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Mechanical Properties of Materials in Microstructure Technology
Published in Bharat Bhushan, Handbook of Micro/Nano Tribology, 2020
Fredric Ericson, Jan-Åke Schweitz
Static fatigue is a slow fracture process that is given many names in the literature: delayed fracture, slow crack growth, subcritical crack growth, secondary crack growth, or stress rupture. These names fairly well describe the phenomenon: at some constant load (below the nominal fracture limit of the body) stress concentrations at local defects in the material may cause an insidious crack growth. When the crack reaches a certain critical size, the effective cross-sectional area of the specimen has been sufficiently reduced as to make the specimen reach its nominal fracture limit, and a disastrous residual failure occurs. This phenomenon is well known in many ceramic materials, e.g., glass and SiO2, and is partly considered to be due to stress corrosion at the crack tip. Static fatigue of Si has been reported, but this result has been disputed by others.
Mirror Materials
Published in Anees Ahmad, Handbook of Optomechanical Engineering, 2017
Trent Newswander, Roger A. Paquin
A difficulty of using glass is its low strength. Strength limits the use of glass to mirrors and nonload bearing structures. In applications where glass is exposed to a chemically reactive environment, its strength can be further reduced. Glass, crystals, ceramics, and brittle materials are susceptible to static fatigue, slow crack growth, or best known as stress corrosion cracking. Stress corrosion cracking occurs when a structure under constant load experiences crack propagation with time and prematurely fails due to a chemical reaction. For glass, most crystals, and most ceramics, exposure to water or water vapor results in a chemically reactive environment giving rise to stress corrosion cracking. Silicon and silicon carbide are a notable crystal and a ceramic, respectively, that do not react with water.8 One example of stress corrosion cracking causing premature failure is the Zerodur segments of the Keck primary mirror. The Keck mirror segments have experienced slow crack growth. The Keck mirrors are exposed to continuous gravitational loading that is cyclic in magnitude as the telescope slews across the sky. Relatively coarsely ground surfaces have left subsurface damage that has led to the generation of subcritical crack growth, and eventual fracture.
Ceramic Biomaterials
Published in Joyce Y. Wong, Joseph D. Bronzino, Biomaterials, 2007
It is of great interest to know whether the inert ceramics such as alumina undergo significant static or dynamic fatigue. Even for the biodegradable ceramics, the rate of degradation in vivo is of paramount importance. Controlled degradation of an implant with time on implantation is desirable. Above a critical stress level, the fatigue strength of alumina is reduced by the presence of water. This is due to the delayed crack growth, which is accelerated by the water molecules [Park and Lakes, 1992]. Reduction in strength occurs if water penetrates the ceramic. Decrease in strength was not observed in samples which did not show water marks on the fractured surface (Figure 2.16). The presence of a small amount of silica in one sample lot may have contributed to the permeation of water molecules that is detrimental to the strength [Park and Lakes, 1992]. It is not clear whether the static fatigue mechanism operates in single crystal alumina. It is reasonable to assume, that static fatigue will occur if the ceramic contains flaws or impurities, because these will act as the source of crack initiation and growth under stress [Park and Lakes, 1992].
Effect of the geometric shapes of repair patches on bonding strength
Published in The Journal of Adhesion, 2021
Hyunhee Lee, Seokwoon Seon, Soyoun Park, Rashiga Walallawita, Kwangju Lee
Adhesive bonding is widely used as a method for joining two or more components and repairing defects in composite structures. The conventional joining methods using bolts or rivets have the disadvantage that high stress is concentrated on the joining part. Thus, failure occurs at stresses lower than expected with conventional joints. Also, in the case of non-metallic composite materials, it is difficult to apply mechanical joining because it is vulnerable to plastic deformation. As a result, joining of composite structures generally uses adhesives. Adhesive bonding is also important in repairing the damaged composite structures. For example, wind turbine blades are often subject to complex loads (static, fatigue and transport/installation loads) under demanding service conditions (temperature, moisture, erosion and lightning strike).[2] If defects are caused by these factors, they are usually reinforced by bonding repair patches.
Time-dependent deformation and fracture evolution around underground excavations
Published in Geomatics, Natural Hazards and Risk, 2020
Teng-Fei Fu, Tao Xu, P.L.P. Wasantha, Tian-Hong Yang, Yoshitaka Nara, Zhen Heng
Either the rheological model or empirical model is often used to study the time-dependent behaviour of rocks (Maranini and Yamaguchi 2001; Shao et al. 2006; Xu et al. 2017; Zhao et al. 2017). The mechanical elements, including spring, dashpot, and slider, are combined in different ways to establish the viscoelastic model or visco-elasto-plastic model (e. g., Maxwell model, Kelvin-Voigt model), characterizing time-dependent deformation of rocks. The empirical models (e.g., the power law, and the hyperbolic law) also successfully describe time-dependent behaviour of rocks. For example, Amitrano and Helmstetter (2006) propose an empirical relation between time to failure and applied stress based on static fatigue laws for the time-dependent deformation and damage of rocks. Time-dependent deformation of the rock mass is mainly attributed to a mechanism of stress corrosion (Das and Scholz 1981; Atkinson 1982; 1984; Atkinson and Meredith 1987). Stress corrosion is driven by reactions that occur preferentially between a chemically activated geological fluid (commonly water) and the strained bonds at crack tips (Scholz 1972; Michalske and Freiman 1982; Freiman 1984; Baud et al. 2000). Stress corrosion theory was incorporated into the numerical modelling to describe the time-dependent deformation at mesoscale better. For example, Potyondy (2007) introduced stress corrosion into the two- and three-dimensional particle flow code (PFC2D/3D) to mimic time-dependent behaviour in silicate rocks.