Measurement of the Anisotropic Dynamic Elastic Constants of Additive Manufactured and Wrought Ti6Al4V Alloys

Additively manufactured (AM) materials and hot rolled materials are typically orthotropic, and exhibit anisotropic elastic properties. This paper elucidates the anisotropic elastic properties (Young’s modulus, shear modulus, and Poisson’s ratio) of Ti6Al4V alloy in four different conditions: three AM (by selective laser melting, SLM, electron beam melting, EBM, and directed energy deposition, DED, processes) and one wrought alloy (for comparison). A specially designed polygon sample allowed measurement of 12 sound wave velocities (SWVs), employing the dynamic pulse-echo ultrasonic technique. In conjunction with the measured density values, these SWVs enabled deriving of the tensor of elastic constants (Cij) and the three-dimensional (3D) Young’s moduli maps. Electron backscatter diffraction (EBSD) and micro-computed tomography (μCT) were employed to characterize the grain size and orientation as well as porosity and other defects which could explain the difference in the measured elastic constants of the four materials. All three types of AM materials showed only minor anisotropy. The wrought (hot rolled) alloy exhibited the highest density, virtually pore-free μCT images, and the highest ultrasonic anisotropy and polarity behavior. EBSD analysis revealed that a thin β-phase layer that formed along the elongated grain boundaries caused the ultrasonic polarity behavior. The finding that the elastic properties depend on the manufacturing process and on the angle relative to either the rolling direction or the AM build direction should be taken into account in the design of products. The data reported herein is valuable for materials selection and finite element analyses in mechanical design. The pulse-echo measurement procedure employed in this study may be further adapted and used for quality control of AM materials and parts.

2021 Reviewers With Distinction Award

The Journal of Mechanical Design Reviewers with Distinction Award is given to reviewers who have made a meritorious contribution to the journal in terms of the number, quality, and turnaround time of reviews completed during the past year.

Equivalent Electronic Circuit of a System of Oscillators Connected With Periodically Variable Stiffness

In this paper, we have shown the electronic circuit equivalence of a mechanical system consists of two oscillators coupled with each other. The mechanical design has the effects of the magnetic, resistance forces and the spring constant of the system is periodically varying. We have shown that the system’s state variables, such as the displacements and the velocities, under the effects of different forces, lead to some nonlinear behaviors, like a transition from the fixed point attractor to the chaotic attractor through the periodic and quasi-periodic attractors. We have constructed the equivalent electronic circuit of this mechanical system and have verified the numerically obtained behaviors using the electronic circuit.

Kinematics analysis and gait planning for a hemiplegic exoskeleton robot

Background: As the world's aging population increases, the number of hemiplegic patients is increasing year by year. At present, in many countries with low medical level, there are not enough rehabilitation specialists. Due to the different condition of patients, the current rehabilitation training system cannot be applied to all patients. so that patients with hemiplegia cannot get effective rehabilitation training. Methods: Through a motion capture experiment, the mechanical design of the hip joint, knee joint and ankle joint was rationally optimized based on the movement data. Through the kinematic analysis of each joint of the hemiplegic exoskeleton robot, the kinematic relationship of each joint mechanism was obtained, and the kinematics analysis of the exoskeleton robot was performed using the Denavit-Hartenberg (D-H) method. The kinematics simulation of the robot was carried out in automatic dynamic analysis of mechanical systems (ADAMS), and the theoretical calculation results were compared with the simulation results to verify the correctness of the kinematics relationship. According to the exoskeleton kinematics model, a mirror teaching method of gait planning was proposed, allowing the affected leg to imitate the movement of the healthy leg with the help of an exoskeleton robot. Conclusions: A new hemiplegic exoskeleton robot designed by Shenzhen Institute of Advanced Technology (SIAT-H) is proposed, which is lightweight, modular and anthropomorphic. The kinematics of the robot have been analyzed, and a mirror training gait is proposed to enable the patient to form a natural walking posture. Finally, the wearable walking experiment further proves the feasibility of the structure and gait planning of the hemiplegic exoskeleton robot.

A portable six-wheeled mobile robot with reconfigurable body and self-adaptable obstacle-climbing mechanisms

Mobile robots can replace rescuers in rescue and detection missions in complex and unstructured environments and draw the interest of many researchers. This paper presents a novel six-wheeled mobile robot with a reconfigurable body and self-adaptable obstacle-climbing mechanisms, which can reconfigure itself to three locomotion states to realize the advantages of terrain adaptability, obstacle crossing ability and portability. Design criteria and mechanical design of the proposed mobile robot are firstly presented, based on which the geometry of the robot is modelled and the geometric constraint, static conditions and motion stability condition for obstacle crossing of the robot are derived and formulated. Numerical simulations are then conducted to verify the geometric passing capability, static passing capability and motion stability and find feasible structure parameters of the robot in obstacle crossing. Further, a physical prototype of the proposed mobile robot is developed and integrated with mechatronic systems and remote control. Using the prototype, field experiments are carried out to verify the feasibility of the proposed design and theoretical derivations. The results show that the proposed mobile robot satisfies all the criteria set and is feasible for applications in disastrous rescuing scenarios.

A Review of Existing Transtibial Bionic Prosthesis: Mechanical Design, Actuators and Power Transmission

Transtibial and transfemoral amputations are the most common amputations in the world, loss of lower extremity result in impaired function extremities and also body balance. A prosthesis is a medical device designed to replace a specific body part to restore function to a body part lost due to an accident or disease. Most doctors strongly recommend the use of a prosthesis so that patients can return to normal activities after undergoing an amputation. Besides functioning to support beauty, the use of prostheses is also to restore the quality of life of prosthetic users, the issue of metabolic energy consumption when walking is also very important in designing transtibial bionic prosthesis because it involves the comfort of the user transtibial prosthesis. Most of the existing transtibial prosthesis products in Indonesia are conventional passive transtibial foot products, and passive prosthesis users show a limp or asymmetrical gait pattern so that conventional passive prosthesis users experience discomfort when walking in the form of pain in the amputated leg and normal foot, which can cause secondary musculoskeletal injuries such as joint disorders. Passive prostheses cannot generate propulsive force during push-off phase (terminal stance and preswing) of the human gait cycle. The use of passive prostheses can also consume 20-30% more metabolic energy while walking so that it can cause fatigue for the user. Transtibial bionic prosthesis research is growing, transtibial bionic prosthesis can overcome the weakness of passive prosthesis because it can produce push-off during gait cycle and several researchers have shown that bionic prostheses are capable of mimicking the human gait, as well as improve the performance in a more natural gait and normal walking. This study aims to study the existing transtibial bionic prosthesis by comparing between 6 existing designs of powered ankle or transtibial bionic prosthesis that have been published in several publications. The discussion focuses on the design and mechanical systems, actuators related to the selection of motors and drive mechanisms as well as power transmission from actuators to moving components.

Defect Detection of Mechanical Design Products Using Deep Neural Network

In industrial production, defect detection is one of the key methods to control the quality of mechanical design products. Although defect detection algorithms based on traditional machine learning can greatly improve detection efficiency, manual feature extraction is required and the design process is complicated. With the rapid development of CNN, major breakthroughs have been made in computer vision. Therefore, building a surface defect detection algorithm for mechanical design products based on DCNNs plays a very important role in improving industrial production efficiency. This paper studies the surface defect detection algorithm of mechanical products based on deep convolutional neural network, focusing on solving two types of problems: defect recognition and defect segmentation. Aiming at the problem of defect recognition, this paper studies a defect recognition algorithm based on fully convolutional block detection. This algorithm introduces the idea of block detection into the ResNet fully convolutional neural network. While realizing the local discrimination mechanism, it overcomes the shortcomings of the traditional block detection receptive field. Compared with the original ResNet image classification algorithm, this algorithm has stronger generalization ability and detection ability of small defects. Aiming at the problem of defect segmentation, this paper studies a defect segmentation algorithm based on improved Deeplabv3+.

Negotiating Uneven Terrain by a Simple Teleoperated Tracked Vehicle with Internally Movable Center of Gravity

We propose a mechanical design for a simple teleoperated unmanned ground vehicle (UGV) to negotiate uneven terrain. UGVs are typically classified into legged, legged-wheeled, wheeled, and tanked forms. Legged vehicles can significantly shift their center of gravity (COG) by positioning their multi-articulated legs at appropriate trajectories, stepping over a high obstacle. To realize a COG movable mechanism with a small number of joints, a number of UGVs have been developed that can shift their COG by moving a mass at a high position above the body. However, these tend to pose a risk of overturning, and the mass must be moved quite far to climb a high step. To address these issues, we design a novel COG shift mechanism, in which the COG can be shifted forward and backward inside the body by moving most of its internal devices. Since this movable mass includes DC motors for driving both tracks, we can extend the range of the COG movement. We demonstrate that a conventional tracked vehicle prototype can traverse a step and a gap between two steps, as well as climb stairs and a steep slope, with a human operating the vehicle movement and the movable mass position.