Evolution Of Strain Fields Research Articles

Effect of Crack Initiation and Life Prediction of Polyacrylonitrile-Reinforced Gussasphalt Surfacing over Steel Bridge Deck under Fiber Content Variation

The crack initiation and life prediction of fiber-reinforced asphalt concrete (FRAC) surfacing for steel bridge decks under a cyclic vehicle load are analyzed from the perspective of damage mechanics. The damage field and the stress and strain field evolution rule of a composite beam in fatigue test are studied, and a fatigue failure criterion is proposed for steel deck FRAC surfacing. Bending fatigue tests are performed on composite beams composed of a steel deck and polyacrylonitrile (PAN)-fiber-reinforced Gussasphalt (GA), i.e., GA-PAN, concrete surfacing under different fiber content and temperature conditions. The damage evolution characteristics of GA-PAN concrete surfacing over the steel deck with different fiber lengths and volume ratios are predicted by analyzing the fatigue life equations. The results show that the steel bridge deck FRAC surfacing model can reflect the comprehensive influence of the fiber content and length on the fatigue performance of steel bridge AC. Specifically, a lower temperature results in the fiber more synergistically affecting the fatigue resistance of AC. Theoretically, the service performance of asphalt concrete increases with the increase of fiber length and content. The optimum fiber length and volume ratio of GA-PAN are found to be 9 mm and 0.46–0.48%, respectively, considering the construction workability.

Fracture of Titanium Alloys at High Strain Rates and under Stress Triaxiality

The present study investigates the effect of stress triaxiality on mechanical behavior and fracture of Ti-5Al-2.5Sn alloy in a practical relevant strain rate range from 0.1 to 1000 s−1. Tensile tests were carried out on flat smoothed and notched specimens using an Instron VHS 40/50-20 servo-hydraulic test machine. High-speed video registration was conducted by Phantom 711 Camera. Strain fields on the specimen gauge area were investigated by the digital image correlation method (DIC). The fracture surface relief was studied using digital microscope Keyence VHX-600D. Stress and strain fields during testing of the Ti-5Al-2.5Sn alloy were analyzed by the numerical simulation method. The evolution of strain fields at the investigated loading condition indicates that large plastic deformation occurs in localization bands. The alloy undergoes fracture governing by damage nucleation, growth, and coalescence in the localized plastic strain bands oriented along the maximum shear stresses. Results confirm that the fracture of near alpha titanium alloys has ductile behavior at strain rates from 0.1 to 1000 s−1, stress triaxiality parameter 0.33 < η < 0.6, and temperature close to 295 K.

Mechanical behavior around double circular openings in a jointed rock mass under uniaxial compression

To better understand the mechanical behavior in a jointed rock mass, a series of uniaxial compression tests were conducted on non-persistently jointed rock specimens with double circular holes. Acoustic emission (AE) and digital image correlation (DIC) techniques were applied to capture micro-crack events and real-time strain field evolution in the specimens. The results indicate that the existence of non-persistent joints has a significant influence on the strength characteristics of the specimens. Specifically, peak strength decreases at first and reaches a minimum at 30° then increases with increase in the joint dip angle. DIC technology has successfully monitored the development of surface strain fields. The fracture evolution process is comprehensively understood. Every sudden change in a strain field is usually accompanied by apparent AE events and stress–strain curves take the form of oscillations. The crack coalescence modes among joints can be summarized as six types and the crack coalescence patterns around holes and joints can be divided into three categories. These results are helpful to understanding further the mechanical properties and fracture mechanism of openings in non-persistently jointed rock masses.

In-Situ Electron Channeling Contrast Imaging under Tensile Loading: Residual Stress, Dislocation Motion, and Slip Line Formation

Elastoplastic phenomena, such as plastic deformation and failure, are multi-scale, deformation-path-dependent, and mechanical-field-sensitive problems associated with metals. Accordingly, visualization of the microstructural deformation path under a specific mechanical field is challenging for the elucidation of elastoplastic phenomena mechanisms. To overcome this problem, a dislocation-resolved in-situ technique for deformation under mechanically controllable conditions is required. Thus, we attempted to apply electron channeling contrast imaging (ECCI) under tensile loading, which enabled the detection of lattice defect motions and the evolution of elastic strain fields in bulk specimens. Here, we presented the suitability of ECCI as an in-situ technique with dislocation-detectable spatial resolution. In particular, the following ECCI-visualized plasticity-related phenomena were observed: (1) pre-deformation-induced residual stress and its disappearance via subsequent reloading, (2) heterogeneous dislocation motion during plastic relaxation, and (3) planar surface relief formation via loading to a higher stress.

Experimental study of macroscopic localization of plastic flow in al-mg alloy under complicated stress-strain state

The main objective of this work is to investigate the spatial-time inhomogeneity due to the jerky flow during plastic deformation of Al-Mg alloy under complex stress conditions. The use of specimens in the form of plates with a rigid circular rim and a reverse curvature rim was considered. Experimental data on the evolution of inhomogeneous strain and temperature fields have been obtained, illustrating the processes of initiation and propagation of deformation bands of the localized plastic flow by using the digital image correlation and infrared thermography.

Experimental study of shear properties of 3D woven composite using digital image correlation and acoustic emission

This work is dedicated to the experimental study of the shear properties of three-dimensional reinforced composites taking into account their structural features, in Iosipescu tests. Shear strains have been determined using Vic-3D non-contact three-dimensional digital optical system. The evolution of inhomogeneous strain fields on the surface of composite specimens of the structure under study has been analyzed. The variants of strain averaging in the specimen working area have been analyzed using Vic-3D tools. AMSY-6 acoustic emission system has been used to assess the structural integrity of composite materials under loading.

Geostatistical analysis of strain localization in triaxial tests on sand

Strain localisation influences the behaviour of sheared soils in a way that the assumption of the continuum loses validity and may become inapplicable. This occurrence affects the interpretation of triaxial tests where samples are usually considered as representative elements of an equivalent continuum and a phenomenological interpretation is carried out to calibrate constitutive models. Considering the inherent and stress-induced heterogeneity of the material as a possible precursor of localisation, the evolution of porosity and strain fields is studied interpreting with statistical and geostatistical analyses, the results of X-ray computerised microtomography. The study is performed on sandy samples sheared in triaxial compression and extension. The analysis carried out before shearing shows a meaningful statistical heterogeneity of the porosity with correlation lengths to the order of a few grain diameters. The evolution on shearing reveals the onset of a second statistical population and of an anisotropic spatial variation dictated by strain localisation.

Impact of cooling on fracturing process of granite after high-speed heating

Only very few studies about the influence of cooling on the thermo-mechanical properties of heated rocks exist. This study focuses on high-speed heating followed by slow as well as rapid cooling of granite samples. Target temperatures of 400 °C and 800 °C were reached with heating rates of 200 °C/min, 300 °C/min and ISO 834 standard fire curve. Numerical simulations are used to investigate the temperature evolution inside the sample. The digital image correlation (DIC) technique was used to analyse the strain field evolution during air-cooling with furnace removed. DIC analysis reveals that the microcracks induced by high-speed heating and high temperatures (i.e. 800 °C) can close progressively due to the material contraction during cooling. The closing of microcracks caused by shrinkage plays a more important role than cracking behaviour caused by thermal gradients (i.e. thermal shock). Both rapid (furnace removed) and slow (furnace in place) cooling do not show obvious influence on the stress-strain behaviours of granite samples heated to 400 °C. The samples heated to 800 °C with rapid cooling have a higher uniaxial compression strength than in slow cooling case. This indicates that the contraction effect during rapid cooling might be more intense than in slow cooling cases.

Multi-Physics and Multi-Scale Meshless Simulation System for Direct-Chill Casting of Aluminium Alloys

This paper represents an overview of the elements of the user-friendly simulation system, developed for computational analysis and optimization of the quality and productivity of the electromagnetically direct-chill cast semi-products from aluminium alloys. The system also allows the computational estimation of the design changes of the casting equipment. To achieve this goal, the electromagnetic and the thermofluid process parameters are coupled to the evolution of Lorentz force, temperature, velocity, concentration, strain and stress fields as well as microstructure evolution. This forms a multi-physics and multi-scale problem of great complexity, which has not been demonstrated before. The macroscopic fluid mechanics, solid mechanics, and electromagnetic solution framework is based on local strong-form meshless formulation, involving the radial basis functions and monomials as trial functions, and local collocation or weighted least squares approximation. It is coupled to the micro-scale by incorporating the point automata solution concept. The entire macro-micro solution concept does not require meshing and space integration. The solution procedure can be easily and efficiently automatically adapted in node redistribution and/or refinement sense, which is of utmost importance when coping with fields exhibiting sharp gradients, which occur in the phase-change problems. The simulation system is coded from scratch in modern Fortran. The elements of the experimental validation of the system and the demonstration of its use for round billet casting in IMPOL Aluminium Industry are shown.

Localized strain fields in honeycomb materials with convex and concaved cells

Characterizing the local deformations and strain fields in ligaments of periodic honeycomb materials is instrumental to applications such as cores of sandwich structure as well as honeycomb shaped tunable phononic crystals, compliant metamaterials, and 4D printed shape memory materials. Such applications employ honeycomb shaped materials whose nonlinear behavior is controlled by localized deformations in their ligaments. These local deformations can exhibit values and complex non-uniform states that far exceed the overall macroscopic deformations and strains sustained by the honeycomb material in value and complexity. Each of the aforementioned applications can be better designed with an improved understanding of the evolution of local strain fields in honeycombs' ligaments during their deformation. This work aims to characterize the local strains and deformations developing in honeycomb ligaments near honeycombs’ macroscopic onset of yielding. Finite element computations and the full field strain measurement technique digital image correlation (DIC) are used to measure and characterize the local deformations in the ligaments of concaved and convex honeycombs. Results show nonlinear strain gradients and significant localization in deformed ligaments. Moreover, complex state of stress comprising biaxial normal stresses and shear stresses exists near plastic zones.

Mixed-Mode Deformation and Failure of a Magnesium Sheet Quantified using a Modified Arcan Fixture

The anisotropic plastic deformation and the failure behaviour of a rolled magnesium sheet is quantified by means of a modified Arcan loading device. Combined mode I/II loadings are applied at room temperature. Anti-buckling guides enforce parallel motion of the fixture. The evolution of local strain fields during loading is analysed by means of digital image correlation. Two target functions are constructed from each test: one including the kinetic, another describing the kinematical response. The failure strain as a function of the loading direction and sheet orientation is quantified. It is shown that the failure mechanism is independent of the loading angle. Guidelines for future constitutive model developments are provided in terms of the exploration of the samples motion, the uniformity of strain fields and their respective significance.

Fatigue damage evolution in thick composite laminates: Combination of X-ray tomography, acoustic emission and digital image correlation

The main purpose of this study is to provide a thorough experimental investigation of fatigue damage mechanisms and evolution in thick carbon/epoxy laminate subjected to bending load. The use of X-ray computed tomography (CT) in this study has allowed the visualization of all damage present in the studied laminates, which made it possible to identify, quantify and locate them precisely and therefore, to identify the physical origin of residual strength decrease and acoustic emissions (AE). Furthermore, the results of the AE analysis have provided very valuable information about the nature and evolution of damage. However, the determination of the depth and size of internal damage was not possible with this technique. The displacement field measured by digital image correlation (DIC) made it possible to determine and monitor the strain field evolution during the experiments. The combination of the results of the three non-destructive techniques used in this work has allowed better characterization of fatigue damage evolution in the studied laminates, and provide a complete and accurate description of the different mechanisms involved during their damage process.

Studying the Development of a Technological Defect in Complex Stressed Construction CFRP Using Digital Image Correlation and Acoustic Emission Methods

The work is devoted to experimentally studying regularities of inelastic strain and fracture in CFRP samples with pre-embedded technological defects corresponding to possible nonpressings and starved joints between material layers within a given bounded domain. The evolution of inhomogeneous strain fields at the surface of CFRP samples with an internal starved-joint type defect in a complex stressed state is analyzed. Faulty structures were located in the composite material using the acoustic emission method. This method allowed analysis of damage accumulation processes in complex loaded samples. The experimental data will be used in further tests to assess the adequacy of signals received from embedded fiber optic sensors subjected to complex-form loads.

Two-pyroxene syenitoids from the Moldanubian Zone of the Bohemian Massif: Peculiar magmas derived from a strongly enriched lithospheric mantle source

In the Variscan Bohemian Massif, (ultra-)potassic plutons are conspicuously associated with the high-grade, lower crustal/upper mantle Gföhl Unit (Moldanubian Zone). They can be subdivided into two contrasting groups: (1) coarse amphibole–biotite melagranite to quartz syenite with conspicuous K-feldspar phenocrysts (the ‘durbachite suite’), and (2) essentially equigranular biotite–two-pyroxene quartz syenites to melagranites (the ‘syenitoid suite’).The latter suite, represented by the Tábor and Jihlava plutons, is characterized by an originally ‘drier’ mineral assemblage orthopyroxene + clinopyroxene + Mg-biotite + plagioclase + K-feldspar + quartz, with accessory zircon, apatite, ilmenite, monazite and/or rutile + chromite. The rich assemblage of both rock-forming and accessory minerals allows testing of numerous geothermobarometers. The resulting P–T data are mutually consistent; they document emplacement of the parental magmas at mid-crustal levels (~5 kbar/19 km for Tábor and 7–8 kbar/26–30 km for Jihlava) and record near-isobaric crystallization from at least 1170 °C to the solidus.New Isotope-Dilution Thermal Ionization Mass-Spectrometry (ID-TIMS) U–Pb ages for zircon (336.9 + 0.6 Ma) and rutile (336.8 + 0.8 Ma) from the Tábor Pluton, together with previously published ages from the Jihlava Pluton, provide temporal constraints for the emplacement and rapid cooling of the syenitoids below c. 600 °C. This supports the hypothesis of post-tectonic emplacement of a hot and dry melt (>1200 °C?) into an essentially consolidated orogenic crust.The two syenitoid plutons have comparable, crust-like radiogenic isotope signatures (87Sr/86Sr337 = 0.7114–0.7133, ε337Nd = −6.8 to −8.0). This, in context of whole-rock geochemical variation and K-feldspar Pb isotopic compositions, is consistent with generation from a strongly enriched lithospheric mantle source. Shortly before melting, the local orogenic mantle was most likely modified by deep subduction and relamination of felsic crustal material of the Saxothuringian provenance, transformed to the felsic high-pressure granulites common in the Moldanubian Zone. In the subsequent evolution of the two-pyroxene syenitoid plutons, crystal fractionation and accumulation played a key role, unlike in the durbachite suite itself, where magma mixing with leucogranitic melts was much more important.Structural relations inside and around the (ultra-)potassic plutons (~343–335 Ma) suggest that – besides different depths and specific processes of magma emplacement – these plutons track different kinematic histories and evolutions of regional strain fields in their high-grade country rocks in this crucial period of the Variscan Orogeny.

Analytical elastoplastic analysis of heteroepitaxial core-shell nanowires

Semiconductor nanowires, grown heteroepitaxially, have many unique properties compared to heteroepitaxial thin films: e.g., the possibility of lateral relaxation, high surface to volume ratio and lower strain energy. While the onset of plastic deformation in flat thin films has been studied extensively, much less is understood about this phenomenon in the nanowire geometry. Here, we report development of a continuum analytical model that predicts not only the onset of plastic deformation for core-shell structures with anisotropic slip system, but also the evolution of stress and strain fields beyond the initial yield. This is the first analytical elastoplastic study of heteroepitaxial core-shell nanowires. Our model is verified against finite element simulations. To illustrate trends predicted by the model, we choose InGaAs for core-shell system as an example. We find that most energetically favorable positions for formation of the first dislocations in the heterostructure have misorientation of 0, π/2, π, and 3/2π from the principal slip planes in zinc-blend structures. We demonstrate that anisotropy in slip systems of the heterostructure reduces the critical misfit strain. Finally, we find that there is a critical ratio (χ) of shell thickness to core radius that maximizes the thickness of the elastoplastic region. This critical ratio is independent of geometry and depends only on material properties such as elastic moduli and yield strength of the heterostructure.

Modeling, testing and calibration of ductile crack formation in grade DH36 ship plates

The initiation of ductile fracture in grade DH36 shipbuilding steel was modeled using the Hosford-Coulomb fracture model. The hardening and ductile fracture characteristics of DH36 were assessed by performing experiments on notched tension, central hole tension, plane strain tension and shear specimens. Detailed finite element analysis of each experiment was performed to evaluate the evolution of local stress and strain fields. The loading paths to ductile fracture initiation were determined in terms of the stress triaxiality and Lode angle parameter histories extracted from finite element analyses with very fine solid element meshes. The Hosford-Coulomb fracture model parameters were identified using the extracted loading paths and adopting a linear damage accumulation law. It has been found that ductile fracture behavior of DH36 is dependent not only on the stress triaxiality but also the Lode angle.

Fracture of notched ductile bulk metallic glass bars subjected to tension-torsion: Experiments and simulations

In bulk metallic glasses (BMGs) that are susceptible to shear band mediated plasticity, crack initiation and growth is a strain-controlled process associated with a unique material specific length scale. In notched specimens subjected to pure modes I and II, and mixed mode I/II loading conditions, this length scale determines the extent to which a crack can grow within a dominant shear band before propagating catastrophically. While a similar fracture mechanism is expected in the anti-plane shear mode (mode III) dominant loading condition, there are few studies that have investigated this aspect. In this paper, an attempt is made to understand the crack growth processes and fracture mechanisms under mode III and mixed mode I/III loading conditions by conducting pure torsion and combined tension-torsion experiments on a Zr-based BMG that exhibits significant room temperature plasticity. Specimens with either high or low notch acuity are tested to assess the effects of plastic constraint on the fracture process. Detailed finite element simulations were performed to study the evolution of stress and strain fields within each specimen before the onset of fracture. These are correlated with the fractographic features to determine the fracture criterion and mechanism. Results indicate that the length scale associated with fracture and the mechanism of crack growth are both sensitive to the application of tensile loads. The fracture toughness of BMGs under different modes of loading are evaluated and compared.

Deformation Field in Diametrically Loaded Soft Cylinders

Deformation fields at the surface of diametrically squeezed shallow cylinders in the large deformation regime are measured experimentally and numerically for different material behaviour in the large deformation regime. By means of a digital image correlation method optimized for large displacements, strain fields are measured and compared with finite element simulations. Assuming a neo-Hookean behaviour for cylinders made of rubber silicone, the strain field is found to be in quantitative agreement with non-linear finite element simulations up to the highest deformations reached in our experiments (15%). For materials that follow an elastoplastic constitutive law, agreement is lost after few percents of deformation and location of the strain field differences are identified up to strains as high as 30%. Strain field evolution is also measured for solid foam cylinders up to 60% of global deformation strain. This method that can be applied to a broad variety of materials, even in the occurrence of large deformations, provides a way to study quantitatively local features of the mechanical contact.

Experimental investigation of shear band and shear strain field evolution during blanking of AA6082 sheet

Interrupted blanking experiments were performed to study the deformation behavior of AA6082 (T6) sheets. An intentional asymmetry was introduced by having the punch and the die of different edge radii. High-resolution microstructural montages of sheet cross-section were used to experimentally measure the shear strain field during each interrupted blanking experiment. This provided a comprehensive experimentally measured spatial–temporal shear strain field for the blanking process. The shear strain was found to be high and localized at the top and bottom surfaces of the sheet which was in contact with the punch and the die. The shear strain was observed to monotonically reduce and get diffused at the interior of the sheet. The strain distribution was also calculated by finite element simulations and found to be in good agreement with the experimentally measured strain distributions. However, the peak strains predicted by the finite element simulations were always marginally lower than those observed by the experimental observations.

Impact of free volume on shear band multiplication and bending plasticity

In-situ electron microscopy mechanical testing is a promising deformation observation technology that has been extensively applied to explore the deformation and fracture behaviour of MGs. In this paper, we adopt it in conjunction with digital image correlation technique to investigate the impact of free volume on shear band multiplication and bending plasticity in MGs. The strain evolutions of the notched Zr-, Ce- and Mg-based MGs are studied respectively. The results indicate that the strain-concentration striations before yielding contain amounts of free volume can improve the shear band multiplication, and are beneficial to the subsequent bending plasticity. Such conclusions successively verify a novel integrated method to explore the in-situ strain field evolution process near the notch of MGs, which is advantage for understanding the deformation and fracture behaviour of MGs in a complex stress state.