Fresh Fracture Surfaces Research Articles

Recycling of additively printed anisotropic Nd-Fe-B bonded magnets

A high potential cost-effective and environmentally friendly method has been applied for recycling anisotropic Nd-Fe-B bonded magnets. Waste additively printed Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) Nd-Fe-B anisotropic bonded magnet was pulverized into composite powder containing Nd-Fe-B particles and nylon binder through cryomilling at a liquid nitrogen temperature (∼77 K) under Ar inert atmosphere. Then, the cryomilled composite powder was warm compacted into a bonded magnet. The magnetic particles were aligned during post-compaction annealing under a magnetic field of 30 kOe. The recycled bonded magnets have a higher density (3 % enhancement), but slightly inferior magnetic properties compared to the original magnets, i.e., the magnetic remanence, coercivity and maximum energy product are reduced by 2 %, 3 % and 8 %, respectively. The scanning electron microscopy revealed that some HDDR Nd-Fe-B powder crumbled into fine particles during cryomilling. Powder X-ray diffraction showed a small amount of Nd-oxide impurity in the cryomilled powder. The slightly deteriorated magnetic properties are ascribed to the oxidation of Nd-Fe-B particles due to formation of fresh fracture surface during cryomilling. The approach enables the direct reuse of end-of-life bonded magnets in an economical and environmentally friendly way.

Oxygen inward diffusion and complex reorganization of the reaction layer in ion-exchanged glasses evidenced by Auger electron spectroscopy

Alkali silicate glasses subjected to low-temperature ion exchange in molten salt are being probed by Auger electron spectroscopy (AES). Spot analyses and elemental line scans across fresh fracture surfaces of broken glass specimens reveal oxygen enrichment of reaction layer where Na+-for-K+ ion exchange occurred and exceptional similarity in shapes of concentration depth profiles of potassium and oxygen. The probe provides the first direct evidence that smaller-for-larger ion replacement is accompanied by inward oxygen transport into reaction zone where potassium cations rearrange former sodium sites in accordance with their steric demands. Such chemical and structural rearrangements entail defect formation and glass network depolymerization and fragmentation, which define stress buildup and relaxation, residual stress intensity and profile in the surface layer of chemically strengthened glasses.

Fatigue crack growth behavior of marine steel under variable amplitude loading-combining DIC technique and SEM observation

This paper experimentally studies the fatigue crack growth (FCG) behavior of AH36 grade marine steel under variable amplitude loading. FCG testing is conducted using compact tension specimens. Digital image correlation technique (DIC) is employed to study the evolution of strain distribution and cyclic fatigue characteristics. Fractographic analysis is performed to explore the overload retardation mechanism. Results indicate that the FCG rate varies significantly under overload application, leading to the extension of fatigue life by 1.03, 1.18 and 2.42 times under the overload ratios (OLRs) of 1.5, 2.0 and 2.5. Reasonable evaluation of the fatigue performance depends on the loading history. Extension of the fatigue life for periodic overload (POL) cases is not a simple superposition of that under two single ones, the interaction effect between the adjacent tensile overloads cannot be neglected. Greater plastic dissipated energy corresponds to more severe plastic deformation at larger OLRs and crack lengths. The larger strain accumulation at the crack tip should account for the greater FCG resistance and more evident overload retardation effect. The fresh fracture surface is destroyed at a certain region after overload due to the premature contact of the crack flanks, confirming that overload induced crack closure exists.

Wide-blocky veins explained by dependency of crystal growth rate on fracture surface type: Insights from phase-field modeling

AbstractVein microstructures contain a wealth of information on coupled chemical and mechanical processes of fracturing, fluid transport, and crystal growth. Numerical simulations have been used for exploring the factors controlling the development of vein microstructures; however, they have not been quantitatively validated against natural veins. Here we combined phase-field modeling with microtextural analysis of previously unexplained wide-blocky calcite veins in natural limestone and of the fresh fracture surface in this limestone. Results show that the wide-blocky vein textures can only be reproduced if ∼10%–20% of crystals grow faster than the rest. This fraction corresponds to the amount of transgranularly broken grains that were observed on the experimental fracture surfaces, which are dominantly intergranular. We hypothesize that transgranular fractures allow faster growth of vein minerals due to the lack of clay coatings and other nucleation discontinuities that are common along intergranular cracks. Our simulation results show remarkable similarity to the natural veins and reproduce the nonlinear relationship between vein crystal width and vein aperture. This allows accurate simulations of crystal growth processes and related permeability evolution in fractured rocks.

First Results on Understanding the Shiny Surfaces of Heat-Treated Chert

ABSTRACT In Europe, intentional heat treatment of silica rocks to improve their knapping quality appeared for the first time during the Solutrean (ca 25.5–23 ka cal BP). The recognition of heat-treated artefacts in assemblages is based on macroscopic criteria like shiny luster. Surface luster or gloss, however, may be prone to observer bias and calls for more objectives measures to quantify it. In this study, we use a laser scanning microscope to measure the micro-relief on surfaces knapped after heat treatment at different temperatures. Our results show the evolution of the roughness of fresh fracture surface as a function of heating temperatures. Roughness appears to be a good proxy for macroscopically observable surface gloss caused by heat treatment. Such Ra measurements even bear the potential to become a new tool for the recognition of heat treatment.

Interpretation of the origin of analcimes with mineralogical, microtextural, and geochemical investigations: a case study from Aktepe region (NE of Kalecik, Ankara, Central Anatolia, Turkey)

The presence of analcime minerals in volcanic rocks is a topic requiring attention during classification of rocks. In fact, when transformation from leucite to analcime is present, the original chemistry of the rock will change and this situation will cause falsification in identification of the original character of the rock. As a result, it is important to determine the origin of the analcime in rocks containing analcime. This study focuses on the investigation of the origin of analcimes occurring in alkaline volcanic rocks (tephrite, phonotephrite) of the Aktepe region, located in the northeastern part of Ankara (Central Anatolia, Turkey). These volcanic rocks consist of abundant clinopyroxene, analcime, Fe-Ti oxides, minor olivine pseudomorphs, and scarce microcrystals of feldspar. The analcime phenocrysts are typically euhedral to anhedral (nearly rounded) in shape with up to 1 cm diameter and there are no other hydrous minerals in the samples. Leucite-like crystals are rarely preserved in the cores of larger analcime crystals and twin lamellas observed in them could demonstrate this preservation. Even partially transformed leucites are rare but not impossible to find in nature because of the rapid kinetics of the transformation of leucite to analcime. Examination of microprobe and EDS spectra, corona-type texture observed around the analcime–leucite interface suggest that some leucite crystals did not transform entirely to analcime crystals and a progressive transformation of leucite into analcime occurred locally in the region. Analcime and preserved leucite-like crystal existence could also be seen from the obtained X-ray diffractograms. The fresh fracture surfaces of separated large analcime crystals have microporous texture indicating a characteristic feature of analcimes that formed by an ion-exchange process from leucite. Geochemically, calculations of gains and losses of Na and K contents from the original bulk-rock chemical composition reveal that Na content increase is counterbalanced by a decrease in K content. This could also be strongly explained by an analcimization process and/or progressive transformation of leucite into analcime. It is concluded that analcimes formed secondarily from leucite by an ion-exchange process after cooling of magma via graded infiltration of Na-bearing fluids into the Aktepe ultrapotassic alkaline volcanic rocks.

Experimental interaction of hydrothermal Na-Cl solution with fracture surfaces of geothermal reservoir sandstone of the Upper Rhine Graben

The Upper Rhine Graben (URG) is a Tertiary rift structure in central Europe offering favorable conditions for geothermal energy utilization. Relatively high heat flow combines with sufficiently high permeability of the hydrothermal reservoirs. One of the target stratigraphic levels is the lower Triassic sandstone formation (Buntsandstein), where hot water resources at temperatures up to 250 °C can be utilized. Extensional neotectonics and hydraulic stimulation form new fracture surfaces in the reservoir rocks (enhanced geothermal system, EGS). The exposed fresh rock fracture surface reacts with the highly saline reservoir brines (Na-Cl up to 200 g l−1) with consequences for the permeability of the reservoir.In order to better understand the dynamic evolution of fault systems caused by fluid-rock interaction, we conducted batch-type experiments in a stirred autoclave system and reacted arkosic sandstone with synthetic 2 molal Na-Cl solution at temperatures of 200 °C and 260 °C. After 45–55 days reaction time altered rock samples were compared with the starting material and the geochemical-mineralogical processes were deduced with the help of XRD, SEM methods and EMP measurements. Fluid compositions were examined by ICP-MS, ICP-OES and IC.The arkosic sandstones show a surprisingly high reactivity during the experiments. Quartz grain surfaces show deep dissolution features and all reaction fluids were saturated with respects to quartz. Illite and kaolinite from the primary sandstone cementation completely dissolved from the sample surface. Perfectly euhedral crystals of metastable analcime formed during the experiment as separate crystals on quartz, as groups or clusters and as surface covering mats. The overall net transfer process dissolves quartz + illite + kaolinite ± K-feldspar and precipitates analcime + chlorite ± albite. The process is accompanied by a total volume increase of the solids of 20–30 vol%. K-feldspar dissolution is hampered by albitization rims shielding K-feldspar and efficiently preventing an equilibration of the Na–K exchange with the fluid. The experiments show changes on the rock surface, leading to an increase of the aperture of a single fracture during the early phases of reaction and later to a decrease as the fluid-rock reaction progresses. Alteration of the fracture surface also generates loose fragments and altered minerals. This fine material may efficiently reduce the fracture aperture at narrow points along the fracture.

Low‐Energy Ion‐Scattering Spectroscopy of Modified Silicate Glasses

Low‐Energy Ion‐Scattering (LEIS) spectroscopy is a technique with a unique sensitivity to the elemental composition of the top atomic layer of a solid surface. LEIS measurements of ternary silicate glasses modified with Na2O, Cs2O, CaO, and BaO show that the compositions of the as‐cast (melt) surface and the in‐vacuum fracture surface often differ. While the as‐cast surface is usually depleted of alkali ions (Na+ or Cs+) compared to the nominal (batch) glass composition, there is often strong accumulation of the same mobile cations on the fresh fracture surface. Depth profiles obtained by sputter etching reveal elemental concentration gradients normal to the glass surface. The final concentrations often fail to reach the nominal glass composition, suggesting the likely presence of preferential sputtering effects and thereby the distortion of the measured concentration gradient. At present, the lack of reliable standards and preferential sputtering effects in the LEIS of multicomponent glasses limit somewhat the absolute chemical composition and structural information that can be obtained with this otherwise unique and powerful method of surface analysis.

Seismic moment tensor and b value variations over successive seismic cycles in laboratory stick‐slip experiments

AbstractThe formation of fault damage due to slip under high normal stresses can rarely be monitored under in situ conditions. To advance our understanding of microfracture processes, we investigated stick‐slip events on Westerly granite samples containing the following: (1) a planar saw cut fault and (2) a fault developed from a fresh fracture surface. We examined temporal changes of seismic moment tensors and b values of acoustic emission (AE) events. During experiment on the saw cut surface, small AEs exhibiting non‐double‐couple components were observed continuously and strong AEs displaying double‐couple components were visible only when approaching the slip onsets. Sliding on naturally fractured surfaces showed, in addition to double‐couple components, significant volumetric contributions, especially during the interslip periods and immediately after stick‐slip events indicating substantial shear‐enhanced compaction within a relatively broad damage zone. The obtained results shed light on how differences in fault structure control the kinematics of microseismicity during different periods of the seismic cycle.

Griffith Cracks at the Nanoscale

When Griffith presented his famous theory of crack stability in elastic materials in the early twentieth century, he was unable to provide much detail on the structure of cracks at the nanometer level of resolution. Now, almost 100 years later, techniques such as transmission electron microscopy, atomic force microscope, nuclear reaction analysis, and nuclear reflection are available to achieve this level of resolution. Here, we review the kind of data obtained using these techniques and the implications of the data vis‐á‐vis cracks in silicate glasses. Measurements by atomic force microscopy provide information on the size of the nonlinear zone at crack tips in glass, on environmental conditions at crack tips, and on the possibility of cavity formation as a mechanism of crack growth. Examination by nuclear reaction analysis and neutron reflection of fresh fracture surfaces formed in water has yielded information on water penetration through the glass surrounding the crack tip, to a resolution of 3–5 nm. Improvement of measurement techniques in the coming years should enable us to study crack tips in glasses to even higher levels of resolution and to answer more detailed questions concerning the level of stress and the size of the nonlinear zone at the crack tip.

Performance and Analysis of Concrete in Sewer Environment: Anisotropy of Damage

The costs associated with the provision and maintenance of drinking water and wastewater infrastructure represents a significant financial demand worldwide. Maintenance costs are disproportionately high, indicating a lack of adequate durability. The aim of this paper is to study the degradation mode and the responsible factors of deterioration concrete pipe, used in the sewage’s city of Rennes (France). Thus, samples taken from different areas of the pipe (Raft, medium and roof) were analyzed using investigation methods in order to illustrate the internal and external rate of damage, caused by effluents and ground. In addition, chemical and mineralogical changes recorded were identified using scanning electron micrographs (SEM). In this research program, chemical and mechanical studies were performed by measuring the compressive and splitting tensile strengths. A phenolphthalein indicator solution is applied to a concrete specimen on a fresh fracture surface to determine the corrosion of steel reinforcement and the diseases advancement (pathologies depth). This study clearly shows the important anisotropy of both, internal and external damage. Finally, recommendations are proposed in line with the environment and possible approaches to sustainability assessment are also imposed.

Unusual elastic behaviour of a wood-like material during bending, milling and nanoindentation

Pistachio hardshells exhibit isotropic composite sclerenchyma at the length-scale of nanoindentation. The cells are composed of the common wood constituents and their macroscopic elasticity is unusual: upon reversible bending to less than 50% of the clear-length the height to length ratio increases up to 10-fold. Such behaviour is not reflected by the reduced elastic modulus value from nanoindentation compared with very close values for brittle walnut and macadamia shell. This may indicate outstanding values of Poisson's ratio for pistachio, but it covers only the sclerenchymatous cell wall. Three-dimensional light microscopy on fresh fracture surfaces revealed highly structured pistachio cells with micrometre-sized protrusions in all directions, in contrast to the smoothness of walnut cells in the outer shell. Neither of the hardshells exhibited a fibrous microstructure. Elasticity and isotropy of pistachio hardshells were also evident in their unusual breaking into uniform brick-type fragments upon milling. The cell interlocking in pistachio shells (three varieties and also a salted roasted brand) may be interpreted as a bionics model for elasticity with potential use for the construction of macroscopic structures, and pistachio shells may be used as elastic feedstock rather than being wasted.

Hierarchical structure of asteriscus and in vitro mineralization on asteriscus substrate

Fish otolith is a kind of typical natural calcium carbonate biomineral, which is composed of a pair of asteriscus, lapillus and sagitta. In this article, asteriscus samples are polished and etched by two reagents: NaOH and Na 2EDTA. Scanning Electron Microscope (SEM) and High Resolution Transmission Electron Microscope (HRTEM) analysis showed that asteriscus has a five-level hierarchical structure. In the in vitro mineralization experiments, protein matrices extracted from asteriscus were added as additives in the solutions; the fresh fracture surfaces of asteriscus were used as substrates to study their influences on calcium carbonate growth. The results showed that asteriscus substrate could mediate vaterite crystals with the same crystal polymorph as those vaterite crystals composing asteriscus. In the presence of water soluble protein matrix and acid soluble matrix, the crystal morphology significantly changed.

Nutshells' mechanical response: from nanoindentation and structure to bionics models

Nut or seed hardshells' mechanical response is studied by vertical nanoindentation in the untreated surface with pyramidal tips. As in uniform materials the exponent of the loading curves is also determined to be 3/2 (but not 2 as Hertzian theory would predict) in all of the nanocomposites with correlation coefficients r > 0.999 (except two cases with r > 0.996 due to minor distortions) for the linear plots with slope k (µN nm−3/2). Unlike hardness H and reduced elastic modulus Er, the pyramidal nanoindentation coefficient k reflects the total mechanical response. It allows for depth calculations according to FN = kh3/2. Thus, the k-values represent the penetration resistance, and k−1 represents the penetratability. Importantly, the penetration depth h follows FN2/3 but not FN1/2 what Hertzian theory had been claiming, despite the importance for wear, mar, mechanical treatments, and tribology. A quantitative mechanical rating is now possible for the first time. The values of k can be converted to different tip tapers. Reliable and repeatable vertical indentations with sharp cube corner were achieved by approaching sufficiently flat summit sites on the rough surfaces under AFM control. The surfaces were not previously destroyed by microtome cutting or abrading and polishing. Detailed knowledge of the architectures of the shells' sclerenchyma was obtained by 3D optical microscopy in color of clean natural surfaces and on fresh fracture surfaces in different directions. Multiple layers, fibrous structures in various alignments and several types of almost isodiametric cells were differentiated. The linearity of the FNversush3/2 plots secures uniformity of the thick-walled cells notwithstanding initial minor deviations due to nanoroughness and surface layers. Comparison with published bone and nacre data is added and discussed. The nanomechanical response of unidirectional indentation at the cellular level influences the cracking resistance of the whole nuts/seeds, but the higher level of wood hierarchy has often the higher weight. The various stabilization tools governing the elastic and brittle responses towards the bending forces that apply upon cracking of the nuts/seeds with their different sizes and shapes are discussed on the basis of the structural data. Importantly, the combination of the mechanical and structural results reveals four new bionics models. These are possible improvements of Yacoe-Davis type dome constructions by stiffeners between the polygons, hedgehog effect for highly elastic materials by efficient interlocking, ellipsoidal cell effect for highly inelastic products, and vertical nanofiber arrays for highly light absorbing very dark surfaces.

The Use of Atomic Force Microscopy to Study Crack Tips in Glass

This article presents a review of the application of atomic force microscopy (AFM) to crack-tip corrosion during subcritical crack growth in glass. The two principal experimental techniques used in this type of study are (1) the direct observation of crack motion by scanning the tip of a crack during crack growth and (2) the examination of fracture surfaces once the specimen has been fractured in two. The first technique has been used to demonstrate and quantify water condensation at crack tips during subcritical crack growth and is particularly useful at low crack velocities. The second technique has been used to quantify the crack-tip corrosion process and the shape of the crack tip during crack growth. In this article, we discuss experimental results showing that the environment that develops at the tips of freshly fractured glass surfaces in soda lime glass can corrode the glass surfaces near the crack tip. Soda lime silicate glass contains mobile alkali ions that will exchange with hydronium ions in solution at the crack tip, forming a highly basic solution that is corrosive to glass. Experimental evidence for such corrosion has been obtained by the atomic force microscope, which demonstrates a displacement of the two fracture surfaces near the crack tip that can be as much as 20 nm, depending on how long the crack is held open at the fatigue limit. Despite the corrosion and displacement of the crack surfaces, the crack tip itself appears to remain sharp, suggesting that the fatigue limit in soda lime silicate glass is not due to crack-tip blunting. Most likely, the fatigue limit is a consequence of ion exchange at the crack tip, in which hydronium ions in the crack-tip solution exchange with sodium ions in the glass. As hydronium ions are larger than sodium ions, this exchange process leaves a compressive stress within the fresh fracture surface of the glass that resists crack motion and results in a stress-corrosion fatigue limit, as first proposed by Bunker and Michalske. In agreement with this mechanism, no fatigue limit is observed for silica glass, which also exhibits no ion exchange. As the crack-tip solution in silica glass is only mildly acidic, pH ≈ 5, corrosion does not occur at crack tips of this glass as supported by the observation that no crack-tip displacements are observed in silica glass by AFM. As the proposed ion exchange mechanism used to explain the stress corrosion limit in glass is at variance with the belief that the fatigue limit in glass is the result of crack-tip blunting, we discuss the possibility of plastic deformation at crack tips in glass and conclude that the available experimental data does not support such a model. At the present time, chemical reaction based crack growth theories are most consistent with the body of crack growth data that is available on glass and are probably the best explanation for the phenomenon.

Properties of fracture surfaces of glassy polymers: Chain scission versus chain pullout

Fresh fracture surfaces formed by tensile failure of craze in molded polystyrene (PS) bars have been compared with the molded surfaces of the same bars, using an atomic force microscope with a thermal probe and operated in local thermal analysis. The results indicate that molecular weight is much higher in the interior of the sample than at the surface. No evidence was found for degradation of the PS chains via chain scission during crazing. Alternative explanations for the low-molecular weights at the molded surface are discussed.

Effect of an Ultrahigh Rubber ABS Impact Modifier Resin on Mechanical Properties of Intumescent Flame-Retardant ABS Composites

The effect of an ultrahigh rubber acrylonitrile-butadiene-styrene (ABS) impact modifier resin (UHR-ABS) on the mechanical properties of an intumescent flame-retardant ABS composite was characterized. Samples were obtained by compounding ABS and an intumescent flame-retardant master batch that was prepared using an intumescent flame-retardant composite (IFRC) with ABS and/or UHR-ABS as well as by direct compounding IFRC, UHR-ABS, and ABS. The incorporation of UHR-ABS resulted in reduction in the storage modulus, damping behavior and glass transition temperature, as evidenced by dynamic mechanical analysis. With increasing mass fraction of the UHR-ABS, the tensile strength and the flexural strength decreased gradually, and the notched impact strength increased, but the increase was more significant for compounding IFRC, UHR-ABS, and ABS than for compounding ABS and IFRC master batches. SEM micrographs of the fresh fracture surface of the composites were used to estimate the mechanism of the increased notched impact strength owing to the incorporation of UHR-ABS.

Growth Mechanism of Penta-twinned Ag Nanowires Synthesized by Polyol Process

Multi-twinned silver nanoparticles and nanowires were synthesized by polyol process in the presence of surfactant poly (vinyl pyrrolidone) (PVP) K30. In order to study the growth mechanism, silver nanowires were synthesized under various conditions by changing n(PVP)/n(AgNO3) and the state of agitation. A comparative study of these nanowires was performed by means of transmission electron microscopy (TEM) and optical absorption spectra. The results showed that there coexisted two different growth modes, and hence the competition between them. According to one mode, the as-synthesized nanowires grew along {111} faces, by which the pentagonal pyramid-shaped tip was enclosed, as the active facets. By another mode, the nanowires grew layer by layer along{110} face, which was perpendicular to the wire axis [110], as the active facet. There were many stacking faults and dislocations in the nanowires deformed plastically during the preparation of the nanowires. The fresh fracture surfaces of the nanowires were the favorable positions where the nucleation and growth of newly formed crystals might take place.

Sub-critical crack growth in some phosphate glasses

The effect of alumina on the sub-critical crack growth is investigated on a set of calcium aluminophosphate glasses of molar composition 50% P2O5–(50−x)% CaO–x% Al2O3 (0% ⩽x⩽10%). The crack propagation is operated using the double-cleavage-drilled-compression (DCDC) method. In this test, the sample is loaded in compression with a mechanical testing machine in an environmentally controlled chamber (both temperature and water vapor pressure). The crack velocity is plotted as a function of stress intensity factor. Regions I and II are characterized by a dependence of crack velocity on the amount of alumina content in the glass. When the alumina content increases, glass shows, as expected, a trend less sensible to the stress corrosion correlated to the slope of the crack growth curves, but in the same time, these curves are shifted toward higher crack velocity values. For different compositions, an unexpected behavior is also observed with temperature and water vapor pressure. We assume the formation of a ‘gel-like product’ on fresh fracture surfaces which modifies the reactivity at the crack tip.

Metallurgical failure analysis of a horse trailer: A criminal investigation

Following the deaths of three people involved in a vehicular collision with an ejected horse, criminal charges were brought against the horse’s owner. Failure analysis was performed on the horse’s trailer to determine its roadworthiness. The failure mode of the hinge between the door and frame became the focus of the investigation. The analysis concentrated on proving whether or not the hinge was broken prior to the collision; that the trailer was not roadworthy; and whether the trailer was capable of safely transporting two horses on a public freeway. Analysis using optical microscopy and the scanning electron microscope indicated the hinge was significantly corroded and mechanically deformed. Additionally, there was evidence of paint on an alleged fresh fracture surface. The results were presented at the trial of the defendant.