Showing papers on "Brittleness published in 2010"

Why are double network hydrogels so tough

Abstract: Double-network (DN) gels have drawn much attention as an innovative material having both high water content (ca. 90 wt%) and high mechanical strength and toughness. DN gels are characterized by a special network structure consisting of two types of polymer components with opposite physical natures: the minor component is abundantly cross-linked polyelectrolytes (rigid skeleton) and the major component comprises of poorly cross-linked neutral polymers (ductile substance). The former and the latter components are referred to as the first network and the second network, respectively, since the synthesis should be done in this order to realize high mechanical strength. For DN gels synthesized under suitable conditions (choice of polymers, feed compositions, atmosphere for reaction, etc.), they possess hardness (elastic modulus of 0.1–1.0 MPa), strength (failure tensile nominal stress 1–10 MPa, strain 1000–2000%; failure compressive nominal stress 20–60 MPa, strain 90–95%), and toughness (tearing fracture energy of 100∼1000 J m−2). These excellent mechanical performances are comparable to that of rubbers and soft load-bearing bio-tissues. The mechanical behaviors of DN gels are inconsistent with general mechanisms that enhance the toughness of soft polymeric materials. Thus, DN gels present an interesting and challenging problem in polymer mechanics. Extensive experimental and theoretical studies have shown that the toughening of DN gel is based on a local yielding mechanism, which has some common features with other brittle and ductile nano-composite materials, such as bones and dentins.

Transition from a strong-yet-brittle to a stronger-and-ductile state by size reduction of metallic glasses

Abstract: Amorphous metallic alloys, or metallic glasses, are lucrative engineering materials owing to their superior mechanical properties such as high strength and large elastic strain. However, their main drawback is their propensity for highly catastrophic failure through rapid shear banding, significantly undercutting their structural applications. Here, we show that when reduced to 100 nm, Zr-based metallic glass nanopillars attain ceramiclike strengths (2.25 GPa) and metal-like ductility (25%) simultaneously. We report separate and distinct critical sizes for maximum strength and for the brittle-to-ductile transition, thereby demonstrating that strength and ability to carry plasticity are decoupled at the nanoscale. A phenomenological model for size dependence and brittle-to-homogeneous deformation is provided. A long-standing goal in engineering is to create better structural materials with enhanced useful properties for particular applications, commonly attained by constructing specific microstructures. Typical examples include martensites for strengthening, reinforced concrete for toughening and cellular structures for energy absorption. It was recently reported that extrinsic size also strongly affects crystalline properties at the submicrometre scale 1,2 , providing the opportunity to use feature size as a design parameter in attaining superior mechanical properties.

“Electrochemical Shock” of Intercalation Electrodes: A Fracture Mechanics Analysis

Abstract: Fracture of electrode particles due to diffusion-induced stress has been implicated as a possible mechanism for capacity fade and impedance growth in lithium-ion batteries. In brittle materials, including many lithium intercalation materials, knowledge of the stress profile is necessary but insufficient to predict fracture events. We derive a fracture mechanics failure criterion for individual electrode particles and demonstrate its utility with a model system, galvanostatic charging of Li x Mn 2 O 4 . Fracture mechanics predicts a critical C-rate above which active particles fracture; this critical C-rate decreases with increasing particle size. We produce an electrochemical shock map, a graphical tool that shows regimes of failure depending on C-rate, particle size, and the material's inherent fracture toughness K Ic . Fracture dynamics are sensitive to the gradient of diffusion-induced stresses at the crack tip; as a consequence, small initial flaws grow unstably and are therefore potentially more damaging than larger initial flaws, which grow stably.

Electron-beam-assisted superplastic shaping of nanoscale amorphous silica.

Abstract: Glasses are usually shaped through the viscous flow of a liquid before its solidification, as practiced in glass blowing. At or near room temperature (RT), oxide glasses are known to be brittle and fracture upon any mechanical deformation for shape change. Here, we show that with moderate exposure to a low-intensity ( 10(-4) per second. We show not only large homogeneous plastic strains in compression for nanoparticles but also superplastic elongations >200% in tension for nanowires (NWs). We also report the first quantitative comparison of the load-displacement responses without and with the e-beam, revealing dramatic difference in the flow stress (up to four times). This e-beam-assisted superplastic deformability near RT is useful for processing amorphous silica and other conventionally-brittle materials for their applications in nanotechnology.

Micromechanical parameters in bonded particle method for modelling of brittle material failure

Abstract: Bonded particle modelling (BPM) is nowadays being extensively used for simulating brittle material failure. In BPM, material is modelled as a dense assemblage of particles (grains) connected together by contacts (cement). This sort of modelling seriously depends on the mechanical properties of particle and contact, which are named here as micro-parameters. However, a definite calibration methodology to obtain micro-parameters has not been so far established; and many have reported some serious problems. In this research, a calibration procedure to find a unique set of micro-parameters is established. To attain this purpose, discrete element code of UDEC is used to perform BPM. This code can be conveniently developed by the user. The proposed BPM is composed of rigid polygonal particles interacting at their contact points. These contacts can undergo a certain amount of tension, and their shear resistance is provided by cohesion and friction angle. The results demonstrate that each material macro-property (i.e. Young's modulus, Poisson's ratio, internal friction angel, internal cohesion, and tensile strength) is directly originated from and distinctly related to the contact properties (i.e. normal and shear stiffness, friction angel, cohesion, and tensile strength). Copyright (C) 2010 John Wiley & Sons, Ltd.

Effect of rhenium on the dislocation core structure in tungsten.

Abstract: Despite exhibiting the highest melting point of all metals, the technological use of tungsten is hampered by its room-temperature brittleness. Alloying with Re significantly ductilizes the material which has been assigned to modified properties of the 1/2(111) screw dislocation. Using density functional theory, we show that alloying induces a transition from a symmetric to an asymmetric core and a reduction in Peierls stress. This combination ductilizes the alloy as the number of available slip planes is increased and the critical stress needed to start plastic deformation is lowered.

Fracture toughness of polycrystalline tungsten alloys

Abstract: Tungsten and tungsten alloys show the typical change in fracture behavior from brittle at low temperatures to ductile at high temperatures. In order to improve the understanding of the effect of microstructure the fracture toughness of pure tungsten, potassium doped tungsten, tungsten with 1 wt.% La 2 O 3 and tungsten rhenium alloys were investigated by means of 3-point bending, double cantilever beam and compact tension specimens. All these materials show the expected increase in fracture toughness with increasing temperature. The experiments demonstrate that grain size, texture, chemical composition, grain boundary segregation and dislocation density seem to have a large effect on fracture toughness below the DBTT. These influences can be seen in the fracture behavior and morphology, where two kinds of fracture occur: on the one hand transgranular and on the other hand intergranular fracture. Therefore, techniques like electron backscatter diffraction (EBSD), Auger electron spectroscopy (AES) and X-ray line profile analysis were used to improve the understanding of the parameters influencing fracture toughness.

Evidence for a Long-Term Strength Threshold in Crystalline Rock

Abstract: The mechanical response of brittle rock to long-duration compression loading is of particular concern in underground disposal of nuclear waste, where radionuclides must be isolated from the biosphere for periods of the order of a million years. Does the strength decrease without limit over such time, or is there, for some rock types, a lower “threshold” strength below which the rock will cease to deform? This paper examines the possibility of such a threshold in silicate crystalline rocks from several perspectives, including: (1) interpretation of the results of short-term creep tests on rock; (2) numerical analysis of the effect of decrease in fracture toughness due to stress corrosion on the strength of a crystalline rock; and (3) evidence from plate tectonics, and observations of in situ rock stress in granite quarries. The study concludes that there isclear evidence of threshold strength. The threshold is of the order of 40% of the unconfined compressive strength or higher for laboratory specimens under unconfined compressive loading, and increases rapidly in absolute value with confinement. Field evidence also leads to the conclusion that the long-term strength of crystalline rock in situ is of comparable magnitude to the laboratory value.

The fracture toughness of bulk metallic glasses

Abstract: Stiffness, strength, and toughness are the three primary attributes of a material, in terms of its mechanical properties. Bulk metallic glasses (BMGs) are known to exhibit elastic moduli at a fraction lower than crystalline alloys and have extraordinary strength. However, the reported values of fracture toughness of BMGs are highly variable; some BMGs such as the Zr-based ones have toughness values that are comparable to some high strength steels and titanium alloys, whereas there are also BMGs that are almost as brittle as silicate glasses. Invariably, monolithic BMGs exhibit no or low crack growth resistance and tend to become brittle upon structural relaxation. Despite its critical importance for the use of BMGs as structural materials, the fracture toughness of BMGs is relatively poorly understood. In this paper, we review the available literature to summarize the current understanding of the mechanics and micromechanisms of BMG toughness and highlight the needs for future research in this important area.

Engineered Cementitious Composites: Can Composites Be Accepted as Crack-Free Concrete?

Abstract: Because conventional concrete is brittle and tends to crack easily under mechanical and environmental loads, there are concerns with durability. During the past decade, the effort to modify the brittle nature of ordinary concrete has resulted in high-performance fiber-reinforced cementitious composites (HPFRCCs), which are characterized by tensile strain-hardening after first cracking. Engineered cementitious composites (ECCs), a special type of HPFRCC, represent a new concrete material that offers significant potential to reduce the durability problem of concrete structures. Unlike ordinary concrete and fiber-reinforced concrete materials, ECC strain-hardens after first cracking, as do ductile metals, and it demonstrates a strain capacity 300 to 500 times greater than normal concrete. Even at large imposed deformation, crack widths of ECC remain small, less than 80 μm. Apart from unique tensile properties, the relationship between crack characteristics and durability—including transport properties (perme...

Effect of Seawater and Warm Environment on Glass/Epoxy and Glass/Polyurethane Composites

Abstract: A study of the durability of fiber reinforced polymer (FRP) materials in seawater and warm environment is presented in this paper. The major objective of the study is to evaluate the effects of seawater and temperature on the structural properties of glass/epoxy and glass/polyurethane composite materials. These effects were studied in terms of seawater absorption, permeation of salt and contaminants, chemical and physical bonds at the interface, degradation in mechanical properties, and failure mechanisms. Test parameters included immersion time, ranging from 3 months to 1 year, and temperature including room temperature and 65°C. Seawater absorption increased with immersion time and with temperature. The matrix in both composites was efficient in protecting the fibers from corrosive elements in seawater; however moisture creates a dual mechanism of stress relaxation—swelling—mechanical adhesion, and breakdown of chemical bonds between fiber and matrix at the interface. It is observed that high temperature accelerates the degradation mechanism in the glass/polyurethane composite. No significant changes were observed in tensile strength of glass/epoxy and in the modulus of both glass/epoxy and glass/polyurethane composites. However, the tensile strength of the glass/polyurethane composite decreased by 19% after 1 year of exposure to seawater at room temperature and by 31% after 1 year of exposure at 65°C. Plasticization due to moisture absorption leads to ductile failure in the matrix, but this can be reversed in glass/polyurethane composites after extended exposure to seawater at high temperature where brittle failure of matrix and fiber were observed.

A Semi-Circular Bend Technique for Determining Dynamic Fracture Toughness

Abstract: We propose and validate a fracture testing method using a notched core-based semi-circular bend (SCB) specimen loaded dynamically with a modified split Hopkinson pressure bar (SHPB) apparatus. An isotropic fine-grained granitic rock, Laurentian granite (LG) is tested to validate this dynamic SCB method. Strain gauges are mounted near the crack tip of the specimen to detect the fracture onset and a laser gap gauge (LGG) is employed to monitor the crack surface opening distance. We demonstrate that with dynamic force balance achieved by pulse shaping, the peak of the far-field load synchronizes with the specimen fracture time. Furthermore, the evolution of dynamic stress intensity factor (SIF) obtained from the dynamic finite element analysis agrees with that from quasi-static analysis. These results prove that with dynamic force balance in SHPB, the inertial effect is minimized even for samples with complex geometries like notched SCB disc. The dynamic force balance thus enables the regression of dynamic fracture toughness using quasi-static analysis. This dynamic SCB method provides an easy and cost-effective way to measure dynamic fracture toughness of rocks and other brittle materials.

Effect of densification on crack initiation under Vickers indentation test

Abstract: Crack initiation in various commercial glass compositions was investigated by measuring “crack resistance”, which is determined by a series of Vickers indentation and counting of cracks around the indentation. The crack resistance of glass does not have clear relationship with hardness, fracture toughness, nor “brittleness” which is a ratio of the hardness to the fracture toughness. However, the crack resistance has a strong relationship with densification. Glass experiencing larger densification around the indentation shows higher crack resistance. Densification is assumed to reduce residual stress around the indentation, resulting in an increase in the crack resistance.

Brittleness of materials: implications for composites and a relation to impact strength

Abstract: Brittleness of materials—whether it occurs naturally or with aging—affects significantly performance and manifests itself in various properties. In the past, brittleness was defined qualitatively, but now a definition of brittleness for viscoelastic materials exists, enabling analysis of all types of polymer-based materials. The quantity brittleness, B, has been evaluated for neat thermoplastics, but here composites and metal alloys are also assessed. The physical significance of brittleness is connected to the dimensional stability of materials. The connections of brittleness to tensile elongation and to fatigue are explored while its relationship to surface properties—specifically wear by repetitive scratching—is examined more closely. The economic impact of wear results in monetary loss associated with failure and reduced service life of plastic parts—thus its connection to brittleness finds use across a broad spectrum of industrial applications which utilize plastics for manufacturing, processing, etc. We also demonstrate a correspondence between impact strength (Charpy or Izod) and brittleness of polymers. It is shown that the assumption hardness is equivalent to brittleness is inaccurate; this fact has important implications for interpreting the results of mechanical testing of viscoelastic materials.

Nanocellulose and Microcellulose Fibers for Concrete

Abstract: A study was conducted in which a reactive powder concrete was reinforced with a combination of nanocellulose and microcellulose fibers to increase the toughness of an otherwise brittle material. These fibers could provide the benefit of other micro- and nanofiber reinforcement systems at a fraction of the cost. An empirical investigation into the effects of several different reinforcement schemes on processing parameters and mechanical properties of a reactive powder concrete mixture was conducted. In particular, notched-beam tests were performed under crack-mouth opening displacement control to measure fracture energy under stable crack-growth conditions. Preliminary results show that the addition of up to 3% micro- and nanofibers in combination increased the fracture energy by more than 50% relative to the unreinforced material, with little change in processing procedure. Splitting tensile tests were also performed for comparison with beam-bending tests. Current work focuses on applying high-resolution ...

Mode II Fracture Toughness of a Brittle and a Ductile Adhesive as a Function of the Adhesive Thickness

Abstract: The main goal of this study was to evaluate the effect of the thickness and type of adhesive on the Mode II toughness of an adhesive joint. Two different adhesives were used, Araldite ® AV138/HV998 which is brittle and Araldite 2015 which is ductile. The end notched flexure (ENF) test was used to determine the Mode II fracture toughness because it is commonly known to be the easiest and widely used to characterize Mode II fracture. The ENF test consists of a three-point bending test on a notched specimen which induces a shear crack propagation through the bondline. The main conclusion is that the energy release rate for AV138 does not vary with the adhesive thickness whereas for Araldite 2015, the fracture toughness in Mode II increases with the adhesive thickness. This can be explained by the adhesive plasticity at the end of the crack tip.

A Probabilistic Damage Model of the Dynamic Fragmentation Process in Brittle Materials

Abstract: Dynamic fragmentation is observed in brittle materials such as ceramics, concrete, glass, or rocks submitted to impact or blast loadings. Under such loadings, high-stress-rate tensile fields develop within the target and produce fragmentations characterized by a high density of oriented cracks. To improve industrial processes such as blast loadings in open quarry or ballistic efficiencies of armors or concrete structures against impact loadings, it is essential to understand the main properties of such damage processes (namely, characteristic time of fragmentation, characteristic density, orientation and extension of cracking, ultimate strength) as functions of the loading rate, the size of the structure (or the examination volume), and the failure properties of the brittle material concerned. In the present contribution, the concept of probability of nonobscuration is developed and extended to predict the crack density for any size, shape of the loaded volume, stress gradients, and stress rates. A closed-form solution is used to show how a brittle and random behavior under quasi-static loading becomes deterministic and stress rate dependent with increasing loading rates. Two definitions of the tensile strength of brittle materials are proposed. As shown by Monte Carlo simulations, for brittle materials, the “ultimate macroscopic strength” applies under high loading rate or in a large domain, whereas the “mean obscuration stress” applies under low stress rate or in a small domain. Next, a multiscale model is presented and used to simulate damage processes observed during edge-on impact tests performed on an ultra-high-strength concrete. Finally, the fragmentation properties predicted by modeling of six brittle materials (dense and porous SiC ceramics, a microconcrete, an ultra-high-strength concrete, a limestone rock, and a soda-lime silicate glass) are compared.

Tensile Deformation and Fracture Mechanism of Bulk Bimodal Ultrafine-Grained Al-Mg Alloy

Abstract: The tensile fractures of ultrafine-grained (UFG) Al-Mg alloy with a bimodal grain size were investigated at the micro- and macroscale using transmission electron microscopy (TEM), scanning electron microscopy (SEM) equipped with focused ion beam (FIB), and optical microscopy. The nanoscale voids and crack behaviors near the tensile fracture surfaces were revealed in various scale ranges and provided the evidence to determine the underlying tensile deformation and fracture mechanisms associated with the bulk bimodal metals. The bimodal grain structures exhibit unusual deformation and fracture mechanisms similar to ductile-phase toughening of brittle materials. The ductile coarse grains in the UFG matrix effectively impede propagation of microcracks, resulting in enhanced ductility and toughness while retaining high strength. In view of the observations collected, we propose a descriptive model for tensile deformation and fracture of bimodal UFG metals.

Mechanical properties of tannin-based rigid foams undergoing compression

Abstract: The mechanical properties of a new class of extremely lightweight tannin-based materials, namely organic foams and their carbonaceous counterparts are detailed. Scaling laws are shown to describe correctly the observed behaviour. Information about the mechanical characteristics of the elementary forces acting within these solids is derived. It is suggested that organic materials present a rather bending-dominated behaviour and are partly plastic. On the contrary, carbon foams obtained by pyrolysis of the former present a fracture-dominated behaviour and are purely brittle. These conclusions are supported by the differences in the exponent describing the change of Young's modulus as a function of relative density, while that describing compressive strength is unchanged. Features of the densification strain also support such conclusions. Carbon foams of very low density may absorb high energy when compressed, making them valuable materials for crash protection.

Stress–strain behaviour and abrupt loss of stiffness of geopolymer at elevated temperatures

Abstract: This paper reports stress versus strain curves of geopolymer tested while the specimens were kept at elevated temperatures, with the aim to study the fire resistance of geopolymer. Tests were performed at temperatures from 23 to 680 °C and after cooling. Hot strengths of geopolymer increased when the temperature increased from 290 to 520 °C, reaching the highest strength at 520 °C, which is almost double that of its initial strength at room temperature. However, glass transition behaviour was observed to occur between 520 and 575 °C, which was characterised by abrupt loss of stiffness and significant viscoelastic behaviour. The glass transition temperature is determined to be 560 °C. Further, the strength reductions occurred during cooling to room temperature. This is attributed to the damage due to brittle nature of the material making it difficult to accommodate thermal strain differentials during cooling phase.

Time Effects Relate to Crushing in Sand

Abstract: Based on previously obtained experimental results, a mechanistic picture of time effects in granular materials is presented. Accordingly, time effects are caused by grain crushing, which in turn is time dependent, as indicated by static fatigue of brittle materials. Triaxial compression tests have been performed on Virginia Beach sand at high pressures, where grain crushing is prevalent, to study effects of initial loading strain rates on subsequent amounts of creep and stress relaxation. Grain size distribution curves were determined after each test and the amount of crushing, as characterized by Hardin's breakage factor, is related to the energy input to the triaxial specimens. A pattern emerges that indicates the importance of crushing for the axial and volumetric strains, while rearrangement and frictional sliding between intact grains play much smaller roles in the stress-strain and volume change behaviors of granular materials at high stresses and shear strains. Because particle crushing is a time-dependent phenomenon described as static fatigue or delayed fracture, the close relation between time effects and crushing in granular materials is established.

Damage of concrete in a very high stress state: experimental investigation

Abstract: This study is intended to characterize the evolution in triaxial behavior of a standard concrete subjected to confining pressures varying from 0 to 600 MPa. Hydrostatic and triaxial tests, with several unloading–reloading cycles, are carried out on concrete samples using a high-capacity triaxial press. These tests serve to identify the evolution of the elastic unloading characteristics of concrete, depending on both confining pressure and axial strain. A number of optical observations are also provided to allow visualizing the evolution in concrete damage mode in the middle of the sample. Experimental results indicate a sizable change in concrete behavior with confining pressure. At low pressure values, Young’s modulus decreases and Poisson’s ratio rises sharply with axial strain. The concrete exhibits brittle behavior with failure caused by a localized damage mechanism. In contrast, at high confining pressures, the concrete becomes a ductile material, and the evolution in its unloading characteristics is negligible. Failure is thus associated with diffuse material damage. The concrete behaves like a granular material controlled by plasticity, meaning that the damage phenomenon observed at low confinement is completely inhibited.