Uniaxial Deformation Research Articles

Material behaviour of unstabilised earth block masonry and its components under compression at varying relative humidity

In this study, the material behaviour of unstabilised earth block masonry consisting of different block and mortar types is analysed with particular regard to the influence of varying relative humidity. The uniaxial compressive strength and deformation characteristics of unstabilised earth blocks and mortars as well as of unstabilised earth block masonry are studied in detail and compared to conventional masonry to evaluate whether the structural design can be made accordingly. An increase of 30 % points in relative humidity leads to a reduction of the masonry´s compressive strength between 33 % and 35 % whereas the Young´s modulus is reduced by 24–29 %. However, the ratio between the Young´s modulus and the characteristic compressive strength of earth block masonry ranges between E33/fk = 283–583 but is largely independent of the relative humidity. The results show that the mechanical properties of the investigated unstabilised earth block masonry are sufficient for load-bearing structures, yielding a masonry compressive strength between 2.3 MPa and 3.7 MPa throughout the range of moisture contents investigated. In general, the design concept of conventional masonry can be adapted for unstabilised earth masonry provided that the rather low Young´s modulus as well as the moisture dependence of both, compressive strength and Young´s modulus, are sufficiently taken into account.

Micro-mechanisms of failure in nano-structured maraging steels characterised through in situ mechanical tests

High-pressure-torsion (HPT) processing introduces a large density of dislocations that form sub-grain boundaries within the refined nano-scale structure, leading to changes in precipitate morphology compared to hot-rolled maraging steels. The impact of such nanostructuring on the deformation and fracture micro-mechanisms is being reported for the first time using in situ characterization techniques along with transmission electron microscopy and atom probe tomography analysis, in this study. Digital image correlation has been used to quantify the full field strain maps in regions of severe strain localization as well as to determine the fracture toughness through critical crack tip opening displacements. It is seen that the phenomenon of planar slip leads to strain softening under uniaxial deformation and to crack branching under a triaxial stress state in hot rolled maraging steels. On the other hand, nano-structuring after HPT processing creates a large number of high angle grain boundaries as dislocation barriers, leading to strain hardening under uniaxial tension and nearly straight crack path with catastrophic fracture under triaxial stress state. Upon overaging, the hot-rolled sample shows signature of transformation induced plasticity under uniaxial tension, which is absent in the HPT processed overaged samples, owing to the finer reverted austenite grains containing higher Ni concentration in the latter. In the overaged fracture test samples of both the hot-rolled and HPT conditions, crack tips show a signature of strain induced transformation of the reverted austenite to martensite, due to the accompanying severe strain gradients. This leads to a higher fracture toughness even while achieving high strengths in the overaged conditions of the nanocrystalline HPT overaged samples. The results presented here will aid in design of suitable heat treatment or microstructure engineering of interface dominated nano-scale maraging steels with improved damage tolerance.

Infill Strategy in 3D Printed PLA Carbon Composites: Effect on Tensile Performance.

Tuning the infill pattern is one of the key features in additive manufacturing to optimise part weight. In this work, the effect of the infill strategy, including rate and pattern type, is studied on the mechanical performance of polylactic acid (PLA)-carbon composite. In particular, three types of patterns and four filling levels are combined. These combinations are evaluated by tensile loading applied on dogbone specimens. In addition, the underlined deformation mechanisms are further explored using filament-based finite element model. The numerical simulation is built from sliced models and converted into 3D meshes to predict tensile performance. The results show that the infill rate has a nonlinear effect on the density of PLA–carbon composites, and its magnitude depends on the complexity of the generated pattern. In addition, tensile loading is found to activate varied modes of shearing and uniaxial deformations depending on the pattern type. This leads to different profiles and rankings of the tensile performance and allows the infill strategy to significantly affect the part performance, along with its density.

Investigation on uniaxial ratcheting fatigue behaviors and microstructure evolution of ultrafine-grained 6061 aluminum alloy

The motivation of the present study is to explore ratcheting-fatigue behavior and the corresponding micro-deformation mechanism of ultrafine-grained aluminum alloys 6061 (UFG AA6061) under two stress-controlled low-cycle fatigue (LCF) loading modes, i.e., mean tension stress (MTS) and mean compressive stress (MCS). The uniaxial ratcheting tests demonstrate that the fatigue-ratcheting damage of UFG AA6061 is prone to occur under MCS-LCF loading, rather than MTS-LCF loading. Detailed microstructure characterizations by Electron backscatter diffraction (EBSD) techniques under these two loading modes show that the grain aspect ratios decrease greatly and the fractions of high angle boundaries increase remarkably in the case of MCS-LCF loading. These observations are mainly ascribed to the room temperature continuous dynamic recrystallization (CDRX), in which low angle boundaries evolve into high angle grain boundaries and the formation of equiaxed grains is accelerated. In contrast, there were no obvious change in the grain aspect ratios or the fractions of high angle boundaries at MTS-LCF loading condition, while the Schmid factor of non-octahedral slip system {100} <110> increases and the texture evolve gradually. The deformation mechanism is grain boundary sliding (GBS) and grain rotation would accommodate the process of GBS. An analysis of the microstructure-properties relationship showed the superposition effect of residual shear deformation and uniaxial LCF ratcheting deformation is responsible for the CDRX at room temperature. And the occurrence of GBS is mainly caused by the activation of non-octahedral slip system.

Rheology of marine sponges reveals anisotropic mechanics and tuned dynamics.

Sponges are animals that inhabit many aquatic environments while filtering small particles and ejecting metabolic wastes. They are composed of cells in a bulk extracellular matrix, often with an embedded scaffolding of stiff, siliceous spicules. We hypothesize that the mechanical response of this heterogeneous tissue to hydrodynamic flow influences cell proliferation in a manner that generates the body of a sponge. Towards a more complete picture of the emergence of sponge morphology, we dissected a set of species and subjected discs of living tissue to physiological shear and uniaxial deformations on a rheometer. Various species exhibited rheological properties such as anisotropic elasticity, shear softening and compression stiffening, negative normal stress, and non-monotonic dissipation as a function of both shear strain and frequency. Erect sponges possessed aligned, spicule-reinforced fibres which endowed three times greater stiffness axially compared with orthogonally. By contrast, tissue taken from shorter sponges was more isotropic but time-dependent, suggesting higher flow sensitivity in these compared with erect forms. We explore ecological and physiological implications of our results and speculate about flow-induced mechanical signalling in sponge cells.

Mechanical properties of a two-phase high-entropy Fe50Mn30Co10Cr10 alloy down to ultralow temperatures

The mechanical properties comprising the stress-strain characteristics under uniaxial tensile deformation, the acoustic properties from mechanical resonance spectroscopy and—in parallel—the microstructural evolution during deformation of a nonequiatomic high-entropy alloy (HEA) Fe50Mn30Co10Cr10 have been studied in a wide temperature range, including ultralow temperatures down to 0.5 K. In the temperature range 300 to 4.2 K, a strong temperature dependence of the tensile strength occurs, hinting at the thermally activated nature of plastic deformation. Within the range of extremely low temperatures (4.2–0.5 K), however, the alloy exhibits anomalies of the yield strength, as well as discontinuous plasticity. Over the whole temperature range, the dynamic Young’s modulus of tensile deformed samples shows a reduction of absolute values compared to those of the undeformed ones, and at temperatures &amp;lt; 30 K a change of the temperature dependence from almost linear to power-law type. At all temperatures down to 0.5 K, the alloy’s plasticity stays as high as 50% as a consequence of a deformation driven martensitic phase transformation from fcc to hcp lattice (TRIP effect). Considering the ultralow deformation temperatures, the tensile strength reaches record values of 1513 MPa at 4.2 K, and still of 1274 MPa at 0.5 K, each being paired with significant strain hardening. These results suggest the HEA Fe50Mn30Co10Cr10 as a promising structural material for use in cryogenic environments down to extremely low temperatures.

Twinning and dynamic recrystallization behaviors during inchoate deformation of pre-twinned AZ31 Mg alloy sheet at elevated temperatures

To investigate the interrelation of dynamic recrystallization and detwinning behaviors, especially during the inchoate deformation stage at elevated temperatures, a commercial rolled AZ31 Mg alloy sheet is pre-twinned along the transverse direction to introduce {10−12} tensile twins, then the uniaxial stretch deformation is conducted. Results reveal that the detwinning dominates the microstructure evolution at room temperature, 100 °C, and 150 °C, while the dynamic recrystallization participates at temperatures of 200 °C and 250 °C. Further, the continuous dynamic recrystallization and detwinning accommodate the strain at 200 °C, while the discontinuous dynamic recrystallization suppresses the detwinning and predominates the deformation at 250 °C. Under different deformation temperatures, the interaction of dynamic recrystallization and detwinning exhibits different influences on the microstructure. Non-basal slips are activated at elevated temperatures, which affects the dynamic recrystallization behaviors and accommodates deformation strain. The texture evolution is closely related to the microstructure evolution mechanisms. The detwinning plays a hardening role in strength while dynamic recrystallization has a softening influence, the eventual effect on strength depends on the temperature.

Viscoelastic Properties of Soft Adhesives

AbstractIn this paper the viscoelastic behavior of different soft adhesives with substantially similar adhesive performance is analyzed in the light of a Slip‐Tube Model. In this model, the junction points between network chains and virtual chains are replaced by slip‐links attached to fluctuating ends of virtual chains. The slip‐links are allowed to slide along the contour of the network chains but not pass through each other. The predictions of the slip‐tube model are in excellent agreement with the experimental data on uniaxial deformation of soft adhesive networks. Furthermore, the study shows that some adhesive performances can be described qualitatively by Small Amplitude Oscillatory Shear (SAOS), and the in‐service properties can be described with adhesives with different viscoelastic spectra. Highly nonlinear uniaxial stretching tests reveal deeper insights into the description of the adhesives performances. Some well‐established models are adopted and slightly modified by introducing the rate‐dependence of their physical parameters to describe the response of soft adhesive to stretching.

The Investigation of Mechanical Properties of Polycrystalline Nb Nanowire Under Applied Tensile Deformation by Molecular Dynamics Simulation

In this study, the change in the mechanical properties of Niobium (Nb) nanowire with different grain numbers under applied uniaxial tensile deformation was tried to be investigated by Molecular Dynamics (MD) simulation method. The Embedded Atom Method (EAM), which includes many-body interactions, was used to determine the force interactions between atoms. To determine the effect of grain number on the mechanical properties of Nb nanowire, stress-strain curve, young modulus, yield strain and atomic images obtained from the common neighbor analysis method (CNA) were used. It has been determined that necking and breaking of the model nanowire occur at the grain boundaries, however, the number of grains has important effects on the mechanical properties.

Poly(L‐lactic acid) and poly(ε‐caprolactone) based ultra‐strong and tough thermoplastic polyurethane‐urea with multi‐urea segments and oriented microstructures

AbstractAlthough poly(L‐lactic acid) (PLA) has been widely studied and used in biomedical areas because of its well‐known biocompatibility, degradability and renewability, its potential application is still greatly limited by its brittleness and poor ductility. In this work, we, for the first time, reported a series of multi‐urea linkage segmented ultra‐strong thermoplastic polyurethane‐urea (PUU) copolymers using PLA and poly(ε‐caprolactone) as the mixed soft segments and water as an indirect in situ chain extender. The tensile strength, elongation at break and tensile modulus of the obtained PUU copolymers are 45–52 MPa, 380–540% and 180–430 MPa, respectively. After a simple one‐step uniaxial tensile deformation was applied to these PUU copolymers, their tensile strength and tensile modulus could be dramatically increased to 200 and 630 MPa, respectively, while maintaining enough good ductility (elongation at break &gt;50%). Scanning electron microscope and Wide‐angle X‐ray Diffraction results showed that the dramatically improved mechanical performance should be mainly attributed to their more oriented microstructures. This study provided an easy strategy to pursuing synthetic PLA based biopolymers combining with ultrahigh mechanical strength and biocompatible and degradable properties.

Deformation Behavior of Two-Phase Gradient Nanograined Fe95Ni5 Alloys under Different Types of Loading

In this paper, we used molecular dynamics simulations to study the atomic mechanisms of phase transformations, plasticity features, and mechanical properties of two-phase Fe95Ni5 (at. %) samples with a gradient nanograined structure under uniaxial deformation and shear. The simulated samples with a uniform distribution of Ni atoms are composed of fcc grains from 10 to 30 nm in size, which in turn contain bcc interlayers in the form of lamellae of various distribution and size. It was shown that uniaxial loading or shear causes the bcc-fcc phase transformation in the lamellae. In the vast majority of cases, phase transformations are initiated at the junction of lamellae and grain boundaries. Deformation-induced phase transformations in lamellae occur at the front of bands propagating from grain boundaries. Grains larger than ~15 nm can have several bands or regions with differently orientated fcc lattices, whose meeting results in grain fragmentation. It was found that the atomic volume increases abruptly during the bcc-fcc structural phase transformation. The Kurdyumov–Sachs orientation relation is valid between the initial bcc and formed fcc structures. It was shown that the volume fraction and spatial distribution of the bcc phase significantly affect the yield stress of the sample. The yield stress can be increased by forming the bcc phase only in large-grained layers. This behavior is associated with the fragmentation of large grains, and consequently with grain refinement, which, in accordance with the Hall–Petch relation, improves the strength of the material.

Influence of icosahedron and icosahedron-like on plastic deformation of Co50Mn50 alloy under compressive conditions

The establishment of the cluster-based structure-property relationship is of great significance to enhance the mechanical properties of metallic glasses (MGs). In this paper, molecular dynamics simulation has been used to obtain the uniaxial compression deformation process at four cooling rates. The results show that the molar volume and atomic average packing density increase with the increase of the cooling rate. The yield strength decreases, and the deformation mode gradually changes from localized deformation to delocalized deformation. The increase of icosahedron and icosahedron-like within the system impedes the plastic flow of MGs. The large-sized nanoclusters composed of icosahedron and icosahedron-like are destroyed during compression, resulting in a concentration of stress localization. The system has high initial resistance to plastic flow. The shear transition zone is concentrated to form shear bands, which reduces the plasticity of MGs. These results have marvelous theoretical guidance for improving the plasticity of MGs.

Electrical Conductivity of Rubber Composites with Varying Crosslink Density under Cyclic Deformation.

Studies addressing electroconductive composites based on rubber have attracted great interest for many engineering applications. To contribute to obtaining useful materials with reproducible behavior, this study focused on understanding the mechanism of conductivity changes during mechanical deformation for rubber composites based on styrene-butadiene rubber (SBR) or ethylene-propylene-diene terpolymer (EPDM) vulcanized for various times. The composites were characterized by static electrical conductivity, tensile testing, dynamic mechanical thermal analysis (DMTA), and crosslink density measurements. The tensile strength and Young’s modulus were found to increase significantly with rising vulcanization time. Higher static conductivity values of the composites were observed with the increase in vulcanization time. The most important aspect of this investigation consisted in the electrical current measurement online with recording the stress-strain curves, revealing the details of the uniaxial cyclic deformation effect on changes in the structure of conductive pathways indirectly. The electrical conductivity during five runs of repeated cyclic mechanical deformations for SBR composites increased permanently, although not linearly, whereas EPDM composites showed a slight increase or at least a nearly constant current, indicating healing of minor defects in the conductive pathways or the formation of new conductive pathways.

Microscopic observation of the effects of elongation on the polymer chain dynamics of crosslinked polybutadiene using quasi-elastic γ-ray scattering.

A synchrotron-radiation-based quasi-elastic γ-ray scattering system has been developed that uses time-domain interferometry to observe microscopic polymer dynamics under uniaxial deformation. The stress-producing mechanism of crosslinked polybutadiene has been studied from a microscopic viewpoint. It was found that the mean relaxation time ⟨τ⟩ of the microscopic polymer motion observed over a relatively high temperature (T) range (i.e. T-1 < 0.0045 K-1) increased with elongation on both the intra- and intermolecular scales. Following an extensive strain dependence study, it was found that the strain dependences of both the intra- and intermolecular ⟨τ⟩ changed with the stress dependence. It was therefore suggested that ⟨τ⟩ increased due to the constraint of the local polymer chain motion caused by elongation. The local molecular dynamics of polymer chains under uniaxial deformation could be evaluated at intra- and intermolecular scales separately for the first time using our method.

The effect of heating stage parameters on AlSi coating microstructure and fracture at high temperatures

AlSi coating fracture during press hardening of boron steel can significantly increase tool wear and reduce the product quality. During heating of AlSi-coated press hardening steel, the coating layer evolves due to diffusion. This results in various Fe − Al intermetallic compounds (such as FeAl, Fe2Al5), voids throughout the coating and rise in surface roughness. The goal of this study is to investigate the effect of heating parameters on AlSi coating micro-structure and thereby on its fracture behavior during deformation at elevated temperatures. Heating and quenching experiments are performed and later the coating layer is inspected under the microscope to check the distribution of intermetallics, voids and surface roughness. Subsequently, the coating fracture for the micro-structures resulting from different heating parameters is investigated during uniaxial tensile deformation at 700 °C. After the test, the severity of coating cracks is correlated in terms of the percentage of Fe-rich compounds and void distributions in the AlSi coating. The results show that by increasing the amount of Fe-rich compounds in the coating, its crack density is significantly reduced. Finally, it is shown that by adjusting the heating stage parameters, it is possible to further improve the ductility of coating micro-structure and minimize coating fracture upon tensile deformation.

PFC Simulation Study on Time-dependent Deformation Failure Properties and Energy Conversion Law of Sandstone under Different Axial Stress

Rock burst mostly happens in deep rock engineering, which obviously performs obvious time-dependent destruction and releases huge energy. Thus, the uniaxial compressive time-dependent deformation failure tests of sandstone under different axial stress are conducted and the optimized Parallel-bonded stress corrosion (PSC) model is established based on PFC method. The results show that tension micro-cracks are the direct reason of rock bearing capacity reduction during the whole failure process. The time-dependent deformation stage is composed of the initial strain stage, the steady-state strain stage and the accelerated strain stage. As the axial stress increases, rock mass enters into the accelerated strain stage more quickly and the strain amount of time-dependent deformation stage decreases. From the comparison of energy conversion between PFC simulation and experiments, the higher elastic strain energy will be accumulated and the less dissipation energy will be produced while the axial stress increases when reaching the failure point. After the failure point, the elastic strain energy has a small increase because the strain rate changes faster than the failure relaxation rate. Based on the calculation results of residual elastic energy index (AEF), the rock burst shows a tendency that from weakness to moderation as the axial stress increases. The research results will provide a certain reference in predicting the rock burst.

The enhancement of exciton-like luminescence in KCl single crystals under local and uniaxial elastic lattice deformation

Radiative annihilation of exciton-like formations (bound excitons) in KCl single crystals doped with various impurity cations (Li, Na, Sr) has been studied using highly sensitive luminescence methods. In KCl:Na with different sodium concentration, X-ray luminescence bands with the maxima at 2.8 and 3.1 eV, which and attributed to exciton-like formations in the region of local lattice deformation by single Na+ and pair Na+-Na+ centers, become significantly more intense with sodium impurity concentration. The luminescence efficiency of such bound anion excitons also increases in the KCl:Na crystals exposed to a uniaxial elastic stress along the 〈100〉 and 〈110〉 crystallographic directions. The influence of temperature as well as lattice symmetry lowering due to applied elastic deformation (external action) or local deformation in the vicinity of impurity ions (internal action) on the efficiency of the recombination formation of bound excitons has been analyzed.

Versatilely Manipulating the Mechanical Properties of Polymer Nanocomposites by Incorporating Porous Fillers: A Molecular Dynamics Simulation Study.

Polymer nanocomposites (PNCs) have been attracting myriad scientific and technological attention due to their promising mechanical and functional properties. However, there remains a need for an efficient method that can further strengthen the mechanical performance of PNCs. Here, we propose a strategy to design and fabricate novel PNCs by incorporating porous fillers (PFs) such as metal-organic frameworks with ultrahigh specific surface areas and tunable nanospaces to polymer matrices via coarse-grained molecular dynamics simulations. Three important parameters─the polymer chain stiffness (k), the interaction strength between the PF center and the end functional groups of polymer chains (εcenterend), and the PF weight fraction (w)─are systematically examined. First, attributed to the penetration of polymer chains into PFs at a strong εcenterend, the dimension of polymer chains such as the radius of gyration and the end-to-end distance increases greatly as a function of k compared to the case of the neat polymer system. The penetration of polymer chains is validated by characterizing the radial distribution function between end functional groups and filler centers, as well as the visualization of the snapshots. Also, the dispersion state of PFs tends to be good because of the chain penetration. Then, the glass transition temperature ratio of PNCs to that of the neat systems exhibits a maximum in the case of k = 5ε, indicating that the strongest interlocking between polymer chains and PFs occurs at intermediate chain stiffness. The polymer chain dynamics of PNCs decreases to a plateau at k = 5ε and then becomes stable, and the relative mobility to that of the neat system as well presents the same variation trend. Furthermore, the mechanical property under uniaxial deformation is thoroughly studied, and intermediates k, εcenterend, and w can bring about the best mechanical property. This is because of the robust penetration and interaction, which is confirmed by calculating the stress of every component of PNCs with and without end functional groups and PF centers as well as the nonbonded interaction energy change between different components. Finally, the optimal condition (k = 5.36ε, εcenterend = 5.29ε, and w = 6.54%) to design the PNC with superior mechanical behavior is predicted by Gaussian process regression, an active machine learning (ML) method. Overall, incorporating PFs greatly enhances the entanglements and interactions between polymer chains and nanofillers and brings effective mechanical reinforcements with lower filler weight fractions. We anticipate that this will provide new routes to the design of mechanically reinforced PNCs.

Study on macro-meso mechanical properties of cemented tailings backfill with high fly ash content.

The use of cement and fly ash (FA) to prepare cemented tailings backfill (CTB) lowers backfill mining costs while also reducing pollution caused by the accumulation of waste materials like tailings and FA, making it a green backfill mining process. While adding FA to CTB may reduce costs, too much FA might weaken CTB's strength property. Mechanical tests were used to explore the effects of FA content and curing time on the uniaxial compressive strength (UCS) and deformation modulus of CTB in this research. The effects of FA content on failure modes, strain energy, and crack evolution of CTB were studied using a numerical model that considered FA content and particle contact mode. The influence mechanism of different FA contents on CTB was also revealed at the microscopic level. The results demonstrate that the UCS of CTB has a quadratic polynomial and a linear relationship with FA content and curing time respectively, and that the elasticity modulus and secant modulus of CTB increase and then decrease with FA content under different curing times. The peak strain energy of CTB increases and subsequently declines with the FA content, and crack propagation inside CTB may be limited by regulating the FA content. A reasonable content of FA can optimize the size and distribution of CTB microscopic defects, enabling them to exhibit superior strength property. This study systematically explores the mechanism of different FA contents on the strength property of CTB from a macro-micro perspective, providing an essential reference value for improving the recycling of FA and waste residues such as tailings.

Predict the evolution of mechanical property of Al-Li alloys in a marine environment

The ocean is one of the essential fields of national defense in the future, and more and more attention is paid to the lightweight research of Marine equipment and materials. This study it is to develop a Machine learning (ML)-based prediction method to study the evolution of the mechanical properties of Al-Li alloys in the marine environment. We obtained the mechanical properties of Al-Li alloy samples under uniaxial tensile deformation at different exposure times through Marine exposure experiments. We obtained the strain evolution by digital image correlation (DIC). The strain field images are voxelized using 2D-Convolutional Neural Networks (CNN) autoencoders as input data for Long Short-Term Memory (LSTM) neural networks. Then, the output data of LSTM neural networks combined with corrosion features were input into the Back Propagation (BP) neural network to predict the mechanical properties of Al-Li alloys. The main conclusions are as follows: 1. The variation law of mechanical properties of 2297-T8 in the Marine atmosphere is revealed. With the increase in outdoor exposure test time, the tensile elastic model of 2297-T8 changes slowly, within 10%, and the tensile yield stress changes significantly, with a maximum attenuation of 23.6%. 2. The prediction model can predict the strain evolution and mechanical response simultaneously with an error of less than 5%. 3. This study shows that a CNN/LSTM system based on machine learning can be built to capture the corrosion characteristics of Marine exposure experiments. The results show that the relationship between corrosion characteristics and mechanical response can be predicted without considering the microstructure evolution of metal materials.