Large Macroscopic Strain Research Articles

In situ synchrotron X-ray diffraction analysis of deformation behavior of a Nb/NiTi composite for biomedical applications

The deformation behavior involved in Nb/NiTi composite for biomedical applications within a large macroscopic strain range was investigated by tensile loading–unloading test and in situ synchrotron X-ray diffraction (SXRD). Experimental results show that during loading, the Nb/NiTi composite experiences the elastic elongation of B2-NiTi austenitic, B19′-NiTi martensitic and β-Nb phases, B2 → B19′ stress-induced martensitic (SIM) transformation and tensile plastic deformation of β-Nb phase. During unloading, the deformation behavior involved in Nb/NiTi composite includes the elastic recovery of B2-NiTi austenitic, B19′-NiTi martensitic and β-Nb phases, reverse phase transformation B19′ → B2 and compressive deformation of β-Nb phase. The martensitic transformation in this composite is almost reversible and occurs in a localized manner. These results might contribute to a comprehensive understanding of the deformation mechanism involved in Nb/NiTi composite and shed some light on design and development of novel composites with a combination of good biocompatibility and excellent superelasticity for biomedical applications.

Crystallization Temperature Dependence of Cavitation and Plastic Flow in the Tensile Deformation of Poly(ε-caprolactone).

In situ small-, ultrasmall-, and wide-angle X-ray scattering measurements were performed to investigate the structural evolution of crystalline lamellae and cavities as a function of deformation ratio during tensile deformation of isothermally crystallized poly(ε-caprolactone). The cavities were modeled as cylinder-shaped objects which are oriented along the stretching direction and randomly distributed in the samples, and their dimensions were evaluated by direct model fitting of scattering patterns. At small deformations, the orientation of these cavities at the onset of cavity formation was related to the isothermal crystallization temperature. Upon further stretching, the cavities were found to cluster in the interfibrillar regions at moderate strains where the long spacing of the newly developed lamellae along the stretching direction remained essentially constant. At large orientations, the cooperative deformational behavior mediated via slippage of fibrils was evidenced, the extent of which depended on the cavity number, which could be traced back to the significantly different coupling forces imposed by chains connecting adjacent fibrils. Furthermore, wide-angle X-ray scattering results revealed that a fraction of the polymer chains with their orientation perpendicular to the stretching direction were still preserved even at large macroscopic strains.

Microscale spatial heterogeneity of protein structural transitions in fibrin matrices.

Following an injury, a blood clot must form at the wound site to stop bleeding before skin repair can occur. Blood clots must satisfy a unique set of material requirements; they need to be sufficiently strong to resist pressure from the arterial blood flow but must be highly flexible to support large strains associated with tissue movement around the wound. These combined properties are enabled by a fibrous matrix consisting of the protein fibrin. Fibrin hydrogels can support large macroscopic strains owing to the unfolding transition of α-helical fibril structures to β sheets at the molecular level, among other reasons. Imaging protein secondary structure on the submicrometer length scale, we reveal that another length scale is relevant for fibrin function. We observe that the protein polymorphism in the gel becomes spatially heterogeneous on a micrometer length scale with increasing tensile strain, directly showing load-bearing inhomogeneity and nonaffinity. Supramolecular structural features in the hydrogel observed under strain indicate that a uniform fibrin hydrogel develops a composite-like microstructure in tension, even in the absence of cellular inclusions.

Metallic muscles and beyond: nanofoams at work

In this contribution for the Golden Jubilee issue commemorating the 50th anniversary of the Journal of Materials Science, we will discuss the challenges and opportunities of nanoporous metals and their composites as novel energy conversion materials. In particular, we will concentrate on electrical-to-mechanical energy conversion using nanoporous metal-polymer composite materials. A materials system that mimic the properties of human skeletal muscles upon an outside stimulus is coined an ‘artificial muscle.’ In contrast to piezoceramics, nanoporous metallic materials offer a unique combination of low operating voltages, relatively large strain amplitudes, high stiffness, and strength. Here we will discuss smart materials where large macroscopic strain amplitudes up to 10 % and strain-rates up to 10−2 s−1 can be achieved in nanoporous metal/polymer composite. These strain amplitudes and strain-rates are roughly 2 and 5 orders of magnitude larger than those achieved in common actuator materials, respectively. Continuing on the theme of energy-related applications, in the summary and outlook, we discuss two recent developments toward the integration of nanoporous metals into energy conversion and storage systems. We specifically focus on the exciting potential of nanoporous metals as anodes for high-performance water electrolyzers and in next-generation lithium-ion batteries.

Magnetic shape memory effect in orbital-spin-coupled system MnV2O4

Magnetic-field(H)-induced shape memory effect without temperature variation is demonstrated by a combination of H-sweep and H-rotation in an orbital-spin-coupled spinel-type insulating system MnV2O4. A rotation of the direction of a magnetic field of 5 T in the low-temperature tetragonal phase gives rise to a large macroscopic strain up to 1%, which stems from a 90° rotation of tetragonal domains. The results show several possible ways of macroscopic shape control in this class of matter, and may open a possibility of high-speed actuators free from the eddy current.

Inhomogeneous microscopic shear strains in a complex crystal lattice subjected to large macroscopic strains (exact solutions)

Nonlinear theory of microscopic and macroscopic strains is developed for the case of large inhomogeneous relative displacements of two sublattices making up a complex crystal lattice; in this case, in addition to an acoustic mode, a pseudooptical, strongly nonlinear mode is excited. The equation of relative motion of the sublattices can be solved exactly for the specific case of a centrosymmetric crystal. The corresponding equilibrium equation is the sine-Helmholtz equation and has a doubly periodic solution. This solution describes fragmentation of the lattice, more specifically, the appearance of a domain superstructure with large periods, whose building blocks contain oppositely sensed rotons separated by topological defects that are opposite in sign. Purely elastic microscopic strains are followed by elastoplastic ones. Both types of strain arise as a result of bifurcation, which causes a change from the initially homogeneous strain field to an inhomogeneous one. The domain sizes take on optimal values when the external homogeneous macroscopic strains reach a certain threshold magnitude.

High temperature mechanical behavior of aluminium titanate–mullite composites

Compressive deformation tests at high temperatures (1300–1450°C) have been performed on aluminium titanate (undoped and modified with MgO and Fe 2O 3)–10 wt.% mullite composites obtained by reaction sintering. Two deformation regimes have been observed, depending on the experimental conditions. At high stresses/strain rates, the samples failed intergranularly at very small failure strains. At low stresses/strain rates, large macroscopic strains were reached without significant microstructural changes. MgO and Fe 2O 3 additions have a pronounced effect on the brittle-ductile transition. In contrast, they have no effect on the steady-state flow, which is characterized by a stress exponent close to unity and an activation energy of 650 kJ/mol. Experimental results are compatible with a mechanism of grain boundary sliding controlled by cation bulk diffusion.

Effect of deformation on the position of X-ray diffraction lines from gold

Abstract X-ray measurements have been made on gold deformed by 75% in compression. The probability of (deformation) stacking faults is less than 0.0015, a value obtained from the shift in positions of the Debye-Scherrer lines. Any change in lattice parameter corresponding to the fractional change in density (1.2 × 10−4) observed by Clarebrough et al. (1962) is masked by a large macroscopic strain (4.7 × 10−4, tensile) in the surface of the specimen.