Development Of Localized Deformation Research Articles

Characterization and growth mechanisms of adhesion‐induced microcavities during debonding of softened glass

AbstractThe glass/mold interaction is crucial for controlling the surface quality of high‐precision glass products and elongating the lifespan of precious molds in hot forming techniques. Here we employ the probe tack test to separate a typical glass molding interface composed of N‐BK7 glass and tungsten carbide molds at different temperatures from 655 to 690°C. The macroscale debonding behavior translates from interfacial fracture to cohesive bulk deformation as temperature increases. The glass surfaces after debonding are covered by numerous randomly distributed cavities in micrometer. With temperature increasing, the maximum depth of microcavities greatly increases from less than 0.5 to over 10 μm; the area fraction overall increases and reaches 15% at maximum. These microcavities could result from the development of localized deformation at the gas‐trapping spots, due to the separation of the adhesive glass/mold interface. A large‐sized cavity evolves from the cyclic growth and coalescence of small cavities. For the interfacial fracture cases, cavities mainly propagate as cracks along the interface, and thus develop into shallow disc‐like shapes. However, for the cohesive cases, cavities prefer to grow in the bulk. The growth bifurcation could be governed by the competition between strain energy release rate and viscoelastic complex modulus.

Influence of stress history, strain rate and end effects on undrained shear behaviour and strain localization of laterite soil

ABSTRACT Recognizing the abundant use of lateritic soils in the construction of embankments and their susceptibility towards extreme erosion, it is necessary to explore the influence of stress history and strain rate on their undrained shear behaviour. The mechanism of development of localized deformation and failure of soil mass are equally important for understanding the stability of laterite soils. Moreover, the use of porous stones in conventional triaxial testing for drainage to investigate the undrained shear behaviour of soils leads to the development of shear stresses near the ends and thereby influences the mechanical response of the soil. In the current research, the effect of stress history, strain rate and boundary effects on the shear strength behaviour and strain localization patterns of naturally available laterite soil is explored. The effect of end boundaries on the initiation and propagation of localized deformations of soil have been also evaluated in this study.

Response of anisotropic polyurethane foam to compression at different loading angles and strain rates

Polyurethane (PU) foam can display mechanical anisotropy because of elongation of cells in the vertical direction when component liquid chemicals are mixed to generate PU foam. Existing studies on the anisotropic mechanical response of PU foam appear limited, particularly for high strain-rate loading. In this investigation, a PU foam with an apparent density of 36.5 kg/m3 was fabricated, and 40 mm cubic specimens were cut at 0°, 30°, 45°, 60° and 90° to the foam-rise direction, and subjected to quasi-static (0.0042–0.42 s−1 global strain rate) and dynamic (∼190 s−1 global strain rate) compression, to identify the effects of loading direction and strain rate on the resulting mechanical response. Dynamic compression tests were performed using a modified Hopkinson bar device, with force equilibrium in the foam specimens examined and validated via finite element modelling. The overall stress–strain response, as well as the deformation modes, varies significantly with loading direction. For compression along the foam-rise direction (0°), deformation of foam cells is dominated by unstable buckling of cell struts; this leads to deformation localization and subsequent layer-by-layer crushing. The corresponding overall stress–strain response displays a brief post-yield strain softening phase, generated by the initiation and development of localized deformation in the first cell layer to collapse. This is followed by a protracted stress plateau, whereby localized crushing propagates layer-by-layer through the specimen. For compression along the transverse direction (90°), deformation of foam cells is characterized by stable bending of cell struts, leading to uniform deformation within the foam specimen, accompanied by gradual post-yield hardening in the overall stress–strain response. For compression at various inclinations (e.g. 30°, 45°, 60°) to the foam-rise direction, foam cells deform via a combination of buckling and bending of cell struts, and the overall stress–strain response lies between that corresponding to compression along the foam-rise and the transverse directions. With respect to the influence of strain rate, a higher deformation rate generates a stiffer and stronger overall stress–strain response. Several material parameters — i.e. overall elastic stiffness, yield stress, densification strain and effective energy absorption capacity — were examined to characterize the mechanical properties of PU foam. The elastic stiffness, yield stress and effective energy absorption capacity all decrease with an increase in loading angle to the foam-rise direction, but they increase with strain rate. However, the densification strain varies little with loading angle or strain rate.

Microstructure, mechanical properties and fracture behavior of ultra-high strength dual-phase steel

Abstract An ultra-high strength dual-phase steel with main components 0.16C–1.38Si–3.20Mn was produced by cold rolling followed by intercritical heat treatment. Influences of different annealing temperatures on the mechanical properties were investigated, focusing on the development of localized deformation as well as crack initiation and propagation. The work hardening behavior of tested steels after annealing was analyzed in terms of the modified C–J analysis. The experimental results indicate that the microstructures of all the tested steels contain ferrite and martensite. Annealing at 800 °C, the best comprehensive performance with a yield strength of 873 MPa, tensile strength of 1483 MPa, total elongation of 11% and yield ratio of 0.58 was achieved. Besides, the mechanisms of damage taking place in the ferrite and crack initiating at the interface between the ferrite and martensite have been observed. The crack initiating perpendicular to the lath of the martensite and then following at or near the narrowest part of a martensite ligament have also been observed. The original crack propagation exhibits a transgranular fracture feature.

Strain‐induced outgassing of three‐phase magmas during simple shear

AbstractA major factor determining the explosivity of silicic eruptions is the removal of volatiles from magma through permeability‐controlled outgassing. We studied the microstructural development of permeability during deformation of highly viscous magma by performing simple shear experiments on bubble (0.12–0.36 volume fraction) and crystal‐bearing (0–0.42 volume fraction) silicate melts. Experiments were performed under torsion, at high temperature and pressure (723–873 K and 150–200 MPa) in a Paterson deformation apparatus at bulk shear strains between 0 and 10. The experimental setup allows for gas escape if bubble connectivity is reached on the sample periphery. Three‐dimensional imaging and analysis of deformed bubbles was performed using X‐ray tomography. The development of localized deformation in all samples, enhanced by crystal content, leads to brittle fracture at bulk strains > 2 and sample‐wide fracturing in samples deformed to strains >5. A decrease in both bubble fraction and dissolved volatile content with increasing strain, along with strain‐hardening rheological behavior, suggests significant shear‐induced outgassing through the fracture networks, applicable to shallow conduit degassing in magmas containing crystal fractions of 0–0.42. This study contributes to our understanding of highly viscous magma outgassing and processes governing the effusive‐explosive transition.

Localized deformation in sands and glass beads subjected to plane strain compressions

In order to investigate shear behavior of granular materials due to excavation and associated unloading actions, load-controlled plane strain compression tests under decreasing confining pressure were performed under drained conditions and the results were compared with the conventional plane strain compression tests. Four types of granular material consisting of two quartz sands and two glass beads were used to investigate particle shape effects. It is clarified that macro stress-strain behavior is more easily influenced by stress level and stress path in sands than in glass beads. Development of localized deformation was analyzed using photogrammetry method. It was found that shear bands are generated before peak strength and shear band patterns vary during the whole shearing process. Under the same test condition, shear band thickness in the two sands was smaller than that in one type of glass beads even if the materials have almost the same mean particle size. Shear band thickness also decreased with increase of confining pressure regardless of particle shape or size. Local maximum shear strain inside shear band grew approximately linearly with global axial strain from onset of shear band to the end of softening. The growth rate is found related to shear band thickness. The wider shear band, the relatively lower the growth rate. Finally, observed shear band inclination angles were compared with classical Coulomb and Roscoe solutions and different results were found for sands and glass beads.

Investigation of localized deformation in partially saturated sand under triaxial compression using microfocus X-ray CT with digital image correlation

In this paper, localized deformation in partially saturated sand was investigated quantitatively using microfocus X-ray computed tomography (CT) and an image analysis of the CT images. Triaxial compression tests on a partially saturated dense Toyoura sand specimen were carried out under a low confining pressure and under drained conditions for both air and water. The development of localized deformation was observed macroscopically using microfocus X-ray CT, and the displacement field over the entire specimen was quantified by an image analysis of the CT images with the digital image correlation (DIC) technique. The progressive development of shear bands is discussed with reference to these images. In addition, the region of localization was observed microscopically by partial CT scanning on a micron scale with high spatial resolution. Changes in the particulate structures are also discussed herein. The DIC image analysis of the partial CT images provided a microscopic displacement field and indicated that very fine localized shear deformation developed before the shear bands had become visible in the macroscopic investigation.

Development of Localized Deformation in AA 2024-O

An experimental study is presented to (1) quantify the rate-sensitive mechanical response and (2) examine the localized deformation behavior under an applied temperature gradient in the alloy AA 2024. Isothermal flow stresses are obtained at temperatures from −100°C to 495°C and strain rates from 10−2/s to 10−5/s using routine compression tests and a novel cyclic test, which expedites the characterization. The material displays two distinct kinetic responses with both being amenable to localization phenomena. The lower temperature/high strain rate regime displays a rate-insensitive yield with Stage III/IV work hardening. At higher temperature/low strain rates, a rate-sensitive response with little work hardening is observed. In order to relate the material constitutive behavior to the development of localized deformation, a temperature gradient test is performed wherein temperature differences of approximately 30°C are enforced between the top and bottom surfaces of a cylindrical compression test specimen. Deformation heterogeneity developed in the two distinct regimes of material response is illustrative of warm and hot working conditions typical of industrial processes, such as rolling.

Computational modeling of deformation bands in granular media. II. Numerical simulations

Development of accurate mathematical models of geomaterial behavior requires a more fundamental understanding of the localization phenomena; in particular, the important factors responsible for the inception and development of localized deformation. The objective of this paper is to implement and test the mathematical formulations presented in a companion paper to better understand the different failure processes in granular media, specifically the formation of deformation bands in geomaterials. Our approach revolves around the use of classical bifurcation theory combined with advanced constitutive modeling and state-of-the-art computation to capture the end members of the failures modes described in the companion paper, namely, simple shear, pure compaction, and pure dilation bands, as well as the combination modes described in the geological framework. The paper revisits the notion of the critical hardening modulus as it applies to the entire range of failure modes, elucidates the role of the third stress invariant and finite deformation effects on the localization properties, and describes some useful properties of the constitutive and algorithmic tangent operators as they relate to the capture of the onset of deformation bands in geomaterials.

Computational modeling of deformation bands in granular media. II. Numerical simulations

Development of accurate mathematical models of geomaterial behavior requires a more fundamental understanding of the localization phenomena; in particular, the important factors responsible for the inception and development of localized deformation. The objective of this paper is to implement and test the mathematical formulations presented in a companion paper to better understand the different failure processes in granular media, specifically the formation of deformation bands in geomaterials. Our approach revolves around the use of classical bifurcation theory combined with advanced constitutive modeling and state-of-the-art computation to capture the end members of the failures modes described in the companion paper, namely, simple shear, pure compaction, and pure dilation bands, as well as the combination modes described in the geological framework. The paper revisits the notion of the critical hardening modulus as it applies to the entire range of failure modes, elucidates the role of the third stress invariant and finite deformation effects on the localization properties, and describes some useful properties of the constitutive and algorithmic tangent operators as they relate to the capture of the onset of deformation bands in geomaterials.

The growth of shear bands in composite microstructures

Abstract Shear band formation in materials with inhomogeneous and composite microstructures is influenced by factors that usually do not come into play in monolithic materials. Experiments and calculations have shown that inhomogeneities in material properties enhance the localization of deformation. This investigation concerns the propagation of shear bands in a two-phase tungsten composite under the conditions of nominally pure shear deformation. Finite element calculations are carried out to delineate the effects of different grain–matrix morphologies. In the numerical models, the initiation of shear bands is triggered by a notch, simulating the effect of defects such as microcracks and microvoids in materials. Calculations demonstrate that phase morphology, particle size and the relative location of initiation site have significant influences on the development of localized deformation. The work and energy evolutions are tracked for each constituent phase in the microstructures. In addition, the exchange of thermal energy through heat flow between the phases is analyzed. The results show that a strong correlation exists between the course of shear band propagation and the thermomechanical coupling between microscopic phases.

The nature of localized deformation in 6111-T6 aluminium alloy and polycarbonate

The nature of localized deformation that occurs both in metal and polymer samples was examined under uniaxial tension by measuring the complete strain distribution of a deforming specimen at a constant crosshead speed. The development of localized deformation in 6111-T6 aluminium alloy and polycarbonate was recorded along both the longitudinal and lateral directions as the specimen was stretched until it fractured. Determination of the true strain at various points on the tensile specimens was made by using an automated, vision-based image processing method using a CCD camera to measure the deformation of a silk-screened grid on the specimen. This vision-based system produced accurate results, allowing for careful analysis of the entire necking process from initiation to propagation. It has been observed that localized deformation develops principally in the longitudinal direction. The major and minor true strain are uniform for the uniform section specimens, however, these strains peak in the centre of the hourglass specimens in the longitudinal direction for both materials. For both specimen geometries, the major strain is uniform while the minor strain is non-uniform in the lateral direction. The longitudinal true strain to lateral true strain ratio remains at about two for the uniform section specimens, but this ratio increases to between 3 and 5 for the hourglass specimens, due to the constraining effect of the sample geometry.