Coalescence Stresses Research Articles

Interfacial engineering of Pickering emulsions stabilized by pea protein-alginate microgels for encapsulation of hydrophobic bioactives

This study aims to investigate the effects of interfacial layer composition and structure on the formation, physicochemical properties and stability of Pickering emulsions. Interfacial layers were formed using pea protein isolate (PPI), PPI microgel particles (PPIMP), a mixture of PPIMP and sodium alginate (PPIMP-SA), or PPIMP-SA conjugate. The encapsulation and protective effects on different hydrophobic bioactives were then evaluated within these Pickering emulsions. The results demonstrated that the PPIMP-SA conjugate formed thick and robust interfacial layers around the oil droplet surfaces, which increased the resistance of the emulsion to coalescence, creaming, and environmental stresses, including heating, light exposure, and freezing-thawing cycle. Additionally, the emulsion stabilized by the PPIMP-SA conjugate significantly improved the photothermal stability of hydrophobic bioactives, retaining a higher percentage of their original content compared to those in non-encapsulated forms. Overall, the novel protein microgels and the conjugate developed in this study have great potential for improving the physicochemical stability of emulsified foods.

Experimental and Numerical Studies on Crack Initiation and Coalescence in Sandy Mudstone with Prefabricated Cross-Flaws Under Uniaxial Compression

The purpose of this paper is to study the crack initiation, propagation, and coalescence of the sandy mudstone sample with two sets of prefabricated cross-flaws under uniaxial compression. This study is different from previous studies on single or multiple parallel prefabricated flaws. The prefabricated cross-flaws are characterized by the dip of the rock bridge with the direction of the main flaw ( β ) and the angle between the direction of main and minor flaws ( γ ). The effects of these two parameters on crack initiation, propagation, coalescence, crack initiation stress, and coalescence stress are analyzed. Moreover, numerical simulation of the uniaxial compression experiments is performed using PFC2D with a flat-joint model, and the simulation results are in good agreement with those from the experiments. The results demonstrate that the dip angle of the rock bridge with the direction of the main flaw ( β ) has strong effects on the crack initiation and coalescence stresses. The larger the angle between the direction of main and minor flaws γ , the greater the crack initiation and coalescence stresses. The crack initiation stress is reduced for the case with cross-flaws compared with that with non-cross-flaws. Meanwhile, the connection type of main flaws and the width of the crack coalescence zone are difficult to observe through the experiments and are discovered from the numerical simulation.

Study on the Effect of Continuity and Load Angle of Prefabricated Cross‐Flaws on Crack Initiation and Coalescence

In this study, a model that is closer to the state of fracture presentation in natural rocks has been developed, which is different from the previous. The cross‐flaws can be characterized by the joint persistency (k) and the angle between the primary flaws and axial load (α). The two parameters were varied individually and, by combining them, nine specimens with different nodal parameters were formed. Laboratory specimens and numerical simulations were performed on these specimens to investigate the crack extension process and the variation of crack initiation and coalescence stresses. It is found that a new category of crack coalescence is discovered according to the experimental results besides those reported before, and the angle α affects whether tensile‐shear cracks appear. Also, α has an impact on the location where crack first occurs. The joint persistency k alters rock failure mode and has a substantial effect on crack initiation stress. However, the effect on the aggregation stress is not significant. The crack initiation stress decreases in the case of cross‐flaws in contrast to flat fissures. In addition, the flat‐joint model in PFC2D is used for numerical simulation. It is possible to conduct a study that is difficult to achieve experimentally by using simulations, i.e., only changing one macroparameter without changing others and thus studying the changes in the effect on cracking during fracture. The simulation results are in good agreement with the experimental results. At the same time, the connection mode and the width of the crack coalescence zone of the primary defect, which is difficult to observe in the experiment, are found out from the numerical simulation.

Strain localization and cracking behavior of sandstone with two gypsum-infilled parallel flaws

The infilled flaws within rocks significantly influence the mechanical behavior of non-through flawed rocks. To investigate the influence of infilling on strain localization in the pre-cracking, cracking and cracking coalesce stages, experimental and numerical investigations were conducted on sandstone containing two gypsum-infilled parallel flaws, but of different flaw geometric configurations. The full-field strain evolution and progressive cracking process are analyzed using the acoustic-optical-mechanical methods, while the stress evolution and stress transfer by infilling were studied by the particle flow code modeling. The results show that the major principal strain of flawed sandstone with infilling changes from the diffused ellipse to highly localized linear strip with the increase of loading level, while the shear strain keeps a diffused ellipse shape invariable. The shear strain localization of infillings with a large flaw inclination initiates at a lower loading level than that of small one. The peak strength and Young’s modulus of flawed sandstone show a first decrease, then increase and finally decrease trend with the ligament angle increasing from 0° to 150°, and both achieve their minimums at 60°. Ten types of cracks were observed, and four patterns of crack coalescence were identified. By comparison with sandstone of open flaws, the flaw infilling simplifies the geometric pattern of produced crack networks, which is quantitatively characterized by fractal dimension analysis. The numerical modeling shows that the infilling enlarges the compressive bond force zone and decreases the tensile bond force zone in the surrounding areas of pre-existing flaw, thus increasing the reinforcement coefficient. Moreover, the flaw geometric configuration has a significant influence on the reinforcement coefficient. It is also found that the increment of reinforcement coefficient defined by crack initiation stress is greater than those defined by either crack coalescence or peak stresses.

Modelling Micro-cracking Behaviour of Pre-cracked Granite Using Grain-Based Distinct Element Model

In this paper, the micro-cracking behaviour of pre-cracked Barre granite is investigated using a grain-based distinct element model (GBM). We investigated and demonstrated a cohesive model in a distinct element code, PFC2D, to mimic the elastic and softening response of the intra-grain contacts in the GBM. The study employed the smooth-joint model to simulate the micro-cracking behaviour of grain interfaces. The grain size distribution, as well as the mineral constituent of Barre granite, was incorporated in the numerical model. The model was calibrated against uniaxial compressive strength and Brazilian split-tensile-strength tests. We found that the GBM framework successfully reproduced the macroscopic physical properties obtained from the laboratory tests. When calibration was complete, the geometries of pre-existing cracks, which were considered in the experimental testing, were imported into the numerical model and used to generate synthetic, pre-cracked Barre granite. The macroscopic cracking process in the generated numerical models was observed by monitoring the evolution of intra- and inter-granular micro-cracks. The cracking and coalescence behaviour of numerical pre-cracked granite revealed that the proposed GBM approach can replicate the macroscopic fracturing pattern of pre-cracked Barre granite with close agreement to the experimental observations. The crack initiation, coalescence, and peak axial stresses were also recorded during numerical testings, and a good agreement was also achieved between these simulated results and the laboratory data. The proposed GBM framework is promising for research into micro-cracking behaviour of pre-cracked crystalline rocks under compressive loading.

Fracture Mechanism of Brazilian Discs with Multiple Parallel Notches Using PFC2D

This study presents crack initiation, propagation and coalescenceat or near pre-existing open cracks in a numericalmodel under Brazilian test. Firstly, Particle Flow Code intwo dimensions (PFC2d) was calibrated with respect to thedata obtained from experimental laboratory tests to ensurethe conformity of the simulated numerical models response.Brazilian discs contain one, two, three, four, and five parallelcentred cracks (45° to the horizontal) under compressiveline loading. Models containing two and three cracks havedifferent joint spacing and joint configuration. In model consistingone flaws, tensile cracks initiated from notch tip andpropagates in direction of compressive loading till coalescewith model edge. By increasing the number of notch, first typeof tensile crack initiated at the tips of outer flaws and coalescedwith model edge. Also second type of tensile cracksinitiates from middle of inner flaws and coalesce with tip ofthe neighbouring flaws. The results show that joint spacingand joint configuration has important effect on the failure patternin model consisting two and three notch. Experimentaland numerical results rendered by other researchers showed agood agreement with the numerical results in the coalescencecharacteristics in cracked model. In addition, crack initiationand coalescence stresses in models were analyzed and comparedwith those in the single-flawed model.

Erratum to: On the Strength and Crack Propagation Process of the Pre-Cracked Rock-Like Specimens Under Uniaxial Compression

The pre-cracked cylindrical specimens of rock-like materials are experimentally tested under compressive loading (using specially made rock-like specimens from portland pozzolana cement). The stress-strain and strength of the specimens are measured showing the decreasing effects of the cracks and their orientation on the final breakage stress of the specimen. The crack initiation and coalescence stresses during the crack propagation process are observed. The breakage process of the specimens is studied by inserting single and double cracks with different inclination angles at the center and applying uniaxial compressive stress at both ends of the specimen. The wing cracks are produced at the first stage of loading and start their propagation toward the direction of uniaxial compressive loading. The secondary cracks may also be produced in form of quasicoplanar and/or oblique cracks in a stable manner. The secondary cracks may eventually continue their propagation in the direction of the maximum principle stress. The same specimens are numerically simulated by an indirect boundary element method known as displacement discontinuity method. One may validate the results by the numerical and experimental results given in this study.

Crack Initiation, Propagation and Coalescence in Rock-Like Material Containing Two Flaws: a Numerical Study Based on Bonded-Particle Model Approach

Cracking and coalescence behavior in a rectangular rock-like specimen containing two parallel (stepped and coplanar) pre-existing open flaws under uniaxial compression load has been numerically studied by a parallel bonded-particle model, which is a type of bonded-particle model. Crack initiation and propagation from two flaws replicate most of the phenomena observed in prior physical experiments, such as the type (tensile/shear) and the initiation stress of the first crack, as well as the coalescence pattern. Eight crack coalescence categories representing different crack types and trajectories are identified. New coalescence categories namely “New 1” and “New 2”, which are first observed in the present simulation, are incorporated into categories 3 and 4, and category 5 previously proposed by the MIT Rock Mechanics Research Group, respectively. The flaw inclination angle (β), the ligament length (L) (spacing between two flaws) and the bridging angle (α) (inclination of a line linking up the inner flaw tips, between two flaws) have different effects on the coalescence patterns, coalescence stresses (before, at or post the peak stress) as well as peak strength of specimens. Some insights on the coalescence processes, such as the initiation of cracks in the intact part of specimens at a distance away from the flaw tips, and coalescence due to the development and linkage of a number of steeply inclined to vertical macro-tensile cracks are revealed by the present numerical study.

An experimental and numerical study of fracture coalescence in pre-cracked specimens under uniaxial compression

This study presents crack initiation, propagation and coalescence at or near pre-existing open cracks or flaws in a specimen under uniaxial compression. The flaw geometry in the specimen was a combination of a horizontal flaw and an inclined flaw underneath. This flaw geometry is different from those reported in the previous studies, where a pair of parallel flaws was used. Three materials were used, PMMA (Poly Methyl MethAcrylate), Diastone (types of molded gypsum), and Hwangdeung granite. Crack initiation and propagation showed similar and different patterns depending on the material. In PMMA, tensile cracks initiated at the flaw tips and propagated to the tip of the other flaw in the bridge area. The cracks then coalesced at a point of the inclined flaw, which is affected by the flaw inclination angle. For Diastone and Hwangdeung granite, tensile cracks were observed followed by the initiation of shear cracks. Coalescence occurred mainly through the tensile cracks or tensile and shear cracks. Crack coalescence was classified according to the crack coalescence types of parallel flaws for overlapping flaw geometry in the past works. In addition, crack initiation and coalescence stresses in the double-flawed specimens were analyzed and compared with those in the single-flawed specimen. Numerical simulations using PFC 2D (Particle Flow Code in two dimensions) based on the DEM (Discrete Element Method) were carried out and showed a good agreement with the experimental results in the coalescence characteristics in Hwangdeung granite. These experimental and numerical results are expected to improve the understanding of the characteristics of cracking and crack coalescence and can be used to analyze the stability of rock and rock structures, such as the excavated underground openings or slopes, tunneling construction, where pre-existing cracks or fractures play a crucial role in the overall integrity of such structures.

Intrinsic stresses in AlN layers grown by metal organic chemical vapor deposition on (0001) sapphire and (111) Si substrates

Stress evolution during metal organic chemical vapor deposition growth of AlN layers on (111) Si and (0001) sapphire substrates was investigated using in situ wafer curvature measurements in order to understand the origin of growth stresses. AlN layers 170±30nm thick were deposited over a temperature range of 600–1100°C at a growth rate of 0.2±0.05nm∕s. On (111) Si, AlN films grow under a constant tensile stress right from the beginning of growth in the temperature range investigated. In contrast, above 900°C on sapphire, an initial compressive growth stress is observed followed by a transition to a final tensile stress, while below 800°C the stress is tensile right from the beginning of growth as observed on Si. The origin of this stress behavior is explained in terms of a combination of epitaxial stress and grain coalescence stress. Calculation of the grain coalescence stresses show that the value is higher for growth on sapphire substrates than on Si substrates. Also, for growth on both substrates, a sharp drop in the value of the coalescence related tensile stress is observed between 900°C and 800°C. Both these features are correlated to the variation in the integral breadth of the (0002) and (101¯1) AlN x-ray rocking curves. It is shown that both the lower tensile stress on Si substrates and the drop in stress between 800°C and 900°C can be attributed to the reduction in grain coalescence stresses due to increasing grain boundary energies in the film resulting from increasing grain misorientation with respect to the substrate/film interface.

Metallic Island Coalescence: Molecular Dynamics Simulations of Boundary Formation and Tensile Strain in Polycrystalline Thin Films

AbstractIsland coalescence during the early stages of polycrystalline film formation is thought to lead to the large values of tensile stress observed in experiments. Continuum models have been developed which are in semi-quantitative agreement with in-situ experimental observations of stress evolution during coalescence. However, the size of coalescing particles is in the nanometer range, so that atomistic treatments might be expected to more accurately reveal the mechanisms leading to coalescence stresses. We have performed molecular dynamics (MD) computational experiments in which small metallic clusters both in free space and on weakly bound substrates are allowed to coalesce. These atomistic simulations complement the finite element modeling (FEM) and analytical work on the tensile strains developed during grain boundary formation by coalescence. Additionally, the atomistic simulations implicitly capture the effects of anisotropy and special orientations. The MD simulations also provide atomic-level detail of the boundary structure that is absent in continuum models. We have focused on simulations of Ag as a high mobility material at and above room temperature and have begun studying Ni as a material that can behave either as a high or low mobility material depending on the deposition temperature(i.e. Ni has a higher activation energy for adatom motion than Ag.). We find that for small islands, the initial boundary formation occurs in times on the order of nanoseconds. For nanometer-sized clusters in free space and on traction-free substrates, the cluster can undergo large rotations prior to boundary formation. The simulated boundary heights fit well with both a FEM model and an analytical model. Work is ongoing to further quantify the results of the simulations and relate the atomistic forces and positions to the stress state. We are pursuing methods for including the effects of traction at the interface, both through continuum and discrete methods.

Coalescence of multiple flaws in a rock-model material in uniaxial compression

A number of specimens made of gypsum with three and 16 flaws have been prepared and tested in compression. Results from these experiments are compared with observations from similar specimens with two flaws. The comparisons indicate that the cracking pattern observed in specimens with multiple flaws is analogous to the pattern obtained in specimens with two flaws. Two types of cracks initiate from the tips of the flaws: wing cracks and secondary cracks. Wing cracks are tensile cracks that initiate at an angle with the flaw and propagate in a stable manner towards the direction of maximum compression. Secondary cracks are shear cracks that initially propagate along their own plane in a stable manner. Two types of secondary cracks are possible: coplanar or quasi-coplanar, and oblique. As the load is increased, wing cracks propagate in a stable manner and secondary cracks may propagate in an unstable manner and produce coalescence , which occurs when two flaws are linked together. Nine types of coalescence have been observed, and each type is characteristic of a particular flaw geometry. The stresses at which wing and secondary cracks initiate and coalescence occurs strongly depend on the geometry of the flaws and on the number of the flaws; as the flaw inclination angle increases, the spacing increases, or the number of flaws decreases, initiation and coalescence stresses increase.

Intrinsic stress, island coalescence, and surface roughness during the growth of polycrystalline films

During film growth by a variety of techniques, intrinsic tensile stresses can be created by the coalescence of neighboring islands. Experimental results with diamond films produced by chemical vapor deposition are compared with a relatively simple model to demonstrate that a realistic interpretation of these coalescence stresses must account for effects that are associated with surface roughness. First, the interpretation of curvature measurements during the early stages of film growth must account for this surface roughness. Also, the experiments show that tensile stresses are induced by grain boundary formation during continuing growth after the initial island coalescence event. This understanding differs from the traditional interpretation that continuing intrinsic stress is produced by “templated” growth onto an already strained crystalline lattice. A kinetic model of stress evolution during postcoalescence growth is also presented.

Modelling of environment assisted cracking

A critical evaluation of mechanistically based modelling of environment-assisted cracking has been made. Models based on anodic reaction, namely slip-dissolution, anodic reaction-induced cleavage and surface mobility are evaluated from both a mechanistic and quantitative perspective. The film-rupture slip-dissolution mechanism is increasingly perceived to be relevant only to intergranular cracking though not as an exclusive mechanism for this process. The basic principle of film-induced cleavage has been demonstrated experimentally but the range of applicability of the mechanism remains contentious and requires further work. The concept of the surface mobility model is interesting but the quantitative crack growth model is based on unsatisfactory assumptions. In relation to models of environment-assisted cracking based on the effect of hydrogen atoms it is evident that for the non-hydride forming systems the hydrogen enhanced localised plasticity mechanism is gaining wider acceptance though studies of Fe-Si single crystals suggest that for some systems a cleavage mechanism of crack advance may prevail. Quantitative modelling of crack growth based on anodic reaction processes is reasonably well developed, though with a need to provide information about crack-tip electrochemistry and strain rates. These models are comparatively simple but modelling of cracking due to hydrogen atoms is more complex, for example, because of the diffusion and trapping of hydrogen atoms in the metal. Accordingly, models of crack growth in this case tend to be based on rate-limiting processes and in corrosion fatigue in particular there is a need to integrate the chemistry, mechanics, transport and materials interaction in an improved quantitative framework. Mechanistic modelling has an important role in understanding and predicting environment-assisted cracking but the application to life prediction may be limited to specific systems because of the complexities of multiple nucleation events, crack coalescence, crack-tip shielding, crack branching and complex stresses. The intelligent combination of empirical modelling supported by insight from mechanistic modelling provides the most effective approach to predicting environment-assisted cracking in metal-environment systems.