Thermal Crack Formation Research Articles

ОЦІНКА НАПРУЖЕНОГО СТАНУ РОБОЧОГО ШАРУ ВАЛКІВ КОЕРЦИТИМЕТРИЧНИМ ЕКСПРЕС-МЕТОДОМ

The purpose of the work is the problem of researching the influence of chemical composition and structure on the indicators of operational stability of centrifugal two-layer rolling rolls made of high chromium cast iron by the coercimetric express method. In order to ensure the achievement of this goal, the following tasks were set and solved: conducting an analysis of the causes of the destruction of the surface layer of rolls; building a mathematical model for studying the influence of the chemical composition and structure of cast iron on the level of hardness and coercive force; using the coercimetric method to analyze the quality of rolled rolls. Conclusions. Forecasting the behavior of the rolls during operation is inextricably linked to the analysis of the main factors on which the level of properties and operational stability of the working layer depend. The properties of high-chromium alloys are closely related to the type of phases formed and their ratio. The amount of residual austenite and martensite in the structure of the alloy has the greatest influence on the properties of the alloy (in the studied interval of changes in the proportion of phases). The operational layer of two-layer high-chromium rolls is characterized by a high level of residual stresses. Exceeding the permissible level of stresses or their unfavorable distribution can cause premature destruction during the operation of the roll. With the help of the coercimetric express method, it is possible to solve a fairly wide range of tasks of quality control of rolls, related to the determination of the level and distribution of residual stresses in the working layer of the rolls. This is because the intensity of the process of formation of thermal cracks under conditions of thermo-cyclic loading largely depends on the level of residual compressive stresses. Too high a level of stresses or their unfavorable distribution can cause premature destruction of the working layer of the roll, because the typical type of destruction due to an unbalanced distribution of stresses is cracking. A local increase in the level of coercive force can be the basis of the prediction of crack formation.

Prevention of Thermal Crack in Steel Slab Using Neural Networks Model to Predict Impact Absorption Energy

To prevent thermal cracks on steel slabs during the stacking, scarfing, and grinding processes after continuous casting, an optimized machine learning (ML) model to predict the impact absorption energy (EIA) of steel slabs is developed. A total of 1,421 experimental EIA data are collected from Charpy impact tests at slab temperatures 25 ≤ TS ≤ 400 °C, then ML models that considered 15 steel‐component variables and temperatures are developed. Four ML models are developed and their accuracies are compared. The optimized deep neural network algorithm predicts EIA most accurately with root mean squared error of 10.82 J and coefficient of determination (R2) of 0.992. Then the predicted EIA is used as the criterion to predict the occurrence of thermal cracks in slabs that are actually produced by a continuous casting process. At TS = 250 °C, cracking does not occur when the steel has predicted EIA > 175 J. The use of the developed model to predict EIA can prevent the formation of thermal cracks in slabs produced by continuous casting, and enable optimization of the cooling method and scarfing methods that precede the next process after continuous casting of slabs.

Ensuring thermal crack resistance of the bridge pier structure

Introduction. In conditions of negative temperatures or in the absence of the possibility to optimize the composition of concrete during the period of strength gain of concrete monolithic reinforced concrete structures, the formation of thermal cracks is possible, the repair of which requires significant additional costs that are not provided before the start of work.The aim of the work is to reduce the cost of repairing these manufacturing defects in the form of pre-operational temperature-shrinkage cracks.Materials and methods. Cost reduction was achieved by assigning several options for concrete care, calculating the thermally stressed state of structures for each of the assigned options and choosing the optimal option for concrete care, providing minimal tensile stresses leading to the appearance of temperature-shrinkage cracks in structures, with the least amount, width of opening and depth of crack propagation.The calculation was carried out using modern calculation programs according to the methodology used in the NIIZHB named after A.A. Gvozdev, JSC Research Center of Construction.The features of the construction conditions were: concreting at a negative ambient temperature, the use of a concrete mixture with a cement consumption of 440 kg/m3; concreting the structure with sections 2.25–5.05 m high with an interval of 12 days between divisions; ensuring heat exchange of the structure with the environment in the initial period of 2–3 days after concreting and regulating the cooling rate of the structure using thermal insulation materials.Results. According to the results of the work, it was possible to practically eliminate the appearance of the defects in question and the cost of repairing cracks.Conclusions. The cost of work and repairs turned out to be about 10 times less than the estimated cost of repairing cracks that usually occur in similar structures and construction conditions.

Influence of Mo micro-particles on crack formation, microstructure, and mechanical behaviour of laser powder bed fusion fabricated CuZrAl bulk metallic glass composites

ABSTRACT In this work, a new design principle, i.e. doping with refractory metal particles with a low diffusion rate to prevent the formation of cracks and to improve the mechanical properties of bulk metallic glass (BMG) composites, was put forward. It was proven that crack-free and dense Mo(p)/Cu47Zr47Al6 BMG composites with enhanced mechanical properties can be produced via LPBF. The dislocations generated in the Mo particles can release thermal stress, thereby inhibiting the formation of thermal-cracks. The fracture patterns of Mo particles show that they can delay rapid of crack expansion, thereby improving the inherent strength and toughness of the material.

THERMAL BEHAVIOR OF MASS CONCRETE STRUCTURES WITH DIFFERENT MATERIAL PROPERTIES

In mass concrete constructions, cement hydration produces much heat at an early age. Because of the different distribution of temperature in their parts, which leads to a significant temperature difference between the concrete block’s center and outer surface. When the temperature difference among structural parts surpasses the permissible temperature difference, thermal cracks can occur. Hence, this article presents the development of the temperature field in mass concrete structures with two different concrete mixes M1 and M2. M1 consists of 100% ordinary Portland cement, and M2 consists of 65% ordinary Portland cement and 35% fly ash. Furthermore, by using the finite element method, the effect of the initial temperature of concrete mixtures (M1, M2) on the temperature field in the structure has been investigated. The researched results can be used to reduce the risk of thermal crack formation in mass concrete.

Effects of rheological stratification and elasticity of lithosphere on subduction initiation

The breaking and bending of rigid and elastic lithosphere was probably essential for the initiation of plate subduction, although how this occurred is still poorly understood. Here we test effects of rheological stratification and elasticity of lithosphere on subduction initiation, which are possibly resulting from thermal cracking and seawater penetration into the lithosphere. In addition to the strong influence of water on rheological behavior, the material rigidity is also sensitive to the development of crack and fluid saturation. Numerical modeling indicates that water-weakening and a low-rigidity lithosphere are essential for the initiation of plate subduction, and such conditions are likely to have arisen on the early Earth due to extensive thermal contraction of the planet. Our results indicate that the formation of thermal cracks and penetration of seawater play an important role on subduction initiation, and are likely to have operated on planets other than Earth. However, if the ocean is disappeared, fluid penetration is likely to cease and plate tectonics would have stopped due to increasing the strength and rigidity of the lithosphere.

Numerical study on fire resistance behavior of EPS sandwich panels

A low-cost, energy-efficient earthquake-resistant building system appears to be an alternative solution for India's critically needed affordable housing. Insulating Building Sandwich Panel (IBSP) is a high-tech pre-fabricated building system, which consists of expanded polystyrene (EPS) insulation core confined by pressurized in-site concrete reinforced with galvanized welded steel mesh at both side tied with shear connecter. It has several advantages being energy efficient, low cost, greater seismic resistant features. However, performance of these IBSP against fire and resistance to bullet penetration are very important criteria to be ensured. Fires are becoming a growing hazard in built environment, particularly for mass housing. Thus, study of the fire resistance of these building systems is significantly important. The present paper aimed to numerical study of these IBSP under fire loading by developing a thermo-mechanical model of the sandwich panel adopting a transient heat-transfer analysis coupled with temperature-displacement analysis. Based on the heat transfer analysis, the cross-sectional temperature gradient of the panel is evaluated. Subsequently, the increased temperature causes in stress redistribution, which may result in deterioration of strength, stiffness and the overall service life. The proposed study also looks at the variations in stress on material fiber caused by different fire exposure. It is observed that the changing non-linear cross-sectional temperature gradient during a fire event can cause adequate distress, which may lead to the formation of thermal cracks in the panel. A three-point bending test is used to record the distribution of stresses on different temperature zones. The ABAQUS software is used for the Finite Element Analysis (FEA) of these panel. It seems that the study of the fire resistance of EPS sandwich panel is quite significant considering the safety and durability of this alternative sustainable building construction for affordable housing.

Prevention in Thermal Crack Formation in Chabazite (CHA) Zeolite Membrane by Developing Thin Top Zeolite and Thick Intermediate Layers.

Chabazite (CHA) zeolite membranes with an intermediate layer of various thicknesses were prepared using planetary-milled seeds with an average particle diameter of 300, 250, 200, 140, and 120 nm. The 120 nm seed sample also contained several smaller particles with a diameter of 20 nm. Such small seeds deeply penetrated into the pore channels of the α-alumina support during the vacuum-assisted infiltration process. During the secondary growth, the penetrated seeds formed a thick intermediate layer exiting between the zeolite layer and support. A decrease in seed size increased the penetration depth of seeds and the thickness of the intermediate layer, while the thickness of seed coating and zeolite layers was decreased. CHA zeolite membranes with a thin top zeoliate layer and a thick intermediate layer showed an excellent water/ethanol separation factor (>10,000) for 90 wt.% ethanol at 70 ℃ with a total flux of 1.5 kg m−2 h−1. There was no observation of thermal cracks/defects on the zeolite separation layer. The thick intermediate layer effectively suppressed the formation of thermal cracks during heating, since the tensile stress induced in the zeolite layer was well compensated by the compressive stress on the support. Therefore, it was successfully proven that controlling the microstructure of top surface and intermediate layers is an effective approach to improve the thermal stability of the CHA zeolite membrane.

Microstructure and Mechanical Properties of ZrN, ZrCN and ZrC Coatings Grown by Chemical Vapor Deposition

As the demands for wear-resistant coatings in the cutting industry are constantly rising, new materials that have the potential to exhibit enhanced coating properties are continuously explored. Chemical vapor deposited (CVD) Zr(N,C) is a promising alternative to the well-established and thoroughly investigated Ti(C,N) coating system, owing to its advantageous mechanical and thermal properties. Thus, within this work, CVD ZrN, ZrCN and ZrC coatings were deposited at 1000 °C, and subsequently their microstructure and mechanical properties were investigated in detail. Scanning electron microscopy, electron backscatter diffraction and X-ray diffraction experiments revealed that all coatings exhibited a columnar structure and a fiber texture, where ZrN and ZrCN displayed a <100> preferred orientation in growth direction and ZrC showed a <110> texture. Tensile residual stresses that arise due to a mismatch in the coefficient of thermal expansion between the cemented carbide substrate and the coating material decreased with the addition of C to the coatings. No stress relaxation through thermal crack formation was observed in the coatings. The highest hardness was determined for the ZrC coating with 28.1 ± 1.0 GPa and the lowest for the ZrN coating with 22.1 ± 0.9 GPa. Addition of C to the ZrN coating increased the hardness to 26.1 ± 1.6 GPa, which can be explained by a more covalent bonding character, as well as by solid solution strengthening. The ZrCN coating exhibited the highest Young’s modulus, followed by the ZrC and ZrN coatings, which can be attributed to differences in their electronic band structure.

Building a nomogram to predict maximum temperature in mass concrete at an early age

During the construction of massive concrete structures, the main factor that affects the structure is temperature. The resulting temperature is the result of hydration of the cement and some other factors, which leads to the formation of thermal cracks at an early age. So, the prediction of temperature history in massive concrete structures has been a very important problem. In this study, with the help of numerical methods, a temperature nomogram was built to quickly determine the maximum temperature in concrete structures with different parameters such as size, cement content, and the initial temperature of the concrete mixture. The obtained temperature nomogram has been compared with the results of the finite element method and the model experiment gives reliable results. It can be used to predict maximum temperature in mass concrete structures to prevent the formation of thermal cracks.

All-MBE grown InAs/GaAs quantum dot lasers with thin Ge buffer layer on Si substrates

A high-performance III–V quantum-dot (QD) laser monolithically grown on Si is one of the most promising candidates for commercially viable Si-based lasers. Great efforts have been made to overcome the challenges due to the heteroepitaxial growth, including threading dislocations and anti-phase boundaries, by growing a more than 2 µm thick III–V buffer layer. However, this relatively thick III–V buffer layer causes the formation of thermal cracks in III–V epi-layers, and hence a low yield of Si-based optoelectronic devices. In this paper, we demonstrate a usage of thin Ge buffer layer to replace the initial part of GaAs buffer layer on Si to reduce the overall thickness of the structure, while maintaining a low density of defects in III–V layers and hence the performance of the InAs/GaAs QD laser. A very high operating temperature of 130 °C has been demonstrated for an InAs/GaAs QD laser by this approach.

Thermal crack formation in TiCN/α-Al2O3 bilayer coatings grown by thermal CVD on WC-Co substrates with varied Co content

Within this work, the thermal stress build-up of chemically vapor deposited TiCN/α-Al2O3 bilayer coatings was controlled by tuning the coefficient of thermal expansion (CTE) of the substrate material. This was implemented through a Co content variation from 6 to 15 wt.% in WC-Co substrates, which exhibit higher CTEs with increasing Co contents and thereby approach the CTE values of TiCN and α-Al2O3. High temperature X-ray diffraction was employed to determine thermal expansion of an α-Al2O3 powder. Crystallographic texture of the α-Al2O3 coating layer was evaluated by electron backscatter diffraction and taken into consideration in order to assign the appropriate in-plane CTE. This consideration indicated a lower CTE mismatch of α-Al2O3 with WC-Co, compared to TiCN with WC-Co. X-ray diffraction was further utilized for the determination of residual stress in TiCN and α-Al2O3, demonstrating a decrease in both layers for Co contents below 12.5 wt.%. Decreasing stress signaled the formation of thermal crack networks confirmed by scanning electron microscopy surface images. Lower residual stresses were determined in TiCN compared to α-Al2O3 layers of bilayer coatings, contradicting finite element simulations of thermo-elastic stress, that were carried out to illustrate the stress relaxation effects caused by thermal cracks. Monolayer TiCN coatings were annealed at 1000 °C, to replicate stress relaxation taking place during α-Al2O3 deposition, exhibiting a similar residual stress state to TiCN base layers of bilayer coatings. Thermal crack formation was found to be the dominating stress relaxation mechanism in α-Al2O3, while TiCN undergoes further relaxation through secondary mechanisms.

The problem of temperature cracking in concrete gravity dams

Introduction. The concreting of solid structures, such as concrete dams, bridge constructions, foundations of buildings and structures, is accompanied by exothermic heating, caused by cement hydration. Heat, emitted by mass concrete blocks, slowly leaves constructions. A substantial temperature difference frequently arises between the solid concrete centre and its surface. If this temperature difference reaches a critical value, thermal cracking occurs, which destroys structural continuity. Temperature problems and those associated with thermal stress state should be resolved to pre-assess and prevent potential cracking. This problem has enjoyed the attention of specialists, and it has been the subject of numerous research projects. Materials and methods. The overview is based on the information about implemented research projects focused on the thermal cracking of mass concrete dams and its prevention. Computer modeling techniques were applied to develop a mathematical model capable of projecting and assessing the potential cracking of mass concrete. Results. The co-authors have compiled an overview of advanced approaches to the assessment of potential thermal crack formation, contemporary problem-solving methods and selected research findings obtained using the finite element method. The co-authors offer a thermal behaviour/thermal stress state projection methodology for solid concrete, as well as a thermal crack formation assessment methodology. Conclusions. The thermal cracking problem has not been solved yet. The proposed methodology and a projection-oriented numerical model can be used as a reference by civil engineers in the process of designing and constructing concrete gravity dams. It may help to reduce cracking probability caused by heat evolution in cement.

Stress relaxation through thermal crack formation in CVD TiCN coatings grown on WC-Co with different Co contents

TiCN coatings were grown by chemical vapor deposition (CVD) on WC-Co substrates with different Co contents, in order to control thermal stress. The driving force for the development of thermal stress is attributed to the difference between room and deposition temperature (ΔT ≈ −780 °C), and the mismatch of the coefficient of thermal expansion (CTE) between substrate and coating. Co contents of 6, 7.5, 10, 12.5, and 15 wt% were utilized to adjust the CTE of the substrate, and therefore tune the stress in TiCN coatings. Dilatometry of the substrates and high temperature X-ray diffraction of a powdered TiCN coating indicate a decreasing CTE-mismatch for increasing substrate Co contents. In consequence, residual stress in TiCN determined by X-ray diffraction increases up to 662 ± 8 MPa with decreasing Co contents down to 10 wt%. For Co contents below 10 wt%, the residual stress decreases. The formation of thermal crack networks in TiCN, analyzed by scanning electron microscopy, coincides with 10 wt% Co. Stress relaxation in TiCN coatings through the formation of thermal cracks becomes evident. A finite element simulation utilized for the calculation of residual stress distributions reveals shielding effects, which occur with the introduction of thermal cracks. Discrepancies between experimental and simulated thermo-elastic stresses imply the presence of secondary relaxation sources. High temperature residual stresses in TiCN, determined up to 1000 °C (i.e. above deposition temperature), suggest additional thermal crack formation for substrate Co contents of 6 wt%.

The Impact of Fly Ash after SNCR Treated with Tannin-Based Products on AAC Production

This paper is focused on the effect of treatment of fly ash after selective non-catalytic reduction (SNCR) with tannin on autoclaved aerated concrete (AAC) production in order to reduce or stop ammonia leakage from the fresh mixture due to its alkalinity. A pure form of tannin and a tannin-based product „Farmatan“ were used as a treatment in dosage ranging from 0,5 g – 3 g of agent per 1 kg of fly ash. Efficient dosage was determined at 2 wt.% of fly ash by the speed of an indicator change due to gaseous ammonia diluted in water. The rheological properties of fresh mixtures were observed by consistency test in Viskomat showing that Farmatan causes delay of hydration. The results of bulk density and compressive strength testing revealed that Farmatan causes an increase of bulk density and at higher amount decreases the compressive strength because of thermal crack formation due to combined effect of delayed hydration and thixotropy. Using x-ray diffraction (XRD) analysis there were no differences in phase composition observed.

Thermal properties of Central Aare granite for temperatures up to 500°C: Irreversible changes due to thermal crack formation

AbstractThermal diffusivity, heat capacity, and thermal conductivity of Central Aare granite are reported in the temperature range from 25°C to 500°C. Each rock sample underwent three consecutive heating and cooling cycles. Significant irreversible changes in the properties due to thermal crack formation could be observed. After the first thermal cycle, both thermal diffusivity and conductivity dropped to about 75% of the initial value at room temperature, whereas the heat capacity did not show an irreversible decay. For subsequent thermal cycles, no further permanent changes of the investigated properties could be observed. From the conducted measurements, accurate correlations are derived, offering a platform for precise high‐temperature experiments and other research on Central Aare granite and similar granitic rocks. The report shows that the assumption of constant thermal properties leads to significant inaccuracies at elevated temperatures, especially if thermal cycles are present.

마모와 제동에 의한 화차륜의 잔류응력 변화

During the past few years, several incidents of freight car wheel failure during operation have occurred due to fatigue crack and overheating from braking. Tensile residual stress on the wheel tread creates an environment conducive to the formation of thermal cracks that may threaten the safety of train operations. It is important to investigate the residual stress on wheels in order to prevent derailment. In the present paper, the residual stress on wheels is measured using the x-ray diffraction system and the residual stress is analyzed using FEM. The result shows that the residual stress on the wheel rim is lower than that on the wheel tread center and the stress on over-braked wheels changes from compression residual stress to tensile residual stress.

Detection and Characterization of Early-Age Thermal Cracks in High-Performance Concrete

Streicker Bridge is a new pedestrian bridge built on the Princeton University campus. Structural health monitoring (SHM) is applied with the aim of transforming the bridge into an on-site laboratory for various research and educational purposes. Two fiber optic sensing technologies are permanently deployed: discrete long-gauge sensing technology based on fiber Bragg gratings (FBG) and truly distributed sensing technology based on Brillouin optical time domain analysis (BOTDA). The sensors were embedded in the concrete during construction. This paper describes the real-time detection and characterization of early-age thermal cracks in the high-performance concrete deck of Streicker Bridge. The deployed monitoring strategy and the monitoring systems, which successfully detected cracking, are described. The observed crack propagation trajectories are presented. Details of a simple finite element model (FEM) of the bridge are given, and the analysis procedure used to demonstrate the formation of thermal cracks using this model is outlined.

Densification, Microstructure, and Wear Property of In Situ Titanium Nitride-Reinforced Titanium Silicide Matrix Composites Prepared by a Novel Selective Laser Melting Process

This work presents the densification behavior, microstructural features, microhardness, and wear property of in situ TiN/Ti5Si3 composite parts prepared by a novel Selective Laser Melting (SLM) process. The occurrence of balling phenomenon at a low laser energy density combined with a high scan speed and the formation of thermal cracks at an excessive laser energy input generally decreased densification rate. The in situ-formed TiN reinforcing phase experienced a successive morphological change: an irregular polyangular shape—a refined near-round shape—a coarsened dendritic shape, as the applied laser energy density increased. The variations in liquid-solid wettability and intensity of Marangoni convection within laser molten pool accounted for the different growth mechanisms of TiN reinforcement. The TiN/Ti5Si3 composite parts prepared under the optimal SLM conditions had a near-full 97.7 pct theoretical density and a uniform microhardness distribution with a significantly increased average value of 1358.0HV0.3. The dry sliding wear tests revealed that a considerably low friction coefficient of 0.19 without any apparent fluctuation and a reduced wear rate of 6.84 × 10−5mm3/Nm were achieved. The enhanced wear resistance was attributed to the formation of adherent strain-hardened tribolayer covered on the worn surface.

Selective Laser Melting of in-situ TiC/Ti 5Si 3 composites with novel reinforcement architecture and elevated performance

A novel Selective Laser Melting (SLM) process was applied to prepare bulk-form TiC/Ti 5Si 3 in-situ composites starting from Ti/SiC powder system. The influence of the applied laser energy density on densification, microstructure, and mechanical performance of SLM-processed composite parts was studied. It showed that the uniformly dispersed TiC reinforcing phase having a unique network distribution and a submicron-scale dendritic morphology was formed as a laser energy density of 0.4 kJ/m was properly settled. The 96.9% dense SLM-processed TiC/Ti 5Si 3 composites had a high microhardness of 980.3HV 0.2, showing more than a 3-fold increase upon that of the unreinforced Ti part. The dry sliding wear tests revealed that the TiC/Ti 5Si 3 composites possessed a considerably low friction coefficient of 0.2 and a reduced wear rate of 1.42 × 10 − 4 mm 3/Nm. The scanning electron microscope (SEM) characterization of the worn surface morphology indicated that the high wear resistance was due to the formation of adherent and strain-hardened tribolayer. The densification rate, microhardness, and wear performance generally decreased at a higher laser energy density of 0.8 kJ/m, due to the formation of thermal cracks and the significant coarsening of TiC dendritic reinforcing phase.