Prestressing failures

Creep tests are ideally suited for the measurement of long-term polymer properties in aggressive environments. Both the time to failure and the ultimate elongation in such creep tests tend to be reduced. Another test to determine plastic behavior in a corrosive atmosphere is a prestressed creep test in which the specimens are prestressed at different loads, which are lower than the creep load, before the final creep test (11). 

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. 

A prestressed roller bearing is used to detect the presence of hydrogen sulfide, but more specifically it is used to test for hydrogen embrittlement tendency of the drilling fluid. When introduced to the environment, the bearing has sufficient residual stresses to cause failure if sufficient hydrogen sulfide concentration is present. 

Crack branching is a common feature in failure of ceramic components. Cracks branch at a critical velocity, which is of the order of half the speed of sound in the specific glass under study. The acceleration of crack initiation to the critical velocity depends on the energy dissipation available from the release of stored energy. The energy source can be applied stress, prestressing or residual stress. 

Failure of prestressing steel is usually induced by atomic hydrogen that penetrates the metal lattice. The conditions required for cracking are a sensitive material, a tensile stress and an environment that produces atomic hydrogen on the steel surface. 

U. Ntirnberger, Corrosion induced 6 failures of prestressing steel ,... 

Particular problems arise when the corrosion problem is the black rust described in Chapter 2 and prestressed, post-tensioned structures where corrosion is difficult to detect as the tendons are enclosed in ducts. Tendon failure can be catastrophic as they are loaded to 50% or more of their ultimate tensile strength and modest section loss leads to failure under load. The particular problems of assessing prestressed concrete structures are addressed in Chapter 4. 

Prestressing steel is loaded typically to around than 80% of its ultimate tensile strength. Therefore modest section loss, particularly due to pits, can lead to crack initiation and rapid, catastrophic failure. There are apocryphal tales of prestressing rods shooting out of buildings due to corrosion induced failures. The following test techniques are used in these situations ... 

The failures of prestressed concrete structures and concern about methods of investigating and rehabilitating them are a major issue in the civil... 

Because if the prestressing steel has corrosion pits in it, those pits are potential stress concentrators and there is a risk of either under protection in the bottom of the pit which could lead to continued corrosion and then to failure of the stressed strands, or of overprotection... 

In addition, FE simulation is applied to determine load-adapted tool layouts by modeling the die insert and the prestressing system. This helps to enhance tool life and to reduce tool failure by overload and fatigue. Especially in 2D axially symmetric FE models, a combined modeling of workpiece and tool, including the prestressing... 

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