Brittle Plastic Research Articles

Brittle–ductile mode cutting of glass based on controlling cracks initiation and propagation

Owing to brittleness and hardness, functional glass is one of the most difficult to cut materials. This paper proposes a new machining method—brittle–ductile mode machining combining both properties of brittle breakage and plastic flow of glass. Edge-indention experiments are first conducted in order to deduce the laws of crack initiation and propagation in the process of glass cutting, then a single-straight tool with big inclination angle is designed for glass cutting based on the laws of crack initiation and propagation and properties of plastic flow. With this new tool, the lateral and subsurface cracks initiation can be suppressed, and media cracks propagate away from machined surface. At the same time, the requirements for machining glass in ductile manner can be fulfilled. Validation experiments show that highly efficient and precise glass cutting can be achieved at the cutting depth of sub-millimeter level, and an integral and crack-free surface with good finish can be obtained. This method overcomes the process restriction on critical cutting depth and tool feed for ductile regime turning technology and can be transferred to mass production.

Effect of drying catalysts on the properties of thermal copolymers from conjugated linseed oil–styrene–divinylbenzene

The effect of addition of a varying concentration of a drying catalyst (cobalt salt as primary drier) and a combination of catalysts such as cobalt, zirconium (secondary drier) and calcium (auxiliary drier) in a fixed concentration (1%) to a 50:20:30 compositions of 87% conjugated linseed oil, styrene (ST), and divinylbenzene (DVB) has been studied by characterizing the resulting polymers from thermal polymerization with various techniques such as soxhlet extraction, 1H NMR ( 1H nuclear magnetic resonance) spectroscopy, dynamic mechanical analysis (DMA), mechanical and thermogravimetric analysis. The thermal polymerization is performed in the temperature range of 85–160 °C. By soxhlet extraction, it is observed that the polymers contain approximately 64–77% crosslinked materials and the crosslinked insoluble fraction increases with an increase in cobalt catalyst concentration. For fixed concentration (1%) of catalysts, the insoluble fraction from the soxhlet extraction is maximum for the cobalt–zirconium mixture and minimum for the cobalt–calcium mixture. The micro-composition of these polymers obtained from the 1H NMR spectroscopy indicates that the crosslinked materials are composed of both soft oily and hard aromatic phases. The polymers with varying cobalt concentrations up to 0.6 wt% exhibit two separate glass transition temperatures, indicating the presence of two separate phases, one soft rubbery phase with sharp glass transition temperature of −50 °C and a hard brittle plastic phase of broadened glass transition temperature of 70–120 °C. On the other hand, instead of sharp peaks, the polymers with 0.8 and 1.0% cobalt salts exhibit two humps and a distinct peak in between the humps in the tan δ plots, indicating the presence of an additional phase comprised of a copolymer of linseed oil–styrene and DVB. For fixed concentration (1%) of catalysts, the cobalt–calcium combination follows the similar trend in the tan δ as that for the polymers with 0.8–1.0% cobalt whereas other combinations exhibit two phases. These polymers possess crosslink densities of 0.63–0.91 × 10 4 mol/m 3 and compressive strengths of 2.0–26.6 MPa. The catalyzed polymers are thermally stable below 300 °C and exhibit a major thermal degradation with a maximum degradation of 82–88% at a temperature of 500 °C.

Nonlinear mechanical properties of lattice truss materials

An increment equivalent continuum method was developed to study the nonlinear mechanical properties of lattice truss composite materials. As the strut material is brittle or elastic-perfectly plastic, the stiffness method is applicable. The stiffness matrix and strengths of the Kagome lattice core material, the pyramid lattice core material and the corresponding octet-truss lattice were deduced. Initial yield surfaces were depicted by polyhedrons in tri-axial stress spaces, pure shearing stress spaces and compared in two-dimensional stress spaces, respectively. As the strut property was nonlinear, the increment method based on the flexibility method was applied to study the plasticity of strain hardening solids. The complete stress–strain equations were deduced by integration. The suggested increment method was valid to model the complete stress–strain curves of the octet-truss composite and the Kagome lattice core of the sandwich panel. The simulations were well consistent with the reference experimental results.

Fiber Reinforced Laminates: Progressive Damage Modeling Based on Failure Mechanisms

In the present article, computational modeling of progressive damage in continuous fiber reinforced laminates is considered. After a general review of modeling approaches and experimentally observed behavior of laminates, the focus is laid on predicting non-linear laminate behavior by models based on continuum damage mechanics. The wide variety of continuum damage models is demonstrated by example of three different damage models from the literature which are described in more detail. Finally, a ply level damage model developed by the authors is presented. The model is based on brittle failure mechanisms postulated by Puck and is able to capture several characteristics of the damage behavior of laminates. Furthermore, this brittle damage model is extended to include plastic shear deformations. It is shown, that the extended model capturing brittle damage and plastic strains leads to significant improvements in the prediction of the non-linear laminate behavior.

Stress and fracture prediction in inverted half-graben structures

Two-dimensional finite element models are used to study the temporal evolution and spatial distribution of stress and strain during half-graben inversion. Modeling is based on forward balanced cross-sections and involves various parameter studies to assess the impact of different syn- and post-rift lithologies as well as scenarios with syn-tectonic deposition or erosion, respectively. In total, 48 different scenarios were analyzed. Modeling results are presented as contour maps of total displacement, brittle plastic strain, mean stress and differential stress as well as vector diagrams showing the orientation of the principal stresses. This information can be combined to predict fracture types and fracture orientations and provide fracture intensity maps throughout the evolving inversion structure. Modeling results demonstrate quantitatively how stress and strain in inverted half-grabens critically depend on the rheology of the syn- and post-rift sediments and their mechanical interaction as well as the stress transfer through the hanging wall. Four end-member scenarios can be identified on the basis of distinct fault reactivation tendencies, deformation styles and fracture patterns. Model predictions are compared to natural examples of inverted half-graben structures using seismic as well as outcrop data. Thus, the geomechanical models can provide templates for a reservoir- (kilometer-) scale prediction of tectonic stresses and fractures for a common type of inversion structure.

Stressed state corresponding to the formation of large-scale brittle failure of rocks

Issue of relations between brittle failure and localized plastic flow, which are realized in the fault zones of the Earth’s crust, is one of the most important problems in geomechanics. New data on values of natural stresses in seismoactive regions allowed us to distinguish the regularities of their distribution, which correspond to the possibility of the appearance of large-scale brittle failure. It was found that areas of medium and low levels of confining pressure are the most hazardous regions in terms of a strong earthquake. Here, a small part of the released energy is consumed for negotiating friction forces. Therefore, brittle failure is more efficient. In the regions of high rock pressure, the dissipation of elastic energy is realized as creep along the fault or cataclastic flow due to a large number of weak earthquakes.

In vitro rapid intraoral adjustment of porcelain prostheses using a high-speed dental handpiece

In vitro rapid intraoral adjustment of porcelain prostheses was conducted using a high-speed dental handpiece and diamond bur. The adjustment process was characterized by measurement of removal forces and energy, with scanning electron microscopic (SEM) observation of porcelain debris, surfaces and subsurface damage produced as a function of operational feed rate. Finite element analysis (FEA) was applied to evaluate subsurface stress distributions and degrees of subsurface damage. The results show that an increase in feed rate resulted in increases in both tangential and normal forces (analysis of variance (ANOVA), P < 0.01). When the feed rate approached the highest rate of 60 mm min −1 at a fixed depth of cut of 100 μm, the tangential force was nearly seven times that at the lowest feed rate of 15 mm min −1. Consequently, the specific removal energy increased significantly (ANOVA, P < 0.01), and the maximum depth of subsurface damage obtained was approximately 110 and 120 μm at the highest feed rate of 60 mm min −1 using SEM and FEA, respectively. The topographies of both the adjusted porcelain surfaces and the debris demonstrate microscopically that porcelain was removed via brittle fracture and plastic deformation. Clinicians must be cautious when pursuing rapid dental adjustments, because high operational energy, larger forces and severe surface and subsurface damage can be induced.

Sliding wear behavior of plasma sprayed Fe 3Al–Al 2O 3 graded coatings

Fe 3Al–Al 2O 3 double-layer coatings (DC), Fe 3Al–Fe 3Al/50%Al 2O 3–Al 2O 3 triple-layer coatings (TC) and Fe 3Al–Al 2O 3 graded coatings (GC) were produced from a series of Fe 3Al/Al 2O 3 composite powders with different compositions on low carbon steel substrate using PLAXAIR plasma spraying equipment. Friction behaviors and wear resistance of the three kinds of coatings have been investigated under different loads. Tests were carried out using an MRH-3 standard machine, in lineal contact sliding under dry condition against hardmetal, at a sliding velocity of about 1.57 ms − 1 . Wear rates under different loads were measured and the friction coefficients were recorded. SEM analysis was carried out to identify the wear mechanisms. The results show that the GC has higher wear-resistance than DC and TC. The tribological characteristics of graded coating were different along the depth of the coatings, and the surface of coatings with pure Al 2O 3 does not show the best wear resistance. The wear rate and friction coefficients were also different under different loads. The failure types of plasma-sprayed Fe 3Al–Al 2O 3 graded coatings in lineal contact were: loosening of ceramic particles, crack nucleation and propagation, brittle fracture, plastic deformation, and adhesive wear.

Evolutionary nature of structure formation in lithospheric material: universal principle for fractality of solids

Destruction of geomaterials and geomedia material, as well as general brittle and ductile materials, have been treated theoretically and experimentally using the general approach of nonlinear dynamic systems. The process of destruction in loaded solids (inelastic deformation, damage accumulation, fracture) is presented as a space-time evolution of a nonlinear dynamic system, which allows interpreting all deformation and fracture within the limits of a single theory. The space-time hierarchies of nonlinear systems were found out to undergo collective effects and self-organization. The experimental and theoretical studies of the evolution of loaded solids revealed their universal fractality and showed brittle fracture and plastic deformation to be self-similar processes at different scales, for which scaling parameters have been estimated. The evolution of inelastic strain and destruction of solids is modeled numerically in terms of hierarchic systems.

Repeating earthquakes, episodic tremor and slip: Emerging patterns in complex earthquake cycles?

AbstractThe lack of regularity in earthquake cycles continues to be a confounding issue in earthquake science. Lately, observations of episodic nonvolcanic tremor and slip (ETS) along a few well‐instrumented tectonic plate boundaries are intriguing: these features recur together with predictable time intervals. Data now trace recurring ETS back to 1990 and no significant earthquake ever followed an ETS episode. This observation and the fact that stress drops associated with episodic slips are low, only on the order of 0.01 MPa, suggests that repeated ETS has little cumulative effects in priming the fault for the next large earthquake. Another known regularity in seismic activity is the so‐called repeating earthquakes that rupture the same patch of fault repetitively. Current hypotheses for repeating earthquakes point to the interaction of continual, aseismic fault slip with locked, seismogenic patches of the fault. Interestingly, ETS also recur near where the transition between brittle faulting and plastic flow is expected, although it is not clear how and why regularities in space and time are interconnected. New data show that ETS recurs throughout the entire length of the Cascadia subduction zone, thus ruling out any special, local factors as necessary conditions for ETS. Recurrence intervals of ETS vary along the Cascadia. Such variations are not governed by the rate of plate motion that ultimately drives the earthquake process, but they do coincide with variations in the geology of the overriding plate that can influence the rheology along the plate interface. To this end, we call attention to the Portevin‐Le Chatelier effect (PLC, or jerky flow) as a potential analog to the earthquake process. The dynamics of the PLC has been extensively studied and shows many intriguing features as the system goes from chaotic to self‐organized critical regimes as strain rate increases. In particular, the PLC exhibits not only stick‐slip behavior (stress serration) over time but also spatial interactions over extended regions—features that are necessary to account for complex spatio‐temporal variations associated with earthquake activities. © 2007 Wiley Periodicals, Inc. Complexity 12: 33–43, 2007

Construction of stess-strain curves for brittle materials by indentation in a wide temperature range

A test method procedure for constructing stress-strain curves by indentation of brittle and low plastic materials under temperature ranging from 20 to 900?C was developed recently by Yu. Milman, B. Galanov et al. According to this test method procedure stress-strain curves ? - ? for Si, Ge, SiC, TiB2 and WC/Co hard alloy were constructed in the above temperature region and mechanical parameters such as elastic point, ?e, yield stress, ?s, etc. were extracted by using the measurement results obtained by a set of trihedral pyramid indenters with different angles at the tip, ?1, ranging from 45 to 85?C.

Symmetry-, time-, and temperature-dependent strength of carbon nanotubes.

Although the strength of carbon nanotubes has been of great interest, their ideal value has remained elusive both experimentally and theoretically. Here, we present a comprehensive analysis of underlying atomic mechanisms and evaluate the yield strain for arbitrary nanotubes at realistic conditions. For this purpose, we combine detailed quantum mechanical computations of failure nucleation and transition-state barriers with the probabilistic approach of the rate theory. The numerical results are then summarized in a concise set of equations for the breaking strain. We reveal a competition between two alternative routes of brittle bond breaking and plastic relaxation, determine the domains of their dominance, and map the nanotube strength as a function of chiral symmetry, tensile test time, and temperature.

Equivalent Mohr–Coulomb and generalized Hoek–Brown strength parameters for supported axisymmetric tunnels in plastic or brittle rock

The evaluation of equivalent Mohr–Coulomb (M–C) strength parameters to the prototype Hoek–Brown (H–B) ones for tunnels has been tackled in different ways for many years. The extension of the H–B criterion to the generalized one has made the challenge even greater. Most of the latest methods did not account for the effect of the support pressure and none gave formulae for equivalent parameters of supported or brittle rock. Here, an almost exact explicit solution for the evaluation of the critical pressure, of a tunnel in a rock mass satisfying the generalized H–B criterion, is initially investigated. Then, formulae are derived for the evaluation of equivalent parameters, of either elastoplastic or elastic–brittle plastic rock. They are based either on a best fitting procedure of the two envelopes or on the equation of selected responses of the models. Supported tunnels in equivalent M–C rock masses are then validated against those excavated in the prototype H–B rock masses.

Novel Conjugated Linseed Oil−Styrene−Divinylbenzene Copolymers Prepared by Thermal Polymerization. 1. Effect of Monomer Concentration on the Structure and Properties

A variety of novel opaque, white polymers ranging from rubbery materials to tough and rigid plastics have been prepared by the thermal polymerization at 85-160 degrees C of varying amounts of 87% conjugated linseed oil, styrene, and divinylbenzene. Gelation of the reactants typically occurs at temperatures higher than 120 degrees C, and fully cured thermosets are obtained after postcuring at 160 degrees C. The fully cured thermosets have been determined by Soxhlet extraction to contain approximately 35-85% cross-linked materials. The microcomposition of these polymers, as determined by 1H NMR spectroscopy, indicates that the cross-linked materials are composed of both soft oily and hard aromatic phases. After solvent extraction, the insoluble materials exhibit nanopores well distributed throughout the polymer matrixes. Dynamic mechanical analysis of these polymers indicates that they are phase separated with a soft rubbery phase having a sharp glass transition temperature of -50 degrees C and a hard brittle plastic phase with a broadened glass transition temperature of 70-120 degrees C. These polymers possess cross-link densities of 0.15-2.41 x 10(4) mol/m3, compressive Young's moduli of 12-438 MPa, and compressive strengths of 2-27 MPa. These materials are thermally stable below 350 degrees C and exhibit a major thermal degradation of 72-90% at 493-500 degrees C.

Synthesis of unsaturated polyester resin from postconsumer PET bottles: Effect of type of glycol on characteristics of unsaturated polyester resin

AbstractPostconsumer PET bottles including water and soft‐drink bottles were depolymerized by glycolysis in excess glycols, such as ethylene glycol, propylene glycol, and diethylene glycol, in the presence of a zinc acetate catalyst. The obtained glycolyzed products were reacted with maleic anhydride and mixed with a styrene monomer to prepare unsaturated polyester (UPE) resins. These resins were cured using methyl ethyl ketone peroxide (MEKPO) as an initiator and cobalt octoate as an accelerator. The physical and mechanical properties of the cured samples were investigated. It was found that the type of glycol used in glycolysis had a significant effect on the characteristics of the uncured and cured UPE resins. Uncured EG‐based UPE resin was a soft solid at room temperature, whereas uncured PG‐ and DEG‐based resins were viscous liquids. In the case of the cured resins, the EG‐based product exhibited characteristics of a hard and brittle plastic, while the PG‐based product did not. The DEG‐based product exhibited characteristics of hard and brittle plastic after strain‐induced crystallization had occurred. In addition, it was also found that no separation of the type of bottles was needed before glycolysis, since UPE resins prepared from water bottles, soft‐drink bottles, and a mixture of both bottles showed the same characteristics. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 788–792, 2003

Failure mode transition in ceramics under dynamic multiaxial compression

An experimental technique based on the Kolsky pressure bar has been developed to investigate the behavior of ceramics under dynamic multiaxial compression. Experimental results for aluminum nitride (AlN), together with data available in the literature, indicate that a Mohr-Coulomb criterion and the Johnson–Holmquist model fit the experimental data for failure in a brittle manner, whereas the ceramic material exhibited pressure insensitive plastic flow at high pressures. A failure surface is constructed which represents the material failure behavior, including brittle failure, brittle/ductile transition and plastic flow, under various pressures. The effect of various material properties on the failure behavior was investigated. The Poisson's ratio is found to be a measure of brittleness for ceramic materials with low spall strength under shock wave loading conditions. Lower value of Poisson's ratio indicates that the material will fail in a brittle manner through axial splitting even under uniaxial strain loading; whereas materials with higher Poisson's ratio may be expected to deform plastically beyond the Hugoniot Elastic Limit (HEL). The applicability of the proposed failure surface to a range of ceramics is explored and the limitations of the model are outlined.

Discontinuous bifurcations in the case of the Burzy?ski-Torre yield condition

Material instability resulting in discontinuous plastic bifurcations was considered in numerous papers for typical yield conditions. Here we discuss this phenomenon for the paraboloidal Burzynski-Torre yield condition, adequate for many brittle plastic materials. This condition is described by a function which is not homogeneous with respect to the stress components and this fact brings some additional difficulties. General formulae are derived and then plane stress states are discussed in detail. Related decohesion phenomena are also mentioned.

An instrument for in situ growth rate characterization of mechanically strained crystals

The crystal growth and dissolution characteristics of a certain material can be considerably influenced by the strain present in a growing (dissolving) crystal. Strain can be induced in various ways. One of the most common and always present in industrial processes, where attrition processes are always accompanied by generation of mechanical strain in a newly formed small crystal fragments, is mechanical stressing, in situ, during preparation and handling. To gain deeper insights into some aspects of this phenomenon, a sophisticated equipment is needed for in situ controlled stressing of extremely brittle and fragile crystals. For this purpose, we have developed an apparatus which comprises a specially designed straining cell coupled with a laser Michelson interferometer for growth rate measurements. The straining cell is designed to accommodate crystals that undergo fracture below 200 microstrains. The stress imposed on a crystal is computer controlled with a precision of approximately 5%. Details of the instrument are given together with two examples of straining in situ brittle paracetamol and plastic sodium nitrate crystals. The measured changes in growth rate of a paracetamol crystal, in the quasi linear region 0–70 kPa are estimated to be (−9.4±0.1)×10−11 m/s/kPa.

Influence of cracking on tensile behavior of brittle materials

This paper deals with mechanical behavior of quasi-brittle materials in tension, such as concrete. Two elementary LEFM models are presented. Attention is focused on the crack phenomenon, highlighting the influence of initial damage on structural response. The first model consists of a plate of finite size with a central crack. The second model corresponds to an infinite plate with an infinite set of collinear cracks of equal length and spaced at a constant distance apart. For both models, an analytical study is conducted which leads to a determination of the link between critical stress and displacement, initially considering only the incremental displacement due to the damage present in the material, and subsequently also taking into account the elastic compliance of the structure. An analysis is also performed in which the concomitance of brittle fracture and plastic collapse is considered, which reveals interesting scale effects.

Percolation theory and physics of compression

The concept of percolation theory is an excellent tool to elucidate the physics of compression. Earlier findings taking into account the percolation theory indicated that the formation of a tablet can be subdivided into a two-stage process with a ‘weak-bond’ percolation effect at a lower percolation threshold p c corresponding to the relative tapped density ϱ r and a ‘strong-bond/site’ percolation effect at an upper percolation threshold p c ∗ , i.e. at a relative density ϱ r ∗ , where brittle fracture and/or plastic flow starts to play an important role for the formation of a stable compact. The new findings which are now presented indicate that the uniaxial compression can be interpreted as a 2-dimensional percolation process where the stress is transmitted by the contact points of the particles. Thus the modified Young's elasticity modulus of the compact can be described by the fundamental equation of percolation theory with a critical exponent q = 1.3 (which is the conductivity exponent) and with a lower percolation threshold of the relative tapped density ϱ r. If the same equation is applied for the tensile strength, high values of the critical exponent result, indicating that a still unknown fractal dimension seems to play a key role.