Regular Crack Research Articles

Desiccation cracks in siliciclastic deposits: Microbial mat-related compared to abiotic sedimentary origin

Siliciclastic sediment colonized by microbial mats yield a set of distinct sedimentary fabrics that are collectively called “mat-related structures (MRS)”. In the rock record, versatile cracks are observed in biostabilized strata, but the mechanisms responsible for their formation remain debated. Microbially stabilized sediments produce desiccation cracks that serve as modern analogs for fossil microbial cracks. However, since both microbial mat shrinkage and clay shrinkage may contribute to the formation of these desiccation cracks, it is difficult to isolate the influence of the microbial mat on the resulting crack formation, distribution and morphology. To address this issue, we conducted a series of desiccation experiments that determine differences between microbially influenced desiccation cracks (i.e. biotic) and those formed in identical, but sterilized (i.e. abiotic) siliciclastic sediment. Three sediment mixtures were used: (1) very fine-sized sand, (2) mixed (ungraded) silt/clay, and (3) normally graded silt/clay. In all of the experiments, the water-rich microbial mat contracted substantially while drying, producing isolated pockets of shallow, but wide cracks, the distribution of which was controlled by heterogeneities in the mat structure and thickness variations of the mat. In the clay-poor substratum, the microbial mat was the only crack-forming mechanism, while in the clay-rich substrata (experiments 2 and 3) desiccation cracks were more strongly influenced by clay shrinkage. The abiotic clay-rich sediment produced a polygonal network of deep cracks intersecting at 90–120o junctions. In the biotic clay-rich experiments, the microbial mat modified these desiccation features by withstanding crack propagation or by producing curled-up crack polygon margins. Even though a microbial mat shrinks substantially with desiccation, its cohesive nature and heterogeneous distribution prevents the formation of a regular crack network, but its shallow penetration into the sediment limits its influence on cracking. The biotic crack formation is best characterized by a heterogeneous distribution of wide, but shallow pockets of radiating cracks that do not connect to one another, while abiotic cracking is best characterized by an orthogonal network of deep, intersecting cracks. By comparing biotic and abiotic shrinkage cracks formed in identical sediment, we improve our understanding of modern desiccation features and thus strengthen our interpretation of the rock record.

Efficient Analysis of 3D Mixed-Mode Cracks of a Pressure Vessel Based on Schwartz-Neuman Alternating Method

The Schwartz-Neuman alternating method is employed to analyze 3D cracks in structural components with complicated geometries.The SIFs of Mode I, II and III for the mixed-mode cracks under complicated stress state are obtained from the alternating computing scheme between finite element method solution for the uncracked body and the symmetric Galerkin boundary element method solution for the crack in an infinite body. The SIFs of the different surface cracks postulated at a regular pipe and elliptical surface cracks at the nozzle-cylinder intersection of a typical reactor pressure vessel are investigated by using the Schwartz-Neuman alternating method. The comparison with other analysis models and results reported in the literature shows that the Schwartz-Neuman alternating method can efficiently and accurately evaluate SIFs of surface cracks of different shapes and depths with much smaller computational models, which indicates that the Schwartz-Neuman alternating method is an efficient method in the evaluation of the SIFs of 3D cracks.

Singular path-independent energy integrals for elastic bodies with Euler–Bernoulli inclusions

This paper is devoted to a rigorous analysis of an equilibrium problem for a two-dimensional homogeneous anisotropic elastic body containing a thin isotropic elastic inclusion. The thin inclusion is modelled within the framework of Euler–Bernoulli beam theory. Partial delamination of the inclusion from the elastic body results in the appearance of an interfacial crack. We deal with nonlinear conditions that do not allow the opposing crack faces to penetrate each other. We derive a formula for the first derivative of the energy functional with respect to the regular crack perturbation along the interface, which is related to energy release rates. It is proved that the energy release rates associated with crack translation and self-similar expansion are represented as path-independent integrals along smooth curves surrounding one or both crack tips. The path-independent integrals consist of regular and singular terms and are analogues of the well-known Eshelby–Cherepanov–Rice J-integral and Knowles–Sternberg M-integral.

Aging characteristics of glass fiber‐reinforced polyamide in hot water and air

This article focuses on the accelerated aging behavior of a glass fiber‐reinforced and heat stabilized polyamide grade. Accelerated aging of injection molded specimens was achieved by hot air exposure (HAE) and pressure cooker testing (PCT) at elevated temperatures. For both aging routines, characteristic aging indicators were found. For hot air exposure, the carbonyl index obtained from infrared spectra revealed superficial thermo oxidation. For pressure cooker testing, the development of a regular crack pattern and successive surface roughening were observed by light microscopy and laser confocal microscopy. This was accompanied by a significant reduction in molar mass assessed by viscometry. In addition, progressing degradation of mechanical performance was ascertained by tensile testing experiments. At the onset of material cracking, the molar mass was about 25% of the initial value. POLYM. COMPOS., 39:997–1005, 2018. © 2016 Society of Plastics Engineers

The interplay between drug-use behaviors, settings, and access to care: a qualitative study exploring attitudes and experiences of crack cocaine users in Rio de Janeiro and São Paulo, Brazil.

BackgroundDespite the growing attention surrounding crack cocaine use in Brazil, little is understood about crack users’ histories, use patterns and the interplay of drug-use behaviors, settings, and access/barriers to care. Qualitative studies seldom cross-compare findings regarding people who use crack from different settings. This study aims to explore the insights of regular crack users in two major Brazilian cities and to examine how social and contextual factors, including stigma and marginalization, influence initial use and a range of health and social issues.MethodsIn-depth interviews and focus groups were conducted with 38 adult crack cocaine users recruited from impoverished neighborhoods in Rio de Janeiro and São Paulo. Interviews and focus groups were audio recorded and transcribed verbatim. Qualitative analysis was carried out, and content was organized and analyzed by recurrent themes relevant to study interests.ResultsFor study participants from both cities, frequent crack cocaine use plays a central role in daily life and leads to a range of physical, psychological, and social consequences. Common concerns among users include excessive crack use, engagement in risky habits, infrequent health service utilization, marginalization, and difficulty reducing use.ConclusionsDisadvantaged conditions in which many crack cocaine users grow up and live may perpetuate risk behaviors and stigma may further marginalize users from necessary health and recovery services. Reducing stigma and moralizing discourse related to drug use, especially among health professionals and law enforcement personnel, may help encourage users to seek necessary care. New harm-reduction-based care and treatment alternatives for marginalized drug users are being developed in parts of Brazil and elsewhere and should be adapted and expanded for other populations in need.

Calciphytoliths (calcium oxalate crystals) analysis for the identification of decayed tea plants (Camellia sinensis L.)

The history of tea is poorly known, mainly due to the questionable identification of decayed tea plants in archaeological samples. This paper attempts to test the utility of calciphytoliths (calcium oxalate crystals) for the identification of tea in archaeological samples. It provides the first survey of the macropatterns of calciphytoliths in several species of Theaceae and common non-Theaceae plants. Crystals were extracted from 45 samples of tea, Theaceae and common non-Theaceae plants, and detected microscopically between crossed polarizers. In tea plants, druse and trichome base are the most distinctive crystals. Druses have the smallest diameter (11.65 ± 3.64 μm), and trichome bases have four distinctive straight and regular cracks, similar to a regular extinction cross. The results provide morphological criteria for distinguishing tea from other plants, specifically the presence of identifiable druses together with calcified trichome bases. The implications are significant for understanding the history of tea and plant exploitation, especially for plants for which the preservation of macrofossils is poor.

Finite Element Analysis on Causes of Crack in Large Underground Garage

The paper aims to solve the serious and regular crack problems in underground garage. ANSYS finite element software is applied to set up the overall finite element model on floor-foundation and foundation coupling beam-foundation soil in underground garage. Combined with engineering field detection, factors influencing underground garage floor like underground water level, soil expansion caused by water content change in expansive soil, soil poisson ratio, foundation settlement are taken into consideration to find out the causes of cracks. The study is expected to provide reference for similar cases in other projects.

Morphology of Edge Cracks of Rolled Magnesium Alloy Sheet

Magnesium alloys show low deformability at low temperature because of hcp structure and inactiveness of basal slip. Manufacturing of thin sheet is difficult in industries. Some approaches, such as small-draft multi-pass rolling, intermediate annealing, isothermal rolling and high-speed rolling were proposed to overcome the deformability. However, small edge cracks are still formed on the sheet. In this study, rolling speed of 1000m/min was employed to warm-roll AZ31B magnesium alloy in a single pass at different temperatures. The edge cracks formed after the rolling were classified into three main groups: minor, regular and zigzag edge cracks. ‘Crack contact length’ are introduced to explain the morphology of edge cracks. The results show that the critical reduction for crack initiation depends on the pre-heating temperature. The spacing between edge cracks increases linearly with the crack contact length regardless of roll diameter, speed and reduction. It is suggested that this approach is useful to understand the formation mechanism of edge cracks and to evaluate the rollability of magnesium alloys.

Influence of a plasma jet on different types of tungsten

The influence of a plasma producing nonstationary thermal loads akin to edge-localized modes in a tokamak on different types of tungsten is investigated. Tungsten is irradiated by a jet of a hydrogen plasma generated in a plasma gun. The plasma density and velocity are on the order of 1022 m−3 and 100–200 km/s, respectively, and the irradiation time is 10 μs. Two plasma flux densities, 0.70 and 0.25 MJ/m2, are used. Structural modifications in irradiated single-crystal and hot-rolled tungsten samples, as well as in V-MP and ITER_D_2EDZJ4 tungsten powders, are examined. It is found that the plasma generates a regular crack network with a period of about 1 mm on the surface of the single-crystal, hot-rolled, and V-MP powder samples, while the surface of the ITER_D_2EDZJ4 powder is more cracking-resistant. The depth of the molten layer equals 1–3 μm, and the extension of intense thermal action is 15–20 μm. The material acquires a distinct regular structure with a typical grain size of less than 1 μm. X-ray diffraction analysis shows that irradiation changes the crystal lattice parameters because of the melting and crystallization of the surface layer. The examination of the V_MP tungsten powder after cyclic irradiation by a plasma with different energy densities shows that high-energy-density irradiation causes the most significant surface damage, whereas low-energy-density irradiation generates defects that are small in size even if the number of cycles is large.

Soft Matter Beats Hard Matter: Rupturing of Thin Metallic Films Induced by Mass Transport in Photosensitive Polymer Films

The interface between thin films of metal and polymer materials play a significant role in modern flexible microelectronics viz., metal contacts on polymer substrates, printed electronics and prosthetic devices. The major emphasis in metal-polymer interface is on studying how the externally applied stress in the polymer substrate leads to the deformation and cracks in metal film and vice versa. Usually, the deformation process involves strains varying over large lateral dimensions because of excessive stress at local imperfections. Here we show that the seemingly random phenomena at macroscopic scales can be rendered rather controllable at submicrometer length scales. Recently, we have created a metal-polymer interface system with strains varying over periods of several hundred nanometers. This was achieved by exploiting the formation of surface relief grating (SRG) within the azobenzene containing photosensitive polymer film upon irradiation with light interference pattern. Up to a thickness of 60 nm, the adsorbed metal film adapts neatly to the forming relief, until it ultimately ruptures into an array of stripes by formation of highly regular and uniform cracks along the maxima and minima of the polymer topography. This surprising phenomenon has far-reaching implications. This is the first time a direct probe is available to estimate the forces emerging in SRG formation in glassy polymers. Furthermore, crack formation in thin metal films can be studied literally in slow motion, which could lead to substantial improvements in the design process of flexible electronics. Finally, cracks are produced uniformly and at high density, contrary to common sense. This could offer new strategies for precise nanofabrication procedures mechanical in character.

How Surface Functional Groups Influence Fracturation in Nanofluid Droplet Dry-Outs

In this study of drying water-based nanofluid droplets, we report the influence of surface functional groups and substrate surface energies on crack formation and dry-out shape. These two key parameters are investigated by allowing nanofluids with several functional groups grafted on polystyrene nanoparticle surfaces to dry on various substrates. These experiments result in a variety of regular crack patterns with identical nanoparticle diameter, material, concentration, and drying conditions. We demonstrate that, despite the various patterns observed, the crack spacing/deposit height ratio is constant for similar substrate surface energies and linearly increases with this parameter. Moreover, this study shows that the crack shape is strongly influenced by surface functional groups as a result of particle interactions (depending on the particle surface potentials) and compaction during solvent evaporation.

Chemically induced fracturing in alkali feldspar

Fracturing in alkali feldspar during Na+–K+ cation exchange with a NaCl–KCl salt melt was studied experimentally. Due to a marked composition dependence of the lattice parameters of alkali feldspar, any composition gradient arising from cation exchange causes coherency stress. If this stress exceeds a critical level fracturing occurs. Experiments were performed on potassium-rich gem-quality alkali feldspars with polished (010) and (001) surfaces. When the feldspar was shifted toward more sodium-rich compositions over more than about 10 mole %, a system of parallel cracks with regular crack spacing formed. The cracks have a general (h0l) orientation and do not correspond to any of the feldspar cleavages. The cracks are rather oriented (sub)-perpendicular to the direction of maximum tensile stress. The critical stress needed to initiate fracturing is about 325 MPa. The critical stress intensity factor for the propagation of mode I cracks, K Ic, is estimated as 2.30–2.72 MPa m1/2 (73–86 MPa mm1/2) from a systematic relation between characteristic crack spacing and coherency stress. An orientation mismatch of 18° between the crack normal and the direction of maximum tensile stress is ascribed to the anisotropy of the longitudinal elastic stiffness which has pronounced maxima in the crack plane and a minimum in the direction of the crack normal.

Rhenium used as an interlayer between carbon–carbon composites and iridium coating: Adhesion and wettability

Iridium (Ir) is one of the most promising materials as a protective coating on carbon/carbon (C/C) composites for very high temperature applications. However, the poor adhesion and thermal stress induced by thermal expansion mismatch between Ir coating and C/C substrate restrict their application. Rhenium (Re) has been selected as the interlayer material between the Ir coating and C/C substrate which can improve the adhesion of Ir coating and relieve the thermal stress. The Re coating was prepared on the C/C substrate by chemical vapor deposition (CVD). The Ir coating was electrodeposited on the C/C and Re/C/C substrates in molten salt. The morphology and microstructure of the Re and Ir coatings were studied by SEM and XRD. The bond strength of the coatings on C/C substrate was measured by coating-pull-off test. The molten salt wettability on the C/C and Re/C/C substrates was determined by drop shape method. The results showed that the CVD Re coating on the C/C substrate had a preferential orientation of <002> with many fine regular cracks. The bond strength between the Re coating and C/C substrate was higher than 15MPa before and after annealing treatment. The contact angles of molten salt on C/C and Re/C/C substrates at 500°C were 128.5° and 43.4°, respectively. The bond strength of as-deposited coatings increased from 3.2MPa for Ir/C/C to 7.9MPa for Ir/Re/C/C. After high temperature annealing treatment, however, the bond strength has been reduced by ~50%, probably due to the infiltration of molten salt into the C/C substrate.

Comparison of the Simulation and Experimental Fatigue Endurance Behaviors in T6-Treated Nanosized SiC Reinforced Al Alloy Composite

The fatigue endurance behaviors of SiC nanoparticle reinforced 6061 Al alloy composites (T6-treated SiCnp/6061Al) were simulated by finite element (FE) method with the help of recently developed cyclic viscoplastic constitutive model and its implementation into a FE code Solidworks simulation. The Rice-Tracey damage parameter was used for the crack path calculation did the prediction agree completely with the observed behavior. FE simulation of the materials showed that the composite samples exhibit regular crack propagation behavior. At early stages of loading the hydrostatic stress was low and the damage evolution was affected by the plastic flow. The damage grew in outer layers where the maximum equivalent plastic strain occurs. By increasing the number of fatigue failure cycles at 3×105cycles, the hydrostatic stress increased and its effect became dominant. In the 200 MPa tests the slope of the tensile portion of the hysteresis loops decreased with fatigue cycling, indicating progressive decrease in the tensile modulus. Stress concentration, due to the presence of nanoparticle reinforcements, produces controlled crack growth and higher stresses, which are related to regular energy release by the material during fracture. The need for higher stresses for a crack to propagate reveals the materials microstructural strength.

Key drug use, health and socio-economic characteristics of young crack users in two Brazilian cities

BackgroundCrack use constitutes a major problem in cities across Brazil. While existing data suggest that crack use is generally concentrated among disenfranchised young people with extensive health problems and crime involvement, extensive data gaps exist. To address this issue, this study aimed to assess key characteristics of young crack users in two Brazilian cities. MethodsN=160 regular and young adult (ages 18–24) crack users were recruited by community-based methods in the cities of Rio de Janeiro (Southeast) and Salvador (Northeast). Assessments included an interviewer-administered questionnaire on key social, drug use, health and service use characteristics, as well as serological testing of HBV, HCV and HIV status, and were conducted anonymously between November 2010 and June 2011. Participants provided informed consent and received transportation vouchers following assessment completion. The study was approved by institutional ethics review boards. ResultsThe majority of participants were: male, with less than high school education, unstably housed (Rio only); gained income from legal or illegal work; arrested by police in past year (Salvador only); had numerous daily crack use episodes and shared paraphernalia (Salvador only); co-used alcohol, tobacco, cannabis and cocaine; had no injection history; rated physical and mental health as ‘fair’ or lower (Salvador only); had unprotected sex; were never HIV tested; were not HIV, HBV or HCV positive; and did not use existing social or health services, but desired access to crack user specific services. ConclusionCrack users in the two Brazilian sites featured extensive socio-economic marginalization, crack and poly-drug use as well as sexual risk behaviours, and compromised health status. Social and health service utilization are low, yet needs are high. There is an urgent need for further research and for targeted interventions for crack use in Brazil.

Fire tests on two-way concrete slabs in a full-scale multi-storey steel-framed building

This paper presents the results from two full-scale furnace tests conducted on two-way concrete slabs supported by composite beams in a three-storey steel-framed building. Each floor of the building consisted of nine panels (three by three) supported by composite beams. In two tests, a corner and an interior panel on the top of the building were heated by two specially designed furnaces respectively. Detailed experimental data in the form of describing slab cracking, the furnace temperatures, temperature distributions within the slab, vertical deflections and horizontal displacements are presented. Comparison of the results for the two tests indicates that the structural fire behaviour of two-way concrete slabs supported by composite beams in a multi-storey steel-framed building is highly dependent on the restraint provided by the adjacent structural members. Observations from the tests indicate that in addition to the extensive cracks formed on the top surface of the heated panels, regular cracks also occurred on the top of the adjacent unheated panels due to structural continuity and the interaction between the concrete slabs and the supporting beams. The test results show that both tested panels had good fire performances even under long duration fire conditions.

Formation of drying crack patterns in soils: a deterministic approach

Soils, as well as most of deformable multiphase porous materials, are likely to suffer from desiccation cracking, leading to the formation of regular crack patterns affecting their permeability. The ensuing crack spacing has often been related to a concept sometimes called “sequential infilling”: it is assumed that desiccation cracks are formed by successive generations. However, such a concept does not consider the pattern of a simultaneous crack formation at a given moment. Using our desiccation cracking test results and their numerical simulation, we propose a consistent explanation for the formation of desiccation crack patterns in soils. We show that the “sequential infilling” concept is suitable only when the position of the crack(s) clearly stems from the stress field. To derive an estimate of the desiccation crack spacing, the overall energy of the system needs to be considered. Statistical variability should be superimposed on the mean deterministic conditions discussed here.

Enceladus: A hypothesis for bringing both heat and chemicals to the surface

The eruptive plumes and large heat flow (∼15GW) observed by Cassini in the South Polar Region of Enceladus may be expressions of hydrothermal activity inside Enceladus. We hypothesize that a subsurface ocean is the heat reservoir for thermal anomalies on the surface and the source of heat and chemicals necessary for the plumes. The ocean is believed to contain dissolved gases, mostly CO2 and is found to be relatively warm (∼0°C). Regular tidal forces open cracks in the icy crust above the ocean. Ocean water fills these fissures. There, the conditions are met for the upward movement of water and the dissolved gases to exsolve and form bubbles, lowering the bulk density of the water column and making the pressure at its bottom less than that at the top of the ocean. This pressure difference drives ocean water into and up the conduits toward the surface. This transportation mechanism supports the thermal anomalies and delivers heat and chemicals to the chambers from which the plumes erupt. Water enters these chambers and there its bubbles pop and loft an aerosol mist into the ullage. The exiting plume gas entrains some of these small droplets. Thus, nonvolatile chemical species in ocean water can be present in the plume particles. A CO2 equivalent-gas molar fraction of ∼4×10−4 for the ocean is sufficient to support the circulation. A source of heat is needed to keep the ocean warm at ∼0°C (about two degrees above its freezing point). The source of heat is unknown, but our hypothesis is not dependent on any particular mechanism for producing the heat.

A model for regular desiccation cracks formation

A desiccation theory is developed providing a rational framework for a three-dimensional analysis of shrinkage and crack formation in a stiff clay surface layer. With the present model, we are estimating the shrinkage stress and the chemical potential variation in a morpheme produced after cracking, in a stiff clay surface layer, in terms of measurable quantities. In regular shrinkage morphemes, the shrinkage stresses are calculated from the shrinkage displacement and moisture content change. Other parameters such as the water mobility are introduced in order to express the chemical potential and the entropy changes in terms of the moisture content change and temperature at various instants of time.

Improved Modified Pressure Imaging and Software for Egg Micro-Crack Detection and Egg Quality Grading

Cracks in the egg shell increase the food safety risk. Eggs with very fine, hairline cracks (micro-cracks) are often undetected during the grading process because they are almost impossible to detect visually. A modified pressure imaging system was developed to detect eggs with micro-cracks and regular cracks alike without adversely affecting egg quality. The modified pressure imaging system forces a change in pressure from the atmospheric pressure to the negative pressure (vacuum) in an enclosure to open the cracked shell surface momentarily, while intensity changes in the open shell surface are recorded by a high-resolution digital camera and detected by an image processing algorithm. In this article, modifications and improvements to the system are reported along with application software development. The previous design for 15 eggs was modified to accommodate 20 eggs and improved to add a stepper motor to image the entire surface of eggs automatically and the data management software was added to aid a professional grader in egg grading. In a three-day study with 3,000 eggs in a commercial egg processing plant, a professional grader operated the 20-egg imaging system after a short training on the first day and another professional grader graded the eggs with hand candling. The accuracy in crack detection of both graders and the system itself was compared in order to test the crack-detection performance of the system and to explore the possibility of the use of the system for egg grading by USDA graders. The system automatically detected checked and cracked eggs with 94.53% accuracy with 0.29% of a false positive rate while a hand candler had 83.58% accuracy in crack detection. The grader operating the system had 98.51% accuracy in crack detection without any false positives. Other quality factors were also graded by both graders. The study demonstrated the potential of the system for egg grading in high accuracy.