Thermal Simulation of an Arbitrary Residual Stress Field in a Fully or Partially Autofrettaged Thick-Walled Spherical Pressure Vessel

The equivalent thermal load was previously shown to be the only feasible method by which the residual stresses due to autofrettage and its redistribution, as a result of cracking, can be implemented in a finite element analysis, of a fully or partially autofrettaged thick-walled cylindrical pressure vessel. The present analysis involves developing a similar methodology for treating an autofrettaged thick-walled spherical pressure vessel. A general procedure for evaluating the equivalent temperature loading for simulating an arbitrary, analytical or numerical, spherosymmetric autofrettage residual stress field in a spherical pressure vessel is developed. Once presented, the algorithm is applied to two distinct cases. In the first case, an analytical expression for the equivalent thermal loading is obtained for the ideal autofrettage stress field in a spherical shell. In the second case, the algorithm is applied to the discrete numerical values of a realistic autofrettage residual stress field incorporating the Bauschinger effect. As a result, a discrete equivalent temperature field is obtained. Furthermore, a finite element analysis is performed for each of the above cases, applying the respective temperature field to the spherical vessel. The induced stress fields are evaluated for each case and then compared to the original stress. The finite element results prove that the proposed procedure yields equivalent temperature fields that in turn simulate very accurately the residual stress fields for both the ideal and the realistic autofrettage cases.

Industrial Constraints Handling With a GA Approach in Multiobjective Optimization of a Composite Material

In this article, complexities related to the multicriteria (multiobjective) optimization of laminated composite structures subjected to technological constraints we will be presented. So, various technological constraints will be presented and a strategy of handling each constraint (in order to use the multiobjective optimization tools based on genetic algorithms) will be also introduced.

An Instrument That Simultaneously Measures Spatiotemporal Gait Parameters and Ground Reaction Forces of Locomoting Rats

We describe an instrument that assesses two features of the gait of rats, spatiotemporal paw movement variables (SPMV) and ground reaction forces (GRF) in the vertical direction. The GRF and the SPMV variables are measured electrically by eight single axis load-cells that support two floor plates. We can derive four gait parameters from the SPMV and GRF measured by the instrument: the stride length of individual limbs, the maximum and mean vertical ground forces, and the intensity of the vibrations created by each paw during locomotion. Measurements of the vertical GRF show errors of less than 3.5%; errors in the determination of the paw positions, used to derive stride lengths, are less than 9 mm. Here we report the stride length, maximum and mean GRF values, and the intensity of the floor vibrations of healthy adult mature rats. Our instrument is capable of evaluating changes in these gait parameters in rat models of injury and disease.

Noninvasive Electromagnetic Evaluation of Bjo¨rk-Shiley Heart Valves: Lifetime Prediction

Prosthetic heart valves of the Bjork-Shiley Convexo-Concave (BSCC) type have long been used extensively in implants; however, there have been reports of cases where one component of the valves failed, leading to the demise of the patient. This paper presents a new method for noninvasive electromagnetic evaluation for this type of valve, using an eddy current transducer with orthogonal coils. In vitro experiments have shown that discontinuities of outlet strut with depths equal or larger than 0.4mm can be detected with a probability of detection (POD) of 86.4%, and in the case of discontinuities with depth equal or larger than 0.6mm with POD of 97%.

Dynamic Vibration Absorber Optimal Design With Emphasis on Complete Machine Dynamics Modelling

The main aim of this paper is improved dynamic vibration absorbers design with taking into account complex rotating machines dynamic The is considered for the complex vibroexitated constructions. Methods of decomposition and the numerical schemes synthesis are considered on the basis of new methods of modal methods. Development of of complicated machines and buildings in view of their interaction with system of dynamic vibration absorbers is under discussion.

Framework for Flaw Detection: Application to Dynamic Crack Detection in Flat Membranes

A computational framework is developed for the detection of flaws in flexible structures. The framework is based on posing the detection problem as an inverse problem, which requires the solution of many forward problems. Each forward problem is associated with a known flaw; an appropriate cost functional evaluates the quality of each candidate flaw based on the solution of the corresponding forward problem. On the higher level, the inverse problem is solved by a global optimization algorithm. The performance of the computational framework is evaluated by considering the detectability of various types of flaws. In the present context detectability is defined by introducing a measure of the distance between the sought flaw and trial flaws in the space of the parameters characterizing the configuration of the flaw. The framework is applied to crack detection in flat membranes subjected to time-harmonic and transient excitations. The detectability of cracks is compared for these two cases.

Analytical Prediction of the Transition to Chaos in Lorenz System

The failure of the linear stability analysis to predict accurately the transition point from steady to chaotic solutions in Lorenz equations motivates this study. A weak non-linear solution to the problem is shown to produce an accurate analytical expression for the transition point as long as the validity condition and consequent accuracy of the latter solution is fulfilled. The analytical results are compared to accurate computational solutions showing an excellent fit within the validity domain of the analytical solution.

Extracting the Elastic Modulus of Compliant Materials Using a Novel Plate Bulge Testing Technique

The mechanical properties of compliant materials such as biological tissues and biocompatible soft polymers are essential in medical research and engineering applications. These properties are often determined using techniques that require costly instrumentation (e.g. pull test machines). Alternative and more accessible methods can significantly aid the characterization process. The bulge test determines a material elastic modulus by analyzing the pressure-deflection response of thin samples made of this material. The technique has been extensively employed in the characterization of metals and semiconductors (modulus ∼ 100 GPa). By employing plate rather than membrane mechanics, the present study extends bulge testing to characterize materials with a modulus that is five orders of magnitude lower (∼ 1 MPa). The novel method is demonstrated analytically using plate theory, numerically using finite element modeling and experimentally by successfully applying it to polydimethylsiloxane (modulus ∼ 1.33 MPa). The introduced technique does not require costly equipment, is simple to implement and presents an appealing alternative to current characterization approaches.

Application of the Complex Envelope Vectorization to a Boundary Element Formulation

The complex envelope vectorization (CEV) is a recent method that has been successfully applied to structural and internal acoustic problems. Unlike other methods proposed in the last two decades to solve high frequency problems, CEV is not an energy method, although it shares with all the other techniques a variable transformation of the field variable. By such transformation involving a Hilbert transform, CEV allows the representation of a fast oscillating signal through a set of low oscillating signals. Thanks to such transformation it is possible to solve a high frequency dynamic problem at a computational cost that is lower than that required by finite elements. In fact, by using finite elements, a high frequency problem usually implies large matrices. On the contrary the CEV formulation is obtained by solving a set of linear problems of highly reduced dimensions. Although it was proved that CEV is in general a successful procedure, it was shown that it is particularly appropriate when the modes of the system have a negligible role on the solution. Moreover, the numerical advantage of the CEV formulation is much more pronounced when full matrices are used. Thus, for the first time it is applied to a boundary element formulation (BEM). Both external and internal acoustic fields of increasing complexity are considered: the internal and external field generated by a pulsating sphere; the external field of a forced box, where the velocity field is determined by finite elements; a set of 4 plates that form an open cavity. The results are compared with those obtained by a BEM procedure (SYSNOISE), highlighting the good quality of the proposed approach. An estimate of the computational advantage is also provided. Finally it is worthwhile to point out that the reduction of the BE matrices allows for an in-core solution even for large problems.

Mass Detection With Nonlinear Nanomechanical Resonators

Nanomechanical resonators having small mass, high resonance frequency and low damping rate are widely employed as mass detectors. We study the performance of such a detector when the resonator is driven into a region of nonlinear oscillations [1]. We predict theoretically that the mass sensitivity of the device in this region may exceed the upper bound imposed by thermo-mechanical noise upon the sensitivity when operating in the linear region. On the other hand, we find that the high mass sensitivity is accompanied by a slow response of the system to a change in the mass. For experimental demonstration we employ homodyne detection (see Fig. 1) for readout of the output signal of an optical displacement detector, which monitors the motion of a doubly clamped nanomechanical resonator made of Pd-Au [2, 3]. The nanomechanical resonator is driven into the region of nonlinear oscillations (see Fig. 2) and the region of bistability is identified (see Fig. 3). As expected theoretically [1] we find that when operating close to the edge of the bistability region the device exhibits strong intermodulation amplification [2] (see Fig. 3). Moreover, strong noise squeezing in the output signal of the homodyne detector is observed in this region [3] (see Fig. 4). An alternative mass detection scheme, in which the resonator is driven into a stochastic resonance, will also be discussed [4].