Mechanical Prestressing

Consider a test piece that prior to fatigue cycling is subjected to an overload by bending that produces traction yielding only on metal fibers farthest from the neutral axis, as schematized in Fig. 3.50a. 

From this load type (1) To this load type (2) Multiply by = Sf Sf i 

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Electric energy is converted into mechanical vibrations in modem machinery using the piezoelectric effect. It concerivs the reversible property of special ceramic materials to deliver an electric voltage when affected by external forces. This characteristic is used for the generation of ultrasonic vibrations in such a way that the applied voltage is converted into mechanical vibrations. Modern sonic converters usually contain several piezoceramic disks of lead zirconate titanate restricted by two final masses that are mechanically prestressed by a centric screwing. 

The bias conditions are defined by the magnetic bias Hq and the mechanical prestress To, applied along the magnetostrictive rod axis, which is referred to as the third axis. Then, considering only the variations around this initial bias state, the material behaves in a quasi-linear manner and follows piezomagnetic laws [34] ... 

As was cited in the case of immersion testing, most SCC test work is accomplished using mechanical, nonelecdrochemical methods. It has been estimated that 90 percent of all SCC testing is handled by one of the following methods (1) constant strain, (2) constant load, or (3) precracked specimens. Prestressed samples, such as are shown in Fig. 28-18, have been used for laboratory and field SCC testing. The variable observed is time to failure or visible cracldng. Unfortunately, such tests do not provide acceleration of failure. 

Additional strain at the cyclopropane ring can also favor the [2n + 2a + 27t] cycloaddition mode. The simplest system exhibiting this mechanism is bicyclopentene (6). ° The <7-bond that is to be broken during the reaction is prestressed by a cyclopropane and a cyclobutene ring, and the two reacting centers are fixed in the cisoid conformation, suitable for cycloaddition. 

An alternative strengthening mechanism is prestressing, called autofrettage, which ensures a more uniform stress distribution under load [16-20]. This intentional over-pressurizing of the vessel leads to a plastic-elastic interface that moves outward as the pressure increases. When the pressure is released, the residual stresses left as a result of radial expansion allow the vessel to be used up to the pressure at which it was subjected to autofrettage, without exceeding the yield point of the material. 

On high-strength steel used in prestressed concrete (but not with common reinforcing steel) under very specific environmental, mechanical loading, metallurgical and electrochemical conditions, hydrogen embrittlement can occur, which may lead to brittle fracture of the material (Chapter 10). 

Besides concrete quality, a minimum value of the concrete cover also has to be specified. Eurocode 2 [3] fixes minimum values ranging from 10 mm for a dry environment up to 55 mm for prestressing steel in chloride-bearing environments, as shown in Table 11.5. It should be kept in mind that these values are minimum values that should be increased to obtain nominal values by 10 mm, to also take into consideration construction variability. Besides the protection of steel to corrosion, further requirements of minimum cover depth are fixed to ensure adequate transmission of mechanical forces and fire resistance. 

Following the mechanical tests at 4.2 K, the Tc of each sample was measured with and without the bronze matrix. From the small ATc observed for the stressed samples (Fig. 2), it was evident that the mechanical straining effectively removed the prestresses. 

H. Bogner and R. Blum The mechanical behaviour of coated fabrics used in prestressing textile engineering Theory, simulation and numerical analysis to be used in a FEM-model, Lecture of the ECCOMAS-Symposium Textile Composites and Inflatable Structures II, Stuttgart, October 2005... 

R. Blum and H. Bogner The mechanical behaviour of coated fabrics and films used in prestressed textile engineering Its importance in the analysis and how to derive a membrane stiffness by simple theoretic considerations, Tensinews Newsletter 10, April 2006... 

Elices, M. (1985) Fracture of steels for reinforcing and prestressing concrete. In Fracture Mechanics of Concrete, pp. 226-271, G.C. Sih and A. DiTommaso (Eds.). Martinus Nijhoff, The Hague. 

The ultimate load characteristics of structures type 1 and 2 were obtained using an ln- house computer program. This program examines a number of possible failure mechanisms In order to find the one associated with minimum potential energy of the vessel. The mechanisms are deflected In Increments, and at each step the vessel pressure which would maintain the mechanism In equilibrium Is found. Shears, deflections, prestress strains and crack sizes are output at each step so that the progressive plastic behaviour of the vessel may be monitored. Three failure criteria are recognised and overall vessel failure Is assumed to occur when one of the three Is violated. 

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