Interfacial crack behavior in the stationary temperature field conditions

The brittle coatings, made of different materials, when subjected to elevated temperatures and in the heat exchange conditions, are susceptible to delamination. Those coatings, as well as thin films, can be used for various therm insulating deposits, e.g. in turbines of thermal power plants. Due to environmental temperature change, in layers made of materials having different thermal expansion coefficients, appear thermal stresses. In this paper driving forces causing delamination of one layer from the other are analyzed i.e. the interfacial fracture in the two-layered, bi-material sample. This analysis was limited to considering the sample behavior when exposed to the stationaiy temperature field. The energy release rate G, which is the driving force for this interfacial fracture, is changing with temperature and that variation is increasing with increase of the temperature difference between the environment and the sample. Analysis of this relation, between the G variation and temperature difference, can be used to predict the maximal temperature difference, which the two-layered sample can be subjected to, without appearance of delamination between layers

Fundamental Problems of Modeling the Fracture Processes in Concrete II: Size Effect and Selection of the Solution Approach

AbstractTheory of concrete fracture, despite all the efforts of numerous researchers, still did not provide the clear answer to the problem of modeling the fracture processes of concrete. Three well known theories are at hand: fracture mechanics, plasticity theory and mechanics of continuous damages. The fundamental assumptions, those theories are based on, do not completely correspond to the nature of concrete. They all are confronted with numerous problems, out of which the four are fundamental: damages micromechanics, damages localization, size effects and the dilemma when to apply the phenomenological and when the micromechanical approach to considering this problem. In this paper are considered the last two of those problems: size effect (scaling laws) and decision making what would be the best way in solving problems of modeling the fracture processes in concrete and concrete structures

Interfacial crack behavior in the stationary temperature field conditions

The brittle coatings, made of different materials, when subjected to elevated temperatures and in the heat exchange conditions, are susceptible to delamination. Those coatings, as well as thin films, can be used for various therm insulating deposits, e.g. in turbines of thermal power plants. Due to environmental temperature change, in layers made of materials having different thermal expansion coefficients, appear thermal stresses. In this paper driving forces causing delamination of one layer from the other are analyzed i.e. the interfacial fracture in the two-layered, bi-material sample. This analysis was limited to considering the sample behavior when exposed to the stationaiy temperature field. The energy release rate G, which is the driving force for this interfacial fracture, is changing with temperature and that variation is increasing with increase of the temperature difference between the environment and the sample. Analysis of this relation, between the G variation and temperature difference, can be used to predict the maximal temperature difference, which the two-layered sample can be subjected to, without appearance of delamination between layers

Influence of temperature on behavior of the interfacial crack between the two layers

In this paper is considered a problem of the semi-infinite crack at the interface between the two elastic isotropic layers in conditions of the environmental temperature change. The energy release rate needed for the crack growth along the interface was determined, for the case when the two-layered sample is cooled from the temperature of the layers joining down to the room temperature. It was noticed that the energy release rate increases with the temperature difference increase. In the paper is also presented the distribution of stresses in layers as a function of the temperature and the layers' thickness variations. Analysis is limited to the case when the bimaterial sample is exposed to uniform temperature

Estimate of a power distributor life span based on the fracture mechanics criteria

Dimensioning of pressure vessels can be done according to various criteria, the basic one being the safety against the fracture by splashing. In this paper two methods for this dimensioning procedure are presented. The first one is based on application of the classical mechanics while the second one uses the fracture mechanics principles. Both methods were applied for checking the exploitation properties and capability of a vapor distributor in a power plant. Based on the in-situ measured damages and calculations by both methods, it was concluded that the vapor distributor could be further used as a part of the boiler installation in the local power plant. The second criterion, based on the application of the fracture mechanics principles, provided more reliable results than the first one, based on the classical mechanics

Estimate of a power distributor life span based on the fracture mechanics criteria

Dimensioning of pressure vessels can be done according to various criteria, the basic one being the safety against the fracture by splashing. In this paper two methods for this dimensioning procedure are presented. The first one is based on application of the classical mechanics while the second one uses the fracture mechanics principles. Both methods were applied for checking the exploitation properties and capability of a vapor distributor in a power plant. Based on the in-situ measured damages and calculations by both methods, it was concluded that the vapor distributor could be further used as a part of the boiler installation in the local power plant. The second criterion, based on the application of the fracture mechanics principles, provided more reliable results than the first one, based on the classical mechanics

Some Aspects of the Three-Dimensional Interface Cracks Analysis

Many problems of interfacial cracks are three dimensional in nature. Three-dimensional cracks at an interface of the two materials are analysed in this paper. For a crack at an interface, the stress intensity factors, load phase angle and energy release rate depend on elastic characteristics of two bonded materials and on geometry and the load conditions of a bimaterial sample. Influence of Dundurs\u27 parameters on stress intensity factors, load phase angle and energy release rate for different bi-material combinations and for the quarter-circular corner crack are discussed in this paper. Results show that elastic properties of materials constituting the interface have significant influence on behaviour of the 3D interface crack. Mode I stress intensity factor KI increases when the crack front approaches the free surface, while KII remains almost constant having the highest values between 10° and 80°, what results in high values of the load phase angle. The KIII stress intensity factor is zero in the symmetry plane, while its value increases as the crack front approaches free surfaces. The energy release rate diagrams show that the crack of a quarter circular front propagates faster closer to free surfaces than in the middle what means that the crack front would have the tendency of straightening

Blood Concentrations of Thyroid Hormones and Lipids and Content of Lipids in the Liver of Dairy Cows in Transitional Period

The aim of the present investigation was to determine the correlation in the blood concentrations of thyroid hormones, lipids and glucose as well as the content of lipids in the liver of dairy cows in the transitional period. The animals (n = 40) were divided into four groups: the first group included late pregnant cows (n = 10) from the 10th to 4th day before calving; the second group included late pregnant cows (n = 10) from the 4th to 1st day before calving; the third group included clinically puerperal healthy cows (n = 10), whereas the fourth group included puerperal cows with clinical symptoms of ketosis (n = 10). Samples of liver and blood tissues were taken from all cows. Pathohistological examination of liver samples showed statistically significantly higher (p p p p p p p p p p p < 0.05) in the group of ketotic cows. Our investigations suggested that a hypothyroidal status was established in ketotic cows and that the blood concentrations of free fatty acids, triacylglycerols, total cholesterol and glucose served as major biochemical indicators in determining liver steatosis in the dairy cows in the transitional period

Changes in Blood Values of Glucose, Insulin and Inorganic Phosphorus in Healthy and Ketotic Dairy Cows after Intravenous Infusion of Propionate Solution

The aim of the present study was to determine the degree of blood glucose utilization by peripheral tissue on the basis of changes in blood concentrations of glucose, insulin and inorganic phosphorus in healthy (n = 10) and ketotic cows (n = 10) after intravenous infusion of propionate solution. Blood samples were taken in both groups of examined cows at the following time intervals: just before (time 0) and 8, 30, 60, 120, 180, 240 and 480 min after the intravenous infusion of 1.84 mol l-1 solution of propionate in the amount of 1 ml kg-1 of body weight. Glucose and insulin blood serum values in both groups of cows increased significantly within 120 min of the experiment (p p p p < 0.05) in blood value of inorganic phosphorus in ketotic cows in comparison with healthy ones. This is linked with the active entry of glucose into glucolytic pathway of peripheral tissues. It can thus be concluded that there is a higher degree of blood glucose utilization by peripheral tissues in ketotic cows

Revidirani kvantitativni indeks osjetljivosti na inzulin: povezanost sa metaboličkim statusom krava tijekom rane laktacije

The revised quantitative insulin sensitivity check index (RQUICKI) is the most commonly used indicator of insulin resistance in dairy cows. The aim of this study was to examine the characteristics of metabolic status in cows with different RQUICKI index values during early lactation. The experiment included 40 Holstein-Friesian cows in the first week of lactation. The cows were classified into four groups according to quartile (Q 1 to 4) values of RQUICKI indexes: Q1 = 0.35-0.41 (most insulin resistant), Q2 = 0.42-0.52, Q3 = 0.53-0.67, Q4 = 0.68-0.77 (most insulin sensitive). Metabolic parameters were significantly different in early lactation cows, classified according to the values of the RQUICKI index. The cows that were the most resistant to insulin (Q1) had higher levels of non-esterified fatty acid (NEFA), cortisol, somatotropic hormone (STH), beta-hydroxybutyrate (BHB), total bilirubin, aspartate aminotransferase (AST), malondialdehyde (MDA) and body condition score (BCS) in comparison to the cows that were the least resistant to insulin (Q4). The cows also had lower levels of insulin-like growth factor I (IGF-I), triiodothyronine (T3), thyroxine (T4), albumin, cholesterol, triglycerides, Ca and P as well as a tendency towards lower insulin and glucose concentrations. Metabolic parameters were strongly regressed by RQUICKI in the most insulin resistant cows (Q1) in relation to the cows in the other groups, Q2-4. The cows with a higher number of metabolic abnormalities in their metabolic profiles had lower RQUICKI values: 0.56 ± 0.045 (no abnormalities); 0.52 ± 0.041 (1 abnormality); 0.47 ± 0.042 (2 abnormalities) and 0.4 ± 0.043 (≥3 abnormalities). We concluded that the RQUICKI index could be applied in order to accurately identify metabolic status in cows during early lactation. However, the kinetics of insulin sensitivity should be further studied using more animals per group, as well as in other breeds of cowsRevidirani kvantitativni indeks provjeravanja osjetljivosti na inzulin (RQUICKI) najčešće se koristi kao pokazatelj otpornosti na inzulin u krava. Cilj ovoga istraživanja bio je ustvrditi karakteristike metaboličkog statusa u krava s različitom vrijednosti RQUICKI indeksa u ranoj laktaciji. Pokus je uključivao 40 krava Holstein-Friesian pasmine. Krave su razvrstane u četiri skupine prema kvartilima (Q 1 do 4) vrijednosti RQUICKI indeksa: Q1 = 0,35-0,41 (najrezistentnije na inzulin), Q2 = 0,42-0,52, Q3 = 0,53-0,67, Q4 = 0,68-0,77 (najosjetljivije na inzulin). Metabolički parametri bili su znakovito različiti u krava u ranoj laktaciji razvrstanima prema vrijednostima RQUICKI indeksa. Krave koje su bile najrezistentnije na inzulin (Q1) imale su i veće razine NEFA, kortizola, STH, BHB, ukupnog bilirubina, AST, MDA i bolju tjelesnu kondiciju u usporedbi s kravama koje su bile najmanje osjetljive na inzulin (Q4). Također, te su krave imale niže koncentracije IGF-I, T3, T4, albumina, kolesterola, triglicerida, Ca i P te sklonost smanjenju koncentracije inzulina i glukoze. U skupini krava koje su najrezistentnije na inzulin (Q1) regresijska analiza je pokazala jaču povezanost između metaboličkih parametara i RQUICKI nego što je to bio slučaj u ostalim skupinama krava (Q2, Q3 i Q4). Krave s većim brojem metaboličkih abnormalnosti profila imale su nižu vrijednost RQUICKI: 0,56 ± 0,045 (bez abnormalnosti); 0,52 ± 0,041 (jedna abnormalnost); 0.47 ± 0.042 (dvije abnormalnosti) i 0.4 ± 0.043 (≥3 abnormalnosti). Zaključujemo da bi se RQUICKI indeks mogao primijeniti za točnije identificiranje metaboličkog statusa krava tijekom rane laktacije. Međutim, kinetika osjetljivosti na inzulin trebala bi se dodatno istražiti na većem broju životinja kao i u krava različitih pasmina