Creation Of Cracks Research Articles

Numerical Analysis of Reinforced Concrete Slab with Subsoil

Abstract The article deals with the interaction of a reinforced concrete slab with subsoil. The paper contains a non-linear analysis based on an experiment of reinforced concrete slabs with dimensions 2000 x 2000 mm and thickness 150 mm. A steel mesh with a diameter of 8/100 mm was used as reinforcement. The calculations and analysis are complemented by a comparison with EC2 design approaches. The research area combines the design of concrete structure and geotechnical tasks. The real behavior of the concrete structure with subsoil is considered for the analysis for advanced design. The selected computational approach of nonlinear analysis allows to capture the change of stiffness after the creation of cracks and modelling the shear punch failure of the slab - the collapse of the structure. The paper focuses on comparing the experiment with the numerical model in select loaded states for various input parameters of subsoil. Based on the experiment and numerical analysis the failure mechanism was determined. It was the punching of the slab. The calculations and the experiment verified that the critically controlled perimeter is at a distance of 1d. The effect of the modulus of elasticity on the slab punch mechanism was verified. In case of low modulus of subsoil, the load-bearing capacity of the slab is significantly reduced. The punching mechanism is influenced not only by the mechanical properties of the concrete but also by the properties of the subsoil. The performed parametric study also verified the influence of the size the nominal cover reinforcement depending on the modulus of elasticity of the subsoil. The deformation variant finite element method and a 3D computational model were used for numerical modelling. Nonlinear analysis was based on the Newton-Rapson method.

Aqueous processing of flexible, free-standing Li4Ti5O12 electrodes for Li-ion batteries

The development of Li-ion batteries for the new flexible technologies should also consider environment and safety issues. Therefore, this research focused on developing an aqueous water-based process for the preparation of free-standing, CNT-free and flexible Li4Ti5O12 (LTO) electrodes, where LTO itself was also obtained through an aqueous route. The electrodes contain carbon black, polyvinyl alcohol (PVA) and polyethylene glycol (PEG) 500 or 6000 g/mol. The SEM and IR-ATR analyses confirmed a good distribution of all components. 2 wt% PEG 6000 or 3 wt% PEG 500 with respect to LTO were the optimum ratios to obtain free-standing flexible electrodes. The suspensions were studied by rheological analysis: the system was found to be stable (G’ modulus above G’’ modulus) and the role of the polymers and carbon in the suspensions has been investigated by viscosity measurements. Excellent electrochemical performances (up to 157mAh/g at 1C rate) were obtained for PEG6000-based current-collector-free electrodes. Mild mechanical stress applied to the electrodes before the measurements allowed to enhance the performances, which could be related to the creation of tiny cracks which led to better penetration of the electrolyte in the electrode.

Microstructural analyses of aluminum-magnesium-silicon alloys welded by pulsed Nd: YAG laser welding

Revealing grains and very fine dendrites in a solidified weld metal of aluminum-magnesium-silicon alloys is difficult and thus, there is no evidence to validate the micro- and meso-scale physical models for hot cracks. In this research, the effect of preheating on the microstructure and hot crack creation in the pulsed laser welding of AA 6061 was investigated by an optical microscope and field emission electron microscopy. Etching was carried out in the gas phase using fresh Keller’s reagent for 600 s. The results showed that the grain size of the weld metal was proportional to the grain size of the base metal and was independent of the preheating temperature. Hot cracks passed the grain boundaries of the weld and the base metal. Lower solidification rates in the preheated samples led to coarser arm spacing; therefore, a lower cooling rate. Despite the results predicted by the micro and meso-scale models, lower cooling rates resulted in increased hot cracks. The cracks could grow in the weld metal after solidification; therefore, hot cracks were larger than predicted by the hot crack prediction models.

Hot cracking in autogenous welding of 6061-T6 aluminum alloy by rectangular pulsed Nd:YAG laser beam

Due to high hot crack susceptibility of Al-Mg-Si alloys, autogenous welding by rectangular pulsed laser beam has not been generally successful in the removal of cracks. In this research, the effect of pulsed Nd:YAG laser parameters and preheating on the creation of hot cracks in the 6061-T6 aluminum alloy was investigated. The sample that was fabricated by the laser parameters including 1 Hz, 0.12 mm/s, 10 ms, and without preheating exhibited the highest cooling rate and the smallest dendrite arm spacing but no hot crack was observed. The tensile test specimens of this sample fractured at a point far from the weld metal and a decrease in the precipitation of silicon and magnesium in the inter-dendritic space and the reduction of thermal stresses resulted in the elimination of hot cracks. However, according to macro-scale models for the creation of hot cracks, preheating decreased the tensile stresses in the BTR (brittle temperature range), but increasing the preheating temperature led to increasing rather than decreasing the hot crack length. In this case, the formation of hydrogen porosity, the segregation of silicon and magnesium, and the creation of low melting point compounds were the important parameters affecting the hot crack initiation and growth.

Young's modulus of kaolinite-illite mixtures during firing

Kaolin (more than 80% of kaolinite) and illitic clay (80% of illite) were used to prepare mixtures with 0–100 mass% of kaolin. Extruded and dried samples were analyzed using thermogravimetry, thermodilatometry and dynamical thermomechanical analysis in the temperature range of 20 °C → 1100 °C → 20 °C, 5 °C/min. The mass loss caused by the release of physically bound water decreased linearly as the amount of kaolin in the mixture increased, from 3.3% mass loss in the samples with 100% illite to 2% mass loss in the samples with 100% kaolin. The mass loss caused by dehydroxylation increased linearly as the amount of kaolin increased, from 3.7% mass loss (samples with 100% of illite) to 10% mass loss (samples with 100% kaolin). The final firing shrinkage depended on the mass fraction of kaolin according to a quadratic function, ranging from 5.8% (samples with 0% kaolin) to 2.4% (samples with 100% kaolin), with the minimum 2% in mixtures with 70 mass% of kaolin. The initial values of Young's modulus varied linearly between 5 GPa (100% illite) and 8 GPa (100% kaolin). Young's modulus of all mixtures increased significantly at low temperatures (up to 300 °C), which was caused by the release of physically bound water. The effect of dehydroxylation on Young's modulus was insignificant, except for samples with 90 to 100 mass% kaolin in the mixture. The final values of Young's modulus depended on the amount of kaolin: from 38.3 GPa for 0% of kaolin to 16.4 GPa for 100% of kaolin. The behavior of Young's modulus during cooling at temperatures below 750 °C (transformation temperature of the glassy phase) was determined by the creation of cracks between the glassy phase and quartz grains. Cracking decreased Young's modulus and continued until the room temperature was reached. The relationships between physical quantities (mass loss, relative thermal expansion, and Young's modulus) and the mass fraction of kaolin identified experimentally were then compared with quantities calculated using the mixture rule. The experimental and calculated values were close; the differences between them were not larger than 3%. The results imply that the physical and chemical processes run in kaolin and illitic clay separately, without mutually influencing each other.

In vitro corrosion studies of stainless-steel dental substrates during Porphyromonas gingivalis biofilm growth in artificial saliva solutions: providing insights into the role of resident oral bacterium.

Stainless-steel AISI 321 is an effective material for fabricating dental crowns and other implants utilized dental restorative protocols for elderly and pediatric populations. This unique clinical application is possible through the mechanical stability and corrosion-resistance properties of this metallic material. However, stainless-steel dental implants eventually fail, leading to the creation of surface cavities and cracks within their microstructures during persistent mechanical stresses and biocorrosion. In this study, the in vitro corrosion behaviour of a medical-grade stainless-steel dental substrate was investigated during Porphyromonas gingivalis biofilm growth process in artificial saliva culture suspension (ASCS). Among the causative bioagents of corrosion, P. gingivalis was chosen for this study since it is also responsible for oral periodontitis and a major contributing factor to corrosion in most dental implants. Increased P. gingivalis growth was observed within the incubation period under study as compact cellular clusters fouled the metal surfaces in ASCS media. This led to the corrosion of steel substrates after bacterial growth maturity within 90 days. Corrosion rate increased with higher CFU and bacterial incubation period for all test substrates due to biocorrosion incited by the volatile sulphide products of P. gingivalis metabolism. The presence of some of these volatile compounds has been observed from experimental evidences. Significant anodic degradation in the forms of localized pitting were also recorded by surface analytical techniques. Residual fluorinated ions within the ASCS media also increased the rate of anodic dissolution due to media acidity. This study has provided extensive insights into the fate of stainless-steel dental crown in oral environments infected by a resident oral bacterium. Influences of oral conditions similar to fluoride-enriched mouthwashes were reflected in a view to understanding the corrosion patterns of stainless-steel dental substrates.

Damage and stiffness research on steel shape steel fiber reinforced concrete composite beams

In this work, an experimental research has been performed on Steel Fiber-Steel Reinforced Concrete (SFSRC)specimens subjected to four-point bending tests to evaluate the feasibility of mutual replacement of steel fibers and conventional reinforcement through studying failure modes, load-deflection curves, stiffness of characteristic points, stiffness degradation curves and damage analysis. The variables considered in this experiment included steel fiber volume percentage with and without conventional reinforcements (stirrups or steel fibers) with shear span depth ratios of S/D=2.5 and 3.5. Experimental results revealed that increasing the volume percentage of steel fiber decreased the creation and propagation of shear and bond cracks, just like shortening the stirrups spacing. Higher crack resistance and suturing ability of steel fiber can improve the stability of its bearing capacity. Both steel fibers and stirrups improved the stiffness and damage resistance of specimens where stirrups played an essential role and therefore, the influence of steel fibers was greatly weakened. Increasing S/D ratio also weakened the effect of steel fibers. An equation was derived to calculate the bending stiffness of SFSRC specimens, which was used to determine mid span deflection; the accuracy of the proposed equation was proved by comparing predicted and experimental results.

The effect of Poisson's ratio on the creation of tensile branches around dynamic faults

It is common for the fractured zone around dynamic faults to contain tensile cracks. The effect of Poisson's ratio (ν) on the creation of these cracks was investigated in this study as a function of the dynamic minimum strain energy density criterion (MSED-Criterion). In addition, the effect of rupture speed (c), friction coefficient (f), fault inclination angle (β) and confining stress (Ko) were taken into account. The MSED-Criterion permits the study of the creation of tensile cracks in brittle and ductile shear zones. It was found that by increasing Poisson's ratio, the rock behavior transfers gradually from brittle to ductile and as a consequence, the branching angle and fracture toughness increase. Whenever the rock mass tends towards ductile behavior, the possibility of creating tensile branches decreases. Ductile shear zones can sustain stresses greater than brittle zones before tensile branches are formed around dynamic faults. The branching angle of tensile cracks is directly proportional to (ν, c, β, f and Ko) and it varies between approximately 77° and 115°. These cracks are formed in the extensional side of a dynamic fault and they tend to grow towards the maximum far-field stress. The results of this study can be used in the field of earth science and earthquakes to identify the formation of tensile branches around dynamic shear ruptures and they were explained depending on experimental results and field observations of other studies.

Stress state analysis in the vicinity of the hydraulic fracturing tool located near the bottom of the well

ABSTRACT Hydraulic fracturing is a widely used method in the mining industry. In the case of underground workings, the formation of parallel fractures across the borehole axis is of great particular interest for many geotechnical issues, for example, improving the production of coal seam gas or making an impermeable screen in massif to eliminate a water inflow. The article presents a hydofrac tool’s scheme with one inflatable packer promoting the creation of transverse crack. We observe in the laboratory the behaviour of the packer to estimate the displacement of its sliding end when the pressure was applied. These data were used to assess the possibility to form a fracture in the desired direction. In addition, numerical studies of the stress state caused by loading, created with the help of the downhole tool, have been carried out. Analysis of parameters necessary for the formation of an initial crack on the borehole wall is presented. Results show that the using of the borehole tool equipped with an anchoring module allows to create conditions for the appearance of a transverse initiating crack. It has been established that a decrease in the length of the anchor leads to an increase in the tensile axial stresses.

The ways to ensure fatigue strength of cold-formed rebars. Foundry production and metallurgy

To ensure safe operation of buildings and structures from reinforced concrete, European norms set requirements for its fatigue strength (resistance). Tests are carried out with oscillation swing amplitude 2sa from 150–200 MPa. Rebar shall withstand from 1 to 2 mln. cycles.The analysis of causes which decrease the fatigue strength of cold-formed steel is made in the article. Fatigue strength is a lot affected by surface defects and stress concentrators. Filler transition in the point where transversal rib crosses a rebar body becomes as a rebar stress concentrator. To detect details of early breakage, analysis the condition of fracture in the area of thickened (marking) ribs was made. Sequence of rebar breakage was established as well as the initial point of a crack creation was detected. This fracture is characterized as a fatigue one, with clear zones of fatigue fracture having fine-grained surface and static fracture zone– the rest of section with fibrous structure. While applying a bar mark, smoothness of filler transition was disturbed in the crossing point where transversal rib crosses a rebar body, and this action caused decrease of fatigue strength of cold-formed rebar.The article describes research on development of requirements for rolling tools used in production of cold-formed rebars.Use of information given in this article during design, purchasing and manufacture of the equipment will allow to minimize factors which have negative influence on rebar’s fatigue test results.

Numerical study of semi-elliptical cracks in the critical position of pipe elbow

Pipelines are considered as a major tool to transport hydrocarbons due to its important role in transporting fluids taking into account the operating conditions. Despite the importance of the elbow which considered as a critical part in the pipelines, the repeated failures and defects became a dangerous and enormously costly issue. In this numerical study, the Fluid-Structure Interaction (FSI) analysis was carried out using Ansys software. This work is divided into four main parts: the first part focused on studying and comparing the effect of bending radius of pipe elbow on the maximum of Von-Mises stress values for each radius with the yield stress of the steel of the pipe. The second part focused on the creation of a semi-elliptical crack for different locations along the elbow angles to show the critical position compared to the stress intensity factors. In the third part, the semi-elliptical crack angle orientation was studied at the critical position to estimate the critical angle. In the last part, the failure assessment diagram (FAD) was used to show the critical crack depth ratios at a critical position and a critical angle.

Hydrogen Effect on the Cyclic Behavior of a Superelastic NiTi Archwire

In this work, we are interested in examining the strain rate effect on the mechanical behavior of Ni–Ti superelastic wires after hydrogen charging and ageing for 24 h. Specimens underwent 50 cycles of loading-unloading, reaching an imposed deformation of 7.6%. During loading, strain rates from 10−4 s−1 to 10−2 s−1 were achieved. With a strain rate of 10−2 s−1, the specimens were charged by hydrogen for 6 h and aged for one day showed a superelastic behavior marked by an increase in the residual deformation as a function of the number of cycles. In contrast, after a few number of cycles with a strain rate of 10−4 s−1, the Ni-Ti alloy archwire specimens fractured in a brittle manner during the martensite transformation stage. The thermal desorption analysis showed that, for immersed specimens, the desorption peak of hydrogen appeared at 320 °C. However, after annealing the charged specimens by hydrogen at 400 °C for 1 h, an embrittlement took place at the last cycles for the lower strain rates of 10−4 s−1. The present study suggests that the embrittlement can be due to the development of an internal stress in the subsurface of the parent phase during hydrogen charging and due to the creation of cracks and local zones of plasticity after desorption.

Formation of ordered defect structures in lithium niobate crystals of different chemical composition at non-equilibrium processes of different nature

Ordered defective structures in lithium niobate crystals have been established. The phenomenon is a result of non-equilibrium processes of a different nature: photo-induced light scattering, laser ablation, and creation of cracks during relieving mechanical stresses. The shape of such structures depends on the nature and energy that affect a crystal. A number of concentric areas with a different density of laser-induced defects with a changed refractive index appear during photoinduced light scattering. Concentric clusters of microdomains appear in a macrodomain of another sign at laser ablation as a result of local switching under impulse laser radiation. A number of microcracks with a fractal structure appear near main macro-crack during cracking under mechanical stress. Thus, appearance of ordered defective structure occurs not only in the local spot of direct exposure to laser radiation, mechanical stress, etc., but also in some volume of a crystal around it.

Intumescent ethylene-vinyl acetate copolymer: Reaction to fire and mechanistic aspects

The concept of intumescence was applied to flame retard ethylene vinyl acetate copolymer (EVA). The paper examines two types of intumescence based on expandable graphite (EG, physical expansion) and on modified ammonium polyphosphate (AP760, chemical expansion). The incorporation of expandable graphite (EG) at relatively low loading (10 wt%) in EVA permits the reduction up to 65% of peak of heat release rate (pHRR) measured by cone calorimetry. The mode of action occurs via the formation of an expanded carbonaceous layer acting mainly as heat barrier limiting heat and mass transfer as evidenced by the temperature measurement as a function of time during cone calorimetry. The incorporation of small amount of ZnCO3 in EVA-AP760 enhances strongly the performance: pHRR was not reduced using the sole AP760 while it is decreased by 54% when only 2 wt% of AP760 is substituted by ZnCO3. A strong synergistic effect was therefore observed. Solid state NMR of 31P and 13C on cone residues prepared at different characteristic times evidenced the mechanism involved is the reinforcement of the protective char by the formation of phosphate glass limiting the creation of cracks and increasing the char strength.

Numerical implementation of the coupled criterion for damaged materials

In this work, we present a numerical model to deal with the fracture of damaged materials within the framework of the finite fracture mechanics (FFM). Continuum damage mechanics combined with the regularization techniques is first applied to determine the damage field. The transition from continuous damage field to discontinuous crack occurs when the coupled criterion (CC) is fulfilled. Special numerical algorithms were developed in crack path tracking and crack creation for the analysis of the entire failure process from damage development to crack creation. Both initiation and growth of one or multiple cracks can reasonably be simulated and efficiency and accuracy of the approach are illustrated through several numerical examples.

Acoustic Emission Response to Erosion-Corrosion and Creep Damage in Pipeline Systems

Pipeline system failures confirm that special attention must be paid to the main components of nuclear power plants in particular. One of the significant degradation factors in terms of integrity and residual life of these components is erosion-corrosion in piping systems and creep in high pressure pipelines of thermal power plants. This article deals with analysis of a set of steel samples with different degrees of degradation using acoustic emission method based on detection of elastic-stress waves in a material. Time domain and frequency domain characteristics of acoustic emission signals generated by different creep mechanisms are analyzed. The main task is to find a relationship between crack creation and propagation and acoustic emission response. Part of the solution is also the design and implementation of a diagnostic method for operation monitoring of the deterioration of the high-pressure piping systems at high temperature. The benefit should be a significant reduction in the risk of damage to important components and reduction of the probability of damaging pipe wall integrity potentially sensitive to erosion-corrosion.

The assessment of Levenberg–Marquardt and Bayesian Framework training algorithm for prediction of concrete shrinkage by the artificial neural network

Shrinkage and creep are the main concrete volume changes over time. This unacceptable concrete deformation leads to stress and cracks creation where eventually reduces the service life of concrete structures. According to this, the prediction of shrinkage and creep strain in concrete structures with acceptable accuracy is the significance essential. The extensive investigation accomplished by several researchers has created different relationships and models for forecasting of shrinkage and creep strain based on experimental and analytical observation. Despite effective efforts in this regard, existing models do not have sufficient accuracy for anticipate of shrinkage strain. According to this, in this research, it has been attempted to provide a shrinkage predicting model based on the artificial neural network technique with the application of RILEM database. Also, it has been tried to determine the accuracy of the proposed model in comparison to the existing standard models by statistical analysis. According to the obtained results, by application of the neural network technique, the shrinkage strain could be predicted with acceptable accuracy especial in the extended period.

Calculating Energy Release Rate as a Function of Crack Length Using a Multiple-Step Crack Closure Technique in Tire Finite Element Models

ABSTRACT When performing predictive durability analyses on tires using finite element methods, it is generally recognized that energy release rate (ERR) is the best measure by which to characterize the fatigue behavior of rubber. By addressing actual cracks in a simulation geometry, ERR provides a more appropriate durability criterion than the strain energy density (SED) of geometries without cracks. If determined as a function of crack length and loading history, and augmented with material crack growth properties, ERR allows for a quantitative prediction of fatigue life. Complications arise, however, from extra steps required to implement the calculation of ERR within the analysis process. This article presents an overview and some details of a method to perform such analyses. The method involves a preprocessing step that automates the creation of a ribbon crack within an axisymmetric-geometry finite element model at a predetermined location. After inflating and expanding to three dimensions to fully load the tire against a surface, full ribbon sections of the crack are then incrementally closed through multiple solution steps, finally achieving complete closure. A postprocessing step is developed to determine ERR as a function of crack length from this enforced crack closure technique. This includes an innovative approach to calculating ERR as the crack length approaches zero.

INTERACTION OF NONLINEAR NUMERICAL MODEL OF SFRC SLAB AND NONLINEAR NUMERICAL SUBSOIL MODEL

Experimental measurements of the concrete slab in interaction with subsoil are compared with numerical analysis based on Finite Element Method FEM. Experimental measurements are conducted with usingan experimental device constructed at the Faculty of Civil Engineering VSB – Technical University of Ostrava. In the article, several types of nonlinearities enter into FEM analysis. Subsoil-structure interaction requires an iterative solution procedure - and therefore structural nonlinearity. Material nonlinearity was used in the numerical model of the slab. This model also allows the creation and development of cracks in concrete - similar as during the experimental load test. Material nonlinearity was also used in the numerical model subsoil - to ensure of apposite subsoil behavior. The purpose of this paper is to compare resulting deformation of the slab with values observed during experimental loading test. It was concluded that the good agreement between the experimental results and numerical simulation was observed.

Evaluation of Thermal History and Defect in Friction Stir Processing of As-Cast Magnesium AZ91

Friction stir processing (FSP) is a thermos-mechanical process for modifying the microstructural and material properties of metals. FSP also can be used for mechanical alloying and producing metal matrix composites. Due to creation of longitudinal cracks and tunneling cavities throughout the processing path, FSP of AZ91 is difficult and also sensitive to processing temperature. In this study, the effect of processing parameters such as the traverse and rotational speeds and the dimension of the tool pin on the temperature history experienced by material were investigated. Additionally, a minimum temperature required to produce a defect-free specimen was presented.