Long-term Loading Conditions Research Articles

Creep crack growth testing of plastics—I. test configurations and test system design

To perform fracture mechanical tests under long-term static loading conditions at varying temperatures and in different environments, several test systems and a test procedure were developed and implemented. The test systems consist of a loading unit for a wide range of load levels and specimen geometries, an environmental chamber, the equipment for temperature control and devices for the measurement of crack lengths and crack opening displacements. A unique feature of these test systems is that crack growth and creep behaviour under static loads for a wide range of materials can be simultaneously characterised in one single test. While Part I of this paper describes the test system design and various test configurations, Part II is concerned with the data acquisition and reduction scheme and also provides experimental results of creep crack growth tests with plastics.

Corrosion-mechanical characteristics of the materials of nonmagnetic shroud rings of turbogenerators. I. 8Mn−8Ni−4Cr and 18Mn−4Cr steels

On the basis of the comprehensive analysis of the corrosion-mechanical characteristics of austenitic nonmagnetic 8Mn−8Ni−4Cr steels, we show that this type of steel is susceptible to corrosion cracking even in low-aggressive aqueous solutions, including humid working environments at a temperature of 80°C. The application of aluminum coatings and mechanical surface treatment increase the time to failure of specimens. Only deep drying of hydrogen used to cool the shrouds eliminates the causes of corrosion cracking of these types of steel. The procedure of cold tensile hardening cannot be applied to 40Kh4G18 steel because it is characterized by high susceptibility to the formation of α-martensite in the process of plastic deformation due to the insufficient stability of austenite. The optimal vanadium contents of steel guaranteeing its high strength and plastic characteristics lie within the range 1.3–1.5%. In the absence of water, 18Mn−4Cr steel does not exhibit any noticeable susceptibility to corrosion cracking. For this reason, the main condition that should be realized to prevent corrosion cracking and corrosion fatigue of shroud rings is the maintenance of the dry state of working environments in the processes of manufacturing, storage, and transportation of these rings, in service, and during downtimes. The presence of Cl− ions, hydrogen sulfide, halides, and nitrates in the working environments significantly enhances the fatigue crack growth rates in the entire investigated range. These ions also promote crack growth under the conditions of long-term static loading.

Dynamic aging of alloyed spring steels of the pearlitic class

A special place among the various methods for increasing the stability and endurance of springs that include heat trea, ment and surface impregnation belongs to the method of dynamic aging (DA), which improves the set of mechanical properties. It increases substantially the resistance to small plastic deformations under conditions of short-term and long-term loading, i.e., the properties that aredecisive for the quality of spring steels and elastic members themselves. The aim of the present work consists in investigating the properties of spring steels after isothermal bainite quenching and dynamic aging by various regimes in order to determine the optimum one.

Geotechnical characteristics and slope stability on the Ebro margin, western Mediterranean

Sedimentological and geotechnical analyses of core samples from the Ebro continental slope define two distinct areas on the basis of sediment type, physical properties and geotechnical behavior. The first area is the upper slope area (water depths of 200–500 m), which consists of upper Pleistocene prodeltaic silty clay with a low water content (34% dry weight average), low plasticity, and high overconsolidation near the seafloor. The second area, the middle and lower slope (water depths greater than 500 m), contains clay- and silt-size hemipelagic deposits with a high water content (90% average), high plasticity, and a low to moderate degree of overconsolidation near the sediment surface. Results from geotechnical tests show that the upper slope has a relatively high degree of stability under relatively rapid (undrained) static loading conditions, compared with the middle and lower slopes, which have a higher degree of stability under long-term (drained) static loading conditions. Under cyclic loading, which occurs during earthquakes, the upper slope has a higher degree of stability than the middle and lower slopes. For the surface of the seafloor, calculated critical earthquake accelerations that can trigger slope failures range from 0.73 g on the upper slope to 0.23 g on the lower slope. Sediment buried well below the seafloor may have a critical acceleration as low as 0.09 g on the upper slope and 0.17 g on the lower slope. Seismically induced instability of most of the Ebro slope seems unlikely given that an earthquake shaking of at least intensity VI would be needed, and such strong intensities have never been recorded in the last 70 years. Other cyclic loading events, such as storms or internal waves, do not appear to be direct causes of instability at present. Infrequent, particularly strong earthquakes could cause landslides on the Ebro margin slope. The Columbretes slide on the southwestern Ebro margin may have been caused by intense earthquake shaking associated with volcanic emplacement of the Columbretes Islands. Localized sediment slides on steep canyon and levee slopes could have been caused by less intense shaking. In general, the slope is stable under present environmental loading conditions and is fundamentally constructional. Nevertheless, rapid progradation caused by high sedimentation rates and other processes acting during low sea-level periods, such as more intense wave loading near the shelfbreak, may have caused major instability in the past.

Role of Particle-Matrix Interface in the Deformation and Fracture Behaviour of Filled Epoxy Resins

As a result of their superior mechanical, chemical, and electrical properties particulate-filled epoxies are finding widespread use in the fabrication of a large number of engineering components, one important example being as electrical insulators for the power industry. Since these materials are mainly subjected to continuous mechanical loads (in addition to a continuous dielectrical stress), there is an apparent need first to understand and then to predict the processes of deformation and fracture during long-term loading conditions. A few years ago, the respective material science knowledge was very limited, and design was mainly based on experience and purely empirical work. During recent years, however, a rather comprehensive understanding of the short and long-term deformation and fracture behaviour of these brittle, highly cross-linked polymers, especially of silica-filled anhydride-cured epoxies, has been accumulated. This holds particularly for the understanding of their creep behaviour, i.e. the deformation-induced volume damage and the mechanisms which are responsible for creep failure. In this article, a survey will given on the applications, and the deformation and fracture behaviour of particulate-epoxies. In particular, the role of particle-matrix interface in connexion with deformation, fracture, and creep behaviour and the related deformation-induced volume damage is discussed. A complete understanding of the role of the particle-matrix interface is still lacking and, therefore, a coherent description is not possible, and the true practical value of a filler surface treatment is not yet known. However, the following can be said: local as well as global debonding between matrix and filler should be prevented; local debonding tends to act as a critical flaw (in a fracture mechanics sense) and leads to brittle failure. Global debonding is equivalent to creep damage and leads to premature failure of the respective component.

Engineering method of predicting creep crack growth in materials

1. We worked out and substantiated an engineering method of temperature-time prediction of the kinetic characteristics of subcritical creep crack growth from the results of crack resistance tests of limited duration by using the criteria of linear fracture mechanics. 2. The suggested method of prediction takes integrally into account the change of the predominant mechanisms of failure in the metal of structural elements of thermal power generating equipment; this makes it possible to solve problems of extrapolation with better physical substantiation. 3. Our calculations make it possible to work out scientifically substantiated recommendations for determining the periods of maintenance and the remaining life of critical structural elements of large power units of thermal power stations operating under conditions of creep. 4. It is necessary to work in the field of experimental investigations of the temperature-time regularities of creep crack nucleation in the zone of a stress raiser with the object of working out scientifically substantiated methods of distant interpolation of crack nucleation; this will make it possible to predict the full life of objects operating under conditions of long-term thermal and mechanical loading.

Evaluation of the performance of thick-walled cylindrical shells made of S�T-1 glass

1. The strength of TSM506, SET-1, and MKR-1 lighting engineering glass varies in range 1225–1410 MPa, i.e., 56–64% of the ultimate strength of 13v technical glass. 2. The aqueous and oil media reduce on the whole the short-term strength of lighting engineering glass. The strength of the effect of the median depends on the type of the stress state in such a manner than the largest (up to 30%) reduction of strength is detected under the effect of tensile stresses. It is evident that this is associated with the general relationships governing the growth of surface cracks in brittle material in compression and tension in surface-active media. 3. In the glass thick-walled cylindrical shells in the examined method of connection with the rigid plug in the conditions of longitudinal compression and external pressure, the lower level of the fracture stresses of compression in the contact zone decreases by a factor of three-four in comparison with the strength of the specimens σcc determined in the tests on the flat support. This is associated with the operation of the scale-technological effect and with the unfavorable variation of the contact conditions and requires the introduction of high (five–six) strength satisfy factors to ensure the reliability of components. 4. Taking into account the real technological initial deviations of the form and dimensions of the support sections of the thick-walled cylindrical glass elements, we designed and tested in short-term and long-term (up to 100 days) effects of the external pressure and experimental batch of transparent shells. The experiments confirm that the experimental-theoretical approach is suitable for designing these objects. The selection of the permissible stresses and working loads with special reference to the devices working in the conditions of long-term static loading was justified.

Renal Tubular Handling of Potassium in Patients With Medullary Sponge Kidney

Renal tubular function, with special emphasis on potassium excretion, was studied in three patients with medullary sponge kidney (MSK). Urinary acidification and concentration abilities were impaired, while glomerular filtration rates remained normal. After short-term intravenous (IV) potassium chloride loading, maximal excretion of potassium was lower in the patients with MSK than in the normal controls. The kaliuretic response to IV sodium sulfate and acetazolamide infusion was maintained. The capacity of the kidney to sustain potassium balance under conditions of either long-term potassium loading or depletion was preserved. We have concluded that the medullary and papillary segments of the human collecting duct, have an important role in the handling of short-term potassium loading. The integrity of these segments, however, is not essential for adaptation to prolonged potassium surfeit or dietary depletion.

Importance of tie molecules in preventing polyethylene fracture under long-term loading conditions

Brittle-type slow crack fracture is the dominant mode of failure in polyethylene pipe used for natural gas distribution. Constant tensile load testing of polyethylene pipe sections indicates a link between this mode of failure and environmental stress cracking. Interlamellar fracture can occur in both the presence and absence of an accelerating environment. The fibre deformation process is reviewed, describing the role of amorphous entangled tie molecules in preventing slow crack-type fracture. Relative resistance to slow crack brittle failure is explained in terms of this model for various materials tested.

Character of the variation in certain properties of 1Cr18Ni9Ti steel under long-term loading conditions at elevated temperatures

The effect of preliminary loading of 1Cr18Ni9Ti steel specimens at 650° C on the stability of their properties was studied. It was shown that curves representing the stability of hardness, elongation, and endurance are analogous in character. Increasing the duration of preliminary loading shifts the stability curves toward the zone of lower stress, but there is no complete correlation between the stability of properties studied.