In this study, the Liu–Murakami (LM) creep damage-coupled model was considered to evaluate the creep properties of martensitic stainless steel. The degree of creep damage was examined at two temperatures (565 ℃ and 593 ℃) to assess mechanically and thermally activated processes. A series of high applied stresses (applied stress/ultimate strength > 0.5) was considered for accelerated creep loadings. A full set of creep constants was determined by combining the Norton and LM models. Constitutive equations were used to quantitatively estimate experimental creep curves. The variation in creep constants was discussed based on stress sensitivity, such as stress triaxiality and applied stress, depending on the power of stress. The creep strain–time curves were successfully estimated. The comparison between the experimental and analytical results was in good agreement in the tertiary regime. In addition, the compensation of the two applied temperatures provides a supplementary explanation of the relationship between the ultimate strength and rupture time in terms of temperature sensitivity. The analytical results show that different applied stresses and temperatures could be compensated to characterize the creep behavior of the material. Thus, the creep strain–time and creep strain rate–certain rupture time curves were finally achieved. The analytical process in this study provides a laboratory-scale assessment of creep properties using the accelerated creep test and LM model.
In a previous work, fatigue experiments performed on Zr55Al10Cu30Ni5 (at.%) bulk metallic glass immersed in distilled water environment have highlighted a causal connection between the generation of corrosion pits and the nucleation of fatigue cracks for the first time in amorphous metallic material. In line with the widely reported results from conventional crystalline materials, additional analyses have suggested an influence of the pit geometry on the occurrence of the fatigue crack nucleation event. Nevertheless, several aspects related to the pit-to-crack transition of the investigated material remain unknown. In the present work, a supplementary investigation based on artificially pitted specimens is undertaken to clarify the effect of the pit aspect ratio on the fatigue behavior of Zr55Al10Cu30Ni5 BMG to assess if the crack nucleation is impacted by the pit geometry. Furthermore, an estimation of the pit geometry effect on the finite fatigue life is also carried out. As an additional involvement, the reason of the slight discrepancy of the fatigue strengths from artificially pitted and smooth specimens is discussed. Experimental results show that the corrosion pit depth governs the fatigue crack nucleation occurrence through the existence of a critical pit geometry, assessed via its aspect ratio in the present work. However, in a noticeable discrepancy with its crystalline counterparts, the influence of the pit geometry on the finite fatigue life of Zr55Al10Cu30Ni5 bulk metallic glass is limited.
The poly(lactic acid) (PLA)-tricalcium phosphate (TCP) composite attracts much attention as a material for a bone fixation device because both have bioabsorbability. However, the applications of the composite bone fixation device are limited to low-loaded regions because of lower strengths. In this study, PLA and TCP/PLA composite cylindrical billets were extruded to improve their strengths, with the aim of clarifying the molecular orientation behavior of the PLA and TCP/PLA billets in extrusion. First, the effect of the orientation behavior in the PLA billets on the extrusion ratio (ER) was calculated. Next, the TCP/PLA billets were analyzed. The molecular orientation obtained by extrusion was investigated by a combination of the finite element method and the chain network model. Consequently, the orientation function of the extruded billet was found to increase with the ER. Furthermore, the orientation function of the TCP/PLA billet was higher than that of the PLA billet, which may be due to the change in the yield stress and n value of the billet by TCP addition. The orientation distribution in the radius direction of the extruded PLA billet showed that the orientation function at the surface region was larger than that at the center region. This orientation distribution might be because the movement distance of the surface region during extrusion was longer than that in the center region. The surface region in the billet was drawn by the difference of those movement distances and might be orientated. The workability of the PLA and TCP/PLA billets was also investigated using pressure during extrusion. The results showed that the extrusion pressure increased with the TCP content and ER. Therefore, extrusion workability decreased with the TCP content and the ER.
The vibration of an elastic toroidal tank induced by nonlinear sloshing is analyzed. A computationally efficient semi-analytical method is presented for sloshing in a toroidal tank with circular meridional cross-section, for which it is customary to resort to numerical methods. This method expresses the solution in term of polar coordinates defined in the meridional cross-section such that their origin is at the intersection of the straight lines that are tangent to the tank at the contact points of the static liquid surface with the tank wall. Numerical results illustrate that the difference between the linear and nonlinear responses of the stress in the tank shell is small although the nonlinearity augments the sloshing motion. To examine the physical reason for this counterintuitive trend, response analysis is conducted for the case where an upper part of the tank is open. The examination shows that the closed circular cross-section of the tank suppresses flexible modes with circumferential wave numbers 2 and 3.
This work applies real-time nonlinear model predictive control (NMPC) to fault-tolerant control problems of an unmanned aerial vehicle (UAV) with failed rotors. In the control problem, a hexacopter with up to three failed rotors out of the six available rotors is considered. The NMPC approach includes a quaternion-based nonlinear model of the hexacopter as well as constraints in the thrusts to consider the inherent nonlinearities of UAVs. The proposed method aims to achieve real-time optimization of the NMPC in the on-board computers without any linearization. We explore all possible scenarios in up to three rotor failures and demonstrate control designs in the NMPC for these scenarios. The simulation results indicate that by using the quaternion model, the position and attitude of a hexacopter can be controlled from a large inclined initial state with a non-zero angular velocity and falling velocity. Moreover, the results reveal that the quaternion model is superior to the Euler angle model in terms of the computation time. We also conduct hardware experiments using an actual hexacopter with a failed rotor to demonstrate the real-time NMPC optimization. The results of the simulations and hardware experiments demonstrate that the NMPC can deal with various operation conditions of a hexacopter in a unified manner, with only minor modifications in the performance index.
Contact-type failures, such as early-stage fatigue cracks, bolt loosening, and welding cracks, are difficult to detect with nondestructive inspection, which uses ultrasonic waves or vibrations. When a structure experiencing contact-type failure vibrates due to ultrasonic vibrations of two different frequencies, low-frequency vibrations with frequencies equal to the difference between the frequencies of the ultrasonic vibrations are generated due to local nonlinearity at the contact interface (frequency down-conversion). The detection methods of contact-type failure based on the frequency down-conversion of elastic waves or vibrations have been proposed. These detection methods can be applied using measurements with low-sampling rates. This study aims to develop a detection and localization method based on the frequency down-conversion of elastic vibrations. Low-frequency vibrations caused by frequency down-conversion are generated only if the structure has contact-type failure. Accordingly, contact-type failure can be regarded as the excitation point of low-frequency vibration caused by frequency down-conversion. Herein, a localization method of contact-type failure using the structural intensity as the localization method of the exciting location was proposed. First, an overview of the frequency down-conversion phenomenon is mentioned. Second, the theoretical investigation of the proposed localization method is explained, and the parametric excitation model that uses the Euler-Bernoulli beam is suggested. The equation transformation of the proposed model clarifies that the contact-type failure can be regarded as the exciting point of low-frequency vibration. Furthermore, from the results obtained herein, the location of contact-type failure was identified as the exciting point of low-frequency vibration caused by the frequency down-conversion. Finally, the basic performance of the proposed localization method was experimentally investigated. The exciting point can be localized from the sign change point of the structural intensity. Thus, the sign change point of the structural intensity of low-frequency vibrations caused by the frequency down-conversion is the contact-type failure location.
A plant with discrete-valued control is considered in this study. In discrete-valued control systems, the control input resolution, which is determined by the minimum value of the amplitude of the discrete-valued input and period, directly affects the control performance. If insufficiently short periods are specified, the control performance decreases due to the poor resolution of the discrete-valued input. To overcome such decrease, multirate control, which employs individual periods for output measurement and control input switching, was adopted in this study. We analyzed a decrease in the discrete-valued control performance caused by the poor control input resolution in pneumatic isolation table control, and numerical simulations and experiments showed that the use of multirate control is effective despite a long output period. The multirate control input was determined based on the Model Predictive Control, and a Kalman filter was employed in the experiments to reduce sensor noise for the pressure sensors.
Multiplexing dynamic vibration absorbers (DVAs) provide better vibration control compared to that obtained with single-mass DVAs; however, their structure is more complicated and they are thus difficult to apply in practice. This paper proposes several optimal design formulas for an electromechanical DVA in which a mechanical single-mass DVA with a piezoelectric element between the primary and absorber masses and a series LR circuit is coupled to the element. The inductor and piezoelectric element act as a virtual mass and a virtual spring, respectively, and thus the circuit works as a kind of double-mass DVA. In the present study, such an electromechanical DVA was optimized based on three design criteria and its design formulas were derived. Such design formulas have been previously reported, but they are very long and impractical. In contrast, the proposed formulas are very simple and the optimal design conditions for a DVA can be easily obtained. The steady-state and free-vibration responses of a three-degree-of-freedom system with an optimized electromechanical DVA are compared with those of a vibratory system with a mechanical series-type double-mass DVA. It is confirmed that the electromechanical DVA exhibits exactly the same vibration suppression performance as that of the mechanical DVA.
This study presents methods to develop a three-dimensional numerical foot model and to identify the loading condition that is used to design a stable sole for running shoes. In a previous study, the authors proposed a method to optimize the shape of the sole to increase stability while maintaining the cushioning property. In the problem formulation, the loading condition was given as a boundary force distributed on the top surface of the sole. The aim of this study is to replace the loading condition with the force and moment at the origin of the ankle joint coordinate (AJC) system by modeling a foot with a finite element model. A finite element model of a foot is constructed using X-ray CT image data, and consists of bony structures, soft tissue, and plantar fascia. The plantar fascia is set at the bottom of the bony structures. The force and moment used in the finite element analysis are identified by inverse dynamic analysis using an experimental measurement in the practical operation of the ground reaction force (GRF) when the GRF in the direction of the foot length becomes minimum. In the finite element analysis, the finite deformation containing the contact condition between the bottom surface of the foot and the ground representing a sole made of resin is considered. For an index of the shoe stability, we define a heel eversion angle (HEA) by the rotational angle of the heel with respect to an axis in the foot length direction and evaluate it by finite element analysis. The validity of the finite element foot model as well as the force and moment obtained in this study are confirmed based on the agreement in the HEA results between the experiment and finite element analysis.
In skateboarding as a sports event, the riders compete in difficulty and completeness of acrobatic motions called “tricks”. As a basic trick, Ollie is performed popularly. However, the basic mechanical principle of the Ollie has not been discussed to date, especially for the Ollie jump (the jumping phase of Ollie). The objective of this study was to elucidate the mechanism of Ollie jump in skateboarding. A simulation model was firstly constructed on a multibody dynamics analysis platform. Next, an experiment using an actual rider was conducted to acquire the motion of the feet during the Ollie jump. By inputting the acquired motion of the feet into the model, a simulation of Ollie jump was carried out. In addition, a parameter study with respect to the geometry of the skateboard and the motion of the rider’s feet was conducted. It was found that the simulated Ollie jump was successful since the skateboard reached a sufficient height and became sufficiently horizontal at the peak height. It was also found that the Ollie jump can be divided into five stages from the mechanical point of view. From the parameter study, it was found that large kick angle of the deck or large distance between two trucks of the deck might cause difficulty in the contact of the tail and the ground, while small kick angle or small distance between two trucks might result in excessive rotating angle of the deck. In addition, three important points for a successful Ollie jump were found to be, to produce sufficiently fast rotational movement of the skateboard around the rear wheels, to separate both feet from the deck before the tail of the deck hits the ground, and to separate the rear foot from the deck at the final stage.