Local Deformation Processes Research Articles

Analysis of changes in the coordinates of the “Tavaksay” geodynamic polygon

This article presents the results of GNSS and laser measurements at the “Tavaksay” geodynamic polygon. Describes the classical methods for determining the change in coordinates based on triangulation and leveling. Ways to improve the accuracy of local microplate tectonics using navigation systems are outlined here.The coordinates of the benchmarks were calculated in the rectangular GaussKruger coordinate system and the spatial system B, L, H for CK42 and WGS84.The trilateration method for determining the distances between points of the geodynamic network is analyzed. An assessment of the accuracy of the coordinates of points and the distances between them, depending on the number of measurement cycles, has been made.The accuracy of the trilateration and GNSS method for this network is graphically presented. The diagonal elements of the covariance matrix of GNSS measurements are investigated. It is proposed to make homogeneous GNSS and laser measurements at the tops of the hills at a certain time interval to study local deformation processes.

Fully integrated multi-scale modelling of damage and time-dependency in thermoplastic-based woven composites

In this work, a multi-scale model established from the concept of periodic homogenization is utilized to predict the cyclic and time-dependent response of thermoplastic-based woven composites. The macroscopic behaviour of the composite is determined from finite element simulations of the representative unit cell of the periodic microstructure, where the local non-linear constitutive laws of the components are directly integrated, namely, the matrix and the yarns. The thermoplastic matrix is described by a phenomenological multi-mechanisms constitutive model accounting for viscoelasticity, viscoplasticity and ductile damage. For the yarns, a hybrid micromechanical–phenomenological constitutive model accounting for anisotropic damage and anelasticity induced by the presence of a diffuse micro-crack network is utilized. The capabilities of the overall multi-scale model are validated by comparing the numerical predictions with experimental data. Further illustrative examples are also provided, where the composite undergoes time-dependent deformations under uni-axial and non-proportional multi-axial loading paths. The multi-scale model is also employed to analyze the influence of the local deformation processes on the macroscopic response of the composite.

Reconstruction of Heat Sources Induced in Superelastically Loaded Ni-Ti Wire By Localized Deformation Processes

Shape memory alloys (SMAs) are phase transforming materials featuring strong thermomechanical couplings. Infrared thermography and heat source reconstruction (HSR) enable to track the calorific signature of deformation processes. The objective was to characterize the transformation processes in a superelastic nickel-titanium SMA wire subjected to a force-controlled superelastic tensile cycle. In-situ recorded thermographs were converted into spatiotemporal maps of heat sources using an in-house developed post-processing method based on the heat diffusion equation resolved numerically for unknown heat sources. Sequentially appearing patterns of localized transformation events of four types were identified and associated with martensite bands nucleations and their subsequent merging upon tensile loading. Analogically, the events associated with austenite bands nucleations and their subsequent merging were identified upon unloading. In addition, weak heat sources observed before and after the localized transformation events were associated with the homogeneous martensitic transformation. The intrinsic dissipation heat associated with the nucleation and merging events is estimated to be ~ 25% of the released/absorbed latent heat.

Reinforcement of polypropylene with alkali-treated sugarcane bagasse fibers: Mechanism and consequences

Polypropylene composites were prepared from neat and alkali-treated sugarcane bagasse fibers. The results showed that alkali treatment leads to an increase in composite stiffness and strength. A maximum is achieved in these properties at around 5 wt% NaOH content of the treating solution. The increase in properties was assigned to the improvement in inherent fiber characteristics. Acoustic emission testing and electron microscopy showed that the two main local deformation processes related to the fibers are their fracture and debonding; the latter is accompanied by the shear yielding of the matrix. Increased inherent strength of the fibers results in an increase in the fracture initiation stress and fracture energy of the composites. Interfacial adhesion has a slight effect on stiffness, but more significant on strength and impact resistance. Changing adhesion modifies the relative importance of local deformation processes, the number of debonding events decreases, while fiber fracture increases with increasing adhesion. Increased interfacial adhesion improves stress transfer and the load bearing capacity of the fibers as well, but suppresses matrix yielding. Alkali treatment increases inherent fiber strength, which can be directly correlated with composite strength.

The role of irradiation on deformation-induced martensitic phase transformations in face-centered cubic alloys

Abstract

Comparative study of fiber reinforced PP composites: Effect of fiber type, coupling and failure mechanisms

A polypropylene (PP) homopolymer was reinforced with carbon fiber, glass fiber and wood flour in order to compare their advantages and drawbacks. Interfacial adhesion does not influence stiffness, but it affects strongly other properties, i.e. tensile strength and impact resistance. Several local deformation and failure processes take place simultaneously or consecutively in the composites during deformation. The fracture of wood fibers is the dominating process at good adhesion, while debonding, fiber pullout and fiber fracture occurs in carbon and glass fiber reinforced composites. Some of the local deformation processes do not result in the failure of the composites, while others lead to catastrophic failure. A close correlation was found between the characteristic stress determined by acoustic emission and the mechanical properties of the composites. Composites with advantageous properties, i.e. large stiffness, acceptable impact resistance and price, can be prepared by the proper selection of fiber type, composition and interfacial adhesion.

Reinforcement of PP with polymer fibers: Effect of matrix characteristics, fiber type and interfacial adhesion

Two polypropylene polymers with considerably differing properties were modified with poly(ethylene terephthalate) (PET) and poly(vinyl alcohol) (PVA) fibers. Fiber content changed from 0 to 60 vol% and interfacial adhesion was modified with the addition of a maleated PP coupling agent. Mechanical properties were characterized by tensile and impact testing, while structure was evaluated by acoustic emission measurements and scanning electron microscopy. Attention was focused on the balance of stiffness and impact resistance and on the relationship of local deformation processes to macroscopic properties. The results showed that different inherent matrix properties result in a dissimilar combination of properties. The strength of interfacial adhesion together with matrix properties determines the extent of reinforcement and the direction in the change of properties as a function of composition. Adhesion influences local deformation processes very strongly and these processes determine the macroscopic properties of the composites. Shear yielding induced by the debonding of fibers consumes much energy, while fiber fracture is less efficient in increasing impact resistance. Fiber characteristics are important for modulus and strength, but less significant for impact resistance. A good combination of stiffness and impact resistance can be obtained in PP homopolymers modified with PVA fibers, while this type of modification is less efficient and/or advantageous in elastomer modified PP.

Structure of sheet blanks from VT6S titanium alloy after local deformation during pulse heating by electric current

The local deformation process of blanks in the form of thin discs from VT6S titanium alloy with the use of pulse heating by electric current and deformation in heated state in the electrodes of the point contact welding machine is studied. The rational values of the process parameters (pulse current, pulse duration and precipitation force) are defi ned. Analysis of the microstructure for the VT6S alloy in its original state and after deformation with heating did not reveal signifi cant increase in grain and the formation of thick alpha layer. The increasing in the plasticity of the VT6S alloy after passing of the current pulse is established. At the same time, in the structure of the deformed zone noted the presence of double.

CONTEMPORARY GEODYNAMICS OF THE GAROMAI ACTIVE FAULT (SAKHALIN ISLAND)

In the northern Sakhalin Island, the tectonic activity of the fault zones is a potential threat to the industrial infrastructure of the petroleum fields. Recently, the background seismicity has increased at the Hokkaido‐Sakhalin fault that consists of several segments, including the Garomai active fault. In the studies of the regional deformation processes, it is important not only to analyze the seismic activity, but also to quantitatively assess the dynamics of deformation accumulation in the fault zones. In order to study the contemporary geodynamics of the Garomai fault, a local GPS/GLONASS network has been established in the area wherein trunk oil and gas pipelines are installed across the fault zone. Based on the annual periodic measurements taken in 2006–2016, we study the features of surface deformation and calculate the rates of displacements caused by the tectonic activity in the fault zone. During the survey period, no significant displacement of the fault wings was revealed. In the immediate vicinity of the fault zone, multidirectional horizontal displacements occur at a rate up to 1.6 mm/yr, and uplifting of the ground surface takes place at a rate of 3.4 mm/yr. This pattern of displacements is a reflection of local deformation processes in the fault zone. At the western wing of the fault, a maximum deformation rate amounts to 1110–6 per year. The fault is a boundary mark of a transition from lower deformation rates at the eastern wing to higher ones at the west wing. In contrast to the general regional compression setting that is typical of the northern Sakhalin Island, extension is currently dominant in the Garomai fault zone. The estimated rates of relative deformation in the vicinity of the Garomai fault give grounds to classify it as ‘hazardous’.

Deformation and failure of sugarcane bagasse reinforced PP

Polypropylene composites were prepared from sugarcane bagasse fibers by extrusion and injection molding. Wood flour was used as reference filler in the study. The fiber content of the composites changed between 0 and 30 wt% in 5 wt% steps. Maleated polypropylene was used as coupling agent to improve interfacial adhesion. Mechanical properties were characterized by tensile and fracture testing, while local deformation processes were followed by acoustic emission and instrumented impact testing, as well as by the analysis of scanning electron micrographs. The results showed that sugarcane bagasse fibers reinforce polypropylene similarly to other natural fibers. They increases stiffness, but decrease tensile yield stress, tensile strength and deformability. Increased interfacial adhesion leads to the considerable improvement of reinforcement. Bagasse fiber and wood flour filled composites have very similar properties. The impact resistance of the composites increased in the presence of both fibers compared to the neat matrix. Debonding is the dominating process in the absence of the coupling agent, while mainly fiber fracture occurs in its presence. Increased plastic deformation after debonding results in slightly improved impact resistance.

Deformation and damage of random fibrous networks

Fibrous networks are encountered in various natural and synthetic materials. Typically, they have random microstructures with complex patterns of fibre distribution. This microstructure, together with significant (in many cases) stretchability of such networks, results in a non-trivial load-transfer mechanism, different from that of continuous media. The aim of this study is to investigate evolution of local deformation, damage and fracture processes in fibrous networks. In order to do this, together with extensive experiments, discontinuous finite-element (FE) models with direct incorporation of microstructural features were developed using a parametric approach for specimens with various dimensions and different types of notches. These models, mimicking a microstructure of the selected fibrous network, were loaded by stretching along a principal direction. Discontinuous FE models provided data not only on a global response of the specimens but also on levels of stresses and strains in each fibre, forming the network. An effect of a notch shape on evolution of fibre strains as well as mechanisms and patterns of damage was investigated using experimental data and simulation results, assessing also toughness of specimens. Strain distribution over selected paths were tracked in notched specimens to quantify strain distributions in the vicinity of notch tips. The growth and patterns of local damage due to axial stretching obtained in advanced numerical simulations with the developed FE models demonstrated a good agreement with experimental observations.

Process mechanism in laser peen texturing artificial joint material

Friction and wear of artificial joints seriously restrict the service life of them. Surface texturing has been recognized as an effective means to reduce friction coefficient and improve wear resistance of artificial joints. In this study, a new and promising technology named laser peen texturing (LPT) is proposed to fabricate dimple textures on artificial joints. Process applicability of LPT on artificial joint material 316 L stainless steel is studied by experiments. Experimental results show that LPT is capable of fabricating dimple arrays on 316 L stainless steel and has good controllability and repeatability. Fundamental process mechanism of LPT is explored by numerical simulation. A 2D axisymmetric and semi-infinite finite element analysis (FEA) model has been developed and validated to predict the stress distribution and deformation process of the biomedical metal. The simulated dimple profiles are well consistent with the measured ones. Stress analysis shows obvious residual compressive stress after LPT. The maximum residual compressive stress and the thickness of the residual compressive stress zone are sensitive to LPT processing parameters. The local dynamic elasto-plastic deformation process of the material is discussed based on the deformation analysis at different time.

Sliding friction on particle filled epoxy: Developing a quantitative model for complex coatings

Epoxy resins represent an important class of thermosetting polymers that are extensively used in demanding applications like in scratch resistant coatings. Usually fillers, either hard (inorganic) or soft (rubbery), are added. Here we test hard and soft particle-filled epoxy systems in single asperity sliding friction experiments, and analyze the results with the hybrid numerical-experimental approach presented earlier. The focus is on the detailed modeling of the local deformation processes and it is confirmed that a rate-independent friction model proves appropriate to quantitatively model this complex process. The constitutive framework developed for amorphous thermoplastic polymers adequately describes also these thermoset systems. The materials response during scratching is likewise. Hard fillers decrease the penetration of the indenter into the surface, and consequently enhance scratch resistance; they cause the lateral friction force to decrease, since less material flows in front of the indenter tip. Soft fillers increase the penetration into the surface, according to expectations, but surprisingly also decrease the friction force.Simulations do not predict this, and suggest an alternative explanation. Migration of rubber particles during sample preparation to the surface could have occurred. Adding a thin rubbery layer to the surface makes the model quantitative, but SEM and TEM pictures of the cross-sections do not confirm this phase separation and instead show the presence of a large number of very small voids. Including these voids in the modeling allows to predict the penetration depth into the surface and lateral force quantitative for all sliding speeds.

Comparison of the reinforcing effect of various micro- and nanofillers in PA6

Composites were prepared from three traditional microfillers, glass beads (GB), wood flour (WF) and glass fibers (GF), as well as sodium montmorillonite (NaMMT) and an organophilic clay (OMMT) in a polyamide (PA) matrix in order to compare their properties and discuss the benefits and drawbacks of using micro- or nanofillers. Structure was characterized by various techniques, interaction estimated by two approaches and local deformation processes were followed by acoustic emission testing and volume strain measurements. The results showed that properties can be modified in a relatively wide range. True reinforcement can be achieved with glass fibers, but the influence of all traditional fillers can be predicted with good certainty because their structure can be controlled quite well. Although the layered silicates reinforce polyamide even better than glass fiber, improvement is achieved only in a very narrow composition range because the structure of their composites cannot be controlled to the desired extent. Numerous local processes take place during the deformation of PA composites, such as cavitation and shear yielding in the matrix, as well as particle related processes. However, it is matrix deformation rather than particle processes that determines composite properties.

Strain rate sensitivity in commercial pure titanium: The competition between slip and deformation twinning

Titanium alloys are widely used in light weight applications such as jet engine fans, where their mechanical performance under a range of loading regimes is important. Titanium alloys are mechanically anisotropic with respect to crystallographic orientation, and remarkably titanium creeps at room temperature. This means that the strain rate sensitivity (SRS) and stress relaxation performance are critical in predicting component life. In this work, we focus on systematically exploring the macroscopic SRS of Grade 1 commercially pure titanium (CP Ti) with varying grain sizes and texture using uniaxial compression. Briefly, we find that Ti samples had positive SRS and samples compressed along the sheet rolling direction (RD) (i.e. soft grains dominant) were less rate sensitive than bars compressed along the sheet normal direction (ND) (i.e. hard grains dominant). We attribute this rate sensitivity to the relative activity of slip and twinning. Within the grain size range of ~317±7μm, we observe an increase in the rate sensitivity, where volume fraction of {101̅2}<101̅1>T1 tensile twins was low, and the twin width at different strain rates were similar. These observations imply that the macroscopic rate sensitivity is controlled by the ensemble behaviour of local deformation processes: the amount of slips accumulated at grain boundaries affects the SRS, which is grain size and texture dependent. We hope that this experimental study motivates mechanistic modelling studies using crystal plasticity, including strain rate sensitivity and twinning, to predict the performance of titanium alloys.

Influence of energy dissipation at the interphase boundaries on impact fracture behaviour of a plain carbon steel

The paper deals with the impact deformation and fracture behaviour of commercial plain carbon pipe steel 17Mn1Si. The explicit account of the internal grain structure, temperature and geometry of the notch have been made in theoretical physical mesomechanics formulation aiming at in depth understanding of the role of strain energy factors in dynamic fracture. Theoretical method of excitable cellular automata and laboratory impact bending tests followed by fractographic analysis were paired with time–frequency analysis of acoustic emission accompanying local deformation and fracture processes. It was shown that formulation of the crack opening criterion under dynamic loading conditions should explicitly account for rotation energy accumulation and incorporate the microscopic temporal and spatial details of defect generation from internal (grain) boundaries. A fairly good agreement has been found between the strain energy characteristics obtained from mechanical loading data and independently measured acoustic emission signal being distinguished in terms of consumed and released energy. The impact toughness almost linearly decreased with temperature, which was consistent with fractographic observations. At the stage of crack initiation, when the energy dissipation processes at the internal structure elements significantly affect the initiation of dynamic fracture, the acoustic emission energy reduced in proportion to the expended mechanical energy, which considerably decreased with temperature. The vital role of the energy release at interface/grain boundaries and its decreased significance with decreasing temperature was demonstrated both in numeric simulations and in dynamic experiments.

The physical and mechanical properties and local deformation micromechanisms in materials with different dependence of hardness on the depth of print

The size hardness effects are studied via the micro- and nanoindentation methods over the wide range of the depth of print h (from dozens of nanometers to several dozen micrometers) for several classes of materials, such as ionic and covalent single crystals (sapphire, silicon, lithium fluoride); metals (single-crystal Al, polycrystalline Cu, Ni, and Nb); ceramics (high-strength nanostructured TZP-ceramic based on the natural zirconium dioxide–baddeleyite mineral); amorphous materials (fused quartz); and polymers (polycarbonate and polytetrafluoroethylene). As is shown, some of them possess severe size hardness effects, whereas the others reveal the weak ones or even a lack of these effects. The thermoactivation analysis is implemented, as well, and the activating and energy characteristics of local deformation processes induced by an indenter are compared with the dominant plasticity micromechanisms of the studied materials at different stages of the print formation and with the size peculiarities. The materials with low hardness coefficients and meeting the requirements of ISО 14577 and GOST R 8.748-2011 standards in the nanohardness measurements are highlighted, as well. In the established load ranges, these materials are the promising candidates for their use as reference samples, which are designed to ensure the uniformity of the hardness measurements at the nano- and microscales, as well as for calibrating and testing the nanoindentometers.

A cohesive-frictional force field (CFFF) for colloidal calcium-silicate-hydrates

Calcium-silicate-hydrate (C-S-H) gel is a cohesive-frictional material that exhibits strength asymmetry in compression and tension and normal-stress dependency of the maximum shear strength. Experiments suggest the basic structural component of C-S-H is a colloidal particle with an internal layered structure. These colloids form heterogeneous assemblies with a complex pore network at the mesoscale. We propose a cohesive-frictional force field (CFFF) to describe the interactions in colloidal C-S-H materials that incorporates the strength anisotropy fundamental to the C-S-H molecular structure that has been omitted from recent mesoscale models. We parameterize the CFFF from reactive force field simulations of an internal interface that controls mechanical performance, describing the behavior of thousands of atoms through a single effective pair interaction. We apply the CFFF to study the mesoscale elastic and Mohr–Coulomb strength properties of C-S-H with varying polydispersity and packing density. Our results show that the consideration of cohesive-frictional interactions lead to an increase in stiffness, shear strength, and normal-stress dependency, while also changing the nature of local deformation processes. The CFFF and our coarse-graining approach provide an essential connection between nanoscale molecular interactions and macroscale continuum behavior for hydrated cementitious materials.

Imaging the Structural Evolution in Nanocrystalline Metals during Mechanical Deformation

Most of our current understanding of the deformation mechanisms active in nanocrystalline (nc) metals stems from in situ deformation experiments on bulk materials using x-ray diffraction (XRD). However, XRD cannot directly resolve the local deformation processes. For a local analysis, these processes are traditionally investigated using BF/DF TEM. However, varying contrast due to local orientation changes, bending and defects during in situ BF-TEM straining experiments make an accurate interpretation for nanometer sized grains difficult. On the other hand, automated crystal orientation mapping (ACOM-TEM) [1] allows to identify the crystallographic orientation of all crystallites with sizes down to around 10 nm, well below the limit of electron back scatter diffraction (EBSD). Performing ACOM-TEM in µp-STEM mode allows to image grain orientations in situ during straining inside a TEM [2, 3]. This combination was key to a new data evaluation based on orientation maps. By tracking individual crystallites through a straining series the change of their orientation can be evaluated in order to distinguish between local crystallite rotation and sample tilting/bending [2, 3]. In addition, twinning/detwinning and grain growth can be directly followed (Fig. 1, 2) and the automatic data evaluation leads to user independent quantitative statistical information such as grain size distribution and grain rotation statistics [3].

Effect of ultrafine grain refinement on hydrogen embrittlement of metastable austenitic stainless steel

Micro-tensile tests were performed on high-pressure-torsion-processed specimens of type 304 steel with grain sizes in the range of 0.1–0.5 μm to clarify the effect of ultrafine grain refinement on the hydrogen embrittlement (HE) of metastable austenitic steel. The ultrafine-grained (UFG) specimens with average grain sizes < ∼0.4 μm exhibited a limited uniform elongation followed by a steady-stress regime in the stress–strain curves, which was attributed to a martensitic transformation. A high yield stress and a moderate elongation to failure were attained for the UFG specimens with an average grain size of ∼0.5 μm in the uncharged state. Hall–Petch relationships well hold between the yield stress and the average grain size for each uncharged and hydrogen-charged specimen. Hydrogen charging increased the friction stress by 40% but did not change the Hall–Petch coefficient. Hydrogen-induced ductility loss was mitigated by ultrafine grain refinement. Ductility loss due to hydrogen charging manifested in the local deformation after a martensitic transformation. This indicates that hydrogen does not significantly affect the martensitic transformation, but shortens the subsequent local deformation process.