Simona Socrate

Constitutive model for the finite deformation stress–strain behavior of poly(ethylene terephthalate) above the glass transition

Abstract A constitutive model for the finite deformation stress–strain behavior of poly(ethylene terephthalate) (PET) at temperatures above the glass transition temperature is presented. In this temperature regime, the behavior of PET is strongly dependent on strain rate and temperature; PET also experiences strain-induced crystallization at these temperatures. The constitutive model accounts for the rate and temperature dependence of the stress–strain behavior by modeling the competition between molecular orientation processes and molecular relaxation processes. The model is fully three-dimensional and is shown to be in good agreement with experimental data over a wide range in strain rates and temperatures as well as under both uniaxial compression and plane strain compression loading conditions.

Changes in the biochemical constituents and morphologic appearance of the human cervical stroma during pregnancy

OBJECTIVE The cervix is the lower portion of the uterus. It is composed of fibrous tissue and its mechanical integrity is crucial for maintaining a healthy gestation. During normal pregnancy, the cervical extracellular matrix (ECM) remodels in preparation for labor. The objective of this study was to investigate the biochemical and morphological changes in cervical stroma associated with physiological remodeling during normal pregnancy. STUDY DESIGN Using human cervical tissue obtained from pregnant and non-pregnant patients, the ECM was analyzed for its biochemical constituents and histologic morphology. The ECM was assayed for hydration, collagen concentration, collagen solubility, total sulfated glycosaminoglycan concentration, and individual disaccharide concentration. The ECM morphology was visualized using conventional histological techniques (Massons trichrome stain, polarized light microscopy) as well as second harmonic generation (SHG) imaging. RESULTS When comparing pregnant to non-pregnant tissue, significant increases were measured for total sulfated glycosaminoglycans, hyaluronic acid, and collagen solubility. The microscopy studies confirmed that the collagenous network of the cervical stroma was anisotropic and pregnancy was associated with a discernable decrease in collagen organization. CONCLUSION Significant changes were seen in the concentration and organization of cervical ECM constituents during normal pregnancy.

Biomechanics of brain tissue

The dynamic behavior of porcine brain tissue, obtained from a series of in vitro observations and experiments, is analyzed and described here with the aid of a large strain, nonlinear, viscoelastic constitutive model. Mixed gray and white matter samples excised from the superior cortex were tested in unconfined uniaxial compression within 15h post mortem. The test sequence consisted of three successive load-unload segments at strain rates of 1, 0.1 and 0.01 s⁻¹, followed by stress relaxation (n=25). The volumetric compliance of the tissue was assessed for a subset of specimens (n=7) using video extensometry techniques. The tissue response exhibited moderate compressibility, substantial nonlinearity, hysteresis, conditioning and rate dependence. A large strain kinematics nonlinear viscoelastic model was developed to account for the essential features of the tissue response over the entire deformation history. The corresponding material parameters were obtained by fitting the model to the measured conditioned response (axial and volumetric) via a numerical optimization scheme. The model successfully captures the observed complexities of the material response in loading, unloading and relaxation over the entire range of strain rates. The accuracy of the model was further verified by comparing model predictions with the tissue response in unconfined compression at higher strain rate (10 s⁻¹) and with literature data in uniaxial tension. The proposed constitutive framework was also found to be adequate to model the loading response of brain tissue in uniaxial compression over a wider range of strain rates (0.01-3000 s⁻¹), thereby providing a valuable tool for simulations of dynamic transients (impact, blast/shock wave propagation) leading to traumatic brain injury.

Micromechanics of toughened polycarbonate

Abstract Numerical studies are presented on micromechanical and macromechanical aspects of deformation mechanisms in toughened polycarbonate. The dependence of the macroscopic stress–strain behavior, and of the underlying patterns of matrix deformation, on void distribution and triaxiality of the loading conditions are discussed. The presence of voids is shown to create stress fields which favor shear yielding over brittle failure mechanisms and thus provide toughness even in the case of highly triaxial stress states. Additionally, we compare predictions obtained using a micromechanical model based on a traditional axisymmetric unit cell, with predictions obtained with an alternative model based on a staggered array of voids. The new model is an axisymmetric equivalent to the Voronoi tessellation of a Body Centered Cubic array of voids (V-BCC model). The V-BCC model appears to be able to better capture essential features of the mechanical behavior of the blends, and provides a more realistic cell-based representation of particle-filled materials in general.

Traumatic brain injury and hemorrhagic shock: evaluation of different resuscitation strategies in a large animal model of combined insults.

ABSTRACT Traumatic brain injury (TBI) and hemorrhagic shock (HS) are the leading causes of trauma-related mortality and morbidity. Combination of TBI and HS (TBI + HS) is highly lethal, and the optimal resuscitation strategy for this combined insult remains unclear. A critical limitation is the lack of suitable large animal models to test different treatment strategies. We have developed a clinically relevant large animal model of TBI + HS, which was used to evaluate the impact of different treatments on brain lesion size and associated edema. Yorkshire swine (42–50 kg) were instrumented to measure hemodynamic parameters and intracranial pressure. A computer-controlled cortical impact device was used to create a TBI through a 20-mm craniotomy: 15-mm cylindrical tip impactor at 4 m/s velocity, 100-ms dwell time, and 12-mm penetration depth. Volume-controlled hemorrhage was started (40% blood volume) concurrent with the TBI. After 2 h of shock, animals were randomized to one of three resuscitation groups (n = 5/group): (a) normal saline (NS); (b) 6% hetastarch, Hextend (Hex); and (c) fresh frozen plasma (FFP). Volumes of Hex and FFP matched the shed blood, whereas NS was three times the volume. After 6 h of postresuscitation monitoring, brains were sectioned into 5-mm slices and stained with TTC (2,3,5-triphenyltetrazolium chloride) to quantify the lesion size and brain swelling. Combination of 40% blood loss with cortical impact and a period of shock (2 h) resulted in a highly reproducible brain injury. Total fluid requirements were lower in the Hex and FFP groups. Lesion size and brain swelling in the FFP group (2,160 ± 202.6 mm3 and 22% ± 1.0%, respectively) were significantly smaller than those in the NS group (3,285 ± 130.8 mm3 and 37% ± 1.6%, respectively) (P < 0.05). Hex treatment decreased the swelling (29% ± 1.6%) without reducing the lesion size. Early administration of FFP reduces the size of brain lesion and associated swelling in a large animal model of TBI + HS. In contrast, artificial colloid (Hex) decreases swelling without reducing the actual size of the brain lesion.

Dynamic mechanical response of brain tissue in indentation in vivo, in situ and in vitro

Characterizing the dynamic mechanical properties of brain tissue is deemed important for developing a comprehensive knowledge of the mechanisms underlying brain injury. The results gathered to date on the tissue properties have been mostly obtained in vitro. Learning how these results might differ quantitatively from those encountered in vivo is a critical step towards the development of biofidelic brain models. The present study provides novel and unique experimental results on, and insights into, brain biorheology in vivo, in situ and in vitro, at large deformations, in the quasi-static and dynamic regimes. The nonlinear dynamic response of the cerebral cortex was measured in indentation on the exposed frontal and parietal lobes of anesthetized porcine subjects. Load-unload cycles were applied to the tissue surface at sinusoidal frequencies of 10, 1, 0.1 and 0.01 Hz. Ramp-relaxation tests were also conducted to assess the tissue viscoelastic behavior at longer times. After euthanasia, the indentation test sequences were repeated in situ on the exposed cortex maintained in its native configuration within the cranium. Mixed gray and white matter samples were subsequently excised from the superior cortex to be subjected to identical indentation test segments in vitro within 6-7 h post mortem. The main response features (e.g. nonlinearities, rate dependencies, hysteresis and conditioning) were measured and contrasted in vivo, in situ and in vitro. The indentation response was found to be significantly stiffer in situ than in vivo. The consistent, quantitative set of mechanical measurements thereby collected provides a preliminary experimental database, which may be used to support the development of constitutive models for the study of mechanically mediated pathways leading to traumatic brain injury.

A study of the anisotropy and tension/compression behavior of human cervical tissue.

The cervix plays a crucial role in maintaining a healthy pregnancy, acting as a mechanical barrier to hold the fetus in utero during gestation. Altered mechanical properties of the cervical tissue are suspected to play a critical role in spontaneous preterm birth. Both MRI and X-ray data in the literature indicate that cervical stroma contains regions of preferentially aligned collagen fibers along anatomical directions (circumferential/longitudinal/radial). In this study, a mechanical testing protocol is developed to investigate the large-strain response of cervical tissue in uniaxial tension and compression along its three orthogonal anatomical directions. The stress response of the tissue along the different orthogonal directions is captured using a minimal set of model parameters generated by fitting a one-dimensional time-dependent rheological model to the experimental data. Using model parameters, mechanical responses can be compared between samples from patients with different obstetric backgrounds, between samples from different anatomical sites, and between the different loading directions for a single specimen. The results presented in this study suggest that cervical tissue is mechanically anisotropic with a uniaxial response dependent on the direction of loading, the anatomical site of the specimen, and the obstetric history of the patient. We hypothesize that the directionality of the tissue mechanical response is primarily due to collagen orientation in the cervical stroma, and provides an interpretation of our mechanical findings consistent with the literature data on preferential collagen alignment.

Micromechanisms of deformation and recovery in thermoplastic vulcanizates

Abstract The micromechanisms of deformation and recovery in thermoplastic vulcanizates (TPVs) are studied using a series of micromechanical models. TPVs are a class of composite material consisting of a relatively large volume fraction of elastomeric particles ( v p =0.40–0.90) in a thermoplastic matrix. A representative TPV with v p =0.77 is selected for the study. Six five-particle representative volume element (RVE) models are constructed where the symmetry of particle distribution and the relative thickness of the matrix ligament bridging particles are systematically varied. The macroscopic stress–strain behavior of the TPV during loading and unloading is successfully predicted by the simulation study as shown by direct comparison with experimental data. The simulation study reveals the important role of relative matrix ligament thickness as well as geometric asymmetry in the formation of a pseudo-continuous rubber phase which provides the rubber-like behavior of TPVs during loading. The study shows the important role of matrix ligament thickness in controlling the initial stiffness and flow stress of the TPV; thinner ligaments lead to earlier matrix yielding and thus earlier formation of the pseudo-continuous rubber phase. Upon formation of the pseudo-continuous rubber phase, the matrix material is seen to accommodate the large straining of the rubber phase by nearly rigid body motion (rotation and translation) of the bulky domains of the matrix; the rubber phase is seen to undergo large contortions as it attempts to deform as an almost continuous network around the “rigid” domains of matrix material. Furthermore, the asymmetry together with the thin matrix ligaments greatly aids the recovery of the material during unloading. Upon unloading, the rubber phase attempts recovery in a rubber-like manner. The bulkier regions of matrix material simply rotate and translate with the recovering rubber domains. The thin ligaments also rotate, but eventually also undergo bending and buckling which enables the large amount of recovery observed in thermoplastic vulcanizates.

Deformation of thermoplastic vulcanizates

Abstract The stress–strain behavior of thermoplastic vulcanizate (TPV) materials is studied experimentally; a constitutive model for the behavior is proposed and found to successfully predict the important features of the observed stress–strain behavior. TPVs are a relatively new class of elastomer-like material consisting of a rather high-volume fraction of elastomeric particles (0.40

Constitutive Modeling of Porcine Liver in Indentation Using 3D Ultrasound Imaging

In this work we present an inverse finite-element modeling framework for constitutive modeling and parameter estimation of soft tissues using full-field volumetric deformation data obtained from 3D ultrasound. The finite-element model is coupled to full-field visual measurements by regularization springs attached at nodal locations. The free ends of the springs are displaced according to the locally estimated tissue motion, and the normalized potential energy stored in all springs serves as a measure of model-experiment agreement for material parameter optimization. We demonstrate good accuracy of estimated parameters and consistent convergence properties on synthetically generated data. We present constitutive model selection and parameter estimation for perfused porcine liver in indentation, and demonstrate that a quasilinear viscoelastic model with shear modulus relaxation offers good model-experiment agreement in terms of indenter displacement (0.19 mm RMS error) and tissue displacement field (0.97 mm RMS error).