Now showing 1 - 10 of 22
  • Publication
    A high rate tension device for characterizing brain tissue
    (Sage Publications, 2012-03-08) ; ;
    The mechanical characterization of brain tissue at high loading velocities is vital for understanding and modeling traumatic brain injury. The most severe form of traumatic brain injury is diffuse axonal injury, which involves damage to individual nerve cells (neurons). Diffuse axonal injury in animals and humans occurs at strains >10% and strain rates >10 s−1. The mechanical properties of brain tissues at these strains and strain rates are of particular significance, as they can be used in finite element human head models to accurately predict brain injuries under different impact conditions. Existing conventional tensile testing machines can only achieve maximum loading velocities of 500 mm/min, whereas the Kolsky bar apparatus is more suitable for strain rates >100 s−1. In this study, a custom-designed high rate tension device is developed and calibrated to estimate the mechanical properties of brain tissue in tension at strain rates ≤ 90 s−1, while maintaining a uniform velocity. The range of strain can o be extended to 100% depending on the thickness of a sample. The same apparatus can be used to characterize the dynamic behavior of skin and other soft biological tissues by using appropriately sized load cells with a capacity of 10 N and above.
      329Scopus© Citations 9
  • Publication
    Extreme softness of brain matter in simple shear
    We show that porcine brain matter can be modelled accurately as a very soft rubber-like material using the Mooney–Rivlin strain energy function, up to strains as high as 60%. This result followed from simple shear experiments performed on small rectangular fresh samples (2.5 cm3 and 1.1 cm3) at quasi-static strain rates. They revealed a linear shear stress–shear strain relationship (R2>0.97), characteristic of Mooney–Rivlin materials at large strains. We found that porcine brain matter is about 30 times less resistant to shear forces than a silicone gel. We also verified experimentally that brain matter exhibits the positive Poynting effect of non-linear elasticity, and numerically that the stress and strain fields remain mostly homogeneous throughout the thickness of the samples in simple shear.
      557Scopus© Citations 60
  • Publication
    Influence of preservation temperature on the measured mechanical properties of brain tissue
    The large variability in experimentally measured mechanical properties of brain tissue is due to many factors including heterogeneity, anisotropy, age dependence and post-mortem time. Moreover, differences in test protocols also influence these measured properties. This paper shows that the temperature at which porcine brain tissue is stored or preserved prior to testing has a significant effect on the mechanical properties of brain tissue, even when tests are conducted at the same temperatures. Three groups of brain tissue were stored separately for at least 1 h at three different preservation temperatures, i.e., ice cold, room temperature (22 °C) and body temperature (37 °C), prior to them all being tested at room temperature (∼22 °C). Significant differences in the corresponding initial elastic shear modulus μ (Pa) (at various amounts of shear, 0≤K≤1.0) were observed. The initial elastic moduli were 1043±271 Pa, 714±210 Pa and 497±156 Pa (mean±SD) at preservation temperatures of ice cold, 22 °C and 37 °C, respectively. Based on this investigation, it is strongly recommended that brain tissue samples must be preserved at an ice-cold temperature prior to testing in order to minimize the difference between the measured in vitro test results and the in vivo properties. A by-product of the study is that simple shear tests allow for large, almost perfectly homogeneous deformation of brain matter.
      692Scopus© Citations 32
  • Publication
    Experimental Characterisation of Neural Tissue at Collision Speeds
    (International Research Council on the Biomechanics of Injury, 2012) ; ;
    Mechanical characterization of brain tissue at high loading velocities is particularly important for modelling Traumatic Brain Injury (TBI). During severe impact conditions, brain tissue experiences a mixture of compression, tension and shear. Diffuse axonal injury (DAI) occurs in animals and humans when both the strains and strain rates exceed 10% and 10/s, respectively. Knowing the mechanical properties of brain tissue at these strains and strain rates is of particular importance, as they can be used in finite element simulations to predict the occurrence of brain injuries under different impact conditions. In this research, we describe the design and operation of a High Rate Tension Device (HRTD) that has been used for tensile tests on freshly harvested specimens of porcine neural tissue at speeds corresponding to a maximum strain rate of 90/s. We investigate the effects of inhomogeneous deformation of the tissue during tension by quasi‐static tests (strain rate 0.01/s) and dynamic tests (strain rate 90/s) using different thickness specimens (4.0, 7.0, 10.0 and 13.0 mm) of the same diameter (15.0 mm). Based on a combined experimental and computational analysis, brain specimens of aspect ratio (diameter/thickness) S = 10/10 or lower (10/12, 10/13) are considered suitable for minimizing the effects of inhomogeneous deformation during tension tests. The Ogden material parameters were derived from the experimental data both at quasi‐static conditions (µ = 440 Pa and α = ‐4.8 at 0.01/s strain rate) and dynamic conditions (µ = 4238 Pa and α = 2.8 at 90/s strain rate) by performing an inverse finite element analysis to model all experimental data. These material parameters will prove useful for the nonlinear hyperelastic analysis of brain tissue.
  • Publication
    Towards a predictive assessment of stab-penetration forces
    Collaborative research between the disciplines of forensic pathology and biomechanics was undertaken to investigate the hyperelastic properties of human skin, to determine the force required for sharp instrument penetration of skin, and to develop a finite element model, which reflects the mechanisms of sharp instrument penetration. These studies have led to the development of a 'stab metric', based on simulations, to describe the force magnitudes in stabbing incidents. Such a metric should, in time, replace the crudely quantitative descriptors of stabbing forces currently used by forensic pathologists.
      415Scopus© Citations 11
  • Publication
    Deficiencies in numerical models of anisotropic nonlinearly elastic materials
    Incompressible nonlinearly hyperelastic materials are rarely simulated in finite element numerical experiments as being perfectly incompressible because of the numerical difficulties associated with globally satisfying this constraint. Most commercial finite element packages therefore assume that the material is slightly compressible. It is then further assumed that the corresponding strain-energy function can be decomposed additively into volumetric and deviatoric parts. We show that this decomposition is not physically realistic, especially for anisotropic materials, which are of particular interest for simulating the mechanical response of biological soft tissue. The most striking illustration of the shortcoming is that with this decomposition, an anisotropic cube under hydrostatic tension deforms into another cube instead of a hexahedron with non-parallel faces. Furthermore, commercial numerical codes require the specification of a 'compressibility parameter' (or 'penalty factor'), which arises naturally from the flawed additive decomposition of the strain-energy function. This parameter is often linked to a 'bulk modulus', although this notion makes no sense for anisotropic solids; we show that it is essentially an arbitrary parameter and that infinitesimal changes to it result in significant changes in the predicted stress response. This is illustrated with numerical simulations for biaxial tension experiments of arteries, where the magnitude of the stress response is found to change by several orders of magnitude when infinitesimal changes in 'Poisson’s ratio' close to the perfect incompressibility limit of 1/2 are made.
    Scopus© Citations 35  588
  • Publication
    Bending instabilities of soft biological tissues
    Rubber components and soft biological tissues are often subjected to large bending deformations while 'in service'. The circumferential line elements on the inner face of a bent block can contract up to a certain critical stretch ratio λcr (say) before bifurcation occurs and axial creases appear. For several models used to describe rubber, it is found that λcr=0.56, allowing for a 44% contraction. For models used to describe arteries it is found, somewhat surprisingly, that the strain-stiffening effect promotes instability. For example, the models used for the artery of a seventy-year old human predict that λcr=0.73, allowing only for a 27% contraction. Tensile experiments conducted on pig skin indicate that bending instabilities should occur even earlier there.
      302Scopus© Citations 76
  • Publication
    Slight compressibility and sensitivity to changes in Poisson's ratio
    (Wiley Blackwell (John Wiley & Sons), 2011-12-12) ; ; ;
    Finite element simulations of rubbers and biological soft tissue usually assume that the material being deformed is slightly compressible. It is shown here that, in shearing deformations, the corresponding normal stress distribution can exhibit extreme sensitivity to changes in Poisson's ratio. These changes can even lead to a reversal of the usual Poynting effect. Therefore, the usual practice of arbitrarily choosing a value of Poisson's ratio when numerically modelling rubbers and soft tissue will, almost certainly, lead to a significant difference between the simulated and actual normal stresses in a sheared block because of the difference between the assumed and actual value of Poisson's ratio. The worrying conclusion is that simulations based on arbitrarily specifying Poisson's ratio close to 1∕2 cannot accurately predict the normal stress distribution even for the simplest of shearing deformations. It is shown analytically that this sensitivity is caused by the small volume changes, which inevitably acy all deformations of rubber-like materials. To minimise these effects, great care should be exercised to accurately determine Poisson's ratio before simulations begin.
      310Scopus© Citations 28
  • Publication
    Characterization of the anisotropic mechanical properties of excised human skin
    The mechanical properties of skin are important for a number of applications including surgery, dermatology, impact biomechanics and forensic science. In this study, we have investigated the influence of location and orientation on the deformation characteristics of 56 samples of excised human skin. Uniaxial tensile tests were carried out at a strain rate of 0.012 s−1 on skin from the back. Digital Image Correlation was used for 2D strain measurement and a histological examination of the dermis was also performed. The mean ultimate tensile strength (UTS) was 21.6±8.4 MPa, the mean failure strain 54%±17%, the mean initial slope 1.18±0.88 MPa, the mean elastic modulus 83.3±34.9 MPa and the mean strain energy was 3.6±1.6 MJ/m3. A multivariate analysis of variance has shown that these mechanical properties of skin are dependent upon the orientation of the Langer lines (P<0.0001−P=0.046). The location of specimens on the back was also found to have a significant effect on the UTS (P=0.0002), the elastic modulus (P=0.001) and the strain energy (P=0.0052). The histological investigation concluded that there is a definite correlation between the orientation of the Langer lines and the preferred orientation of collagen fibres in the dermis (P<0.001). The data obtained in this study will provide essential information for those wishing to model the skin using a structural constitutive model.
      407Scopus© Citations 494
  • Publication
    Third- and fourth-order elasticities of biological soft tissues
    (Acoustical Society of America, 2010) ; ;
    In the theory of weakly nonlinear elasticity,Hamilton et al. [J. Acoust. Soc. Am.116, 41–44 (2004)] identified W=μI2+(A/3)I3+DI22 as the fourth-order expansion of the strain-energy density for incompressible isotropic solids. Subsequently, much effort focused on theoretical and experimental developments linked to this expression in order to inform the modeling of gels and soft biological tissues. However, while many soft tissues can be treated as incompressible, they are not in general isotropic, and their anisotropy is associated with the presence of oriented collagen fiber bundles. Here the expansion of W is carried up to fourth order in the case where there exists one family of parallel fibers in the tissue. The results are then applied to acoustoelasticity, with a view to determining the second- and third-order nonlinear constants by employing small-amplitude transverse waves propagating in a deformed soft tissue.
      265Scopus© Citations 46