessive vibration of the whole-spacecraft isolation system. The AVS-VI is composed of horizontal stiffness spring, positive stiffness spring, parallelogram linkage mechanism, piezoelectric actuator, acceleration sensor, viscoelastic damping, and PID active controller. Based on the AVS-VI, the generalized vibration transmissibility determined by the nonlinear output frequency response functions and the energy absorption rate is applied to analyze the isolation performance of the whole-spacecraft system with AVS-VI. The AVS-VI can conduct adaptive vibration suppression with variable stiffness to the whole-spacecraft system, and the analysis results indicate that the AVS-VI is effective in reducing the extravagant vibration of the whole-spacecraft system, where the vibration isolation is decreased up to above 65% under different acceleration excitations. Finally, different parameters of AVS-VI are considered to optimize the whole-spacecraft system based on the generalized vibration transmissibility and the energy absorption rate.

In response to the identification problem concerning multi-degree of freedom (MDOF) nonlinear systems, this study presents the extended forward orthogonal regression (EFOR) based on predicted residual sums of squares (PRESS) to construct a nonlinear dynamic parametrical model. The proposed parametrical model is based on the non-linear autoregressive with exogenous inputs (NARX) model and aims to explicitly reveal the physical design parameters of the system. The PRESS-based EFOR algorithm is proposed to identify such a model for MDOF systems. By using the algorithm, we built a common-structured model based on the fundamental concept of evaluating its generalization capability through cross-validation. The resulting model aims to prevent over-fitting with poor generalization performance caused by the average error reduction ratio (AERR)-based EFOR algorithm. Then, a functional relationship is established between the coefficients of the terms and the design parameters of the unified model. Moreover, a 5-DOF nonlinear system is taken as a case to illustrate the modeling of the proposed algorithm. Finally, a dynamic parametrical model of a cantilever beam is constructed from experimental data. Results indicate that the dynamic parametrical model of nonlinear systems, which depends on the PRESS-based EFOR, can accurately predict the output response, thus providing a theoretical basis for the optimal design of modeling methods for MDOF nonlinear systems.

Similitude design plays a vital role in the analysis of vibration and shock problems encountered in large engineering equipment. Similitude design, including dimensional analysis and governing equation method, is founded on the dynamic similitude theory. This study reviews the application of similitude design methods in engineering practice and summarizes the major achievements of the dynamic similitude theory in structural vibration and shock problems in different fields, including marine structures, civil engineering structures, and large power equipment. This study also reviews the dynamic similitude design methods for thin-walled and composite material plates and shells, including the most recent work published by the authors. Structure sensitivity analysis is used to evaluate the scaling factors to attain accurate distorted scaling laws. Finally, this study discusses the existing problems and the potential of the dynamic similitude theory for the analysis of vibration and shock problems of structures.

A mathematical model of equivalent bending radius was used to analyze the effect of bending strain on the critical current of yttrium barium copper oxide (YBCO) tapes in distorted bending mode. The relationship between critical current and twist pitch under different bending radii was measured. For enough larger twist pitch, even though the bending radius was smaller than the critical bending radius, the equivalent bending radius was always greater than the critical bending radius. Only when the bending radius was less than the critical bending radius and the twist pitch was reduced to a certain extent, the equivalent bending radius was smaller than the critical bending radius. As a consequence, the critical current dropped sharply and cannot be restored. The model and analytical equation was feasible and can be used for the design of solenoid YBCO magnet.

Because of the end effect, a linear induction motor (LIM) runs in an asymmetrical state even though the winding of each phase is symmetric. Based on the basic principle of the LIM, a new approach was proposed to calcu- late the thrust of the LIM using the instantaneous current value. A three-phase LIM model with 12 slots and a single- layer winding was designed to validate this method. The experiments show that when the current is small, the calculated results basically agree with the experiments. The agreement becomes worse with the increase of the current because of the saturation of the primary iron core. The proposed formula is suitable when the iron core of the LIM primary is in an unsaturated state.