Determination Of Allowable Stresses Research Articles

Secondary Creep Analysis of FG Rotating Cylinder with Exponential, Linear and Quadratic Volume Reinforcement.

Creep is an irreversible time-dependent deformation in which a material under constant mechanical stress and elevated temperature for a considerably prolonged period of time, starts to undergo permanent deformation. Creep deformation occurs in three stages namely, primary, secondary and tertiary. Out of these three stages, secondary or steady state creep is particularly an area of engineering interest as it has almost a constant creep rate. Creep deformation plays a significant role in understanding effective service life of an engineering component working under high temperature conditions as such components such as super-heater and re-heater tubes and headers in a boiler, jet engines operating at temperature as high as 1200 C, usually experience a failure or rupture due to creep phenomenon. Design engineers keep a close attention on working stress conditions and elevated temperature under which an engineering component is expected to work as these conditions determine the onset of creep behavior in an engineering component. By recognizing the parameters of material response to creep behavior, engineers can analyse the useful service life and hazardous working conditions for an engineering components. Recognizing the creep phenomenon as high temperature design limitation, ASME Boiler and Pressure Vessel Code have provided guidelines on maximum allowable stresses for materials to be used in creep range. One of the criteria for determination of allowable stresses is 1% creep deformation of material in 100,000 h of service. Thus, the study of creep behavior in engineering components pertaining to high stress and temperature working conditions is very important as it affects the reliability and performance of the engineering components. The aim of our study is to understand the behavior of secondary creep deformation so that an advanced reinforced functionally graded material with better creep resistance, can be designed. In this paper, a secondary creep analysis of functionally graded (FG) thick-walled rotating cylinder under internal and external pressure is conducted. The novelty of the model intends to specify secondary creep stresses and strains by employing exponential, linear and quadratic volume reinforcement for ceramic in metal matrix in radial direction. This will help us to understand the effect of volume reinforcement in FG cylinder under internal/external pressure and rotating centrifugal body force by obtaining secondary creep stresses and strains. The response of the FG cylinder with isotropic material is analyzed and the solution for stress–strain rates in radial and tangential directions are obtained in closed form. Comparison of steady state creep stresses and strains under exponential, linear and quadratic volume reinforcement profiles are discussed and presented graphically.

К вопросу о совершенствовании расчетов рельсов на прочность

From the middle of the last century, beginning with the works of Professor M. F. Verigo, rejection of yield strength at static loading and transition to the fatigue strength under cyclic loading were accepted and made as the main conceptual provisions in calculations of rails for strength when choosing allowable stress levels. Thus, calculation of rails for the strength should be carried out as all strength calculations of machine parts under loadings, variable in time. Further progress in the development of methods for calculating the strength of rails under varying stresses correspond with the implementation in practice of calculations of probability concepts. In article three types of calculations of rails for strength, having the greatest practical value now, are considered namely: calculation of cyclic strength of rails at prevention of formation of fatigue cracks in a foot and a neck; calculation of cyclic contact strength at prevention within a resource (or probability regulations) of formations of cracks of contact fatigue in a head of regulated shape and size, and also calculation of durability of rails taking into account existence of fatigue cracks at prevention of complete brittle fracture of all section. The main attention is paid to determination of allowable stresses when calculating cyclic strength, as well as to the accounting of a large-scale factor when calculating cyclic contact strength, and also to determination of duration of development of fatigue cracks and their critical size when calculating durability. The existing normative documents concerning calculations of rails on strength have become outdated and demand development of new ones.

Experimental Determination of Allowable Stresses for Bamboo <i>Guadua Angustifolia Kunth</i> Structures

The earthquake on 1999 in Armenia city (Colombia) showed the good seismic performance of structures using bamboo guadua as main material but also the mistakes caused by improper techniques of construction and the lack of parameters design for this type of structures. This work presents the experimental methodology used for the determination of allowable stresses for the design of bamboo guadua structures, used in the project "Validation of Guadua Angustifolia as a structural material for design, by the allowable stresses methodology" financed by the Ministry of Agriculture and Rural development of Colombia (Ministerio de Agricultura y Desarrollo Rural de Colombia) and developed by National University of Colombia (Universidad Nacional de Colombia) since the year 2008 when there were no allowable stress design standards for this structures in Colombia. For the determination of the allowable stresses were carried out tests of samples in compression, tension, and shear parallel to the fiber, compression perpendicular of the fiber and bending beams with bamboo guadua from three different areas of Colombia. It was found also the flexural elastic modulus for the material.

Standardization of the strength of metals under conditions of low-temperature instability of plastic deformation and the action of strong magnetic fields

We propose an approach to determination of allowable stresses by a static-strength criterion for materials of structures that operate at very low temperatures (T≤20 K). This approach takes into account the qualitative change in the nature of deformation of the material, which has a significant effect on the mechanical characteristics and the specific work of deformation. The choice of critical stresses for these conditions is substantiated. The calculations are performed by introducing corrections for low-temperature hardening of the material in the presence of the intermittent-flow effect. We discuss the possibility of taking into account the influence of strong magnetic fields on the magnitude of allowable stresses.

Reduction of metal consumption in engineering structures attainable by the energy-based method for strength analysis

The main disadvantages of the traditional stress-based method for static strength analysis are shown which prevent effective usage of plastic, hardening and cold-resistant steels and alloys. The energy-based method for strength analysis whose keystone is the determination of allowable stresses offers the possibility of a differentiated increase in the level of those stresses for many structural materials owing to the analytically developed mechanism of quantitative consideration of the available plasticity, strain- and low-temperature hardening and fracture toughness (crack growth resistance) and transformation of those characteristics from qualitative to calculated ones. This is equivalent to an automatic correction of safety factors when instead of being the values strictly standardized, they become the values dependent on the main properties of specific materials. The development of the new energy-based method is a result of the evolution of the mechanics of a deformable solid which is implemented directly in the practice of strength analyses. Its application, instead of the stress-based method which is a particular case of the energy-based one, provides a complete consideration of actual strength reserves for different elastic, hardening and cold-resistant metallic materials and their reduced consumption under the principle “the higher the quality, the lower the consumption”. The validity of the energy-based method is supported by the test results for welded pressure vessels under static and cyclic loading by internal pressure at room and cryogenic temperatures.

Energy-based method for determination of allowable stress—Part I

Energy parameters have been developed which enable consideration of plasticity, strain and low-temperature hardening of engineering materials when determining allowable stresses. Materials have been systematized according to their level of hardening and a universal diagram of material hardening has been constructed which allows one to work rationally towards the improvement of the basic mechanical properties of materials in order to increase the level of allowable (calculated) stresses using metallurgical and technological means.

Energy-based method for determination of allowable stress—Part II

An energy-based method for determining allowable stresses, the main link in strength analysis, which for the first time takes account of a basic set of mechanical properties of structural materials, namely resources of plasticity, hardening (strain and low-temperature) and crack resistance, has been developed instead of the classical mechanical method involving the material strength characteristics only. By taking into account the basic mechanical properties, an increase in allowable stress according to the new method is achieved: from fractions of a percent for high-strength low-plasticity materials to 30% at room temperature and up to 50% at cryogenic temperatures for plastic hardening and nonembrittling materials and this allows reduced metal consumption in structures and machines without lowering the level of their reliability. Realization of the energy-based method for determining allowable stresses (calculated resistances) in the strength analysis opens up prospects of resolving the problem of making maximum use of actual strength for all steels and alloys used in machine building. This makes the method of great practical use.

Systematization of constructional materials based on the level of strain hardening and development of an energy method of determination of the allowable stresses and safety factors. Part 2. Determination of allowable stresses taking into consideration the reserves of strain hardening materials

An energy method of determination of allowable stresses and undetermined strength safety factors and analytically based obtaining of higher values of allowable stresses as the result of taking into consideration reserves of strain hardenability, plasticity, and toughness (crack resistance) materials in addition to their strength properties has been developed. As the result of the proposed method determination of allowable stresses (safety factors), until now based primarily on accumulated engineering experience and traditions, is acquiring a scientific basis and the metallurgical industry, the machine building branches of industry, and construction the capability of using reserves of strength of strain hardening materials and as the result of reducing the metal consumption for production of them, including semifinished products, parts, and structures, without reducing the level of their reliability. The reliability of the method has been confirmed by the results of tests at normal and cryogenic temperatures of large models and full-size welded pressure vessels produced from materials of various types.

Determination of allowable stresses in designing vessels for low-cycle fatigue

The author studies the practical introduction of allowable stresses for computation of low-cycle fatigue. This is useful in view of the consideration that the computation becomes identical to the static-strength computation after determination of allowable stress, and also the possibility arises for the development of an algorithm for the design computation. The allowable stress in low-cycle-fatigue computations depends not only on the characteristics of the material, but also on the loading regime.

Prediction of long-term whipping vibration stress due to slamming of large full ship in rough seas

Several characteristics of whipping vibration stress due to slamming impact in regular waves are referred to and the long-term predictions of whipping vibration and wave bending stresses are carried out for the large full ship of L = 247 m, C//B = 0.824. The whipping vibration stress is as large as the wave bending stress in rough seas. This paper clarifies that the reasonable determination of allowable stress or safety factor for the longitudinal strength of ship body must contain the transient whipping vibration and springing stresses. Some theoretically predicted results are compared with the experimental results and a fairly good agreement can be seen between them.