What is Fatigue Limit Of a Material?

What is Fatigue Limit?

The fatigue limit, often called the endurance limit, refers to the maximum stress a material can handle repeatedly without ever failing due to fatigue, even after countless loading cycles.

Some metals, like ferrous alloys and titanium alloys, display a clear-cut fatigue limit. This means if you keep the applied stress below that threshold, the material can theoretically last forever without cracking from fatigue.

On the other hand, materials such as aluminum and copper do not exhibit this distinct boundary. No matter how low the stress is, if you keep cycling it long enough, these materials will eventually break down.

For materials that lack a definite fatigue limit, we use terms like “fatigue strength” or “endurance strength.” These refer to the highest completely reversed bending stress the material can withstand for a certain, specified number of cycles before showing signs of fatigue failure.

Several factors can influence how long a material will survive under cyclic loading—things like the magnitude of applied stresses, leftover (residual) stresses from manufacturing, inherent material properties, internal flaws, grain size, temperature, geometry of the component, quality of the surface finish, as well as exposure to oxidation or corrosion.

For specific materials like steel and titanium, there’s a theoretical stress amplitude below which fatigue failure simply won’t occur, no matter how many times the load is repeated. This value is sometimes called the fatigue limit, endurance limit, or fatigue strength, depending on the context.

When engineers need to estimate how long a material will last before succumbing to fatigue, they have several approaches. One widely used method is the stress-life method, often represented by what’s known as an S-N curve or Wöhler curve.

The S-N curve plots the applied stress (S) on one axis and the number of cycles to failure (N) on the other. What you typically see is that as the applied stress decreases from a high starting point, the expected life of the component increases first slowly, and then much more rapidly as the stress gets lower.

It’s worth noting that fatigue behavior, much like brittle fracture, can be quite unpredictable. The data used to construct S-N curves often shows a fair bit of scatter, which is mostly because fatigue is sensitive to so many variables in the test and the material itself many of which are impossible to control perfectly.

So, engineers usually handle this scatter by treating the data statistically, acknowledging that there’s always some degree of unpredictability in fatigue life.

Who Discover a Fatigue Limit?

Back in 1870, August Wöhler came up with the idea of the endurance limit. At the time, it seemed like a breakthrough for understanding how metals handle repeated stress. But if we fast-forward to what researchers are finding today, the story gets a bit more complicated.

Recent studies actually challenge this old notion, showing that metals might not have a true endurance limit after all. In other words, no matter how low the stress is, if you repeat the cycle enough times, fatigue failure is pretty much inevitable.

So, even a tiny, seemingly harmless stress can eventually cause a metallic material to fail if it’s repeated long enough.

Definitions

The following terms are defined for the S-N curve:

• Fatigue Limit: The fatigue limit—sometimes referred to as the endurance limit represents the maximum stress level that a material can withstand without eventually failing due to fatigue. This property is specific to certain metals, mainly ferrous (iron-based) and titanium alloys. For these materials, if you look at their S–N curves (which show the relationship between stress and the number of cycles to failure), you’ll notice the curve flattens out at higher cycle counts, indicating a threshold below which failure doesn’t occur no matter how many cycles are applied. Not all metals behave this way, though. Materials like aluminum and copper don’t have a clear-cut fatigue limit. Instead, even low-level repeated stresses will ultimately cause these metals to fail if given enough time. For steels, the fatigue limit is typically about half of their ultimate tensile strength, but generally does not exceed 290 MPa (or 42 ksi).
• Fatigue Strength: Fatigue strength, denoted as SNf by ASTM standards, is a little different. It’s defined as the amount of stress a material can handle for a set number of cycles before failing. For example, if you’re working with annealed Ti-6Al-4V titanium alloy, its fatigue strength is about 240 MPa when subjected to 10 million (10⁷) cycles, assuming a stress concentration factor of 3.3. This measure gives you a concrete stress value for a given lifespan rather than a perpetual limit.
• Fatigue Life: Fatigue life is all about how long a material can last under repeated loading. More specifically, it’s the total number of cycles a material can take at a certain stress level before breaking, and you can usually find this number directly from an S–N curve.

How Fatigue Failure Happens?

Fatigue failure isn’t an instant event it unfolds in three main stages:

1. Crack Initiation: This is where things start to go wrong. A tiny crack appears, typically at a point where the stress is especially high—often right on the surface of a part where there’s some imperfection or notch.
2. Crack Propagation: Once a crack forms, it slowly grows a bit with each load cycle. Most of the material’s “fatigue life” is actually spent in this stage, as the crack steadily gets larger but hasn’t yet caused the part to break.
3. Final Failure: Eventually, the crack reaches a critical size. At this point, the remaining cross-section can’t handle the load, and failure happens quickly.

Almost always, these fatigue cracks begin at the surface of a part, especially where there are stress concentrators—edges, scratches, or notches, for example. Anything that increases local stresses or helps a crack form will shorten the fatigue life.

That’s why surface quality matters. Parts that are finely polished rather than roughly ground tend to last longer under cyclic loading. Similarly, treatments that increase the hardness and strength of a part’s surface can also make it more resistant to fatigue. strength and hardness of the surface layers of metal components will also improve fatigue life.

Typical values

For steels, the typical endurance limit (Se) is about half of the material’s ultimate tensile strength, but it generally does not exceed 290 MPa (or 42 ksi). When it comes to iron, aluminum, and copper alloys, their endurance limits are usually around 40% of their ultimate tensile strength. To put it in perspective, irons commonly max out at 170 MPa (24 ksi), aluminum at 130 MPa (19 ksi), and copper at 97 MPa (14 ksi).

It’s important to mention that these numbers are based on tests using smooth, un-notched specimens. If the material sample has notches or any kind of geometric irregularity, its endurance limit tends to be much lower.

With polymeric materials, things work a bit differently. The fatigue limit here is closely tied to the basic strength of the covalent bonds within the polymer chains—the bonds that need to be broken for a crack to grow.

As long as no other thermal or chemical processes interfere and weaken the polymer chains, these materials can actually keep going without crack growth, provided the applied loads stay below that fundamental bond strength.

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