Main Cause Of Cracks Research Articles

Multi-field coupling numerical simulation and experimental research of submerged arc surfacing process on the descaling rolls with different wall thickness

Iron and steel industry descale rolls long-term high-pressure water erosion, high temperature, easy to wear, corrosion failure and other problems, the continuous production of steel rolling impact, descale rolls base wall thickness is the main cause of cracking of the surfacing gradient lap organization. This article combines numerical simulation and experimental testing, the use of CALPHAD method to calculate the surfacing material temperature change physical parameters, based on the double ellipsoid heat source model, the establishment of the descale roller submerged arc surfacing process of multi-field coupling three-dimensional numerical model, calculations and reveal the descale roller submerged arc surfacing process of the temperature field, the stress field, the phase change field and the flow field of the transient evolution of the law, and further analyze the descale roller wall thickness of the surfacing process of the influence of the mechanism. Calculation results show that the depth of melting of the surfacing matrix is positively correlated with the width of the double ellipsoid heat source, and the larger the wall thickness, the faster the cooling rate, the greater the formation of residual stress. The austenite and martensite phase transformation of the overlay welding process is greatly influenced by the wall thickness. At the same time, SEM, EDS, XRD material science microcharacterization experiments were carried out to observe the micro-morphology, phase and elemental composition of the cladding layer. This study lays an important theoretical foundation for optimizing the submerged arc surfacing process of descale roller.

Cracking Resistance of Steam-Cured Precast Concrete Using High Alite Cement with Modified Fly Ash

Cement with fly ash has rarely been used in Japan, mainly because its strength development is slower than ordinary Portland cement. In this research, the effect of the new type of fly ash (which was modified by the electrostatic belt separation method) with high alite (C3S) cement on cracking resistance of precast concrete prepared by steam curing was studied. The mechanical and shrinkage properties of the proposed fly ash concrete were compared with those of concrete made using OPC cement without fly ash. In order to study the cracking tendency of precast concrete with the proposed cement with fly ash, thermal stress analysis was conducted, taking into consideration the experimental data of concrete properties with the different concrete mix proportions. A standard precast concrete box culvert model was used in this 3D FEM analysis, and the distribution of temperature and relative humidity in the cross-section and induced restraint stress during and after steam curing were discussed. Steam-cured concrete with fly ash and high alite cement developed higher compressive strength on the first day of age than concrete with OPC. The proposed fly ash concrete developed high cracking resistance in the early days. On the other hand, the results showed that the drying shrinkage at later ages was the main cause of cracking.

Failure Analysis of Cracking on the Outer Pipe of 12Cr1MoV Water Wall in Thermal Power Plants

In order to identify the main causes of cracking and leakage of 12Cr1MoV water wall tubes outside the furnace in thermal power plants, this paper uses macroscopic morphology observation, stereomicroscope observation, alloy composition analysis, Brinell hardness test, metallographic structure inspection, and other means to detect and analyze the failed water wall tube. The alloy composition content of the water wall tubes submitted for inspection is within the standard requirements, and the Brinell hardness value of the water wall tubes also meets the requirements of relevant standards, The matrix metallographic structure of the water wall tube is normal. The initial leakage point is the cracking of the fillet weld at the end of the water wall tube fin, while other leakage points are caused by the blowing of the medium leaked from the initial leakage point. The initial leakage point is formed by the fatigue crack initiated at the fillet weld of the fin end and the corrosion crack on the inner wall extending towards each other. It is recommended to further inspect whether there is still cracking at the end of adjacent water wall tube fins to prevent similar cracking from happening again. Due to the frequent changes in the “peak shaving” period and amplitude in recent years, it is necessary to design the peak shaving period or amplitude reasonably to ensure the safe operation of the boiler.

Hot salt corrosion behavior of the Ti-44Al-4Nb-1.5Mo-(B,Y) alloy in the temperature range of 750–950 °C

Hot salt corrosion behavior of TNM alloy (Ti-44Al-4Nb-1.5Mo-(B, Y) (at.%)) was investigated in cyclic conditions at 750 °C, 850 °C and 950 °C for 100 h in the 75 wt% Na2SO4 + 25 wt% NaCl salt mixture. The hot salt corrosion resistance of γ、α2 and β phase in the alloy was studied by short-term quasi-in-situ hot corrosion experiments. Results show that the hot salt corrosion resistance of the three phases decreases in the following order: γ、α2、β. The weight changes of TNM alloy after hot salt corrosion at 750 °C, 850 °C and 950 °C for 100 h are 13.8 mg/cm2, -19.6 mg/cm2 and -26.7 mg/cm2. Under the condition of hot salt corrosion at 750 °C, the corrosion scales are mainly composed of a mixed corrosion scale of TiO2 and Al2O3, Al2O3 scale, and Al depletion zone, similar to the morphology of a high-temperature oxidation scale. The corrosion scales are seriously peeled off and destructive transverse cracks are generated inside the corrosion scales under the conditions of hot salt corrosion at 850 °C and 950 °C. The acid-base corrosion dissolution of oxides in the corrosive medium and the formation of volatile chlorides are the main causes of cracks in the TiO2 and Al2O3 scales. In addition, the Nb element in the TNM alloy is mainly enriched beneath the corrosion scales to form a NaNbO3 layer, which can inhibit the decomposition and penetration of the corrosive medium. Mo element reacts with the S element to form MoS2, which anchors the S element to a certain extent.

Towards low-temperature shrinkable synthetic fibers for internally self-prestressing concrete

Matrix cracking caused by temperature stress during the hydration heating/cooling phase of cement is one of the main causes of cracking of bulk concrete. In this study, low-cost, low-temperature shrinkable (LTS) fibers were developed, which can be stimulated to shrink by cement hydration heat in bulk concrete. The contraction stress of LTS fibers then transfers to the cement matrix through interfacial bonding, therefore, a uniform, three-dimensional self-prestressing executes on the bulk concrete matrix, to improve its cracking resistance. The modified polyester (MPET) with a low melting point was used to prepare LTS fibers with an activating temperature of about 60 °C, and their shrinkage behaviour can be activated in the temperature range caused by hydration heat in bulk concrete. The results confirmed that prestressing the cement matrix by activating LTS fibers’ axial shrinkage is an efficient approach for improving the mechanical properties of the cement matrix. This kind of fibers have demonstrated great potential in self-prestressing bulk concrete to improve its cracking resistance in engineering structures.

POSSIBLE REASONS FOR CRACKING OF COMPOSITE SCINTILLATORS AFTER IRRADIATION IN AN ELECTRON ACCELERATOR AT A HIGH DOSE RATE

Possible reasons for cracking of composite scintillators after irradiation in an electron accelerator at a high dose rate are proposed. The main causes of cracking of composite scintillators, such as ionizing radiation and thermal expansion, are considered. Ionizing radiation can affect the polymer part of the composite scintillator, while thermal expansion mainly affects the crystal grains. A possible mechanism of cracking the composite scintillator during high dose rate irradiation is described.

Case analysis of sudden cracking in buildings near a landfill pit in China

The causes of cracks in a six-storey building are studied. Based on analyses of crack morphology, settlement observation and geological survey, it is determined that the landfill pit and uneven settlement are the main cause of cracks. The 20 July Zhengzhou rainstorm in China caused a sharp rise in groundwater level, so that the uneven settlement of the office building near the landfill pit intensified, and then, wall cracking followed. The interaction between buildings and landfill pits under different water levels is studied by three-dimensional numerical simulation. As the water level rises, the horizontal displacement of soil in the landfill pit increases, and the lateral constraint of landfill soil to foundation soil decreases. The result is that the settlement of the column foundation near the landfill pits increases and the differential settlement intensifies. Based on the analysis results, the further development of differential settlement and cracks are evaluated and treatment suggestions are given.

Reducing Thermal Crack Risks Caused by Restraint in Young Concrete - A Case Study on Walls of Water Tanks

Abstract Concrete cracks in structures such as water tanks and nuclear power stations cause anxiety to owners, contractors and engineers. These cracks may significantly increase the structure’s permeability and thus increase leakage, reduce durability, and eventually lead to loss of structural functionality. Therefore it is important to minimize their occurrence and size. To identify effective ways of minimizing cracking in young concrete segments, a parametric study was conducted using the finite element method (FEM). Parameters considered include casting sequence, joint position, wall height, and cooling. The study examined continuous and jumped casting approaches to the casting of a cylindrical reinforced concrete tank for a sewage-treatment plant, with and without the application of the ‘kicker’ technique in which the lower part of the wall is cast with the slab. The main cause of cracking is thermal change and restraint imposed by adjacent older structures, and the FEM predictions agree well with experimental observations. Continuous casting is most effective at minimizing cracking because it creates only two contact edges between newly cast and existing structures producing the lowest level of restraint. The kicker technique is shown to be very effective for reducing restraint and consider rephasing.

Prevention of Sharp Fracture Caused by Large Size Inclusion in Cold Heading Steel

The cracking of cold heading steel during drawing was researched in this paper. The defects were detected by scanning electron microscope and EDS energy spectrum analysis, the direct causes of cracking were discussed and analysed, and the corresponding improvement measures were put forward. Results showed that the large size inclusions with high alumina content in steel were the main cause of cracks in cold heading steel due to their poor deformation ability. The plasticity of inclusions could be controlled through optimization of refining slag to reduce both the CaO and Al2O3 content.

Re-use in road construction of a Karal-type clay-rich soil from North Cameroon after a lime/cement mixed treatment using two different limes

Swelling clays characterized by their shrinkage and swelling phenomenon are known to be the main cause of cracking of paved roads. So, the road construction on swelling soils require knowledge and specific techniques, including the chemical stabilization with lime or cement addition. This technique favors the sustainable development by avoiding waste production especially soils. Indeed a maximum reuse of materials located in the building project area allows reaching the "zero loan, zero deposit" objective.In this paper, the efficiency of mixed treatment with lime and cement was tested on swelling karal-type soils collected at Pitoa in the North Cameroon region. The soil samples were characterized by applying microstructural and geotechnical tests (XRD, SEM, Particle Size Analysis, Atterberg Limits, Modified Proctor, CBR test, and Unconfined Compressive Strength (UCS)). The results showed that the mixed treatment of karal soils using a 2% of hydrated lime mixed with CaO or a 3% of low carbonated hydrated lime and 3, 5 or 7% of cement, reduced drastically the soil volume variation under water content change and improved the soil physico-mechanical properties. Finally, soils after treatment reached the minimum characteristics required for recovery in road construction.

Cracking Analysis of Shield Construction Segments Based on Boss Unevenness

As the most basic structural unit of shield tunnel, the segment will cause tunnel quality problems and ultimately affect the service life of the tunnel. Based on the actual project, this paper takes the segmental cracking phenomenon in the a large diameter shield tunnel as the research object, and analyzes the influence of the three protruding bosses of the cracked segment itself and the adjacent small ring on the segment cracking. Mechanical analysis, and finite element simulation of the force form of the tube under the unevenness of the boss. Through analysis, it is found that the three bosses of the cracked tube itself have little effect on the cracking of the segment, and the unevenness of the annular seam is the main cause of cracking of the segment. The analysis of this paper can take effective control measures to reduce the impact of cracks in shield tunnels on the normal construction of shield tunnels.

Correlation between the cracking pattern of historical structure and soil properties: the case of the church in Ko\u017cuch\xf3w

The paper presents a diagnosis of damage to heritage structures based on the case of the historic church in Kożuchów. The Church of the Purification of the Blessed Virgin Mary dates back to the thirteenth century and is an important sacral building in south-west Poland. Renovation and strengthening works have been conducted in recent years with the goal of structurally stabilizing the heritage building. The paper includes a short history of the building, as well as a review of contemporary diagnostic methods used in heritage buildings with emphasis on the methods used in the diagnosis of the discussed object. Correct diagnosis of the heritage building is the key to selecting an optimal design solution for supporting and strengthening the building structure. Pre-design analysis was based on a geotechnical assessment of ground conditions, identifying cracking patterns of walls and vaults, preparing a digital model using Heritage—Building Information Modeling technology and performing structural analysis. The main cause of cracking of the walls and vaults of the church is related to uneven subsidence and localized stability loss of the building’s foundations, resulting from differentiated soil and water conditions beneath the building. Based on the research and analysis, final conclusions and proposals for strengthening the structure were presented, including jet-grouting columns, tie rods and Fibre Reinforced Cementitious Matrix systems.

Fabrication of steel-Inconel functionally graded materials by laser melting deposition integrating with laser synchronous preheating

This paper examines a functionally graded material, fabricated by fiber laser melting deposition and laser synchronous preheating that incrementally grades from 316L to Inconel625. The cracking behaviour, microstructure evolution and microhardness were determined as the variation of position within the graded materials. The cracks in the non-preheated samples occurred at the deposited layers from 90% 316L/10% Inconel625 to 70% 316L/30% Inconel625, and no cracks were produced in the preheated samples. Precipitation of Nb- and Mo-enrich phases was experimentally identified to be the main cause of cracking. The microstructure of the preheated samples changed from the cellular dendrites to prominent columnar dendrites, and the precipitated phases were transformed from (γ + Laves) eutectic phase to (γ + NbC) phase with the increase of Inconel625. Microhardness presented an approximate linear distribution along the gradient direction. Work shows that microstructure and internal stress can be improved by laser synchronous preheating, thereby contributing to suppress cracking for the functionally graded material.

Computational Model of Rib-Reinforced Plate

The article provides a methodology for constructing a calculation model that allows determining displacements in rectangular plates, reinforced by transverse and longitudinal stiffeners. The calculation model is based on the solution of the Sophie Germain’s equation by the Bubnov-Galerkin methods and successive approximations with the representation of the reactive action of the ribs on the plate in the form of a concentrated forces system. Based on the described computational model, the results of the corresponding algorithm implementation and estimation of its convergence are presented. The need to develop a calculation model is explained by the fact that the main cause of cracks in the asphalt concrete coatings of metal bridges with an orthotropic plate is the wheel load. It reaches its maximum values for short periods of time and does not allow the stresses arising in asphalt concrete to relax. At this stage of the development of the calculation model a single-layer plate is considered supported by longitudinal stiffeners. However, since the bridge bed is a multilayer structure, further development of the design model of the multilayer slab reinforced by longitudinal and transverse stiffeners will be necessary. It will help in the joint work of metal flooring and pavement. In addition, it is necessary to take into account the temperature stress factor.

Early‐Age Cracking Analysis of a HVFA Concrete Structure Based on Thermo‐Hygro‐Mechanical Modeling Combined with XFEM

Due to the low hydration rate of high‐volume fly ash (HVFA) concrete at early age, the temperature gradient between the concrete core and surfaces could be effectively reduced. However, the low hydration rate results in a lack of hydration degree for early‐age HVFA concrete. Thus, during curing, compared to the strength of ordinary Portland cement (OPC), a subsequent lower one of HVFA concrete leads to a more sensitive response to inner stresses induced by thermal and moisture loads. Based on ABAQUS, in this paper, user subroutines in the temperature and moisture fields were developed, with regard to the hydration degree, for simulation of the temperature and moisture influences on concrete. Additionally, the Double Power Law (DPL) model was used to depict early‐age deformations of concrete in the mechanical field. Combined with the extended finite element method (XFEM), another subroutine for early‐age cracking analysis in the mechanical field was then developed. Together with aforementioned subroutines, a thermo‐hygro‐mechanical model is derived. For evaluation of early‐age cracking initiation and propagation of a pier composed of HVFA concrete, the model was implemented with XFEM. The obtained results show that (1) temperature and restraints are the main causes of cracking; (2) moisture loss affects surface cracks on structures at early age; and (3) although the temperature difference between the core and surfaces is not much obvious due to the reduced rate of heat dissipation from hydration, cracking of early‐age HVFA concrete is still likely to happen for its low early strength. Thus, timely curing is critical to prevent early cracking.

Failure Analysis of Clamp Crack on TA Equipment in Converter Station

Through macro inspection, chemical composition analysis, hardness test, tensile performance test, microstructure inspection and scanning electron microscope analysis, the causes of clamp crack on TA equipment in the 500kV AC filter field of the ± 800kV converter station were analyzed. The results show that improper selection of the manufacturing process of the equipment clamps will cause more micro-defects inside it, leading to poor mechanical properties and excessive internal stress, which are the main causes of cracking. Corresponding measures are put forward to improve the manufacturing quality of equipment clamps, which can avoid similar failures from happening again.

An experimental and simulation study on the wet-dry action to crack cause of pier concrete in a tidal river bridge

An inspection of a tidal river concrete bridge in the Mekong River Delta discovered a large number of map cracks in most piers within the tidal range. These map cracks distribute nearly vertically and horizontally with a distance of about 15 and 20 cm. Many of them have a width over 1 mm and a depth exceeding the thickness of the reinforcement protection concrete layer. Considering the location and the pattern of cracks, the most acceptable hypothesis on their cause was the strain gradient in concrete induced by the change of moisture content during tide rise and fall, i.e. by the effect of wet – dry action. To verify this hypothesis, experiments on the time dependent change of concrete moisture content and volume were conducted. Based on the results of these experiments, a computer simulation was performed. The simulated crack map and pattern agreed very well with the observed ones. With the obtained results, it is reasonable to conclude that strain gradient in pier concrete induced by the wet – dry action may one of main causes of cracks in such bridge piers

An experimental and simulation study on the wet-dry action to crack cause of pier concrete in a tidal river bridge

An inspection of a tidal river concrete bridge in the Mekong River Delta discovered a large number of map cracks in most piers within the tidal range. These map cracks distribute nearly vertically and horizontally with a distance of about 15 and 20 cm. Many of them have a width over 1 mm and a depth exceeding the thickness of the reinforcement protection concrete layer. Considering the location and the pattern of cracks, the most acceptable hypothesis on their cause was the strain gradient in concrete induced by the change of moisture content during tide rise and fall, i.e. by the effect of wet – dry action. To verify this hypothesis, experiments on the time dependent change of concrete moisture content and volume were conducted. Based on the results of these experiments, a computer simulation was performed. The simulated crack map and pattern agreed very well with the observed ones. With the obtained results, it is reasonable to conclude that strain gradient in pier concrete induced by the wet – dry action may one of main causes of cracks in such bridge piers

Failure analysis in a steam turbine stop valve of a thermal power plant

In the current study, the problem of cracking on the stop valve body in a steam turbine related to a thermal power plant is assessed. The problem usually occurs consecutively and almost in identical locations for the power plant's stop valves. To do so, the steam flow path from the developed area at the boiler output to the turbine input is divided into three separate computational domains and numerically simulated. The obtained results of the simulation have been successfully verified using the actual data of the thermal power plant. The common factors affecting the formation of crack consist of contacting shell with cooled steam, expansion and contraction movement of a turbine, insufficient insulation, stop valve spring force, internal pressure, and fluid flow momentum are assessed. The von Mises stresses caused by each factor have been determined. It has been attempted to identify the main cause of cracking by comparing the obtained stress concentration points with the real coordinates of the crack. The results of the simulation revealed that the expansion and contraction movements of the turbine were the main cause of fatigue and crack in the stop valve body. Given the key role of the stop valve in generating power of turbines, the findings of this simulation can help to design sensitive equipment more accurately.

About the main causes of cracks in-situ cement concrete pavements in joints

In recent years, the use of concrete (cement concrete) bases and coatings of roads and industrial sites has begun to revive in road construction. Employees of the Department of «roads» BGIT together with students for several years under surveillance for cracks in the monolithic cement concrete layers of coatings. The article presents the results of field surveys of cement concrete coatings of highways of the Bryansk region and industrial sites in order to identify the causes of cracks in the expansion joints. We have examined the sections of roads with cement concrete coating on some plates near the seam and found the formed transverse cracks. They were located mainly along the seam, the maximum opening width reached 5 mm. the Maximum distance from the seam 30 cm. Cracks along the seam were located unevenly and had a diverse shape. We have also calculated several variants of pavement with cement concrete coating on different bases. As a result of the survey were tested: the coefficient of compaction of the base material, the compressive strength of concrete in the coating plates, the technology of works on the device of coatings and expansion joints in them, as well as the control of the project materials submitted by the design organization, including the project of works. In conclusion, the following main causes of cracks in monolithic cement concrete coatings of roads and industrial sites can be identified: errors made in the calculation and design of the pavement structure; errors made in the calculation of the length of the plates, the appointment of reinforcing bars in the seams; errors in the construction of cement concrete pavement construction organization (if device transverse control joints and expansion joints in compression, buckling and longitudinal). The study material is generalized, road and construction organizations are given recommendations for the elimination of cracks, some measures to prevent their occurrence are indicated.