Residual Stresses in Surgical Growing Rods.

The treatment of early onset scoliosis using surgical growing rods suffers from high failure rate. Fatigue resistance can be improved by inducing compressive residual stresses within the near surface region. An in-depth investigation of the residual stresses profile evolution is performed through the sequence of material processing steps followed by surgeons handling operations, in connection to material properties. The final goal is to guide further improvements of growing rod lifetime. Residual stress evaluation was carried out on Ti-6Al-4V rods using digital image correlation applied to microbeam ring-core milling by focused ion beam. This provided experimental stress profiles in shot-peened rods before and after bending and demonstrated that compressive residual stresses are maintained at both concave and convex rod sides. A finite element model using different core and skin conditions was validated by comparison to experiments. The combination of an initial shot peening profile associated with a significant level of backstress was found to primarily control the generation of compressive stresses at the rod surface after bending. Guidelines to promote larger compressive stresses at the surface were formulated based on a parametric analysis. The analysis revealed the first order impact of the initial yield strength, kinematic hardening parameters and intensity of the shot peening operation, while the bending angle and the depth of shot peening stresses were found to be of minor importance. Materials exhibiting large kinematic hardening and low yield strength should be selected in order to induce compressive residual stresses at key fatigue initiation site.

Collagen Induces Anisotropy in Fingertip Subcutaneous Tissues During Contact.

The subcutaneous mechanical response of the fingertip is highly anisotropic due to the presence of a network of collagen fibers linking the outer skin layer to the bone. The impact of this anisotropy on the fingerpad deformation, which had not been studied until now, is here demonstrated using a two-dimensional finite element model of a transverse section of the finger. Different distributions of fiber orientations are considered: radial (physiologic), circumferential, and random (isotropic). The three variants of the model are assessed using experimental observations of a finger pressed on a flat surface. Predictions relying on the physiological orientation of fibers best reproduce experimental trends. Our results show that the orientation of fibers significantly influences the distribution of internal strains and stresses. This leads to a sudden change in the profile of contact pressure when transitioning from sticking to slipping. Interpreted in terms of tactile perception or sensation, these variations might represent important sensory cues for partial slip detection. This is also valuable information for the development of haptic devices.

Room-Temperature Magnetic Switching of the Electric Polarization in Ferroelectric Nanopillars.

Magnetoelectric layers with a strong coupling between ferroelectricity and ferromagnetism offer attractive opportunities for the design of new device architectures such as dual-channel memory and multiresponsive sensors and actuators. However, materials in which a magnetic field can switch an electric polarization are extremely rare, work most often only at very low temperatures, and/or comprise complex materials difficult to integrate. Here, we show that magnetostriction and flexoelectricity can be harnessed to strongly couple electric polarization and magnetism in a regularly nanopatterned magnetic metal/ferroelectric polymer layer, to the point that full reversal of the electric polarization can occur at room temperature by the sole application of a magnetic field. Experiments supported by finite element simulations demonstrate that magnetostriction produces large strain gradients at the base of the ferroelectric nanopillars in the magnetoelectric hybrid layer, translating by flexoelectricity into equivalent electric fields larger than the coercive field of the ferroelectric polymer. Our study shows that flexoelectricity can be advantageously used to create a very strong magnetoelectric coupling in a nanopatterned hybrid layer.