Mircea Arcan

A METHOD TO PRODUCE UNIFORM PLANE-STRESS STATES WITH APPLICATIONS TO FIBER-REINFORCED MATERIALS

This work is concerned with a new method for testing material properties under uniform plane-stress conditions by means of a specially designed plane specimen. Photoelastic analysis showed that in the significant section of the specimen it is possible to produce uniform plane stress, with high accuracy, subject to the limitation that the principal stresses are of different signs. An important special case of loading produces pure shear on the significant section. The specimen is of particular importance for fiber-composite testing.The experimental results presented are encouraging.

A myosignal-based powered exoskeleton system

This paper studies the integration of a human arm with a powered exoskeleton (orthotic device) and its experimental implementation in an elbow joint, naturally controlled by the human. The human-machine interface was set at the neuromuscular level, by using the neuromuscular signal (EMG) as the primary command signal for the exoskeleton system. The EMG signal along with the joint kinematics were fed into a myoprocessor which in turn predicted the muscle moments on the elbow joint. The moment-based control system integrated myoprocessor moment prediction with feedback moments measured at the human arm/exoskeleton and external load/exoskeleton interfaces. The exoskeleton structure under study was a two-link, two-joint mechanism, corresponding to the arm limbs and joints, which was mechanically linked by the human operator. Four indices of performance were used to define the quality of the human/machine integration and to evaluate the operational envelope of the system. Experimental results indicate the feasibility of an EMG-based power exoskeleton system as an integrated human-machine system using high-level neurological signals.

Performances of Hill-type and neural network muscle models—toward a myosignal-based exoskeleton

Muscle models are the essential components of any musculoskeletal simulation. In addition, muscle models which are incorporated in neural-based prosthetic and orthotic devices may significantly improve their performance. The aim of the study was to compare the performances of two types of muscle models in terms of predicting the moments developed at the human elbow joint complex based on joint kinematics and neuromuscular activity. The performance evaluation of the muscle models was required to implement them in a powered myosignal-driven exoskeleton (orthotic device). The experimental setup included a passive exoskeleton capable of measuring the joint kinematics and dynamics in addition to the muscle myosignal activity (EMG). Two types of models were developed and analyzed: (i) a Hill-based model and (ii) a neural network. The task, which was selected for evaluating the muscle models performance, was the flexion-extension movement of the forearm with a hand-held weight. For this task the muscle model inputs were the normalized neural activation levels of the four main flexor-extensor muscles of the elbow joint, and the elbow joint angle and angular velocity. Using this inputs, the muscle model predicted the moment applied on the elbow joint during the movement. Results indicated a good performance of the Hill model, although the neural network predictions appeared to be superior. Relative advantages and shortcomings of both approaches were presented and discussed.

A fundamental characteristic of the human body and foot, the foot-ground pressure pattern.

Abstract A new method and instrument developed by the authors allows a simultaneous recording of the pressure distribution between each foot and the ground during either standing or walking. The light reflected from a special sandwich plate produces an interference pattern which is a function of the contact pressure. The pressure distribution is displayed simultaneously on the whole contact surface as a diagram: the foot-ground pressure pattern (FGP). Quantitative data and new parameters representative of the mechanics of the foot for standing posture are proposed. These will provide a better description and understanding of the mechanics of the foot and the human body. The method may yield important data for the evaluation of prosthetic-orthotic appliances, in post-operative follow-up of orthopedic patients, and in the mechanics of rehabilitation.

A mixed mode fracture specimen for mode II dominant deformation

Abstract Several theories have been proposed for the failure of metals, as well as for the angle of crack propagation in mixed mode loading. In order to demonstrate the validity of these theories, the majority of tests have been carried out with an oblique crack placed in a uniaxial stress field. Better testing conditions may be achieved by placing a crack in a uniform bidimensional stress field. A specimen which was recently developed for K IIC measurement may be readily adapted to achieve a bidimensional stress field and be used for mixed mode testing for the case in which shear deformation is dominant. The main aims of this study are to examine both the cracked and uncracked specimen by means of photoelasticity and finite elements in order to analyze the capabilities and limitations of this specimen for mixed mode testing. It will be demonstrated that there exists a nearly uniform biaxial field in the uncracked specimen. Moreover, calibration formulas will be presented for K I and K II .

Failure of unidirectional fiber-reinforced materials—New methodology and results

AbstractAn experimental procedure has been developed in order to determine a failure envelope for unidirectional fiber-reinforced materials (FRM). This method provides failure data for a wide range,

Toward a pure shear specimen for KIIc determination

- 0.47< \sigma _{22} /\sigma _{12}< 0.47

Dynamic contact stress analysis using a compliant sensor array

. These results provided a way to build a failure envelope for investigated material as well as to verify failure criteria. The proposed method of inducing bidimensional state of stress may be used for a wide range of other materials as well as FRM.