Tensile Stress Buildup Research Articles

Visualization Guided Mitigation of Mechanical Membrane Degradation in Fuel Cells

Dynamic operational power demands from a fuel cell, especially in transportation applications, are known to causefluctuations in its relative humidity (RH) and temperature [1]. This leads to repetitive in-plane compressive and tensile stress build-up in the confined perfluorosulfonic acid (PFSA) ionomer membranes due to constraining of their cyclic swelling and shrinkage under hydrated and dehydrated states, respectively. The ionomer’s dynamic water sorption characteristics during these hygrothermal variations lead to catalyst coated membrane (CCM) deformation and cyclic mechanical stresses resulting in structural fatigue and micro-crack development at high stress accumulation sites [2]. Lately, laboratory-based X-ray computed tomography (XCT) was used for non-destructive four-dimensional (4D) visualization and microstructural analysis of fuel cell parts showing novel insights on their failure processes [3,4]. The objective of the present work is to leverage 4D XCT visualization to design and demonstrate novel mitigation strategies for mechanical membrane degradation in fuel cells. A custom designed X-ray transparent fixture housing a small-scale single cell fuel cell was subjected to 2 min. each of wet and dry cycles with N2 gas at anode and cathode compartments, thereby causing fatigue-based membrane damage [5]. 3D tomographic data sets of same membrane electrode assembly (MEA) locations were acquired as a function of degradation time, which facilitated to study the damage development and growth. As a baseline for this work, a non-bonded MEA with high surface roughness gas diffusion layers (GDLs) demonstrated that the primary membrane failure mode was CCM buckling into GDL voids/pores [6]. To overcome this failure mode, two separate mitigation approaches are explored in the present work: (i) a commercial GDL with low surface roughness; and (ii) bonding of MEA to arrest buckling. Furthermore, a multi-physics finite element method (FEM) model is developed to simulate the contact interaction at the catalyst layer | GDL interface and non-uniformity in the GDL microstructure. The XCT findings are correlated with the numerical model to perceive the specific impact of GDL surface features on the development and distribution of membrane failures.Similar to the baseline MEA, membrane crack development was observed to be the key failure mode for both mitigation approaches. When the two mitigation designs are compared at similar cycling stages with the baseline (Fig. 1), however, the size and density of membrane cracks is drastically reduced. Since both mitigation strategies are tested using the same accelerated stress test protocol as the baseline, it is hereby established that each approach provides substantial mitigation against fatigue driven mechanical membrane degradation, predominantly due to reduced severity of CCM buckling deformation, resulting in a doubling of the membrane lifetime in each case. Complementary finite element simulations corroborate the experimental findings and further estimate the critical GDL void sizes to prevent CCM buckling and the required interfacial MEA adhesion quality to stabilize the MEA for overall membrane durability. Acknowledgements Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program.

Effect of Pressure and Temperature on Microstructure of Self-Assembled Gradient AlxTi1−xN Coatings

The correlation between structural properties of Al-rich self-assembled nano-lamellar AlxTi1−xN coatings and process parameters used during their chemical vapor deposition (CVD) remains unexplored. For this article, two gradient AlxTi1−xN coatings were prepared by a stepwise increase in temperature and pressure in the ranges of 750–860 °C and 1.56 to 4.5 kPa during the depositions at a constant composition of the process gas mixture. The cross-sectional properties of the coatings were analyzed using X-ray nanodiffraction (CSnanoXRD) and electron microscopy. Experimental results indicate that the variation of the process parameters results in changes in microstructure, grain morphology, elastic strain, nanolamellae’s chemistry and bi-layer period. At temperatures of ~750–800 °C and pressures of 2.5–4.5 kPa, preferably cubic nanolamellar grains are formed, whose microstructure correlates with the build-up of tensile stresses, which become relaxed in coating regions filled with nanocrystalline grains. CSnanoXRD superlattice satellite reflections indicate the period of the cubic Al(Ti)N-Ti(Al)N bilayers, which changes from 6.7 to 9 nm due to the temperature increase from 750 to ~810 °C, while it remains nearly unaffected by the pressure variation. In summary, our study documents that CVD process parameters can be used to tune microstructural properties of self-assembled AlxTi1−xN nanolamellae as well as the coatings’ grain morphology.

Nanoscale mechanics of thermally crystallized GST thin film by in situ x-ray diffraction

The thermal crystallization of Ge2Sb2Te5 (GST) thin film is investigated by in situ x-ray diffraction (XRD). The combination of several x-ray diffraction techniques (a) in-plane XRD, (b) out of plane XRD, (c) high resolution XRD on the substrate, and (d) 2D high energy XRD allows the characterization of the mechanical behavior of GST upon thermal crystallization. A new method is proposed for the evaluation of experimental stress vs strain dependence in thermally crystallized GST. Nanoscale strain, macroscale stress, and nucleation/crystallization are fully described by the correlation of these techniques. Upon crystallization, a progressive tensile stress build-up is observed. Concomitant stress build-up is also observed both in the in-plane and out of plane directions of the film. The 2D high energy XRD demonstrates a homogeneous nucleation process and a progressive crystallization of the GST composed partially of amorphous and crystalline parts in the film. The GST nanomechanics is then characterized at the nanoscale (crystallites scale) and at the macroscale (film scale). By plotting the stress vs strain and assuming a Poisson ratio of 0.28, the mixture of phase results in a Young modulus between 9 GPa and 37 GPa for amorphous and crystalline matrices, respectively. Intermediate states with a partial amorphous/crystalline ratio results in intermediate values of the Young modulus. Finally, cross correlation between all XRD techniques gives EGST ≈ 34 GPa and υGST ≈ 0.34 for fcc crystalline GST.

Stress Buildup Upon Crystallization of GeTe Thin Films: Curvature Measurements and Modelling.

Phase change materials are attractive materials for non-volatile memories because of their ability to switch reversibly between an amorphous and a crystal phase. The volume change upon crystallization induces mechanical stress that needs to be understood and controlled. In this work, we monitor stress evolution during crystallization in thin GeTe films capped with SiOx, using optical curvature measurements. A 150 MPa tensile stress buildup is measured when the 100 nm thick film crystallizes. Stress evolution is a result of viscosity increase with time and a tentative model is proposed that renders qualitatively the observed features.

Electron-beam-induced cracking in organic-inorganic halide perovskite thin films

The curious phenomenon of cracking in organic-inorganic halide perovskite (OIHP) thin films for solar cells during scanning electron microscopy (SEM) can be seen in literally thousands of published SEM micrographs. Here we demonstrate, for the first time, the mechanisms responsible for this e-beam-induced damage in OIHP thin films, which is precluding their detailed SEM-characterization and understanding. The e-beam-induced rapid volatilization of the organic species from the OIHP surface in the SEM results in localized shrinkage and buildup of tensile stresses. These stresses drive grain-boundaries cracking, resulting in a ‘mud-cracking’ pattern that is influenced by the thin-film grain size.

Interfacial Silicide Formation and Stress Evolution during Sputter Deposition of Ultrathin Pd Layers on a-Si.

Synchrotron experiments combining real-time stress, X-ray diffraction, and X-ray reflectivity measurements, complemented by in situ electron diffraction and photon electron spectroscopy measurements, revealed a detailed picture of the interfacial silicide formation during deposition of ultrathin Pd layers on amorphous silicon. Initially, an amorphous Pd2Si interlayer is formed. At a critical thickness of 2.3 nm, this layer crystallizes and the resulting volume reduction leads to a tensile stress buildup. The [111] textured Pd2Si layer continues to grow up to a thickness of ≈3.7 nm and is subsequently covered by a Pd layer with [111] texture. The tensile stress relaxes already during Pd2Si growth. A comparison between the texture formation on SiOx and a-Si shows that the silicide layer serves as a template for the Pd layer, resulting in a surprisingly narrow texture of only 3° after 800 s Pd deposition. The texture formation of Pd and Pd2Si can be explained by the low lattice mismatch between Pd(111) and Pd2Si(111). The combined experimental results indicate a similar interface formation mechanism for Pd on a-Si and c-Si, whereas the resulting silicide texture depends on the Si surface. A new strain relaxation mechanism via grain boundary diffusion is proposed, taking into account the influence of the thickness-dependent crystallization on the material transport through the silicide layer. In combination with the small lattice mismatch, the grain boundary diffusion facilitates the growth of Pd clusters, explaining thus the well-defined thickness of the interfacial silicide layer, which limits the miniaturization of self-organized silicide layers for microelectronic devices.

20 Hz synchrotron X-ray diffraction analysis in laser-pulsed WC-Co hard metal reveals oscillatory stresses and reversible composite plastification

The lifetime of WC-Co inserts used in cutting processes, such as milling, is limited by millisecond temperature and mechanical pulses, which occur as a result of interrupted tool-workpiece contact, thermal fatigue and wear. In the current work, synchrotron X-ray diffraction (XRD) was used in conjunction with a pulsed laser heating set-up to characterise the time-dependent development of stresses and microstructure in locally irradiated WC-Co inserts coated by chemical vapour deposition with 6.5 and 3.5 μm thick TiCN and α-Al2O3 films, respectively. Diffraction data from the WC phase were used to evaluate the time and temperature-dependent evolution of in-plane stresses, thermal strains and integral breadths of WC diffraction peaks in experiments with a single and five successive laser shocks applied within 2.2 and 20 s, respectively, using a laser spot diameter of ~5.8 mm and an X-ray beam size of 1 × 1 mm2. The laser heating induces the formation of compressive stresses in the inserts' substrates. Above a temperature of ~750 °C, at the onset of WC-Co composite plastification, compressive stresses relax and then vanish in WC at the maximal applied temperature of ~1300 °C, followed by the build-up of tensile stresses. The applied cyclic heating up and cooling down led to the repetitive formation of compressive and tensile stresses, with temperature dependencies oscillating with the number of applied laser pulses. The observed relatively high tensile stress level of ~1100 MPa in WC was a consequence of the stabilising function of the coating, which hindered the initiation of surface hot cracks and stress relaxation. The stress evolution was coupled with changes in XRD peak broadening, which however strongly depended on the particular hkl reflections and showed oscillatory behaviour within a single temperature cycle. In summary, the unique diffraction set-up revealed stress levels and provides insight into the WC-Co composite plastification mechanism governing the stress build-up and relaxation in locally thermo-shocked WC-Co inserts at millisecond time resolution.

In situ monitoring of stress change in GeTe thin films during thermal annealing and crystallization

Abstract Stress changes in GeTe thin films on silicon have been studied in situ as a function of temperature by optical curvature measurements. Crystallization of the initially amorphous layers is evidenced by a steep tensile stress buildup. The crystallization temperature is shown to be thickness-dependent for the thinner films. Various annealing conditions, such as cooling/re-heating steps and isothermal stages, allow exploring the thermo-mechanical behavior of the films. A non-thermoelastic temperature-dependent behavior is observed in the amorphous phase before crystallization.

Direct Observation of the Thickness-Induced Crystallization and Stress Build-Up during Sputter-Deposition of Nanoscale Silicide Films.

The kinetics of phase transitions during formation of small-scale systems are essential for many applications. However, their experimental observation remains challenging, making it difficult to elucidate the underlying fundamental mechanisms. Here, we combine in situ and real-time synchrotron X-ray diffraction (XRD) and X-ray reflectivity (XRR) experiments with substrate curvature measurements during deposition of nanoscale Mo and Mo1-xSix films on amorphous Si (a-Si). The simultaneous measurements provide direct evidence of a spontaneous, thickness-dependent amorphous-to-crystalline (a-c) phase transition, associated with tensile stress build-up and surface roughening. This phase transformation is thermodynamically driven, the metastable amorphous layer being initially stabilized by the contributions of surface and interface energies. A quantitative analysis of the XRD data, complemented by simulations of the transformation kinetics, unveils an interface-controlled crystallization process. This a-c phase transition is also dominating the stress evolution. While stress build-up can significantly limit the performance of devices based on nanostructures and thin films, it can also trigger the formation of these structures. The simultaneous in situ access to the stress signal itself, and to its microstructural origins during structure formation, opens new design routes for tailoring nanoscale devices.

Intense pulsed light annealing of copper zinc tin sulfide nanocrystal coatings

A promising method for forming the absorber layer in copper zinc tin sulfide [Cu2ZnSnS4 (CZTS)] thin film solar cells is thermal annealing of coatings cast from dispersions of CZTS nanocrystals. Intense pulsed light (IPL) annealing utilizing xenon flash lamps is a potential high-throughput, low-cost, roll-to-roll manufacturing compatible alternative to thermal annealing in conventional furnaces. The authors studied the effects of flash energy density (3.9–11.6 J/cm2) and number of flashes (1–400) during IPL annealing on the microstructure of CZTS nanocrystal coatings cast on molybdenum-coated soda lime glass substrates (Mo-coated SLG). The annealed coatings exhibited cracks with two distinct linear crack densities, 0.01 and 0.2 μm−1, depending on the flash intensity and total number of flashes. Low density cracking (0.01 μm−1, ∼1 crack per 100 μm) is caused by decomposition of CZTS at the Mo-coating interface. Vapor decomposition products at the interface cause blisters as they escape the coating. Residual decomposition products within the blisters were imaged using confocal Raman spectroscopy. In support of this hypothesis, replacing the Mo-coated SLG substrate with quartz eliminated blistering and low-density cracking. High density cracking is caused by rapid thermal expansion and contraction of the coating constricted on the substrate as it is heated and cooled during IPL annealing. Finite element modeling showed that CZTS coatings on low thermal diffusivity materials (i.e., SLG) underwent significant differential heating with respect to the substrate with rapid rises and falls of the coating temperature as the flash is turned on and off, possibly causing a build-up of tensile stress within the coating prompting cracking. Use of a high thermal diffusivity substrate, such as a molybdenum foil (Mo foil), reduces this differential heating and eliminates the high-density cracking. IPL annealing in presence of sulfur vapor prevented both low- and high-density cracking as well as blistering. However, grain growth was limited even after annealing with 400 flashes. This lack of grain growth is attributed to a difficulty of maintaining high sulfur vapor pressure and absence of alkali metal impurities when Mo foil substrates are used.

The Tarsometatarsus of the Ostrich Struthio camelus: Anatomy, Bone Densities, and Structural Mechanics.

BackgroundThe ostrich Struthio camelus reaches the highest speeds of any extant biped, and has been an extraordinary subject for studies of soft-tissue anatomy and dynamics of locomotion. An elongate tarsometatarsus in adult ostriches contributes to their speed. The internal osteology of the tarsometatarsus, and its mechanical response to forces of running, are potentially revealing about ostrich foot function.Methods/Principal FindingsComputed tomography (CT) reveals anatomy and bone densities in tarsometatarsi of an adult and a young juvenile ostrich. A finite element (FE) model for the adult was constructed with properties of compact and cancellous bone where these respective tissues predominate in the original specimen. The model was subjected to a quasi-static analysis under the midstance ground reaction and muscular forces of a fast run. Anatomy–Metatarsals are divided proximally and distally and unify around a single internal cavity in most adult tarsometatarsus shafts, but the juvenile retains an internal three-part division of metatarsals throughout the element. The juvenile has a sparsely ossified hypotarsus for insertion of the m. fibularis longus, as part of a proximally separate third metatarsal. Bone is denser in all regions of the adult tarsometatarsus, with cancellous bone concentrated at proximal and distal articulations, and highly dense compact bone throughout the shaft. Biomechanics–FE simulations show stress and strain are much greater at midshaft than at force applications, suggesting that shaft bending is the most important stressor of the tarsometatarsus. Contraction of digital flexors, inducing a posterior force at the TMT distal condyles, likely reduces buildup of tensile stresses in the bone by inducing compression at these locations, and counteracts bending loads. Safety factors are high for von Mises stress, consistent with faster running speeds known for ostriches.Conclusions/SignificanceHigh safety factors suggest that bone densities and anatomy of the ostrich tarsometatarsus confer strength for selectively critical activities, such as fleeing and kicking predators. Anatomical results and FE modeling of the ostrich tarsometatarsus are a useful baseline for testing the structure’s capabilities and constraints for locomotion, through ontogeny and the full step cycle. With this foundation, future analyses can incorporate behaviorally realistic strain rates and distal joint forces, experimental validation, and proximal elements of the ostrich hind limb.

Detection of Corrosion-Induced Tensile Stress in Aluminum

The enhancement of mechanical degradation by corrosion is the basis for damage processes such as stress corrosion cracking. Corrosion-induced degradation has been attributed to a wide variety of mechanisms.1 Here we show that high-resolution in situ stress measurements during Al corrosion reveal corrosion-induced tensile stress generation, leading to surface plasticity. This finding is evidence that corrosion can directly bring about plasticity, and may be relevant to mechanisms of corrosion-induced degradation. The present measurements are possible because of the unprecedented curvature resolution achievable through the present phase-shifting curvature interferometry method, which permit the monitoring of stress during extended periods of corrosion of thick metal samples.2,3 Al samples were 1 mm thick hard 5N Al sheets or 25 mm thick 5N annealed foils bonded to Si wafers. Some sheet samples were cold worked to 35 % thickness reduction. A reflective gold film was applied to the back side of each sample, which was in contact with air while the front side was exposed to solution. Curvature changes were measured by monitoring interference between two laser beams, each of which reflected twice on the gold surface. Curvature was converted to force per sample width using the Stoney equation.2 The force per width is the integrated in-plane stress through the sample thickness. The obtainable curvature resolution of 10-3 km-1is about 10 times higher than state-of-the-art deflectometry techniques. Thus, while thin metal films on wafers are typically employed in the latter experiments, stress in bulk samples can be measured with curvature interferometry.Fig. 1 shows that the Al samples exhibited large rates of alkaline corrosion-induced tensile stress increase, qualitatively similar to those measured previously on evaporated thin films.4At the outset of dissolution, force increased approximately linearly with time until a plateau was reached at 10-20 min. The plateau force was highest for the cold-worked sheet and smallest for the annealed foil. The yield stress of these samples was determined from Vickers hardness measurements, and found to scale linearly with the plateau stress (inset). Therefore, the plateaus in Fig. 1 apparently correspond to the onset of plastic flow. The preceding linear increase of force may correspond to the buildup of elastic stress to the yield stress. Similar force transients were observed in 1 M NaOH using Al samples with increased impurity content, and with 5N Al samples in solutions of lower pH. The plateau stress increased with impurity concentration and pH (i. e. metal dissolution rate).Tensile stress buildup during corrosion was modeled in terms of diffusion of Al atoms from near-surface dislocations to the metal-oxide interface. The force evolution was predicted based on the requirement of zero net strain resulting from additive contributions from elastic strain, plastic strain associated with power-law creep and diffusion-induced strain. Fitting the model to the force transients yielded values of the Al diffusivity. The elevated values of the fit diffusivity suggested significant rates of metal vacancy injection during the corrosion process. ACKNOWLEDGMENTThis work was supported by the National Science Foundation through NSF-CMMI-100748.REFERENCES1. R. C. Newman, in Corrosion Mechanisms in Theory and Practice,P. Marcus, Editor, p. 499 (2002).2. O. O. Çapraz, K. R. Hebert, and P. Shrotriya, J. Electrochem. Soc., 160,D501 (2013).3. O. O. Çapraz, P. Shrotriya and K. R. Hebert, J. Electrochem. Soc., 161,D256 (2014).4. K. R. Hebert, J. H. Ai, G. R. Stafford, K. M. Ho and C. Z. Wang, Electrochim. Acta, 56, 1806 (2011).Figure 1. Force per width evolution during dissolution of 5N Al samples in aqueous 1 M NaOH solutions at room temperature. In the inset, the force per width at the plateau is plotted vs. the sample yield stress.

Substrate and current density effects on electrodeposited aluminium coatings

Coatings of aluminium were electrodeposited on copper and mild steel substrates in a room temperature ionic liquid (RTIL) using [EMIm]Cl (95%) and AlCl3 (98%) in a glove box having a controlled atmosphere. Formation and purity of aluminium were confirmed by X-ray diffraction, X-ray fluorescence and energy dispersive spectroscopy measurements. Microcrystalline grain morphology of deposited Al was observed in scanning electron microscopy. The present study showed that Al coating electrodeposited in an RTIL exhibited similar morphology on copper and mild steel substrates, and cathode current efficiency depended on purity of the electrolyte and also on the deposition current density. There were indications for build-up of tensile stress in the coating even at low deposition current density.

Stress inversion from initial tensile to compressive side during ultrathin oxide growth of the Si(100) surface

We report the real-time observation of the stress change during sub-nanometer oxide growth on the Si(100) surface. Oxidation initially induced a rapid buildup of tensile stress up to −1.9 × 108 N m−2 with an oxide thickness of 0.25 nm, followed by gradual compensation by a compressive stress. The compressive stress saturated at 5 × 107 N m−2 for an oxide thickness of 1.2 nm. The analysis, assisted by theoretical study, indicates that the observed initial tensile stress is caused by oxygen bridge-bonding between the Si dimers. Atomistic model calculations considering mutually orthogonal orientations of the Si(100) surface structure reproduce the stress inversion from the tensile to the compressive side.

Alignment of cellular motility forces with tissue flow as a mechanism for efficient wound healing

Recent experiments have shown that spreading epithelial sheets exhibit a long-range coordination of motility forces that leads to a buildup of tension in the tissue, which may enhance cell division and the speed of wound healing. Furthermore, the edges of these epithelial sheets commonly show finger-like protrusions whereas the bulk often displays spontaneous swirls of motile cells. To explain these experimental observations, we propose a simple flocking-type mechanism, in which cells tend to align their motility forces with their velocity. Implementing this idea in a mechanical tissue simulation, the proposed model gives rise to efficient spreading and can explain the experimentally observed long-range alignment of motility forces in highly disordered patterns, as well as the buildup of tensile stress throughout the tissue. Our model also qualitatively reproduces the dependence of swirl size and swirl velocity on cell density reported in experiments and exhibits an undulation instability at the edge of the spreading tissue commonly observed in vivo. Finally, we study the dependence of colony spreading speed on important physical and biological parameters and derive simple scaling relations that show that coordination of motility forces leads to an improvement of the wound healing process for realistic tissue parameters.

A relation between relative density, alloy composition and sample shrinkage for nanoporous metal foams

Nanoporous metal foams synthesized by dealloying inherently undergo dimensional changes (shrinkage). If these changes are unaccounted for in the measurement of relative density, this can be underestimated, causing up to an order of magnitude error in weight-normalized properties. In constrained samples the shrinkage leads to build-up of tensile stress that modifies the porosity and may lead to crack formation. A relationship between shrinkage, relative density and atomic fraction is proposed and experimentally verified.

Intrinsic stress in ZrN thin films: Evaluation of grain boundary contribution from in situ wafer curvature and ex situ x-ray diffraction techniques

Low-mobility materials, like transition metal nitrides, usually undergo large residual stress when sputter-deposited as thin films. While the origin of stress development has been an active area of research for high-mobility materials, atomistic processes are less understood for low-mobility systems. In the present work, the contribution of grain boundary to intrinsic stress in reactively magnetron-sputtered ZrN films is evaluated by combining in situ wafer curvature measurements, providing information on the overall biaxial stress, and ex situ x-ray diffraction, giving information on elastic strain (and related stress) inside crystallites. The thermal stress contribution was also determined from the in situ stress evolution during cooling down, after deposition was stopped. The stress data are correlated with variations in film microstructure and growth energetics, in the 0.13–0.42 Pa working pressure range investigated, and discussed based on existing stress models. At low pressure (high energetic bombardment conditions), a large compressive stress is observed due to atomic peening, which induces defects inside crystallites but also promotes incorporation of excess atoms in the grain boundary. Above 0.3–0.4 Pa, the adatom surface mobility is reduced, leading to the build-up of tensile stress resulting from attractive forces between under-dense neighbouring column boundary and possible void formation, while crystallites can still remain under compressive stress.

Factors Controlling the Size of Graphene Oxide Sheets Produced via the Graphite Oxide Route

We have studied the effect of the oxidation path and the mechanical energy input on the size of graphene oxide sheets derived from graphite oxide. The cross-planar oxidation of graphite from the (0002) plane results in periodic cracking of the uppermost graphene oxide layer, limiting its lateral dimension to less than 30 μm. We use an energy balance between the elastic strain energy associated with the undulation of graphene oxide sheets at the hydroxyl and epoxy sites, the crack formation energy, and the interaction energy between graphene layers to determine the cell size of the cracks. As the effective crack propagation rate in the cross-planar direction is an order of magnitude smaller than the edge-to-center oxidation rate, graphene oxide single sheets larger than those defined by the periodic cracking cell size are produced depending on the aspect ratio of the graphite particles. We also demonstrate that external energy input from hydrodynamic drag created by fluid motion or sonication, further reduces the size of the graphene oxide sheets through tensile stress buildup in the sheets.

Fundamentals of desiccation cracking of fine-grained soils: experimental characterisation and mechanisms identification

This paper presents the results of a comprehensive experimental study of the desiccation of fine-grained soils. Air drying of initially saturated soil slabs in controlled conditions is investigated by performing three kinds of tests: free desiccation tests, constrained desiccation tests (prevented shrinkage), and crack pattern tests. Strains, suction, water content, and crack geometry are investigated. Results reveal that unconstrained drying exhibits two stages: a domain with large, mostly irrecoverable deformations and degree of saturation close to 100%, followed by a domain with lower deformations at a decreasing degree of saturation. Homogeneous soil macroscopic cracking is possible only in the presence of boundary constraints and (or) moisture gradients, inducing the build-up of tensile stresses. Results also show that, for the initially saturated remoulded soils tested here, in the whole sample and near a crack initiation point, the degree of saturation remains very close to 100% until cracking, while cracking onset, the air-entry suction, and the shrinkage limit are close to each other. Cavitation nuclei and the formation of an irregular drying front at cracking initiation are commented upon in light of this observation. Finally, results suggest that the crack pattern geometry is the result of energy redistribution. A quantification of the process is proposed.

Fracture initiation associated with chemical degradation: observation and modeling

The fracture initiation in engineering thermoplastics resulting from chemical degradation is usually observed in the form of a microcrack network within a surface layer of degraded polymer exposed to a combined action of mechanical stresses and chemically aggressive environment. Degradation of polymers is usually manifested in a reduction of molecular weight, increase of crystallinity in semi crystalline polymers, increase of material density, a subtle increase in yield strength, and a dramatic reduction in toughness. An increase in material density, i.e., shrinkage of the degraded layer is constrained by adjacent unchanged material results in a buildup of tensile stress within the degraded layer and compressive stress in the adjacent unchanged material due to increasing incompatibility between the two. These stresses are an addition to preexisting manufacturing and service stresses. At a certain level of degradation, a combination of toughness reduction and increase of tensile stress result in fracture initiation. A quantitative model of the described above processes is presented in these work. For specificity, the internally pressurized plastic pipes that transport a fluid containing a chemically aggressive (oxidizing) agent is used as the model of fracture initiation. Experimental observations of material density and toughness dependence on degradation reported elsewhere are employed in the model. An equation for determination of a critical level of degradation corresponding to the offset of fracture is constructed. The critical level of degradation for fracture initiation depends on the rates of toughness deterioration and build-up of the degradation related stresses as well as on the manufacturing and service stresses. A method for evaluation of the time interval prior to fracture initiation is also formulated.