适应极高静水压力的软体智能材料与软体机器人

实验表明由软硬融合方式构造机电系统、力电耦合软体智能材料驱动的软体机器人无需耐压外壳即可适应极高静水压力(110 MPa)并实现驱动.      

Using strain gages to measure both strain and temperature

A transducer is proposed that measures both temperature and strain by using two different strain gages. The theoretical basis for designing such a transducer is developed. Experimental results indicate that the temperature signal can be measured satisfactorily. Paper was presented at the 1991 SEM Spring Conference on Experimental Mechanics held in Milwaukee, WI on June 10–13.     

Embedded piezoelectric sensors to measure peel stresses in adhesive joints

Although peel stresses are believed to be responsible for failure in many adhesive joint geometries, the measurement of these peel stresses has been elusive. In this work, embedded poly(vinylidene fluoride) piezoelectric sensors were used to measure peel stresses in adhesively bonded joints. Piezoelectric KYNAR^® film was etched to produce multi-area stress sensors which were bonded into adhesive joints. Calibration results and results for single-lap and elastomeric butt joints are presented. The elastomeric butt joint was compared to an analytical solution for the bond-normal stresses, and the single-lap joint results were compared to finite-element analysis. Promising features and liminations of this technique are discussed.     

Two new optical techniques to measure strain

A grating engraved on the surface to be analyzed is utilized to measure strain. The grating is illuminated by a monochromatic, plane wavefront. A lens produces the diffraction spectrum of the grating, and an opaque screen is located in the plane of the spectrum. A window allows a single diffraction order to go through the screen. Two techniques are presented. In the point-by-point technique, the illuminating wavefront is reduced to a very small area. Behind the window a light sensor detects the changes of light intensity that are produced by loading the specimen. In the field technique, an image of the specimen is produced, and the light sensor is located in the image plane. In both cases, changes of light intensity are related to strains. Using crossed gratings, a rosette strain gage is obtained. Examples of application of both techniques are presented, showing a very good agreement between the strains measured by the proposed methods and by independent means.     

Microradiographic strain measurement using markers

A method for measuring strain on interior planes of some real structural materials is presented. X-ray images are formed of small (10–40 micron) gold markers placed on selected interior planes of optically opaque X-ray transparent materials. The use of well collimated monochromatic synchrotron radiation makes possible high contrast images of the small markers. Images of the particles before and after straining are recorded photographically. Photographs are enlarged 33X and measured using a simple electro-optical setup. In calibration experiments using approximately a 300-micron gage length, the strain measured by this method agreed with extensometer measured strains to within 100 microstrain. Example applications in a graphite-epoxy composite are presented, including measurement of the strain drop off near the free edge, strain concentration around a hole, and the strain field on a particular plane near a hole after local delamination. The technique is currently limited to materials no less X-ray transparent than titanium.Paper was presented at the 1992 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 8–11.     

The prediction of time-dependent non-linear stresses in viscoelastic materials: I. Selection of constitutive equation and strain measure

The theories for the prediction of time-dependent, non-linear stresses in viscoelastic materials such as polymers are reviewed, and it is noted that the commonly observed stress non-linearity may be ascribed either, as is usually done, to memory-function non-linearity or, alternatively, to strain-measure non-linearity. To investigate the latter alternative whilst retaining a general memory-function non-linearity, a single-integral constitutive equation of the Bird—Carreau type is employed but with an arbitrary strain measure I in place of the normally employed Finger tensor F. This model includes as special cases a large proportion of the constitutive equations previously employed for predictive purposes and in particular with a linear memory function it is shown to be indistinguishable, with the normally conducted shear experiments, from the successful BKZ model.In the new model the shear component I₁₂ of the strain measure can be found from experimental results obtained in the startup of steady shear flow, without specification or restriction of memory-function non-linearity. The form of I₁₂ found from experiment is quite non-linear in shear a for ¦a¦> 2, and hence differs from the F tensor for which F₁₂ = a. The same form for I₁₂ found for a variety of polymer solutions and a polymer melt and consequently a simple function describing I₁₂ is proposed as a new, material-independent, strain measure.     

ANCF curvature continuity: application to soft and fluid materials

The continuity of the position-vector gradients at the nodal points of a finite element mesh does not always ensure the continuity of the gradients at the element interfaces. Discontinuity of the gradients at the interface not only adversely affects the quality of the simulation results, but can also lead to computer models that do not properly represent realistic physical system behaviors, particularly in the case of soft and fluid material applications. In this study, the absolute nodal coordinate formulation (ANCF) finite elements are used to define general curvature-continuity conditions that allow for eliminating or minimizing the discontinuity of the position gradients at the element interface. For the ANCF solid element, with four-node surfaces, it is shown that continuity of the gradients tangent to an arbitrary point on a surface is ensured as the result of the continuity of the gradients at the nodal points. The general ANCF continuity conditions are applicable to both reference-configuration straight and curved geometries. These conditions are formulated without the need for using the computer-aided-design knot vector and knot multiplicity, which do not account properly for the concept of system degrees of freedom. The ANCF curvature-continuity conditions are written in terms of constant geometric coefficients obtained using the matrix of position-vector gradients that defines the reference-configuration geometry. The formulation of these conditions is demonstrated using the ANCF fully parameterized three-dimensional solid and tetrahedral elements, which employ a complete set of position gradients as nodal coordinates. Numerical results are presented in order to examine the effect of applying the curvature-continuity conditions on achieving a higher degree of smoothness at the element interfaces in the case of soft and fluid materials.

     

Macroscopic strain potentials in nonlinear porous materials

By taking a hollow sphere as a representative volume element (RVE), the macroscopic strain potentials of porous materials with power-law incompressible matrix are studied in this paper. According to the principles of the minimum potential energy in nonlinear elasticity and the variational procedure, static admissible stress fields and kinematic admissible displacement fields are constructed, and hence the upper and the lower bounds of the macroscopic strain potential are obtained. The bounds given in the present paper differ so slightly that they both provide perfect approximations of the exact strain potential of the studied porous materials. It is also found that the upper bound proposed by previous authors is much higher than the present one, and the lower bounds given by Cocks is much lower. Moreover, the present calculation is also compared with the variational lower bound of Ponte Castañeda for statistically isotropic porous materials. Finally, the validity of the hollow spherical RVE for the studied nonlinear porous material is discussed by the difference between the present numerical results and the Cocks bound.     

Instrumentation to control strain rate for materials undergoing plastic flow

An instrument is described which controls the head velocity and, therefore, the strain rate of specimens tested in electromechanical universal testing machines. Within the available speed limits of these machines, a predetermined strain rate is achieved by automatic adjustment of a variable resistance in the speed-control circuit. This adjustment is achieved by coupling a potentiometer to the crosshead movement with a cam. An exact mathematical solution is derived for general cam profiles which give either nondecreasing or nonincreasing strain rates. Numerical results are presented for cams which achieve constant true strain rate in tensile and compression testing. For these cases, instrument calibration curves and tensile stress-strain curves for iron at room temperature are presented.     

Magnetoelastic fracture of soft ferromagnetic materials

Effects of magnetic field on fracture toughness of soft ferromagnetic materials were studied using experimental techniques and theoretical models. The manganese–zinc ferrite with a single-edge-notch-beam (SENB) were chosen to be the specimen and the Vickers’ indentation specimen subjected to a magnetic field were chosen to be the specimens. Results indicate that there is no significant variations of the measured fracture toughness of the manganese–zinc ferrite ceramic in the presence of the magnetic field. The theoretical model involves an anti-plane shear crack with finite length in an infinite magnetostrictive body where an in-plane magnetic field prevails at infinity. Magnetoelasticity is used. The crack-tip elastic field is different from that of the classical mode III fracture problem. Furthermore, the magnetoelastic fracture of the soft ferromagnetic material was studied by solving the stress field for a soft ferromagnetic plane with a center-through elliptical crack. The stress field at the tip of a slender elliptical crack is obtained for which only external magnetic field normal to the major axis of the ellipse is applied at infinity. The results indicate that the near field stresses are governed by the magnetostriction and permeability of the soft ferromagnetic material. The induction magnetostrictive modulus is a key parameter for finding whether magnetostriction or magnetic-force-induced deformation is dominant near the front an elliptically-shaped crack. The influence of the magnetic field on the apparent toughness of a soft ferromagnetic material with a crack-like flaw can be regarded approximately in two ways: one possesses a large induction magnetostrictive modulus and the other has a small modulus. Finally, a small-scale magnetic-yielding model was developed on the basis of linear magnetization to interpret the experimental results related to the fracture of the manganese–zinc ferrite ceramics under magnetic field. Studied also is the fracture test of the soft ferromagnetic steel with compact tension specimens published in the existing literature.     

Acoustomechanical constitutive theory for soft materials

Acoustic wave propagation from surrounding medium into a soft material can generate acoustic radiation stress due to acoustic momentum transfer inside the medium and material, as well as at the interface between the two. To analyze acoustic-induced deformation of soft materials, we establish an acoustomechanical constitutive theory by com-bining the acoustic radiation stress theory and the nonlinear elasticity theory for soft materials. The acoustic radiation stress tensor is formulated by time averaging the momen-tum equation of particle motion, which is then introduced into the nonlinear elasticity constitutive relation to construct the acoustomechanical constitutive theory for soft materials. Considering a specified case of soft material sheet subjected to two counter-propagating acoustic waves, we demonstrate the nonlinear large deformation of the soft material and ana-lyze the interaction between acoustic waves and material deformation under the conditions of total reflection, acoustic transparency, and acoustic mismatch.     

Plastic shearing of materials exhibiting strain hardening or strain softening

We consider the plastic shearing of a strain-rate dependent material exhibiting strain hardening or strain softening, subjected to steady shearing. We establish the existence of classical solutions and study the stability of uniform shearing. For materials exhibiting strain hardening or a moderate degree of strain softening we show that, as t , every solution approaches, at specific rates of convergence, uniform shearing; thus shear bands do not form.     

An investigation of the use of strain gages to measure welding-induced residual stresses

The measurement of weld-induced residual stress is important in structures that are subjected to cyclic loading during their service life. Depending on their magnitude, stresses can influence the rate of crack growth under cyclic loading and hence affect the life of the structure. Because the level of residual stress may change during service, measurement of these changes is necessary for accurate life prediction of the structures. The measurement of welding-induced residual stress using strain gages poses significant problems, the most important being the potential damage to the gages by high temperatures generated in the welding process. This laboratory study was undertaken to assess the suitability and signal stability of commercially available resistive strain gages for the measurement of postweld residual stresses in a submarine hull structure. Adhesively bonded and weldable-type strain gages were attached to the surface of a 35 mm thick steel plate, which was then subjected to thermal cycles similar to those encountered during welding construction of a submarine pressure hull. This paper describes the strain gage application procedure, changes in the strain gage output at end of each experimental stage and the history of changes in the residual stress.     

Preface: Mechanical characterization of soft materials

正Determining the mechanical properties of soft materials(e.g.,biological soft tissues,polymeric gels,and soft elastomers)across multiple length scales represents an activity of increasing importance in understanding the deformation behavior of soft materials under various stimuli.Much effort has been made in the past in this field.This theme issue of the Acta Mechanica Sinica contains a number of innovative papers regarding the mechanical characterization of soft materials from five diferent groups.