Brittle State Research Articles

Scontrone (central Italy), signs of a 9-million-year-old tragedy

The fossilierous bonebeds of Scontrone (Abruzzo region, central Italy) are preserved in tidal-flat aeolian calcarenites at the base of the Lithothamnion Limestone, a Miocene carbonate ramp widespread in the central-southern Apennines. The site bears evidence of a catastrophic event at 9 Ma. Reported are the results of the palaeobiological and taphonomic analysis conducted on the rich vertebrate assemblage, particularly on the remains of Hoplitomeryx (Mammalia, Artiodactyla, Ruminantia), recovered from the so-called Scontrone calcarenites between 1992 and 2012. This is the first taphonomic study of a Late Miocene continental bone assemblage preserved in coastal deposits. The bones are not in primary context. They were likely exhumed during the initial phase of a marine transgression after a period of primary ‘storage’ within a possibly flood-generated deposit in an estuarine environment. The mortality patterns indicate that the carcasses accumulated in a short time (within a year). The bones of the disarticulated skeletons were then removed, broken in a dry and brittle state, scattered over wide carbonate ramps along an arid to semi-arid, wind-exposed coastline and eventually buried again in aeolian calcarenites that drape transgressive tidal-flat creek deposits. The analysis also reveals that hoplitomerycids were possibly seasonal reproducers and that the land they inhabited, the so-called Apulia Platform, was probably swept by sudden, disastrous, storm-supplied flash floods.

Embrittlement of structural steel by stress concentrators

The ease of embrittlement characterizes the resistance of a metal to transition from the plastic to the brittle state. The physical significance of the mechanical stability is analyzed in conditions of stress concentration. A method is developed for predicting the ease of embrittlement K msc of structural steel under the action of a stress concentrator on the basis of standard uniaxial tensile test data. The structural quality \(\mu _{P_{ms} }^\sigma \) may be assessed in terms of the residual mechanical stability P ms, which characterizes the remaining resistance to brittleness in conditions of stress concentration. The change in the ease of embrittlement under the action of stress concentrators is analyzed in the case of uniaxial extension and static flexure.

Design of polymeric capsules for self-healing concrete

Up to now, glass capsules, which cannot resist the mixing process of concrete, have been mostly used in lab-scale proof-of-concept to encapsulate polymeric agents in self-healing concrete. This study presents the design of polymeric capsules which are able to resist the concrete mixing process and which can break when cracks appear. Three different polymers with a low glass transition temperature Tg have been extruded: Poly(lactic acid) (PLA) (Tg=59°C), Polystyrene (PS) (Tg=102°C) and Poly(methylmethacrylate/n-butyl methacrylate) (P(MMA/n-BMA)) (Tg=59°C). After heating the capsules prior to mixing with other components of the mix, to shift from a brittle state to a rubbery state, their survival ratio considerably increased. Moreover, a part of the capsules, which previously survived the concrete mixing process, broke with crack appearance. Although some optimization is still necessary concerning functional life of encapsulated adhesives, this seems to be a promising route.

X-ray reflectivity study of the glass transition temperature of thin films

The glass transition takes place in amorphous materials (like polymers) during heating or cooling, and can be described as reversible transition from a hard and brittle state into a rubber-like state. Although physical properties of the material change significantly during the glass transition, this is not a phase transition of the material. The temperature at which the transition between the glassy and rubbery state occurs is called the glass transition temperature, and this temperature is always lower than the melting temperature. Thermodynamically, the glass transition is associated with transfer of heat between the system and its surrounding and with an abrupt volume change. Previously it was shown that the glass transition temperature of nano-films is different from that of bulk materials [1], which signifies the importance of determining this parameter for such systems. In the current work, we use quick X-ray reflectivity (qXRR) measurements to determine the glass transition temperature of polyvinyl acetate (PVAc). PVAc is rubbery synthetic polymer with the formula (C4H6O2), a density of 1.18 g/cm3, and a glass transition temperature for bulk material of 30oC [2]. Regular X-ray reflectivity measurements are based on θ/2θ scans at grazing incidence and typically require 0.5-1.5 h for a single scan. The qXRR technique is based on simultaneous measurement of the whole angular x-ray reflectivity profile and is suitable for in-situ measurement without moving the sample and/or the x-ray optics. Thus, the qXRR technique allows for very fast measurement of the x-ray reflectivity curves (duration of each scan is typically 0.1–20 sec [3]), which permits studying the time evolution of chemical, thermal, and mechanical changes at the surface and interface of different materials. X-ray reflectivity measurements give information about both density and thickness of thin films, and are suitable for studying glass transition phenomena. Nano-thickness PVAc layers on a Si substrate were examined with the qXRR technique, with x-ray reflectivity scans (each 10-seconds in duration) being recorded while temperature was changed from 20 to 50oC (total of 331 scans over 7 hours and 46 minutes). In the current paper, the experimental setup, the data-processing, and the analysis of the results from the qXRR measurements will be presented.

Fracture toughness evaluation of bituminous binders at low temperatures

When asphalt roads turn brittle at low temperatures, they are no longer able to relax completely from traffic induced stresses and may also suffer damage through crack initiation and propagation. In order to avoid early structural damages, the bituminous binders must be selected very carefully. However, conventional test methods for assessing performance of bituminous binders at low temperatures are often unsatisfactory reliable, in particular in case of polymer modified bituminous binders. In this study, an alternative experimental method based on fracture mechanical principles was performed on pre-notched specimens in the brittle state inside a cooling media. This fracture toughness test was evaluated in terms of its suitability for distinguishing different types of polymer modified and unmodified bituminous binders. In addition, the feasibility to evaluate different ageing states was also analysed by testing samples after artificial short-term and long-term ageing procedures. The repeatability obtained from the experimental results showed that the fracture toughness test is a suitable candidate for being introduced in a standardization framework.

Atomic-scale characterization of prior austenite grain boundaries in Fe–Mn-based maraging steel using site-specific atom probe tomography

The embrittlement and de-embrittlement behavior of an Fe–10Mn–1Pd (wt.%) maraging steel upon isothermal aging at 500°C is related to microstructural changes at prior austenite grain boundaries (PAGBs). Site-specific atom probe tomography measurements were performed to analyze the local chemistry of the PAGBs. Tensile tests and hardness measurements were conducted of the ternary alloy and of a binary non-hardenable Fe–10Mn alloy for comparison. Isothermal aging of the binary steel led to a decrease in strength along with a considerable increase in uniform elongation. The Pd-containing alloy, on the other hand, showed significant age-hardening, and an embrittlement and de-embrittlement transition was revealed. Ductile behavior was observed in the initial as-quenched and over-aged states, but there was zero tensile elongation in the intermediate under- and peak-aged regions, where intergranular fracture along the PAGBs occurred. In the brittle peak-aged state a large number of small nanometer-sized particles rich in Mn and Pd formed inside the grains and decorated the PAGBs. The precipitates grew in size on prolonged aging. Mn segregation to the PAGBs was revealed; the Mn concentration level at the boundaries varied with aging time and was highest in the peak-aged condition. Embrittlement and de-embrittlement mechanisms are discussed and compared to these observations.

The depth variations in the b-value of frequency-magnitude distribution of the earthquakes in the Garm region of Tajikistan

The depth changes in the bvalue and density of the number of earthquakes in different magnitude bins (M ≥ 1.8, M ≥ 3.0, M ≥ 3.5) are analyzed using highly accurate seismological observations carried out in 1955-1991 at the Garm prognostic area in Tadjikistan. It is found that the observed bvalues are controlled by the variations in the proportion between weak and strong earthquakes. Two horizons with different patterns of the bvalue are identified in the Earth's crust above and below a depth of 15-16 km. The bvalue in the upper and lower horizons is close to 0.8 and 1.2, respectively. The lower horizon is marked by almost complete absence of relatively strong earthquakes with M ≥ 3.0. The observed changes in the bvalue with increasing depth could probably be due to the increase in the strength of crustal material caused by the growth in tem� perature and confining pressure in the depth interval from 0 to 15 km. The transitional interval between the upper and lower crustal horizons (~13-18 km), which is characterized by a sharp drop in seismic activity, can probably be associated with the zone of the phase transition of crustal material from an elastic brittle state to a plastic state, as suggested by some authors. Typically, the top of this zone hosts the hypocenters of the stron� gest earthquakes in a given territory. The correlation is established between the crustal areas with low bvalues and the locations of the strongest earthquakes in the region. It is suggested that the threedimensional map� ping of the bvalue can be helpful for estimating the locati on, depth, and maximal magnitude of the probable strong earthquakes in seismically active regions and can be used to assess seismic risks.

Low temperature characterization of bituminous binders with a new cyclic shear cooling (CSC) failure test

Low temperature cracking of asphalt pavements is associated to the thermal stresses created when asphalt binder changes from a ductile into a brittle state during cooling. Under repetitive traffic loads, this change of properties can become a significant problem, resulting in the formations of large cracks on the driving surface, thus requiring early pavement repair. Nevertheless, the conventional test methods for assessing low temperature properties of bitumen are often insufficient and subject to low reproducibility. Therefore, a reliable method is necessary to characterize the performance of bituminous binders at low temperature. This work presents the cyclic shear cooling failure test (CSC-failure test) as an alternative method based on a failure test with the dynamic shear rheometer (DSR) at decreasing temperature until fracture occurs. Operational parameters, like shear strain amplitude or loading frequency, have been analysed in order to develop a reproducible procedure applicable for different types of bituminous binders. Moreover, a failure criterion has been defined by analysing the different characteristic temperatures from the failure curves obtained in the test.

Relationship Between Pre-failure and Post-failure Mechanical Properties of Rock Material of Different Origin

Under compression, gathering data related to the post-failure part of the stress–strain curve requires stiff servo-controlled testing systems. In unconfined conditions, data related to the post-peak region of the intact rock parameters are not common as pre-peak and peak state parameters of stress–strain behavior. For problems involving rock in the failed state around structures, proper choice of plastic constitutive laws and post-failure parameters is important for the modeling of the failed state. The aim is to relate commonly used intact rock parameters of pre-failure (tangent modulus Ei and secant modulus Es) and peak strength (σci) states to parameters of the post-failure state under unconfined compression. Post-failure parameters are the drop modulus (Dpf), representing the slope of the falling portion in brittle state, residual strength (σcr), and dilatancy angle (ψ°). Complete stress–strain curves were generated for various intact rock of different origin. Seventy-three post-failure tests were conducted. Samples included in the testing program were chosen to represent rocks of different origin. Specimens of granite, rhyodacite, dunite, quartzite series, glauberite, argillite, marl, and lignite were used in the tests. The results from the pre-failure and peak state testing parts were processed and compared to the post-failure stress–strain parameters. For the estimation of post-failure parameters in terms of the pre-peak and peak states, the functional relations were assessed. It was found that the drop modulus Dpf increases with rock strength σci, following a power function with an approximate power of two. With an exponential trend, the Dpf/Es ratio increases with decreasing Ei/σci ratio. Relations estimating the residual strength and dilatancy from the pre-peak and peak state parameters are in logarithmic and exponential functional forms, respectively.

Evapotranspiration―Soil Structure Relationship in West Marshes of France

The soil potentials, facing to the crop fields, are commonly estimated through the calculation of their available water capacity based on the ETP or ETM estimation. The present work introduces the comparison between theoretical and real available water capacity profiles calculated down to 1.00 m depth. The evapotranspiration data are used to the calculation of ETP in an undrained grassland and ETM in two drained corn fields located in the French Atlantic marshlands. The studied soils have acquired specific properties in response to the reclaiming of the clay; dominant primary sediments began since the Middle Age and late drainage works. The theoretical and real available water capacity profiles are calculated from the ETP and ETM data and from the soil moisture profiles respectively, from June to October 2013. The theoretical and real profiles are confronted to the tensiometric pressure recording at 30, 60 and 90 cm. The tensiometric pressure behavior and associated premature disconnections of the tensiometric plugs are explained thanks to the soil structure-hydromechanical property relationships: i.e. from ductile state in depth to brittle state in surface. The vertical evolutions of the real profiles are explained facing to the plant growing, pluviometry and water nape levels. Their behavior and their shifts from the linear “theoretical” ETP or ETM profiles clearly show the advance of the desiccation front and consequently the kinetics of water consumption by plants. This simple method of calculation and comparison between the real and theoretical ETM or ETP profiles allows the quantitative discussion: 1) on the role of the soil microstructure behavior on the root growing and, 2) on the realism of the crop coefficient taken into account in the ETP or ETM estimation. In these coastal marsh fields, it also argues on the difficulty of management facing to the water and/or salt stresses.

Assessing structural steel in terms of its strength

The ease of embrittlement of a metal characterizes its resistance to transition from a plastic to a brittle state. It is of interest to establish the relation between the ease of embrittlement and the strength, which is the primary mechanical property of structural steel. The mechanical stability K ms supplements σ0.2 in assessing the metal’s susceptibility to embrittlement under the action of external factors that affect the strength. Together with K ms , the mechanical characteristics σ0.2, σ2, and σB completely control the ease of embrittlement—in particular, for structural steel and alloys.

Annealing embrittlement of Fe78Si9B13 (METGLAS-2605S2)

The ductile to brittle transition that occurs in amorphous Fe78Si9B13 (METGLAS-2605S2) has been investigated using mechanical measurements over the temperature range 250–370 °C. The fracture toughness values, K Ic , have been determined for a range of annealing times (5–30 min) and cooling rates of 15–45 °C/min. A pronounced ductile to brittle transition is observed around 310(10) °C although no obvious structural changes are evident as indicated by x-ray diffraction. Comparison of transmission and back-scattered conversion electron 57Fe Mossbauer spectra for the bulk as-received ribbon in the ductile state ( $K_{Ic}=52~{\rm MPa} \cdot \sqrt{m}$ ) and the ribbon annealed to the brittle state ( $K_{Ic}\sim10~{\rm MPa} \cdot \sqrt{m}$ ) indicates magnetic texture effects in both the bulk and on the surface of these amorphous ribbons, related to the magnetostriction resulting from the quenched-in stress during the ribbon production process, and the ensuing stress-relief upon annealing.

An apparatus for the determination of the workability and plastic limit of clays

An apparatus is described that replicates Atterberg's procedure of rolling a thread of clay for the determination of the plastic limit of soils with minimal operator interference or judgement of the crumbling condition required. From nominal stresses applied during rolling and changes in diameter measured for each rolling traverse the toughness of the clay is determined from the stress–strain curves, as work/unit volume for a specified amount of deformation. Threads are tested at water contents ranging from near the sticky limit through the workable plastic region to the brittle state. An abrupt ductile–brittle or plastic–brittle transition is found allowing the plastic limit to be defined as the transition water content. A less abrupt but distinct stiffness transition is frequently obtained that separates the workable plastic region into a soft-plastic and a stiff-plastic region. Good relationships between toughness and water content are achieved from which the maximum toughness, Tmax, at the plastic limit is derived together with other new and useful properties such as the water content at the stiffness transition, wS and at zero toughness, the toughness limit, wT. Also derived are the gradients of the relationships, the toughness coefficients, TC, the range of water contents over which the clay is plastic and tough, the toughness index, IT and the water content in relation to the toughness index, the workability index, Iw. Each clay is found to have a closely correlated toughness–water content relationship depending mainly on the clay content and clay mineralogy demonstrating that control of the plasticity of the clay and its associated water contents are achievable.

Tunable Tensile Ductility in Metallic Glasses

Widespread adoption of metallic glasses (MGs) in applications motivated by high strength and elasticity combined with plastic-like processing has been stymied by their lack of tensile ductility. One emerging strategy to couple the attractive properties of MGs with resistance to failure by shear localization is to employ sub-micron sample or feature length scales, although conflicting results shroud an atomistic understanding of the responsible mechanisms in uncertainty. Here, we report in situ deformation experiments of directly moulded Pt57.5Cu14.7Ni5.3P22.5 MG nanowires, which show tunable tensile ductility. Initially brittle as-moulded nanowires can be coerced to a distinct glassy state upon irradiation with Ga+ ions, leading to tensile ductility and quasi-homogeneous plastic flow. This behaviour is reversible and the glass returns to a brittle state upon subsequent annealing. Our results suggest a novel mechanism for homogenous plastic flow in nano-scaled MGs and strategies for circumventing the poor damage tolerance that has long plagued MGs.

Mechanisms of tensile improvement in caliber-rolled high-carbon steel

The microstructural evolution and tensile characteristics of caliber-rolled plain carbon steels were quantitatively investigated and compared with those of the as-received plain carbon steels. The caliber-rolled steels exhibited a similar microstructure consisting of an ultrafine-grained ferritic matrix and dispersed fine cementite particles. In contrast to the general trend of caliber-rolling processes showing significant reductions in elongation while retaining a high strength, the present caliber-rolled high-carbon steel exhibited simultaneously improved strength and elongation. This distinctive tensile enhancement of the caliber-rolled high-carbon steel was attributed to the increased strain hardening rate caused by the high fraction of submicron cementite particles and to the transition of the fracture mode from a brittle state to a ductile state.

NiAl powder alloys: II. Compacting of NiAl powders produced by various methods

The technological properties of granulated NiAl powders produced by gas spraying of melts and NiAl powders produced by calcium hydride reduction (CHR) of mixtures of nickel and aluminum oxides are compared. The possibilities of production of compact workpieces from these powders using hydrostatic pressing, hot pressing, hot isostatic pressing, and hot extrusion are estimated. To improve compressibility, preliminary milling and/or mechanical activation of the powders are proposed. The strength properties of NiAl rods with a diameter of 20 mm extruded from a temperature of 1100°C and made from the granulated powders are slightly higher than those made from the CHR powders. At temperatures higher than 800°C the properties becomes similar. Transition point td.b from the ductile to brittle state of samples made from powders sprayed in nitrogen and argon is 100–150°C higher than those made from the CHR powders. The difference in the mechanical properties is caused by the structural and chemical microheterogeneity of granules (microingots), which is inherited in the rods after hot deformation and annealing at 1200–1400°C and is (0.67–0.88)Tm NiAl (Tm is the melting point, K).

Effect of the strain rate on the mechanical properties of metalloceramic molybdenum in the ductile-brittle transition temperature range

Metalloceramic molybdenum is tested in the temperature range from −100 to +350°C and the strain rate range from 3.4 × 10−3 to 4 × 103 s−1. This wide strain rate range, which was used for the first time to study molybdenum, makes it possible to find the laws of changes in its strength and deformation characteristics. It is found that the ductile-brittle transition temperature increases from −90 to +250°C in this strain rate range, the dependences of the lower and upper transition temperatures on the strain rate intersect at \( \dot \varepsilon \) = 105.7 s−1, and the effect of the test temperature on the yield strength weakens with increasing strain rate and disappears at \( \dot \varepsilon \) = 105.9 s−1. The closeness of these two strain rates calculated using two different approximations suggests the presence of a certain limiting critical strain rate beginning from which molybdenum is in a brittle state.

Механическая стабильность — универсальная мера сопротивления переходу в хрупкое состояние металла

Механическая стабильность — универсальная мера сопротивления переходу в хрупкое состояние металла

An apparatus for the plastic limit and workability of soils

An apparatus is described that replicates Atterberg's rolling technique for the determination of the plastic limit of soils by detecting the brittle–plastic transition. There is much less operator interference, and judgement of the crumbling condition is practically eliminated. In addition, by means of a loading device, stresses are applied during rolling, the diameter is measured for each rolling traverse, and plots of stress against strain are derived. The workability or toughness of the soil is determined as the product of stress and strain, or work/unit volume, for soil threads prepared at moisture contents ranging from near the sticky limit through the workable plastic region to the brittle state. From the results an abrupt brittle–plastic transition is found, allowing the plastic limit to be defined as the moisture content at this transition. Good relationships between toughness and moisture content are obtained, from which the maximum toughness Tmax at the plastic limit can be derived together with other potentially useful properties such as the toughness limit wT, the moisture content at zero toughness, the toughness coefficients TC, the gradients of the relationships, the toughness index, the workability index and the stiffness transition.

A concept of mechanical stability of structural steels

We have developed a new approach to the evaluation of the capability of steels to resist brittle fracture based on the physical ideas concerning the micromechanism of transition of metals from the plastic state into the brittle state. Within the framework of this approach, we propose new microscopic characteristics of stability of the metal, namely, the parameter of mechanical stability and the coefficient of mechanical stability. The procedure of experimental evaluation of these parameters is described and the relationship between the coefficient of mechanical stability and standard characteristics, such as plasticity under uniaxial tension and impact toughness, is analyzed in detail. We introduce a new characteristic called the force equivalent of embrittlement, which enables us to describe, on a single scale, the embrittling action of the following factors of different physical nature: complex stressed state, stress concentration, low temperatures, and dynamical loading. We propose a criterion aimed at the description of stability of the plastic state of the metal at the tip of a macrocrack and develop a procedure of experimental determination of the value of force equivalent of embrittlement for a standard cracked specimen. For the typical representatives of low-, medium-, and high-strength structural steels considered as an example, we study the regularities of the influence of the strength of a steel on the value of the coefficient of mechanical stability. On the basis of these data, we compare structural steels from the viewpoint of their stability under the embrittling action of cracklike defects.