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Viscoelastic properties of materials
Published in Roderic S. Lakes, Viscoelastic Solids, 2017
Metals as they are used in most applications are polycrystalline. The boundaries between crystals or grains in metals give rise to additional damping as discussed in §8.7. Peaks of this type, known as grain boundary peaks (Fig. 7.9), have been observed in a variety of metals, including aluminum [7.2.3, 7.4.9, 7.4.10]. Grain boundary peaks occur at relatively high temperatures, a significant fraction (referred to absolute zero, degrees Kelvin) of the melting temperature. The ratio of sample absolute temperature to melting temperature is called the homologous temperature. The effect of the grain boundaries differs if the grain and specimen sizes are comparable (macrocrystalline materials); the grain boundaries are called “bamboo boundaries” in this case [7.4.11–7.4.13]. In macrocrystalline materials, the loss peak height is proportional to the number of grains; by contrast, if the grains are small, the loss peak height is independent of grain size.
Sources of Stress and Service Failure Mechanisms
Published in Colin R. Gagg, Forensic Engineering, 2020
‘At ambient and elevated temperatures, most materials can fail at a stress which is much lower than its ultimate strength. This group of failure modes are time-dependent, and termed creep deformation and creep rupture. More generally, materials or components undergoing continuous deformation over time under a constant load or stress’ is said to be creeping.[14,15] Therefore, creep of materials and structures is generally considered a high-temperature progressive deformation at constant stress. Creep is commonly found in boiler applications, gas turbine engines and ovens. These types of service application have components that will experience creep under normal use. Generally, failure by the process of creep is readily identifiable by the extensive deformation that will occur. However, failures may appear ductile or brittle, and cracking may be either trans-granular or inter-granular. Elastic, plastic, visco-elastic and visco-plastic deformations will also be encompassed within the process of creep, being dependent on the material, service temperature and time of creep deformation. The designer encounters most problems from the creep of metals or polymers above their glass transition temperature. Again, temperature is a critical parameter, and for metals, creep must be considered when values of the homologous temperature (Th) exceed about 0.4, where ThK0=T0K/Tm0K
Nanoindentation Creep of Enamel and Dentine Tissues
Published in Arjun Dey, Anoop Kumar Mukhopadhyay, Nanoindentation of Natural Materials, 2018
Nilormi Biswas, Latika Khurana, Arjun Dey, Anoop Kumar Mukhopadhyay
In the general purview of materials engineering, such time-dependent deformation is termed as creep. In the engineering research and development community, though, the time-dependent deformation of a material under a given applied load, especially at temperatures much higher than the homologous temperature of a given material, is called creep, more specifically “thermal creep.” Such a thermal creep can be induced due to sudden change of temperature at a constant applied load. It can be also induced by a sudden change in applied stress at a given high temperature that is well above the homologous temperature of the given material. The rate of this time-dependent deformation is called creep rate. This rate is also a time-dependent property of a given material. However, due to such creep rate dependent deformation, the material cross section and its structural integrity may be often adversely affected, thereby leading to premature failure while in service. Generally creep has certain stages: primary, secondary, and tertiary. The primary stage lasts as long as creep rate decreases with time. It leads to the secondary or steady state creep wherein the creep rate remains constant with respect to increase in time. However, after a certain critical time is elapsed, the third or the tertiary stage of creep happens wherein the creep rate increases rapidly with time, leading ultimately to structural failure of the component. The creep mechanisms are often different for different materials, for example, metals, ceramics, polymers, plastics, rubber, and concrete. The primary mechanisms include bulk diffusion, grain boundary diffusion, dislocation climb, and thermally activated glide. Thus, viscoelastic creep can also happen even at low temperature in biological materials (e.g., bone). In such situations the stress depends not only on the strain but also on the time history of the strain. Such behavior can manifest itself as creep. This leads to a gradual increase in accumulated strain under constant stress. Under certain special condition, stress relaxation may also occur, which means the occurrence of a gradual decrease in stress in a specimen held at constant strain. Further, there can occur the loading-rate dependence of the stiffness. As a consequence, there may be attenuation of sonic or ultrasonic waves when energy dissipation happens in bone loaded dynamically.
Diffusional evolution of σ-phase to resolve a long-standing dissimilar material joining issue in supercritical boiler
Published in Philosophical Magazine, 2022
Suvam Chatterjee, Manas Kumar Mondal, Joydeep Maity
The solidus temperature of T23 steel being 1773 K [14], the boiler service temperature in real practice vis-à-vis the tensile test temperature (571 °C) in the present investigation is very close to the half of absolute solidus temperature (886.5 K; i.e. 613.5 °C). Therefore, with regard to the existing knowledge base, grain boundary sliding is quite expected at this temperature under tensile loading at a strain rate of 10−4 s−1. In support of the argument, the investigation carried out by Sugino et al. [45] is quite relevant, where a substantial grain boundary sliding is observed in ferritic grade steel during high-temperature tensile test at a strain rate of 10−4 s−1 (similar to the present situation). Therefore, the evolution of coherent σ-phase, as exemplified in the present investigation, has its own significance to obviate type IV failure from HAZ by resisting grain boundary sliding at the service temperature. Indeed, the high-temperature tensile test, as carried out at service temperature in the present investigation, is often given the necessary significance for rapid and robust assessment of the dissimilar steel joint performance in the supercritical boiler. Of course, the creep test has its own significance of long exposure near the homologous temperature at constant load that may be useful in future work on this topic.
Experimental investigations into electric discharge grinding and ultrasonic vibration-assisted electric discharge grinding of Inconel 601
Published in Materials and Manufacturing Processes, 2018
Virendra Mishra, Pulak M. Pandey
Super alloys are also called as “super-stainless” alloys.[1] Super alloys can be useful at the temperature of 0.6 Tm (Tm is the melting temperature of material in Kelvin) namely homologous temperature. In oxidizing environmental conditions, super alloys retain its property of stability to bear severe mechanical stresses and strains.[2] Super alloys include iron, nickel, and cobalt-based materials. Inconel 600, Inconel 601, Inconel 625, and Inconel 718, etc. are some of the nickel-based super alloys with superior properties. They have superior creep strength among all the grades of super alloys. The major creep strengthening properties are due to intermetallic compound Ni3 (Ti, Al). An additional strengthening effect arises from precipitation hardening of carbides of chromium, i.e., Cr23C6.[3] Recent advancements in various technological fields demand the development and use of materials which can bear external loads at extremely high temperatures and work at highly corrosive environment. These materials include metal matrix composites used in aerospace, titanium and its alloys used in defense, ceramics used in gas turbines, stainless steels used in chemical and petrochemical industries, and super alloys used in jet engines and power plants.