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09.03.2014

Meaning of fatigue strength, how to treat vascular tinnitus - For You

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The flexural fatigue test is used to characterize the fatigue life of HMA at intermediate pavement operating temperatures.
The basic flexural fatigue test subjects a HMA beam to repeated flexural bending in a controlled atmosphere (Figure 1). In HMA pavements, fatigue cracking occurs when repeated traffic loads ultimately cause sufficient damage in a flexible pavement to result in fatigue cracking (Figure 2).
The concept of a fatigue life centers around the universal idea that most materials undergo a gradual deterioration under repeated loads that are much smaller than the ultimate strength of the material.
A classic fatigue crack starts at the bottom of a HMA pavement layer (or structure) and grows towards the surface. Work on NCHRP Project 9-38, Endurance Limit of Hot Mix Asphalt Mixtures to Prevent Fatigue Cracking in Flexible Pavements, is underway to identify the existence of a fatigue endurance limit and measure it for selected HMA mixtures.
The flexural fatigue test is performed by placing a beam of HMA in repetitive four point loading at a specified strain level. In recent years fatigue testing has been conducted using both constant stress or constant strain load applications. The decision on when to terminate a flexural fatigue test depends on the test mode and purpose.


Beam fatigue testing is performed at intermediate temperatures, usually 68°F (20°C), because fatigue cracking is thought to be a primary HMA distress at these intermediate temperatures. The beam fatigue test provides a measure of the fatigue life and fatigue energy of HMA pavements.
This characterization is useful because it provides estimates of HMA pavement layer fatigue life under repeated traffic loading. A number of factors can influence a pavement’s ability to withstand fatigue, including pavement structure (thin pavements or those that do not have strong underlying layers are more likely to show fatigue cracking than thicker pavements or those with a strong support structure), age of the pavement, and the materials used in construction. The flexural fatigue test is essentially a research test and is not used in specifications.
In a well designed pavement, strains in the pavement are low enough so that fatigue is not a problem. Because of the testing equipment complexity and long testing times, the flexural fatigue test is primarily a research test and is not a standard test in Superpave mix design or quality assurance testing. The flexural fatigue test is used to investigate fatigue as it relates to HMA construction materials. More recent studies seem to suggest that at low levels of strain (around 70 microstrain), HMA mixtures have, in effect, an infinite fatigue life.


However, when pavements are under-designed strains are sufficiently high to cause fatigue failures under repeated loads. These failures ultimately result in fatigue cracking which will cause disintegration of the pavement if not maintained in time. The number of loading cycles to failure can then give an estimate of a particular HMA mixture’s fatigue life. Hence, it may take several days to several weeks to develop sufficient fatigue data to allow analysis of a given mixture. Another important value that can be obtained from the beam fatigue test is the dissipated energy of the specimen.
Note that beyond a certain point the plot is essentially horizontal – indicating an infinite fatigue life.



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