Time-dependent Deformation Response Research Articles

An experimentally validated anisotropic visco-hyperelastic constitutive model for knitted fabric-reinforced elastomer composites

A new anisotropic visco-hyperelastic constitutive model has been developed to predict the time-dependent deformation response of flexible, knitted fabric-reinforced elastomer composites. These composites exhibit a time-dependent response due to the viscoelastic behavior of the elastomer, while the anisotropic reinforcement effect is attributed to the knitted fabrics. The proposed modeling approach features an invariant-based strain energy density function that incorporates the architecture of knitted fabrics and the hyper-viscoelastic behavior of elastomers. In this study, the elastomer is considered as a viscous neo-Hookean material, and the knitted fabrics are modeled as cords with negligible stiffness in bending. The proposed material model is implemented in commercial Finite Element Analysis (FEA) software, Abaqus (version 2017), through a user-defined material subroutine, UMAT. This approach enables the determination of the constitutive behavior of the composite based on the constituent elastomer, fiber properties, and fabric architectures. The proposed model is verified through a discrete 3D FEA model, in which the elastomer and fabric reinforcement are modeled explicitly. The approach is further validated through physical testing including uniaxial tension, torsion, and three-point bending. Since the proposed strain energy-based constitutive model can predict the deformation response of knitted fabric-reinforced elastomer composites based on the fabric architecture, it can be employed as an efficient modeling tool to aid the design of knitting pattern for target mechanical properties.

Anizotropija deformacijskih svojstava meke stijene

Bedding, being a typical feature of sedimentary rocks, is a cause of inherent anisotropy of deformation properties of rock mass. Results of uniaxial and triaxial, short-term and creep tests conducted by the authors on the specimens of a soft sedimentary rock - marl are presented in this paper. Interpolation of results of the short-term tests enabled for determining the parabolic dependence of secant modulus of deformability on angle (a) between load and natural bedding planes in the interval from 0 to 90 degrees. The time-dependent deformation response is described by determining the dependence of parameters of the modified Wallner’s model on the angle a in the interval from 45 to 90 degrees, based on the results of the performed creep tests.

Deformation behaviour of aged coronal dentin.

This study aimed to identify the changes in the time-dependent deformation response of coronal dentin with ageing and its relationship with changes in chemical composition. The structural behaviour of dentin with ageing is affected by changes in the density and diameter of its dentinal tubules (ie porosity), as well as changes in chemical composition throughout the tooth. However, little is known about the time-dependent deformation behaviour of aged dentin and the importance of its hierarchical structure and variations in chemical composition. The spherical indentation response of aged coronal dentin was analysed in the outer, middle and inner regions, and its time-dependent deformation response was modelled in terms of its microstructure and chemical composition using a model recently proposed for young dentin. The viscous deformation behaviour of aged dentin followed a power-law response with a decrease in the stress exponent when compared to young dentin. These results can be explained by cross-linking of the collagen present in the tissue. A decrease in the deformation ability of aged dentin was found. This behaviour could be a result of a dissolution process and reprecipitation of the minerals present in intertubular dentin into the dentinal tubules.

A Theoretical Characterization of Time Dependent Materials by Using a Hyperlogistic-Type Model

In this work, the classical mechanical Voigt model is modified and extended to finite deformations by using a rational elastic spring force function to describe accurately the nonlinear time-dependent deformation response of some viscoelastic materials. As theoretical results, a hyperlogistic-type function has been found as the deformation versus time relationship. This growth model appeared powerful to reproduce mathematically as shown by numerical works, any S-shaped experimental data. Compared with some previous models, the present one-dimensional formulation gives the advantage to assure or to control via an explicit material parameter, to speak, via the coefficient of inertia, the nonlinearity of the model. The proposed model demonstrated then the importance to consider in the material modeling the inertial coefficient.

Time-dependent failure of amorphous poly-d,l-lactide: Influence of molecular weight

The specific time-dependent deformation response of amorphous poly(lactic acid) (PLA) is known to lead to rapid failure of these materials in load-bearing situations. We have investigated this phenomenon in uniaxial compression on P(L)DLLA samples with various molecular weights. The experiments revealed a strong dependence of the yield stress on the applied strain rate. Lower molecular weights showed identical deformation kinetics as higher molecular weights, albeit at lower stress values. This dependence on molecular weight was incorporated into an Eyring-equation by introducing mobility through a virtual temperature that is shifted by the deviation of the Tg from Tg,∞. Stress-dependent lifetime of polymer constructs was described by the use of this modified Eyring-equation, combined with a critical plastic strain. This model proves useful in predicting the molecular weight dependence of the time to failure, although it slightly overestimates life time at low stress levels for a material with very low molecular weight. The versatility of the model is demonstrated on e-beam sterilized PLDLLA, where the resulting reduction in molecular weight induces a substantial decrease in lifetime. A single Tg measurement provides sufficient information to predict the decrease in lifetime.

Dynamic pressurization method for measuring permeability and modulus: II. cementitious materials

In a companion paper, the theory behind a novel method for measuring permeability and bulk modulus is derived. The new method is termed ‘dynamic pressurization’ (DP). In this paper, an apparatus for the new method is described, and the test procedure is applied to cement pastes and concretes. The test involves rapidly applying a hydrostatic pressure to a saturated cylinder, then measuring the time-dependent deformation response of the material. The test is found to be repeatable, and the fitting function described in the companion paper is found to adequately fit the measured data. The measured permeability of the cement pastes compares well to measured values on identical materials using a different technique. The measured bulk moduli of the solid phase of the hardened cement pastes and concretes obtained in the DP test are in the range of expected values based on literature values for the Young’s moduli of clinker and hydration products.