Showing papers on "Deformation (engineering) published in 2016"

Mechanical behavior of additive manufactured, powder-bed laser-fused materials

Abstract: Mechanical behavior of four metallic alloys fabricated with layered, laser-heated methods of additive manufacturing (AM) was compared to that of similar alloys produced with conventional methods (wrought and machined). AM materials were produced by a leading commercial service provider, as opposed to incorporating material specimens produced by unique or specially-adapted equipment. The elastic moduli were measured in flexure, stress–strain characteristics were measured in tensile deformation, and fatigue strengths were measured in fully reversed bending. The effects of fabrication orientation, surface polishing, and hot isostatic pressing upon mechanical behavior were studied. The fatigue strengths exhibited by SLM AlSi10Mg and DMLS Ti6Al4V in the as-fabricated condition proved to be significantly inferior to that of conventional material. These lower fatigue strengths are a consequence of multiple fatigue cracks initiating at surface defects, internal voids and microcracks, and growing simultaneously during cyclic loading. Measured fatigue strengths of DMLS 316L and 17-4PH approached those of corresponding wrought materials when subjected to principal stresses aligned with the build planes. When cyclic stresses were applied across the build planes of the DMLS stainless steels, fatigue fractures often developed prematurely by separation of material. Post-processing the DMLS Ti6Al4V and SS316L with hot isostatic pressure elevated the fatigue strength significantly. Measurements of surface roughness with an optical profilometer, examinations of the material microstructures, and fractography contribute to an understanding of the mechanical behavior of the additive materials.

A transparent bending-insensitive pressure sensor

Abstract: Measuring small normal pressures is essential to accurately evaluate external stimuli in curvilinear and dynamic surfaces such as natural tissues. Usually, sensitive and spatially accurate pressure sensors are achieved through conformal contact with the surface; however, this also makes them sensitive to mechanical deformation (bending). Indeed, when a soft object is pressed by another soft object, the normal pressure cannot be measured independently from the mechanical stress. Here, we show a pressure sensor that measures only the normal pressure, even under extreme bending conditions. To reduce the bending sensitivity, we use composite nanofibres of carbon nanotubes and graphene. Our simulations show that these fibres change their relative alignment to accommodate bending deformation, thus reducing the strain in individual fibres. Pressure sensitivity is maintained down to a bending radius of 80 μm. To test the suitability of our sensor for soft robotics and medical applications, we fabricated an integrated sensor matrix that is only 2 μm thick. We show real-time (response time of ∼20 ms), large-area, normal pressure monitoring under different, complex bending conditions. A composite fibrous material made of carbon nanotubes and graphene responds to small pressure but not to bending deformation.

Structure and mechanics of interfaces in biological materials

Abstract: Hard biological materials — for example, seashells, bone or wood — fulfil critical structural functions and display unique and attractive combinations of stiffness, strength and toughness, owing to their intricate architectures, which are organized over several length scales. The size, shape and arrangement of the ‘building blocks’ of which these materials are made are essential for defining their properties and their exceptional performance, but there is growing evidence that their deformation and toughness are also largely governed by the interfaces that join these building blocks. These interfaces channel nonlinear deformations and deflect cracks into configurations in which propagation is more difficult. In this Review, we discuss comparatively the composition, structure and mechanics of a set of representative biological interfaces in nacre, bone and wood, and show that these interfaces possess unusual mechanical characteristics, which can encourage the development of advanced bioinspired composites. Finally, we highlight recent examples of synthetic materials inspired from the mechanics and architecture of natural interfaces. The mechanical performance of hard biological materials is not only governed by their composition and architecture but also by the interfaces they contain. This Review discusses the composition, structure and mechanics of key interfaces within nacre, bone and wood, and their role in deformation and toughness.

Effects of Acoustic Emission and Energy Evolution of Rock Specimens Under the Uniaxial Cyclic Loading and Unloading Compression

Abstract: Characteristics of energy accumulation, evolution, and dissipation in uniaxial cyclic loading and unloading compression of 30 sandstone rock specimens under six different loading rates were explored. Stress–strain relations and acoustic emission characteristics of the deformation and failure of rock specimens were analyzed. The densities and rates of stored energy, elastic energy, and dissipated energy under different loading rates were confirmed, and an effective approach for the equivalent energy surface was presented. The energy evolution of rock deformation and failure were revealed. It turns out that the rock deformation behavior under uniaxial cyclic loading and unloading compression remained almost unchanged compared with that of uniaxial compression. The degree of match between reloading stress–strain curves and previous unloading curves was high, thereby demonstrating the memory function of rock masses. The intensity of acoustic emission fluctuated continually during the entire cyclic process. Emissions significantly increased as the stress exceeded the unloading level. The peak of acoustic emission increased with increasing loading stress level. Relationships between energy density and axial load indicate that the rock mass possesses a certain energy storage limitation. The energy evolution of rock masses is closely related to the axial loading stress, rather than to the axial loading rate. With increasing axial loading stress, stored energy varied most rapidly, followed by that of the elastic energy, then dissipated energy. Energy accumulation dominates prior to the axial load reaching peak strength; thereafter, energy dissipation becomes dominant. The input energy causes the irreversible initiation and extension of microcracks in the rock body. Elastic energy release leads to sudden instability of rock bodies and drives rock damage.

A phase-field model for ductile fracture at finite strains and its experimental verification

Abstract: In this paper, a phase-field model for ductile fracture previously proposed in the kinematically linear regime is extended to the three-dimensional finite strain setting, and its predictions are qualitatively and quantitatively compared with several experimental results, both from ad-hoc tests carried out by the authors and from the available literature. The proposed model is based on the physical assumption that fracture occurs when a scalar measure of the accumulated plastic strain reaches a critical value, and such assumption is introduced through the dependency of the phase-field degradation function on this scalar measure. The proposed model is able to capture the experimentally observed sequence of elasto-plastic deformation, necking and fracture phenomena in flat specimens; the occurrence of cup-and-cone fracture patterns in axisymmetric specimens; the role played by notches and by their size on the measured displacement at fracture; and the sequence of distinct cracking events observed in more complex specimens.

A Strain-Insensitive Stretchable Electronic Conductor: PEDOT:PSS/Acrylamide Organogels

Abstract: UNLABELLED Organogel-based stretchable electronic conductors exhibit electrical conduction even under a large stretching deformation of 300% without electrochemical reactions at DC voltages. The resistance change with stretching is almost strain-insensitive up to 50% strain and it remains at each deformation up to 1000 fatigue cycle. The polymeric conductive paths of PEDOT PSS are well preserved during the mechanical deformation.

The influence of cell morphology on the compressive fatigue behavior of Ti-6Al-4V meshes fabricated by electron beam melting.

Abstract: Additive manufacturing technique is a promising approach for fabricating cellular bone substitutes such as trabecular and cortical bones because of the ability to adjust process parameters to fabricate different shapes and inner structures. Considering the long term safe application in human body, the metallic cellular implants are expected to exhibit superior fatigue property. The objective of the study was to study the influence of cell shape on the compressive fatigue behavior of Ti-6Al-4V mesh arrays fabricated by electron beam melting. The results indicated that the underlying fatigue mechanism for the three kinds of meshes (cubic, G7 and rhombic dodecahedron) is the interaction of cyclic ratcheting and fatigue crack growth on the struts, which is closely related to cumulative effect of buckling and bending deformation of the strut. By increasing the buckling deformation on the struts through cell shape design, the cyclic ratcheting rate of the meshes during cyclic deformation was decreased and accordingly, the compressive fatigue strength was increased. With increasing bending deformation of struts, fatigue crack growth in struts contributed more to the fatigue damage of meshes. Rough surface and pores contained in the struts significantly deteriorated the compressive fatigue strength of the struts. By optimizing the buckling and bending deformation through cell shape design, Ti-6Al-4V alloy cellular solids with high fatigue strength and low modulus can be fabricated by the EBM technique.

Precise and reversible band gap tuning in single-layer MoSe2 by uniaxial strain†

Abstract: We present photoluminescence (PL) spectroscopy measurements of single-layer MoSe2 as a function of uniform uniaxial strain. A simple clamping and bending method is described that allows for application of uniaxial strain to layered, 2D materials with strains up to 1.1% without slippage. Using this technique, we find that the electronic band gap of single layer MoSe2 can be reversibly tuned by −27 ± 2 meV per percent of strain. This is in agreement with our density-functional theory calculations, which estimate a modulation of −32 meV per percent of strain, taking into account the role of deformation of the underlying substrate upon bending. Finally, due to its narrow PL spectra as compared with that of MoS2, we show that MoSe2 provides a more precise determination of small changes in strain making it the ideal 2D material for strain applications.

Influence of plastic deformation heterogeneity on development of geometrically necessary dislocation density in dual phase steel

Abstract: The present investigation examined the evolution of geometrically necessary dislocation (GND) structure following tensile deformation in a commercially produced dual phase steel, DP 590. GND measurements were made using electron back scatter diffraction (EBSD). The average GND density increased with imposed macroscopic strain, however the rate of increased slowed with increasing strain. GND density was found to be influenced by the ferrite grain size and orientation of the ferrite grains. Small ferrite grains generally had a higher GND density. For this steel the highest GND density was measured for {011}[111] orientations. Analysis of these data using the classical Ashby model for GND content shows that GND density is increasing linearly with strain. The discrepancy between measured and predicted GND density is attributed to the plastic deformation of martensite reducing the requirement of compatibility between ferrite and martensite and dynamic recovery of the dislocation structures decreasing the rate of GND storage with strain.

Grain neighbour effects on twin transmission in hexagonal close-packed materials

Abstract: Materials with a hexagonal close-packed (hcp) crystal structure such as Mg, Ti and Zr are being used in the transportation, aerospace and nuclear industry, respectively. Material strength and formability are critical qualities for shaping these materials into parts and a pervasive deformation mechanism that significantly affects their formability is deformation twinning. The interaction between grain boundaries and twins has an important influence on the deformation behaviour and fracture of hcp metals. Here, statistical analysis of large data sets reveals that whether twins transmit across grain boundaries depends not only on crystallography but also strongly on the anisotropy in crystallographic slip. We show that increases in crystal plastic anisotropy enhance the probability of twin transmission by comparing the relative ease of twin transmission in hcp materials such as Mg, Zr and Ti.

Mechanical plasticity of cells.

Abstract: Under mechanical loading, most living cells show a viscoelastic deformation that follows a power law in time. After removal of the mechanical load, the cell shape recovers only incompletely to its original undeformed configuration. Here, we show that incomplete shape recovery is due to an additive plastic deformation that displays the same power-law dynamics as the fully reversible viscoelastic deformation response. Moreover, the plastic deformation is a constant fraction of the total cell deformation and originates from bond ruptures within the cytoskeleton. A simple extension of the prevailing viscoelastic power-law response theory with a plastic element correctly predicts the cell behaviour under cyclic loading. Our findings show that plastic energy dissipation during cell deformation is tightly linked to elastic cytoskeletal stresses, which suggests the existence of an adaptive mechanism that protects the cell against mechanical damage.

Photoactuators for Direct Optical-to-Mechanical Energy Conversion: From Nanocomponent Assembly to Macroscopic Deformation.

Abstract: Photoactuators with integrated optical-to-mechanical energy conversion capacity have attracted growing research interest in the last few decades due to their unique features of remote control and their wide applications ranging from bionic robots, biomedical devices, and switches to motors. For the photoactuator design, the energy conversion route and structure assembly are two important parts, which directly affect the performance of the photoactuators. In particular, the architectural designs at the molecular, nano-, micro-, and macro- level, are found to play a significant role in accumulating molecular-scale strain/stress to macroscale strain/stress. Here, recent progress on photoactuators based on photochemical and photothermal effects is summarized, followed by a discussion of the important assembly strategies for the amplification of the photoresponsive components at nanoscale to macroscopic scale motions. The application advancement of current photoactuators is also presented.

Austenite stability and its effect on the toughness of a high strength ultra-low carbon medium manganese steel plate

Abstract: A novel two-step intercritical annealing process was designed for an ultra-low carbon medium manganese steel plate. Excellent mechanical properties with yield strength of 590 MPa, tensile strength of 840 MPa, total elongation of 28.5% and high impact energy of 106 J at −80 °C were obtained. The microstructure comprised of ultra-fine grained ferrite and retained austenite together with a small amount of martensite after the two-step intercritical annealing. Both lath-like and blocky retained austenite with volume fraction of ~25% and relatively poor stability were obtained. The submicron-sized lath-like retained austenite exhibited Nishiyama-Wassermann (N-W) orientation relationship with the neighboring martensitic ferrite lath. The fine grain size played a crucial role in stabilizing austenite during phase transformation by significantly lowering Ms temperature and increasing the elastic strain energy. The overall stability of retained austenite during deformation was considered to be mainly governed by the chemical composition of the studied steel. The mechanism of toughening was elucidated. The superior low-temperature toughness was associated with TRIP effect of metastable retained austenite, which relieved the local stress concentration, enhanced the ability to plastic deformation and delayed the initiation and propagation of microcracks.

Tunable Structural Color Surfaces with Visually Self-Reporting Wettability

Abstract: Functional materials with wettability of specific surfaces are important for many areas. Here, a new lubricant-infused elastic inverse opal is presented with tunable and visually “self-reporting” surface wettability. The elastic inverse opal films are used to lock in the infused lubricating fluid and construct slippery surfaces to repel droplets of various liquids. The films are stretchable, and the lubricating fluid can penetrate the pores under stretching, leaving the surface layer free of lubrication; the resultant undulating morphology of the inverse opal scaffold topography can reversibly pin droplets on the fluidic film rather than the solid substrate. This mechanical stimulation process provides an effective means of dynamically tuning the surface wettability and the optical transparency of the inverse opal films. In particular, as the adjustments are accompanied by simultaneous deformation of the periodic macroporous structure, the inverse opal films can self-report on their surface status through visible structural color changes. These features make such slippery structural color materials highly versatile for use in diverse applications.

Hot deformation behavior and constitutive equation of a new type Al–Zn–Mg–Er–Zr alloy during isothermal compression

Abstract: Isothermal compression tests of a new type Al–Zn–Mg–Er–Zr alloy are carried out on a Gleeble-3800 thermal simulator at temperatures varying from 300 to 460 °C and strain rates ranging from 0.001 to 10 s −1 . A comprehensive and scientific constitutive models based on the Arrhenius type equation have been developed from the experimentally measured data. The deformation temperature and strain rate have significant effect on flow stress and the material constants, such as α, β, n, ln A and Q are the functions of the strain. The flow stress calculated by the developed constitutive equation shows a close agreement with the experimental value, which indicates that the proposed constitutive equation can precisely analyze the hot deformation behavior of the Al–Zn–Mg–Er–Zr alloy. Due to the presence of coherent L1 2 -structured Al 3 (Er,Zr) precipitates, the dominant softening mechanism is dynamic recovery during isothermal compression.