Fatigue, in the context of materials science and engineering, refers to the weakening or failure of a material caused by repeatedly applied loads, even if these loads are below the strength limit of the material. Fatigue strength in concrete is a critical property that influences its durability and longevity. Concrete can fail in two ways due to fatigue. Static fatigue or creep rupture occurs under a constant load or one that increases slowly. The other failure mode is due to cyclical or repeated loads, known as fatigue.
Interestingly, the compressive strength of concrete, typically measured in a short span of a few minutes, varies with the rate at which the load is applied and the duration of the test—for instance, a slower loading rate or a longer test duration results in lower measured strength. At the same time, rapid application of load enhances the strength but reduces the strain capacity of the material. Static fatigue arises at low loading rates when stress exceeds about 70 to 80 percent of the short-term strength, initiating rapid microcrack formation and leading to failure. As concrete ages, its strength and fatigue strength increase in tandem, indicating that fatigue failure occurs at a consistent percentage of its maximum strength, regardless of the number of cycles.
In concrete, fatigue can lead to two distinct forms of failure. The first occurs under a consistent load or one that increases gradually; this is referred to as static fatigue or creep rupture.
The second form of failure happens under cyclical or repeated loads, called cyclic fatigue.
Compressive strength is typically measured in a short span of 2 to 4 minutes. As the loading rate decreases or the test duration increases, the measured strength dips below that observed in standard tests.
Conversely, if the load is applied rapidly or nearly instantaneously, the resulting strength is higher, yet the material's strain at failure, or strain capacity, is reduced.
At low loading rates, static fatigue in concrete happens when stress surpasses approximately 70 to 80 percent of the short-term strength. This level marks the beginning of swift microcrack formation, leading to failure.
Concrete becomes stronger as it ages, and its capacity to endure repeated loads, or fatigue strength, increases proportionally. This means concrete fails under fatigue at stress levels proportional to the ultimate strength, regardless of age.
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