Mechanical Deformation Behavior Research Articles

Interface dominated deformation transition from inhomogeneous to apparent homogeneous mode in amorphous/amorphous nanolaminates

• A length-scale-dependent mechanical properties were observed in the A/ANLs. • The hardness of A/ANLs was closely related with their deformation mode. • The deformation mode of A/ANLs was determined by the interface. The amorphous/amorphous nanolaminates (A/ANLs) have aroused great attentions owing to their tunable structure and enhanced mechanical properties. However, the plastic deformation mechanism of A/ANLs have yet been clarified. Here, we systematically examined the mechanical properties and deformation behavior of series of NiNb/ZrCuNiAl A/ANLs via nanoindentaion test. It was found that both the amount and morphology of amorphous/amorphous interface (A/AIs) played crucial roles in the plastic deformation of A/ANLs. Less and straighter A/AIs facilitated multiple shear banding deformation, of which the hardness increased with decreasing layer thickness, as the A/AIs hindered the propagation of shear bands (SBs). Whilst, more and wavier A/AIs promoted homogeneous deformation, of which the hardness stayed at a much lower value and was relatively irrelevant with the layer thickness, for the promoted activation of shear transformation zones by A/AIs. Our results provide guidance for modifying the mechanical properties of amorphous alloys with interface engineering design.

Mechanical response of inclined TBM tunnel due to drainage settlement of deep sandstone aquifer

Drainage of deep aquifers can cause stratum settlement, which has significant effects on the architectural structures within them. Especially for inclined TBM tunnels, the settlement may lead to the longitudinal differential deformation and damage of segments. However, most previous studies on the effects of aquifer drainage settlement on deep structures have focused on the vertical shaft in coal mines, and little attention has been paid to inclined TBM tunnels in deep sandstone aquifers. Through the physical model test and numerical simulation, in this study, the mechanical characteristics and deformation behavior of an inclined TBM tunnel in the deep water-bearing sandstone and impermeable mudstone before and after the aquifer drainage were investigated. The distribution and variation of stresses, internal forces, and deformation on tunnel segments before and after drainage were systematically analyzed along the tunnel longitudinal and circumferential directions. Results show that the whole tunnel is always under compression and mainly subjected to the coupling effect of bending moment and axial compression in the longitudinal direction. After drainage, the longitudinal and circumferential stresses as well as circumferential axial force increase remarkably at the tunnel sidewall but decrease at the tunnel crown and invert. While for the circumferential bending moment, by contrast, it increases at the three positions. In addition, the tunnel experiences a noticeable bending at the junction of the two strata after drainage due to the synchronous settlement with the sandstone, resulting in severe stress concentrations and the formation of the longitudinal shear band at the stratigraphic junction.

High-temperature plastic deformation of langle 110rangle-oriented BaTiO3 single crystals

BaTiO3 single crystals were deformed in compression along the langle 110rangle crystal axis to study the plastic deformability and dislocation structures at high temperatures under different loading conditions. The yield strength is determined from stress–strain curves under strain rate control, load control, strain rate cycling tests, and under step-wise loading conditions to elucidate the impact of measurement approach in yield strength behavior. A comparison between the chosen methods based on stress-dependent strain rate plots indicates that load control measurements are a suitable alternative to the commonly used strain rate-control experiments in metals. This allows avoiding overloading and providing an estimate of the overall achievable strain rates in a ceramic. Activation energies and activation volumes in the temperature range of 1100–1170 °C indicate a similar mechanical deformation behavior to SrTiO3.Graphical abstract

Isogeometric sizing and shape optimization of 3D beams and lattice structures at large deformations

A computational method for optimizing the shape of the centerline curve and the spatial variation of geometric and material sizing parameters of the cross-sections of elastic, 3-dimensional beams and beam structures subject to large deformations is presented in this work. The approach is based on the concept of isogeometric analysis, i.e., the representation of geometry and the discretization of the numerical solution using spline functions. Here, mixed isogeometric collocation methods are used to discretize the geometrically exact 3D beam model. These spline representations are extended to the parameterization of the design variables, which are the initial centerline curves of the beams, as well as cross-sectional sizing properties, which may be varying along the beam axis and can be functionally graded through the cross-sections. To tailor the mechanical deformation behavior of a beam or beam structure, a nonlinear optimization problem is formulated and solved using gradient-based methods. For this purpose, all required gradients and sensitivities are derived analytically. The potential of this holistic design optimization approach is demonstrated in application to tailoring of elastic metamaterials and beam lattice structures, as well as 4D printing of multi-material laminate beams.

Micro-tensile testing of the bond line in hot isostatic pressed aluminum

Considerable effort is being devoted to development and regulatory qualification of low enriched fuels for research and test reactors by many agencies worldwide. One promising fuel configuration being examined for United States higher power research and test reactors (USHPRRs) are plate-type fuels composed of a metallic uranium-molybdenum foil clad in an aluminum alloy. The two pieces of aluminum alloy cladding are bonded using a hot isostatic pressing method. The mechanical properties of the resulting bond line in the aluminum alloy cladding will vary by the HIP'ing parameters, requiring a need to characterize the bond line. Small scale mechanical testing can provide a path for evaluating the mechanical properties and deformation behavior of the bond line both prior to and following irradiation. In this research, room temperature micro-tensile specimens of non-irradiated and irradiated samples containing an Al alloy (AA 6061) bond line were tested to evaluate its strength and deformation behavior. Observations indicated that the strain rate did not affect the deformation behavior or strength and most of the micro-tensile specimens failed in a ductile mode in grains around the bond line. There was no indication that the microstructural features from the bond line affected the mechanical properties of the micro-tensile specimens. An initial examination was performed on irradiated material but further systematic studies of the effects of irradiation can be performed in the future.

Deformation and fracture behaviour of alumina-zirconia multi-material nanocomposites reinforced with graphene and carbon nanotubes

An effort to improve the low fracture toughness of monolithic alumina (Al2O3) ceramic demands microstructural manipulation with nanostructured materials such as zirconia (ZrO2), Graphene (GN) and Carbon Nanotubes (CNTs). Despite these attempts, the fundamental understanding of the mechanical properties and deformation behaviour of multiple combinations of these second-phase additives within the advanced Al2O3 structure are still being explored. In this respect, Al2O3-based nanocomposites reinforced with optimum amounts of ZrO2(10 wt%), GN(0.5 wt%) and CNTs(2 wt%) were evaluated via hot-pressing at 1600 °C, preceding colloidal mixing of the Al2O3 matrix and nanostructured additives. Mechanical properties such as fracture toughness (KIC) and bending strength (σf) measured using three-point bending techniques including single edge notched beam (SENB) test, and direct crack measurement (DCM) methods were calculated from the near-dense fabricated multi-material nanocomposites and benchmarked against the monolithic Al2O3 material under similar experimental conditions. From the conventional SENB test, the fracture toughness values increased by up to 60% on the addition of only 10 wt%ZrO2 to the monolithic Al2O3, whilst a drastic increase by up to 159% was achieved with the incorporation of both GN and CNTs within the Al2O3–ZrO2 structure, as compared to the monolithic Al2O3. The KIC values obtained from SENB tests were typically lower than the DCM method, but similar trend in the toughness behaviour were realized regardless. Bending strength of the prepared monolithic Al2O3 was also increased by up to 46% with the combined additions of ZrO2, GN and CNTs. Toughening and strengthening mechanisms including pull-outs by GN and CNTs, crack arrest and cack bridging were identified as the main source of enhancement in the mechanical properties of the multi-material nanocomposites. The addition of carbon additives also influenced monoclinic and tetragonal ZrO2 phase transformations, which also contributed to the overall mechanical behaviour of the Al2O3-10 wt%ZrO2 nanocomposites.

Effect of graphene on the mechanical properties of metallic glasses: Insight from molecular dynamics simulation

The effect of the graphene on the mechanical properties of the Cu50Zr50 metallic glass (MG) is investigated by molecular dynamics simulation method. The results show that the introduction of graphene can increase the strength of the MG and also enhance its plastic deformation ability. The results also indicate that the mechanical properties and deformation behavior of the MG/graphene nanolaminates (MGGNLs) are closely related to the graphene embedded position. It is worth highlighting that there is a threshold for the embedding position of graphene, which makes the plastic deformation of the MGGNL reach uniform deformation. With the increase of graphene insertion distance, the plastic deformation mode of the MGGNL changes from the interaction of multiple shear bands (SBs) to uniform deformation, and ultimately to a dominant SB propagation. The results indicate that the high-strength and high-plasticity MGGNLs can be obtained by introducing graphene and optimizing its insertion position.

Hydrogen-Related with the Change of Mechanical Properties and Deformation Behavior of 316l Austenite Stainless Steel

Hydrogen-Related with the Change of Mechanical Properties and Deformation Behavior of 316l Austenite Stainless Steel

Compressive properties of imperfect Ti-6Al-4V lattice structure fabricated by electron beam powder bed fusion under static and dynamic loadings

Mechanical response and deformation behavior of Ti-6Al-4V lattice structure are investigated by quasi-static and low-speed impact compression experiments. The lattice specimens with SC-BCC-FCC cells are designed and fabricated by electron beam powder bed fusion (EB-PBF). The nominal stress-strain and stress-time histories of the printed lattice structures under different loading speeds are determined by an electronic universal testing machine and a drop hammer system, respectively. The corresponding global collapse evolution of the specimens is recorded and analyzed. Meanwhile, the high-resolution scanning electron microscope (SEM) and X-ray computed tomography (CT) are adopted to capture the morphology, occurrences, and distributions of the geometric defects induced by the printing process. Based on the distribution characteristics of the defects, a novel statistical finite element model is proposed by considering the strut waviness and thickness variation to predict the mechanical performance and reveal the detailed deformation mechanism of the lattice. A good agreement is obtained between the statistical model predictions and experimental results. Furthermore, the role of each geometric defect on the initial peak stress of the lattice is investigated. The results demonstrate that the strut thickness variation in the lattice structure presents a more significant influence on initial peak stress than the strut waviness under quasi-static and low-speed impact loadings.

Surface-Condition-Dependent Deformation Mechanisms in Lead Nanocrystals.

Serving as nanoelectrodes or frame units, small-volume metals may critically affect the performance and reliability of nanodevices, especially with feature sizes down to the nanometer scale. Small-volume metals usually behave extraordinarily in comparison with their bulk counterparts, but the knowledge of how their sizes and surfaces give rise to their extraordinary properties is currently insufficient. In this study, we investigate the influence of surface conditions on mechanical behaviors in nanometer-sized Pb crystals by performing in situ mechanical deformation tests inside an aberration-corrected transmission electron microscope (TEM). Pseudoelastic deformation and plastic deformation processes were observed at atomic precision during deformation of pristine and surface-oxidized Pb particles, respectively. It is found that in most of the pristine Pb particles, surface atom diffusion dominates and leads to a pseudoelastic deformation behavior. In stark contrast, in surface-passivated Pb particles where surface atom diffusion is largely inhibited, deformation proceeds via displacive plasticity including dislocations, stacking faults, and twinning, leading to dominant plastic deformation without any pseudoelasticity. This research directly reveals the dramatic impact of surface conditions on the deformation mechanisms and mechanical behaviors of metallic nanocrystals, which provides significant implications for property tuning of the critical components in advanced nanodevices.

Dynamic mechanical response and deformation behavior of a novel hierarchical cellular structure material – The thousands of eyes Bodhi

Natural cellular structures encourage researchers to seek new design inspirations for impact protection engineering materials. The thousands of eyes Bodhi (TEB) is a novel hierarchical cellular structure material, which includes the first order filled-cells, the second order novel-closed-cells, and the third order open-cells. In this study, dynamic compressive tests at different high strain rates in the longitudinal and transverse directions were conducted by a split Hopkinson pressure bar (SHPB), respectively. A high-speed camera was employed to capture the macroscopic deformation mechanisms and a scanning electron microscope (SEM) was employed to study the microscopic failure mechanisms. The dynamic mechanical response and strain rate effects of TEB samples in the transverse and longitudinal directions were investigated. The anisotropic behavior of TEB samples is observed under dynamic compression tests. The tests results show that the dynamic mechanical properties of TEB samples and dynamic deformation behavior of each order cellular structure exist a significant strain rate dependence. The TEB material dissipates the impact energy through the dynamic compression mechanisms of each order cellular structure which improves the energy absorption and impact resistance. Thus, it provides a bio-inspired template for the design of energy absorption and crashworthiness engineering materials.

Effects of thermo-oxidative aging on 3-D deformation field and mechanical behaviors of 3-D angle-interlock woven composites

The effect of thermo-oxidative aging on mechanical properties is a key factor in designing composites structures. This paper aims to explore the effects of thermo-oxidative aging on the mechanical behavior, surface deformation and damage evolution of 3-D angle-interlock woven composites (3DAWC) under compression. A bilinear stress-strain curve with high failure strength and compressive modulus were obtained for the unaged sample. The transition region of the stress-strain curve disappeared for the aged sample due to epoxy resin degradation and cracks. The transition strain decreases as the interface damage starts earlier after thermo-oxidative aging. The surface strain, off-plane displacement and Poisson's ratio distribution show structural correlation during compression. Both the surface average strain and the average Poisson's ratio decrease with aging time. A finite element model has been established to explore the mechanical degradation mechanisms.

Constitutive modeling of multiscale polycrystals considering grain structures and orientations

Traditional macro-scaled plasticity theories may not be fully valid and accurate in study of meso-/micro-scaled deformation due to the existence of size effects (SEs). In meso/microforming, there are only several to ten grains in the deformation zone. The anisotropy of individual grains significantly affects the mechanical responses and deformation behaviors of polycrystalline metallic materials. Meanwhile, grain boundaries are isotropic and generally considered as barriers to plastic deformation. In this research, the constitutive modeling considering the orientation and boundary-interior difference of grains was developed and validated by the SE-based experiments and simulations with different specimens and grain sizes. In compression tests, discontinuous shear bands are found to be generated with a zig-zag distribution in the coarse-grained specimen, but long and continuous shear bands appear in the fine-grained specimen. In tensile and meso-heading tests, microcracks initiate at grain boundary regions and grow along grain boundaries. More and smaller microcracks exist in the fine-grained material, while coarse grains facilitate the formation of larger cracks. Furthermore, compared with the crystal plasticity simulation, the simulation using the proposed modeling loses a little accuracy in prediction of freeform geometry in compression but has the advantages of high simulation efficiency and accurate prediction of flow stress. This modeling is thus suitable for addressing the meso-/micro-scaled engineering deformation problems of polycrystalline materials.

Finite Element Analysis on Initial Crack Site of Porous Structure Fabricated by Electron Beam Additive Manufacturing.

Ti6Al4V specimens with porous structures can be fabricated by additive manufacturing to obtain the desired Young’s modulus. Their mechanical strength and deformation behavior can be evaluated using finite element analysis (FEA), with various models and simulation methodologies described in the existing literature. Most studies focused on the evaluation accuracy of the mechanical strength and deformation behavior using complex models. This study presents a simple elastic model for brittle specimens followed by an electron beam additive manufacturing (EBAM) process to predict the initial crack site and threshold of applied stress related to the failure of cubic unit lattice structures. Six cubic lattice specimens with different porosities were fabricated by EBAM, and compression tests were performed and compared to the FEA results. In this study, two different types of deformation behavior were observed in the specimens with low and high porosities. The adopted elastic model and the threshold of applied stress calculated via FEA showed good capabilities for predicting the initial crack sites of these specimens. The methodology presented in this study should provide a simple yet accurate method to predict the fracture initiation of porous structure parts.

Investigations of mechanical properties and deformation behaviors of the Cr modified Ti–Au shape memory alloys

In view of the population aging issues, the functional materials, Ti–Au–Cr shape memory alloys (SMAs), have been explored in this study for the crucial biomaterial applications. This ternary system was chosen for solving the hypersensitivity problem in the conventional SMAs containing Ni and also for its high X–ray contrast, which is an essential requirement in medical applications. Physical metallurgy methods were utilized for the specimen preparation and the fundamental properties and functional performances were also evaluated. Cold–workability was greatly improved with the Cr addition. The parent β–phase was stabilized by 4.5 at.% Cr alloying concentration, while the discrimination of the α′–martensite phase and the α′′–martensite phase at low Cr addition concentrations was clarified. Lattice parameters and the deformation strains were further calculated based on the results of the X–ray diffraction analysis. Most of the specimens possessed shape recovery upon heating, while the Ti–4.0Au–5.5Cr alloy showed an almost perfect shape recovery rate of 96.7%. Pseudoelastic behavior was observed in most of the alloys. The austenite transformation temperatures were determined by the step–by–step heating processes.

COUPLED HYDRO-MECHANICAL MODELING OF GAS FLOW IN SHALE MATRIX CONSIDERING THE FRACTAL CHARACTERISTICS OF NANOPORES

To economically develop shale gas reservoirs, hydraulic fracturing treatments must be applied to artificially introduce pathways for gas to flow out. After hydraulic fracturing, both the induced hydraulic fractures and activated natural fractures provide high permeable channels for gas transport. However, the long-term production performance of shale gas reservoirs is determined by the physical properties of matrix instead of the fractures. In this work, a coupled hydro-mechanical model is developed to investigate gas flow in heterogeneous shale matrix at the microscale. This model considers the mechanical interaction and gas mass transfer between organic matter (OM) and inorganic matter (iOM) during gas depletion. The interesting results show that due to matrix heterogeneity, i.e. co-existence of OM and iOM, the gas flow and mechanical deformation behaviors in shale exhibit regional differences. Specifically, during reservoir depletion, pore pressure, Knudsen number and volumetric strain in iOM are smaller than that in OM while the intrinsic and apparent permeability of OM are smaller than that of iOM. Thus, matrix heterogeneity should be taken into account when characterizing shale gas flow. In addition, sensitivity analysis is conducted to study the influence of fractal parameters on shale apparent permeability evolution. These fractal parameters include nanopore radius fractal dimension, tortuosity fractal dimension and maximum nanopore radius. The analysis indicates that the variation of fractal parameters in iOM results in the change of apparent permeability of both OM and iOM, while the variation of fractal parameters in OM only leads to the change of apparent permeability of OM. The modeling results offer new insights into gas flow behaviors in shale matrix.

Microstructure and mechanical properties of low-carbon steel fabricated by electron-beam additive manufacturing

The microstructure and mechanical properties of a billet obtained by electron-beam additive manufacturing (EBAM) using an industrial welding wire of low-carbon steel were studied. After EBAM, the steel billet possesses the phase composition constant in volume (ferrite with carbides). Deformation behavior of samples of steel obtained by the additive methods depends on their position in the billet. In its lower part, which is characterized by a high cooling rate in the EBAM-process, predominantly equiaxed ferrite grains with an average size of 15 μm are formed. The mechanical properties and deformation behavior (the presence of a yield plateau, the stages of plastic flow) of this part of the billet are close to those of normalized low-carbon steel obtained by conventional methods of metallurgy and thermomechanical processing. In the middle and top parts of the billet, the coarse non-equiaxed ferritic grains (hundreds of micrometers) form. The mechanical properties in this part weakly depend on the position of the samples and their orientation relative to the building direction of the billet — the yield plateau disappears; the yield stress, the ultimate tensile strength, the elongation become lower than those of the normalized low-carbon steel obtained by the conventional method. Despite the predominance of ferritic grains (with carbides), a small portion of grains with a lamellar ferrite morphology resembling martensite or bainite is observed in all parts of the billet. The latter is the result of a complex thermal history of the billet, which can undergo multiple phase transformations during successive heating and cooling cycles.

СОВРЕМЕННЫЕ ТЕНДЕНЦИИ В РАЗВИТИИ МЕТОДОВ И СРЕДСТВ ЭКСПЕРИМЕНТАЛЬНОЙ МЕХАНИКИ. ЧАСТЬ 1

The article considers a number of new directions in the development of methods and means of experimental mechanics. A brief analysis of the trends is given. They are: a) the size reduction of objects (models, samples) of laboratory tests, their unification, as well as the associated miniaturization of equipment and measuring instruments, b) the emergence and development of methods for experimental research of the mechanical characteristics of nanomaterials. When analyzing the trend (a), the characteristics of the EM-6705 installation developed at the Planar JSC are given. The installation is designed to control the strength of wire bridges in electronic products, control the shear strength of bulk leads and determine the relative elongation of the wire. It is as well intended for use in a personal testing center for wear-fatigue testing of materials created for universities by joint efforts of Belarusian State University, S&P GROUP TRIBOFATIGUE Ltd. and the Joint Institute of Mechanical Engineering of the NAS of Belarus within the framework of the task of the State Scientific and Technical Program “Standards and Scientific Instruments”. A brief description is given for the methods of the experimental study of mechanical properties and deformation behavior at the micro- and nanoscale structural levels.

Effects of long-term thermal aging on elevated temperature deformation behaviors of wrought 316LN stainless steel by small punch test

The deformation behaviors of as-received and thermal aged wrought 316LN austenitic stainless steel (ASS) were investigated at 350 °C by small punch test (SPT) in order to evaluate the effects of long-term thermal aging on the elevated temperature deformation behaviors of the wrought ASS. The results showed that long-term thermal aging at 400 °C had obvious influences on the density and structure of dislocations, hence affecting the deformation mechanism and mechanical behaviors. There were many planar slip bands and tangled dislocations in the as-received wrought 316LN ASS, while, a low-density dislocation multiples, coplanar dislocation arrays, stacking faults and extended dislocations were formed due to the microstructure recovery during the long-term thermal aging. Correspondingly, both dislocations slip and deformation twinning played an important role in plastic deformation in the as-received wrought 316LN ASS, while interaction of dislocations and stacking faults inclined to dominate the plastic deformation process of the thermal aged 316LN ASS under the elevated temperature. As a result, long-term thermal aging resulted in the change of the mechanical properties of the wrought 316LN ASS. The hardness and SPT maximum load of 316LN ASS had a slight increase, while the SPT total energy exhibited a reverse trend with increasing aging time, indicating that the 316LN ASS showed a tendency of hardening after the long-term thermal aging.

Investigation on microstructures, compressive properties and energy absorption capacity of carbon nanotubes/aluminum composite foam-filled tubes

This study presents the results of quasi-static compressive tests of aluminum (Al) foam-filled tubes (A-FFTs) and Carbon Nanotubes (CNTs)/Al composite foam-filled tubes (C-FFTs) prepared by in-situ filling space-holder method. The effects of processing parameters and interface interaction between foam fillers and tubes on mechanical properties and deformation behaviors of the C-FFTs are analyzed, through both the macro- and micro-scopic characterizations. The results show that both cold pressing and sintering can improve the interface connection between foam fillers and tubes. Besides, the addition of CNTs not only increases the strength of foam fillers, but also changes the deformation mode of foam fillers from layer-by-layer shear failure to overall shear failure, which in turn influences the mechanical properties of foam-filled tubes (FFTs). Meanwhile, the interface between foam fillers and tubes (macro interface) of the A-FFTs shows a metallurgical bonding because of the atom diffusion during cold pressing and sintering processes, and as a result, the energy absorption capability of the A-FFTs and 2.5 wt% C-FFTs are increased by 51.3% and 45.8%, respectively. Based on the current findings, a distinctive deformation behavior associated with the interaction effect is proposed.