Showing papers on "Stiffness published in 1994"
TL;DR: The homogenization sampling procedure consistently predicted the influence of trabecular bone architecture on effective stiffness and can also provide trabECular tissue stress and strain estimates for arbitrary global loading of whole bones.
342 citations
TL;DR: In this paper, an analytical model is developed to describe the effective elastic properties of a cemented granular material that is modeled as a random packing of identical spheres, and the solution reveals a peculiar distribution pattern of normal and shear stresses at the cemented grain contacts.
335 citations
TL;DR: In this article, the spring constants of a V-shaped atomic force microscopy cantilever beam are given in terms of the beam geometry and material properties, and the normal and longitudinal stiffness are obtained from a few simple equations.
Abstract: For a V‐shaped atomic force microscopy cantilever beam, the spring constants in the three principal directions are given in terms of the beam geometry and material properties. For the lateral stiffness, a closed‐formed expression is presented. Also, the normal and the longitudinal stiffness are obtained from a few simple equations. The results are compared with a finite element study and found to be very accurate. All spring constants depend strongly on the cantilever thickness, which is difficult to measure. In addition, the lateral and longitudinal stiffness are sensitive to the location and the height of the attached pyramid.
318 citations
TL;DR: Regression equations were formulated to define stiffness and energy stored for input to biomechanical models that examine low back function and for bioengineers designing hardware for stabilization and bracing or investigation of traumatic events such as automobile collision.
Abstract: This work investigated the passive bending properties of the intact human torso about its three principal axes of flexion: extension, lateral bending, and axial rotation. Additionally, the effects of wearing an abdominal belt and holding the breath (full inhalation) on trunk stiffness was investigated. The torsos of 22 males and 15 females were subjected to bending moments while "floating" in a frictionless jig with isolated torso bending measured with a magnetic device. Belts and breath holding appear to stiffen the torso about the lateral bending and axial rotation axes but not in flexion or extension. Torsos are stiffer in lateral bending and capable of storing greater elastic energy. Regression equations were formulated to define stiffness and energy stored for input to biomechanical models that examine low back function and for bioengineers designing hardware for stabilization and bracing or investigation of traumatic events such as automobile collision.
197 citations
TL;DR: In this article, a method was developed for the estimation of resilient modulus of compacted fine-grained subgrade soils, which takes into account the influence of soil physical state, stress state, and soil type.
Abstract: A method has been developed for the estimation of resilient modulus of compacted fine-grained subgrade soils. The method takes into account the influence of soil physical state, stress state, and soil type. The effect of soil physical state is quantified by combinations of two equations relating resilient modulus to moisture content. One equation is for paths of constant dry density and the other is for paths of constant compactive effort. The effect of stress state is determined by equations relating resilient modulus at optimum moisture content to deviator stress so that the equation parameters represent the effect of soil type and its structure. Means to estimate the resilient modulus at optimum moisture content are suggested in the absence of actual test data. Examples of applications of this method showed that it is simple and versatile and also gives consistency between predicted resilient modulus and resilient modulus test results.
196 citations
TL;DR: In this article, the properties of composite materials with periodic microstructure were analyzed using the Fourier series technique and assuming the homogenization eigenstrain to be piecewise constant, and the coefficients of the overall stiffness tensor of the composite material were expressed analytically in terms of the elastic properties of the constituents.
182 citations
TL;DR: In this paper, a unified theoretical framework for elastic stiffness degradation based on the concept of loading surface is presented, and the most significant scalar damage models found in the literature are reinterpreted in the context of this unified theory.
133 citations
TL;DR: In this paper, the bending and vibration response of thin and moderately thick laminated beams with ply damage or delaminations is studied using a new discrete-layer laminated beam finite element.
122 citations
Journal Article•
TL;DR: In this review the advantages and disadvantages of different variants of compression testing of trabecular bone are discussed and factors affecting the precision and the accuracy of mechanical properties of trABecularBone derived from such tests are analysed.
Abstract: In this review the advantages and disadvantages of different variants of compression testing of trabecular bone are discussed. Factors affecting the precision and the accuracy of mechanical properties of trabecular bone derived from such tests are analysed. Below are listed some of the important conclusions which can be drawn. Conclusions based on the author's previous studies (I-IX) are shown in italic. 1) Trabecular bone is a viscoelastic solid. 2) Stiffness, strength, ultimate strain, and failure energy are derived from a standard compression test to failure. Viscoelastic properties such as energy dissipation and the relative energy loss (loss tangent) can be obtained from non-destructive cyclic tests. 3) A non-destructive test conducted between a lower load level (zero strain) and an upper strain limit of about 0.8% specimen strain has been developed. The reproducibility of such a test technique has been assessed at different conditions. The reproducibility was best after a number of conditioning cycles in order to achieve a viscoelastic steady state. Orthotropic properties can be determined by non-destructive testing in different directions of cubic specimens. The reproducibility of such testing has been established. 4) The stiffness derived from non-destructive tests will be lower than that obtained from a destructive test because of the non-linearity of the load-deformation curve, but the stiffnesses will be strongly correlated. 5) Stiffnesses derived from destructive and non-destructive tests have an elastic and a viscoelastic contribution. Since the viscoelastic contribution is time dependent, the results will be dependent on strain rate and loading frequency in cyclic tests. 6) Standard testing of small trabecular bone specimens is associated with systematic errors. The most significant of these errors are believed to be related to trabecular disintegrity at the surface of the specimen and to friction at the specimen-platen interface. Structural disintegrity causes an axial strain inhomogeneity resulting in a overestimation of axial strain and a corresponding underestimation of specimen stiffness. Friction at the interface causes an uneven stress and strain distribution in the layer nearest to the test platen resulting in a overestimation of stiffness. The net result of these systematic errors is a 20-40 per cent underestimation of stiffness. 7) The specimen geometry has a highly significant influence on mechanical properties such as stiffness, ultimate strain and energy absorption. A cube with a side length of 6.5 mm and a cylindrical specimen with a length of 6.5 mm and a diameter of 7.5 mm are suggested as standard geometries providing comparable results.
121 citations
TL;DR: In this article, a computational scheme for the integration of rate-dependent constitutive relations for the high strain-rate finite inelastic deformation and failure of metallic materials is introduced.
117 citations
TL;DR: Improved classical laminate theory (CLT) models are developed for plain weave, 5-, and 8-harness satin weave composites in this article, which can be used to predict and rank the performance of woven fabric composites starting from the properties of the constituents.
Abstract: Improved classical laminate theory (CLT) models are developed for plain weave, 5-, and 8-harness satin weave composites. These models retain the simplicity of the fiber undulation model and the bridging model developed by Chou and Ishikawa but do not have the assumptions made to simplify the analysis. Closed-form expressions are derived for the CLT matrices for the woven fabric composites. These expressions are then used to predict moduli, Poisson's ratios, and the coefficients of thermal expansion (CTE) using the properties of the constituents. The results from the present CLT model are compared to the results from the fabric analysis method, three-dimensional (3D) finite element mosaic models, and other CLT models. The present model results for moduli and Poisson's ratio are, in general, in good agreement with those of other models and the very limited experimental results available in the literature. However, the comparisons for the thermal coefficients of expansion are not satisfactory. The models presented in this paper can be used to predict and rank the performance of woven fabric composites starting from the properties of the constituents.
TL;DR: In this article, a micromechanistic deformation model is proposed where loading direction and weave parameters are allowed to vary and the model provides improved estimates for the linear elastic stiffness.
Abstract: A micromechanistic deformation model, that could realistically be incorporated into structural finite element codes, is proposed where loading direction and weave parameters are allowed to vary. Comparisons are made to previous models and experimental results for woven materials, indicating that the proposed model provides improved estimates for the linear elastic stiffness. The model further provides predictions for internal stresses in the longitudinal, transverse, and interlace regions of the woven laminate which qualitatively correspond to the experimentally observed failure mechanisms
TL;DR: Using a single series elastic element in muscle modelling is not appropriate to describe muscle behaviour under all conditions that occur during in vivo activation, and the series elastic component of skeletal-muscle-tendon complex is probably located in more than two morphologically identifiable elements.
Journal Article•
TL;DR: In this paper, the relationship between the track modulus and track performance is discussed, and the effects of superstructure and substructure factors on track performance are illustrated and the means of altering track moduli are suggested.
Abstract: Track modulus is a measure of the vertical stiffness of the rail foundation. Another parameter, track stiffness, is a measure of the vertical stiffness of the entire track structure. Both are related to the track performance. In order to provide a basis for assessing and modifying track performance, the definitions of track modulus and track stiffness are reviewed, and four methods of determining track modulus or track stiffness are discussed. On the basis of analysis with a track structure model, the effects of superstructure and substructure factors influencing track modulus are illustrated and the means of altering track modulus are suggested. Finally, the relationship between the track modulus and track performance is discussed. The subgrade soil conditions are shown to have the greatest influence on track modulus and stiffness. Next in importance are the combined ballast-subballast thickness and the vertical tie-fastener stiffness.
TL;DR: In this paper, a method for predicting the elastic properties of textile-reinforced composites is presented with applications, which is a modification of a Fabric Geometry Model (FGM).
Abstract: A method for predicting the elastic properties of textile-reinforced composites is presented with applications. The method is a modification of a Fabric Geometry Model (FGM) [1–3] that relates fiber architecture and material properties of textile-reinforced composites to its global stiffness matrix through micromechanics and stiffness averaging technique. The FGM, although proven to be a quick and successful method [4], suffers two major drawbacks: 1. incompatibility of the basic transverse isotropy assumption with the theoretical mathematical derivation, (i.e., the mathematical derivation produces elastic constants that do not exhibit transverse isotropy) and 2. inconsistency of the transformation matrices associated with the stiffness calculations (i.e., the technique is not sufficiently robust to handle all cases). In this paper, these problems are discussed and solutions are presented. Comparison between stiffness and compliance averaging approaches is investigated. Moreover, predictions using the Self-Consistent FGM are compared with experimental data available in literature.
TL;DR: In this article, the authors deal with determination of macroscopic fracture characteristics of random particle systems, which represents a fundamental but little explored problem of micromechanics of quasibrittle materials.
Abstract: This paper deals with determination of macroscopic fracture characteristics of random particle systems, which represents a fundamental but little explored problem of micromechanics of quasibrittle materials. The particle locations are randomly generated and the mechanical properties are characterized by a triangular softening force-displacement diagram for the interparticle links. An efficient algorithm, which is used to repetitively solve large systems, is developed. This algorithm is based on the replacement of stiffness changes by inelastic forces applied as external loads. It makes it possible to calculate the exact displacement increments in each step without iterations and using only the elastic stiffness matrix. The size effect method is used to determine the dependence of the mean macroscopic fracture energy and the mean effective process zone size of two-dimensional particle systems on the basic microscopic characteristics such as the microscopic fracture energy, the dominant inhomogeneity spacing (particle size) and the coefficients of variation of the microstrength and the microductility. Some general trends are revealed and discussed.
TL;DR: In this article, a finite strip method for the elastic analysis of anisotropic laminated composite plates is developed according to higher-order shear deformation theory, which accounts for the parabolic distribution of the transverse shear strains through the thickness of the plate.
TL;DR: In this paper, the structural stiffness and damping coefficients of the bump foil strip in a journal bearing or damper are predicted based on the perturbation of the journal center with respect to its static equilibrium position.
Abstract: Compliant foil bearings operate on either gas or liquid, which makes them very attractive for use in extreme environments such as in high-temperature aircraft turbine engines and cryogenic turbopumps. However, a lack of analytical models to predict the dynamic characteristics of foil bearings forces the bearing designer to rely on prototype testing, which is time-consuming and expensive. In this paper, the authors present a theoretical model to predict the structural stiffness and damping coefficients of the bump foil strip in a journal bearing or damper. Stiffness is calculated based on the perturbation of the journal center with respect to its static equilibrium position. The equivalent viscous damping coefficients are determined based on the area of a closed hysteresis loop of the journal center motion. The authors found, theoretically, that the energy dissipated from this loop was mostly contributed by the frictional motion between contact surfaces. In addition, the source and mechanism of the nonline...
TL;DR: In this article, the authors compared different models based on variational analysis with respect to the thermo-elastic properties of cross-ply laminates with transverse cracks in the 90°-layer.
Abstract: Different models based on variational analysis are compared with respect to the thermo-elastic properties of cross-ply laminates with transverse cracks in the 90°-layer. This approach, pioneered by Hashin, applies the principle of minimum complementary energy to describe the stress state in a cross-ply laminate with transverse cracks. Apart from material data, only laminate geometry data and stress distribution assumptions are required. The model developed in the present study contains stress distribution assumptions that are closer to reality than previous models. Non-uniform stress distributions are included through-the-thickness of both the 0°- and the 90°-layers. An importan result is the improved agreement with stiffness reduction data. As expected from the nature of the model, predictions generally provide lower bounds to the stifness of the cracked laminate. However, at high crack densities, this is not the case. This additional stiffness reduction at high crack densities indicates the presence of new damage types, such as branched cracks and local delamination.
TL;DR: In this paper, two types of axial loading devices for displacement-controlled compression tests on geomaterials (i.e., natural soils and rocks, and treated soils) in the laboratory are described.
Abstract: Two types of axial loading devices having high and low capacities of axial load, respectively, for use in displacement-controlled compression tests on geomaterials (i.e., natural soils and rocks, and treated soils) in the laboratory are described. They were designed so that the loading direction can be reversed without any noticeable time lag under a highly constant axial strain rate while using a gear system. Results of representative triaxial and plane strain compression tests on a sand, a sedimentary soft rock, and a clay are presented. These results were obtained from very slow monotonic (or one-way) loading tests with several unload/reload cycles measuring axial strains locally along the lateral surfaces of a specimen continuously for a strain range from the order of 0.0001% (10−6) to that at failure. It is demonstrated that initial stiffness at strains less than about 0.001% (10−5) during primary loading is the same as stiffness during small unload/reload cycles applied immediately after the start of loading. Initial deformation properties are rather linear and elastic.
TL;DR: In this paper, an upper limit of the transverse shear modulus across the corrugations is calculated assuming rigid facings, and the reduction of the effective shearing modulus due to deformations of the facings is quantified.
TL;DR: In this article, the authors presented a method to determine dynamic fracture toughness using a notched three-point bend specimen using a modified Hopkinson pressure bar, where two-point strain measurement has been used to evaluate force and displacement for times greater than the transit time for elastic waves in the Hopkinson bar.
Abstract: The paper presents a method to determine dynamic fracture toughness using a notched three-point bend specimen. With dynamic loading of a specimen there is a complex relation between the stress-intensity factor and the force applied to the specimen. This is due to effects of inertia, which have to be accounted for to evaluate a correct value of the stress-intensity factor. However, the stress-intensity factor is proportional to the load-point displacement if the fundamental mode of vibration is predominant in the specimen. The proportionality constant depends only on the geometry and stiffness of the specimen. In the present method we have measured the applied force and load-point displacement by a modified Hopkinson pressure bar, where two-point strain measurement has been used to evaluate force and displacement for times greater than the transit time for elastic waves in the Hopkinson bar. We have compared the method with the stress-intensity factor derived from strain measurement near the notch tip and good agreement was obtained. The method is well suited for high-temperature testing and results from fracture toughness tests of brittle materials at ambient and elevated temperatures are presented.
TL;DR: In this paper, a coupled tendon-driven robot hand with stiffness control capability was designed and implemented on a laboratory apparatus, the POSTECH Hand II, and an active stiffness control algorithm was developed to control the fingertip force.
Abstract: This paper presents the methods of design and control for a coupled tendon-driven robot hand with stiffness control capability. By using the tendon as a force transmission mechanism, compact design of the robot hand is achieved with the developed controller. Problems specific to the tendon characteristics are considered, such as the slacking problem of tendons when the joint is disturbed, and the tendon elongation problem when the collocated position sensing method is used for compact design. To cope with these problems, two fundamental algorithms are developed and implemented on a laboratory apparatus, the POSTECH Hand II. First, a position estimation algorithm is developed to evaluate the accurate position of the hand, leading to an antagonistic tendon controller. Secondly, an active stiffness control algorithm is developed to control the fingertip force. It is shown that the finger produces excellent linear stiffness characteristics which justifies the effectiveness of the proposed algorithm. The object stiffness control is also implemented to exert desired force to the environment when the hand grasps an object, and is evaluated via experiments. >
Patent•
31 May 1994TL;DR: In this paper, a controlable composite structure or structural elements enclose magnetotheological fluids as a structural component between opposing containment layers to form at least a portion of any variety of extended mechanical systems, such as plates, panels, beams and bars or structures including these elements.
Abstract: Controllable composite structure or structural elements enclose magnetotheological fluids as a structural component between opposing containment layers to form at least a portion of any variety of extended mechanical systems, such as plates, panels, beams and bars or structures including these elements. The control of the stiffness and damping properties of the structure or structural elements is accomplished by changing the shear and compression/tension moduli of the magnetorheological fluid by varying the applied magnetic field. The composite structures of the present invention may be incorporated into a wide variety of mechanical systems for control of vibration and other properties.
TL;DR: A model of articular cartilage is developed in which the continuum stiffness tensor is related to the tissue's microstructure and predicts increased axial stiffness with increasing stretch due to fiber reorientation and reduced axial and shear stiffness with slipping between fiber and matrix.
TL;DR: In this article, a theoretical model was developed to calculate equivalent viscous damping coefficients and structural stiffness of a bump foil strip in a journal bearing or damper, and a computer program was also developed to compute the eccentricity and attitude angle of the journal static equilibrium position as well as the deflections, displacements, reacting forces, and equivalent friction coefficient of each bump on the strip.
Abstract: This paper presents the results of the second part of the investigation on structural stiffness and Coulomb damping in compliant foil journal bearings. In the first part, a theoretical model was developed to calculate equivalent viscous damping coefficients and structural stiffness of a bump foil strip in a journal bearing or damper. A computer program was also developed to compute the eccentricity and attitude angle of the journal static equilibrium position as well as the deflections, displacements, reacting forces, and equivalent friction coefficient of each bump on the strip. This model and program enabled further parametric studies to be conducted in the second part of the investigation, the results of which are the subject of this paper. The design parameters studied were static eccentricity (bearing load), pad angle (load angle), sliding friction coefficients, and perturbation amplitude (dynamic load). In addition, more effective methods of achieving both Coulomb damping and optimum structural stif...
TL;DR: In this article, the dynamic response of a beam with a single-sided crack subject to a moving load on the opposite side is analyzed using Euler beam theory and the assumed mode method.
Abstract: The dynamic response of a beam with a single-sided crack subject to a moving load on the opposite side is analyzed using Euler beam theory and the assumed mode method. The beam is modeled as two separate beams divided by the crack. Two different sets of admissible functions which satisfy the respective geometric boundary conditions are assumed for these two fictitious sub-beams. The rotational discontinuity at the crack is modeled by a torsional spring with an equivalent spring constant for the crack. The transverse deflection at the crack is matched by a linear spring of very large stiffness. Results of numerical simulations are presented for various combinations of constant axial velocity of the moving load and the crack size.
TL;DR: In this article, an alternative approach derived from an empirical method is then proposed for the determination of the effective period shift and equivalent damping ratio for an equivalent elastic system to the base-isolated bridge.
TL;DR: In this paper, the authors report simulations of two-dimensional fiber networks of random geometry and show that the initial loss of stiffness at small strain can be explained with an exponential frequency distribution of microscopic stresses, and the asymptotic stiffness at large external strain agrees with mean-field predictions.
Abstract: We report simulations of two‐dimensional fiber networks of random geometry. The stress distribution along a fiber agrees with the mean‐field Cox prediction, but the stress transfer factor is determined by the properties of the whole fiber and not by just the local segment stiffness as suggested by micromechanical models. This leads to a linear density dependence of the Young’s modulus of a network. The initial loss of stiffness at small strain can be explained with an exponential frequency distribution of microscopic stresses, and the asymptotic stiffness at large external strain agrees with mean‐field predictions. The simulated behavior is independent of the microscopic fracture mechanism in both regions.