Showing papers on "Deformation (engineering) published in 2020"

Mechanical properties and deformation mechanisms of gradient nanostructured metals and alloys

Abstract: Inspired by the gradient structures of biological materials, researchers have explored compositional and structural gradients for about 40 years as an approach to enhance the properties of engineering materials, including metals and metallic alloys. The synthesis of various gradient nanostructured materials, such as gradient nanograined, nanolaminated nd nanotwinned metals and alloys, has provided new opportunities to understand gradient-related mechanical behaviour. These emerging gradient materials often exhibit unprecedented mechanical properties, such as strength–ductility synergy, extraordinary strain hardening, enhanced fracture and fatigue resistance, and remarkable resistance to wear and corrosion, which are not found in materials with homogeneous or random microstructures. This Review critically assesses the state of the art in the field of gradient nanostructured metallic materials, covering topics ranging from the fabrication and characterization of mechanical properties to underlying deformation mechanisms. We discuss various deformation behaviours induced by structural gradients, including stress and strain gradients, the accumulation and interaction of new dislocation structures, and unique interfacial behaviour, as well as providing insight into future directions for the development of gradient structured materials. Gradient nanostructured metals and alloys are an emerging class of materials that exhibit a combination of excellent mechanical properties that are not possessed by their homogeneous counterparts. This Review assesses the fabrication, characterization and deformation behaviour of these materials, as well as the challenges and future directions of the field.

Cooperative deformation in high-entropy alloys at ultralow temperatures

Abstract: High-entropy alloys exhibit exceptional mechanical properties at cryogenic temperatures, due to the activation of twinning in addition to dislocation slip. The coexistence of multiple deformation pathways raises an important question regarding how individual deformation mechanisms compete or synergize during plastic deformation. Using in situ neutron diffraction, we demonstrate the interaction of a rich variety of deformation mechanisms in high-entropy alloys at 15 K, which began with dislocation slip, followed by stacking faults and twinning, before transitioning to inhomogeneous deformation by serrations. Quantitative analysis showed that the cooperation of these different deformation mechanisms led to extreme work hardening. The low stacking fault energy plus the stable face-centered cubic structure at ultralow temperatures, enabled by the high-entropy alloying, played a pivotal role bridging dislocation slip and serration. Insights from the in situ experiments point to the role of entropy in the design of structural materials with superior properties.

Effects of particle size on crushing and deformation behaviors of rockfill materials

Abstract: Strength and deformation behaviors of rockfill materials, key factors for determining the stability of dams, pertain strongly to the grain crushing characteristics. In this study, single-particle crushing tests were carried out on rockfill materials with nominal particle diameters of 2.5 mm, 5 mm and 10 mm to investigate the particle size effect on the single-particle strength and the relationship between the characteristic stress and probability of non-failure. Test data were found to be described by the Weibull distribution with the Weibull modulus of 3.24. Assemblies with uniform nominal grains were then subjected to one-dimensional compression tests at eight levels of vertical stress with a maximum of 100 MPa. The yield stress in one-dimensional compression tests increased with decreasing the particle size, which could be estimated from the single-particle crushing tests. The void ratio-vertical stress curve could be predicted by an exponential function. The particle size distribution curve increased obviously with applied stresses less than 16 MPa and gradually reached the ultimate fractal grading. The relative breakage index became constant with stress up to 64 MPa and was obtained from the ultimate grading at the fractal dimension ( α = 2.7 ). A hyperbolical function was also found useful for describing the relationship between the relative breakage index and input work during one-dimensional compression tests.

Blueprinting Photothermal Shape-Morphing of Liquid Crystal Elastomers.

Abstract: Liquid crystal elastomers (LCEs) are an attractive platform for dynamic shape-morphing due to their ability to rapidly undergo large deformations. While recent work has focused on patterning the director orientation field to achieve desired target shapes, this strategy cannot be generalized to material systems where high-resolution surface alignment is impractical. Instead of programming the local orientation of anisotropic deformation, an alternative strategy for prescribed shape-morphing by programming the magnitude of stretch ratio in a thin LCE sheet with constant director orientation is developed here. By spatially patterning the concentration of gold nanoparticles, uniform illumination leads to gradients in photothermal heat generation and therefore spatially nonuniform deformation profiles that drive out-of-plane buckling of planar films into predictable 3D shapes. Experimentally realized shapes are shown to agree closely with both finite element simulations and geometric predictions for systems with unidirectional variation in deformation magnitude. Finally, the possibility to achieve complex oscillatory motion driven by uniform illumination of a free-standing patterned sheet is demonstrated.

Temperature dependence of elastic and plastic deformation behavior of a refractory high-entropy alloy.

Abstract: Single-phase solid-solution refractory high-entropy alloys (HEAs) show remarkable mechanical properties, such as their high yield strength and substantial softening resistance at elevated temperatures. Hence, the in-depth study of the deformation behavior for body-centered cubic (BCC) refractory HEAs is a critical issue to explore the uncovered/unique deformation mechanisms. We have investigated the elastic and plastic deformation behaviors of a single BCC NbTaTiV refractory HEA at elevated temperatures using integrated experimental efforts and theoretical calculations. The in situ neutron diffraction results reveal a temperature-dependent elastic anisotropic deformation behavior. The single-crystal elastic moduli and macroscopic Young’s, shear, and bulk moduli were determined from the in situ neutron diffraction, showing great agreement with first-principles calculations, machine learning, and resonant ultrasound spectroscopy results. Furthermore, the edge dislocation–dominant plastic deformation behaviors, which are different from conventional BCC alloys, were quantitatively described by the Williamson-Hall plot profile modeling and high-angle annular dark-field scanning transmission electron microscopy.

Elucidating the G″ overshoot in soft materials with a yield transition via a time-resolved experimental strain decomposition.

Abstract: Materials that exhibit yielding behavior are used in many applications, from spreadable foods and cosmetics to direct write three-dimensional printing inks and filled rubbers. Their key design feature is the ability to transition behaviorally from solid to fluid under sufficient load or deformation. Despite its widespread applications, little is known about the dynamics of yielding in real processes, as the nonequilibrium nature of the transition impedes understanding. We demonstrate an iteratively punctuated rheological protocol that combines strain-controlled oscillatory shear with stress-controlled recovery tests. This technique provides an experimental decomposition of recoverable and unrecoverable strains, allowing for solid-like and fluid-like contributions to a yield stress material’s behavior to be separated in a time-resolved manner. Using this protocol, we investigate the overshoot in loss modulus seen in materials that yield. We show that this phenomenon is caused by the transition from primarily solid-like, viscoelastic dissipation in the linear regime to primarily fluid-like, plastic flow at larger amplitudes. We compare and contrast this with a viscoelastic liquid with no yielding behavior, where the contribution to energy dissipation from viscous flow dominates over the entire range of amplitudes tested.

Dynamic compressive behavior of a modified additively manufactured rhombic dodecahedron 316L stainless steel lattice structure

Abstract: Dynamic compression properties of the modified rhombic dodecahedron (RD) lattice structures were investigated by using a Split Hopkinson Pressure Bar (SHPB) system. All the deformation processes were recorded and a digital image correlation (DIC) technique was employed to analyse the strain distribution during the compression process. Experimental results indicated that the modified lattice structure showed better compressive strength, plateau stress and energy absorption when compared with the original one under both quasi-static and dynamic loading conditions. The effect of the shape parameter on the deformation mechanisms of the RD lattice structure was discussed, where three different deformation modes were observed. Finite element analysis was also conducted to simulate the dynamic response of the modified lattice structure. The finite element results are in good agreement with the experimental results. Then, a mode classification map was plotted and discussed based on the numerical results.

Stress–Energy Mechanism for Rock Failure Evolution Based on Damage Mechanics in Hard Rock

Abstract: To investigate the quantification of the extent of damage by considering the energy during rock failure, the pattern of energy dissipation and energy conversion, and the stress–energy mechanism for induced rock failure were analysed under cyclic loading/unloading. Based on damage mechanics, rock mechanics, and energy conservation theory, the test data were analysed. The results showed that the characteristics of hard rock compression are small deformation, high energy, and sudden failure; an elastic–plastic damage constitutive model and a stress–energy–rigidity–damage multi-criteria model for rock failure were established for hard rock. We compared the numerical curves and the experimental curves and found that they coincide. Rock failure is a combination of the results of elastic strain accumulation and dissipation by stress propagation. The key to inducing the energy storage capacity of rock before failure is closely related to the rock damage evolution. The pattern of energy release and dissipation through stress during rock failure was revealed from the perspective of energy using the constitutive model and multi-criteria model established for rock failure; these theoretical studies are very helpful in elucidating the mechanism of rock failure.

Structural Multi-Colour Invisible Inks with Submicron 4D Printing of Shape Memory Polymers

Abstract: Four-dimensional (4D) printing of shape memory polymer (SMP) imparts time responsive properties to 3D structures. Here, we explore 4D printing of a SMP in the submicron length scale, extending its applications to nanophononics. We report a new SMP photoresist based on Vero Clear achieving print features at a resolution of ~300 nm half pitch using two-photon polymerization lithography (TPL). Prints consisting of grids with size-tunable multi-colours enabled the study of shape memory effects to achieve large visual shifts through nanoscale structure deformation. As the nanostructures are flattened, the colours and printed information become invisible. Remarkably, the shape memory effect recovers the original surface morphology of the nanostructures along with its structural colour within seconds of heating above its glass transition temperature. The high-resolution printing and excellent reversibility in both microtopography and optical properties promises a platform for temperature-sensitive labels, information hiding for anti-counterfeiting, and tunable photonic devices.

Tacky Elastomers to Enable Tear-Resistant and Autonomous Self-Healing Semiconductor Composites

Abstract: Mechanical failure of π‐conjugated polymer thin films is unavoidable under cyclic loading conditions, due to intrinsic defects and poor resistance to crack propagation. Here, the first tear‐resistant and room‐temperature self‐healable semiconducting composite is presented, consisting of conjugated polymers and butyl rubber elastomers. This new composite displays both a record‐low elastic modulus (<1 MPa) and ultrahigh deformability with fracture strain above 800%. More importantly, failure behavior is not sensitive to precut notches under deformation. Autonomous self‐healing at room temperature, both mechanical and electronic, is demonstrated through the physical contact of two separate films. The composite film also shows device stability in the ambient environment over 5 months due to much‐improved barrier property to both oxygen and water. Butyl rubber is broadly applicable to various p‐type and n‐type semiconducting polymers for fabricating self‐healable electronics to provide new resilient electronics that mimic the tear resistance and healable property of human skin.

Fiber-Reinforced Viscoelastomers Show Extraordinary Crack Resistance That Exceeds Metals

Abstract: Soft fiber-reinforced polymers (FRPs), consisting of rubbery matrices and rigid fabrics, are widely utilized in industry because they possess high specific strength in tension while allowing flexural deformation under bending or twisting Nevertheless, existing soft FRPs are relatively weak against crack propagation due to interfacial delamination, which substantially increases their risk of failure during use In this work, a class of soft FRPs that possess high specific strength while simultaneously showing extraordinary crack resistance are developed The strategy is to synthesize tough viscoelastic matrices from acrylate monomers in the presence of woven fabrics, which generates soft composites with a strong interface and interlocking structure Such composites exhibit fracture energy, Γ, of up to 2500 kJ m-2 , exceeding the toughest existing materials Experimental elucidation shows that the fracture energy obeys a simple relation, Γ = W · lT , where W is the volume-weighted average of work of extension at fracture of the two components and lT is the force transfer length that scales with the square root of fiber/matrix modulus ratio Superior Γ is achieved through a combination of extraordinarily large lT (10-100 mm), resulting from the extremely high fiber/matrix modulus ratios (104 -105 ), and the maximized energy dissipation density, W The elucidated quantitative relationship provides guidance toward the design of extremely tough soft composites

4D printed electro-induced continuous carbon fiber reinforced shape memory polymer composites with excellent bending resistance

Abstract: Four-dimensional (4D) printing technology of continuous carbon fiber reinforced shape memory polymer composites is a potential manufacturing process for lightweight and high-strength intelligent composite structures. In this study, a 3D printer with dual feed channels based on the fused deposition modeling (FDM) was designed to fabricate continuous carbon fiber reinforced shape memory poly(lactic acid)-based composites (CFRSMPC). The impact of various printing parameters on the bending strength and flexural modulus of 4D printed CFRSMPC was evaluated by the three-point bending test. Meanwhile, mathematical prediction models of bending strength and modulus based on the existing experimental data were established. The electro-induced shape memory effect of 4D printed CFRSMPC was investigated by the electric heating shape recovery test. The shape recovery rate of the specimens was more than 95%, indicating that the resistance heating method is stable and feasible. The quantitative effect of bending angle and temperature on the resistance of CFRSMPC during the programming and recovery process was further investigated. The results demonstrated that the real-time deformation of the CFRSMPC could be monitored by the resistance measurement method. It can be concluded that the CFRSMPC fabricated using the 4D printing method can serve as potential building blocks for electrically activated and deployable structures.

Study on microstructure and tensile properties of 316L stainless steel fabricated by CMT wire and arc additive manufacturing

Abstract: An austenitic stainless steel 316L part was fabricated by cold metal transfer wire and arc additive manufacturing (CMT-WAAM), and its microstructure, microhardness and tensile properties were investigated. Results showed that the as-built 316L part exhibited a multilayered structure along the building direction. In the transverse direction (perpendicular to scanning direction) of each layer, there was also a multilayered structure of alternating overlapping zone (OA) and re-melting zone (RA). Compared with the OA, the RA had higher ferrite content, smaller austenite dendrite size, more dispersed orientation and lower residual stress. The overall multilayered structure and the intra-layer non-equilibrium microstructure exhibit a great influence on the mechanical properties of as-built 316L part. Along the building and transverse direction, the microhardness distribution in the OA was uniform, while the RA showed a trend of lower hardness in the middle and higher hardness on both sides of the RA layer. The effect of multilayered structure on tensile properties was stronger in the transverse direction than that in the building direction. The deformation feature was obviously inconsistent between the OA and RA. Local necking and fracture always occurred in the OA. Microvoids trended to initiate at silicate impurity particles, grow into large cracks, and finally lead to material failure during tension.

Size-dependent stochastic tensile properties in additively manufactured 316L stainless steel

Abstract: Recent work in metal additive manufacturing (AM) suggests that mechanical properties may vary with feature size; however, these studies do not provide a statistically robust description of this phenomenon, nor do they provide a clear causal mechanism. Because of the huge design freedom afforded by 3D printing, AM parts typically contain a range of feature sizes, with particular interest in smaller features, so the size effect must be well understood in order to make informed design decisions. This work investigates the effect of feature size on the stochastic mechanical performance of laser powder bed fusion tensile specimens. A high-throughput tensile testing method was used to characterize the effect of specimen size on strength, elastic modulus and elongation in a statistically meaningful way. The effective yield strength, ultimate tensile strength and modulus decreased strongly with decreasing specimen size: all three properties were reduced by nearly a factor of two as feature dimensions were scaled down from 6.25 mm to 0.4 mm. Hardness and microstructural observations indicate that this size dependence was not due to an intrinsic change in material properties, but instead the effects of surface roughness on the geometry of the specimens. Finite element analysis using explicit representations of surface topography shows the critical role surface features play in creating stress concentrations that trigger deformation and subsequent fracture. The experimental and finite element results provide the tools needed to make corrections in the design process to more accurately predict the performance of AM components.