A family of metric strains and conjugate stresses, prolonging usual material laws from small to large transformations

AbstractA new family of simple generalized strains and conjugate stresses based on the material metric (right Cauchy–Green) tensor is proposed. It includes an interesting quasilinear pair. It is a close approximation of the Seth–Hill family, with the advantage of being easier to calculate. It extends the realm of application of the classical theories of linear elasticity and perfect plasticity from small to large transformations for isotropic and anisotropic materials without any modification

A combined atomic force microscopy and nanoindentation technique to investigate the elastic properties of bone structural units

To our knowledge, this study applied for the first time a recently developed combination of atomic force microscopy (AFM) and nanoindentation on trabecular and compact bone tissue. The major aim was to check the advantage of the available AFM-mode over the conventionally used optical microscope. First, we investigated if removal of the water content helped to prevent enzymatic degradation of the bone tissue and preserve its mechanical properties during a week. After the positive issue of this test, we quantified the intrinsic mechanical properties of single bone structural units (BSU). Bone specimens were obtained from the femoral neck of an 86 year old female. Four BSU were randomly selected and tested each with 24 indents of 5 mN maximum force. The available AFM mode proved to be a very useful tool for surface characterization and precise selection of the indentation area. The elastic modulus ranged from 18 ± 1.7 GPa for a BSU of compact bone to 22.5 ± 3.1 GPa for a BSU of trabecular bone. Hardness showed values between 0.6 ± 0.11 GPa for compact bone and 1.1 ± 0.17 GPa for trabecular bone. The results suggest that the micromechanics of bone tissue may also be described as an assembly of distinct structural units with rather homogeneous material properties

How is the indentation modulus of bone tissue related to its macroscopic elastic response? A validation study

International audienceThis work consists of the validation of a novel approach to estimate the local anisotropic elastic constants of the bone extracellular matrix using nanoindentation. For this purpose, nanoindentation on two planes of material symmetry were analyzed and the resulting longitudinal elastic moduli compared to the moduli measured with a macroscopic tensile test. A combined lathe and tensile system was designed that allows machining and testing of cylindrical microspecimens of approximately 4 mm in length and 300 μm in diameter. Three bovine specimens were tested in tension and their outer geometry and porosity assessed by synchrotron radiation microtomography. Based on the results of the traction test and the precise outer geometry, an apparent longitudinal Young's modulus was calculated. Results between 20.3 and 27.6 GPa were found that match with previously reported values for bovine compact bone. The same specimens were then characterized by nanoindentation on a transverse and longitudinal plane. A longitudinal Young's modulus for the bone matrix was then derived using the numerical scheme proposed by Swadener and Pharr and the fabric–elasticity relationship by Zysset and Curnier. Based on the matrix modulus and a power law effective volume fraction, an apparent longitudinal Young's modulus was predicted for each microspecimen. This alternative approach provided values between 19.9 and 30.0 GPa, demonstrating differences between 2% and 13% to the values provided by the initial tensile test. This study therefore raises confidence in our nanoindentation protocol of the bone extracellular matrix and supports the underlying hypotheses used to extract the anisotropic elastic constants

Conflict-induced behavioural adjustment: a clue to the executive functions of the prefrontal cortex.

The behavioural adjustment that follows the experience of conflict has been extensively studied in humans, leading to influential models of executive-control adjustment. Recent studies have revealed striking similarities in conflict-induced behavioural adjustment between humans and monkeys, indicating that monkeys can provide a model to study the underlying neural substrates and mechanisms of such behaviour. These studies have advanced our knowledge about the role of different prefrontal brain regions, including the anterior cingulate cortex (ACC) and the dorsolateral prefrontal cortex (DLPFC), in executive-control adjustment and suggest a pivotal role for the DLPFC in the dynamic tuning of executive control and, consequently, in behavioural adaptation to changing environments