Characterization of Soft-Tissue Material Properties: Large Deformation Analysis

Stress analysis of thick, curved composite laminates

A new assumed strain quadrilateral element with highly accurate in‐plane bending behaviour is presented for plane stress and plane strain analysis. The basic idea of the formulation consists in identification of various modes of deformation and then in proper modification of the strain field in some of these modes. In particular, the strain operator corresponding to the in‐plane bending modes is modified to simulate the strain field resulting from the assumptions usually made in structural mechanics. The modification of the strain field leads to the assumed strain operator on the element level. As a result, the so‐called shear and membrane locking phenomena are alleviated. The element exhibits remarkable success in bending‐ dominated problems even when severely distorted and high aspect ratio meshes are used. Another advantage of the present assumed strain element is that locking for nearly incompressible materials is also mitigated. While this assumed strain element passes the patch test only for the parallelogram shapes, the element provides convergent solutions as long as the initially general form of the element approaches a parallelogram shape with the refinement of the mesh.

: The Olmsted Locks and Dam will be located on the Ohio River and the locks will be a W-frame type of structure, which is currently unprecedented within the Corps of Engineers. Because of this fact and due to the high fly ash concrete mixture planned for the project, and extensive nonlinear, incremental structural analysis (NISA) was performed. Parameters evaluated included two different placing schemes, plane stress and plane strain analyses, two different concrete mixtures, creep, and shrinkage. The report contains two phases. In the first phase, parameters such as plane stress and plane strain, block and strip placement schemes, and mixtures 6 and 11 were evaluated. The second phase included performance of several analyses based on the load case combinations contained in ETL 1110-2-324, 'Special Design Provisions for Massive Concrete Structures', and included evaluation of the effects of creep and shrinkage. The second phase results were also used to validate the analyses performed in the first phase

The program is applicable to linear problems and has been developed as the foundation of a nonlinear finite-element program for the analysis of two- and three-dimensional structures. It was developed as part of an investigation of prestressed concrete reactor vessels. Hence it is oriented towards the analysis of reinforced and prestressed concrete structures. The operational part of the program can be used to obtain elastic stress and strain distributions in a two-dimensional continuum; namely plane stress, plane strain and axisymmetric stress conditions.

Large-Strain 3-D in Vivo Breast Ultrasound Strain Elastography Using a Multi-compression Strategy and a Whole-Breast Scanning System

State of knowledge in structural mechanics of penile erection, and some areas of ignorance

The accuracy of the rubber constitutive model characterizing experiment data has a crucial influence on the mechanical analysis of rubber structures. In this paper, a new improved hyperelastic constitutive model is proposed, and the model is derived into the stress-strain forms of uniaxial tension, equibiaxial tension and pure shear. Based on the experimental data of filled rubber, the material parameters of each deformation state are obtained by using the newly proposed rubber hyperelastic constitutive model, and the uniaxial tensile (UT), Equibiaxial tension (ET) and Pure shear (PS) specimens are simulated and calculated in the finite element software. the stress state of each finite element specimen is analyzed and the obtained simulation data are compared with the experimental data. It is found that the new model can accurately characterize the hyperelastic mechanical properties of the experimental specimens in different deformation states. At the same time, the reasons for the deviation from the experimental data in the process of plane tensile simulation are analyzed and explained comprehensively. The reliability and accuracy of the classical rubber constitutive relations of polynomial models and eight-chain model are studied. the results show that different hyperelastic models have different ability to describe the hyperelastic behavior in different deformation states. the hyperelastic constitutive model proposed in this paper can be easily embedded into finite element software and has the advantages of accurate results, few material parameters and simple testing.

Several authors have proposed different parameters to include the softening effect in hyperelastic models; however, for a number of materials, softening parameters could be further improved. This article proposes a new softening parameter to include Mullins effect in hyperelastic material models. The methodology employed can be also used in cases with hysteresis or damage in a hyperelastic material, however this methodology modifies the behavior of the material differently from damage theories. Common hyperelastic constitutive models do not include dissipation effects and so the present work intends to fill this gap. Experimental data for silicone in uniaxial tensile test, equibiaxial, and pure shear tests were modeled in order to calibrate the models. The softening parameters essentially changes the constitutive law from the loading to the unloading path. Therefore, it is still necessary to use a hyperelastic model, and here Ogden and Hoss-Marczak material models were used. The obtained results show good agreement with experimental data even when simulating with a compressible finite element code and it can model isotropic Mullins effect.

A plane stress model has been developed using one-dimensional elements for which a stiffness matrix can be generated having a relatively narrow band of nonzero terms. A digital computer program has been developed for this type of matrix for which the computing times are short enough and the round-off errors are small enough that solutions can be obtained for systems having several thousand degrees of freedom. This program will handle problems in plane stress analysis for any boundary conditions or thermal effects and any static loading conditions. The complete solution includes the displacements and the state of stress at all nodes of the model. The program will also handle framed structures with nonprismatic members. The results of several problems in plane stress analysis, including plates that contain holes and notches, are presented. Computing times are a matter of a few minutes per hundred equations using the IBM 7090 Data Processing System, and a few minutes per thousand equations using the IBM 7094 Data Processing System.

The general deformation problem with material and geometric non-linearities is typically divided into a number of subproblems including the kinematic, the constitutive, and the contact=friction subproblems. These problems are introduced for algorithmic purposes; however, each of them represents distinct physical aspects of the deformation process. For each of these subproblems, several well-established mathematical and numerical models based on the 9nite element method have been proposed for their solution. Recent developments in software engineering and in the 9eld of object-oriented C++ programming have made it possible to model physical processes and mechanisms more expressively than ever before. In particular, the various subproblems and computational models in a large inelastic deformation analysis can be implemented using appropriate hierarchies of classes that accurately represent their underlying physical, mathematical and=or geometric structures. This paper addresses such issues and demonstrates that an approach to deformation processing using classes, inheritance and virtual functions allows a very fast and robust implementation and testing of various physical processes and computational algorithms. Here, speci9c ideas are provided for the development of an objectoriented C++ programming approach to the FEM analysis of large inelastic deformations. It is shown that the maintainability, generality, expandability, and code re-usability of such FEM codes are highly improved. Finally, the e=ciency and accuracy of an object-oriented programming approach to the analysis of large inelastic deformations are investigated using a number of benchmark metal-forming examples. Copyright ? 1999 John Wiley & Sons, Ltd.

Micromechanics of brain white matter tissue: A fiber-reinforced hyperelastic model using embedded element technique

Many genetic mutations adversely affect the structure and function of load-bearing soft tissues, with clinical sequelae often responsible for disability or death. Parallel advances in genetics and histomechanical characterization provide significant insight into these conditions, but there remains a pressing need to integrate such information. We present a novel genotype-to-biomechanical phenotype neural network (G2Φnet) for characterizing and classifying biomechanical properties of soft tissues, which serve as important functional readouts of tissue health or disease. We illustrate the utility of our approach by inferring the nonlinear, genotype-dependent constitutive behavior of the aorta for four mouse models involving defects or deficiencies in extracellular constituents. We show that G2Φnet can infer the biomechanical response while simultaneously ascribing the associated genotype by utilizing limited, noisy, and unstructured experimental data. More broadly, G2Φnet provides a powerful method and a paradigm shift for correlating genotype and biomechanical phenotype quantitatively, promising a better understanding of their interplay in biological tissues.

Ductile fracture theories for pressurised pipes and containers

Residual stress in coatings has significant effect on their performance. In cold-sprayed coatings, in which particles impact the substrate at high velocity in solid state, in-plane residual stresses are usually conceived to be compressive. In this research, analysis of residual stresses in cold-sprayed deposits is performed by analytical and numerical modeling. The influence of various parameters such as the dimensions and elastic properties of the coating and the substrate on the residual stress are analyzed. In addition, the amount of heat input as a key parameter in the build-up of the residual stress is examined. It has been found that the heat input and the associated thermal history have a major influence on the final distortion and the residual stress, to an extent that the in-plane stress can in some cases change from compressive to tensile. Based on these results, a simple model is put forward for the prediction of the final state of the stress and distortion in cold-sprayed flat components.

We characterize the material properties of a woven, multi-layered, hyperelastic composite that is useful as an envelope material for high-altitude stratospheric airships and in the design of other large structures. The composite was fabricated by sandwiching a polyaramid Nomex® core, with good tensile strength, between polyimide Kapton® films with high dielectric constant, and cured with epoxy using a vacuum bagging technique. Uniaxial mechanical tests were used to stretch the individual materials and the composite to failure in the longitudinal and transverse directions respectively. The experimental data for Kapton® were fit to a five-parameter Yeoh form of nonlinear, hyperelastic and isotropic constitutive model. Image analysis of the Nomex® sheets, obtained using scanning electron microscopy, demonstrate two families of symmetrically oriented fibers at 69.3°± 7.4° and 129°± 5.3°. Stress-strain results for Nomex® were fit to a nonlinear and orthotropic Holzapfel-Gasser-Ogden (HGO) hyperelastic model with two fiber families. We used a linear decomposition of the strain energy function for the composite, based on the individual strain energy functions for Kapton® and Nomex®, obtained using experimental results. A rule of mixtures approach, using volume fractions of individual constituents present in the composite during specimen fabrication, was used to formulate the strain energy function for the composite. Model results for the composite were in good agreement with experimental stress-strain data. Constitutive properties for woven composite materials, combining nonlinear elastic properties within a composite materials framework, are required in the design of laminated pretensioned structures for civil engineering and in aerospace applications.