S-N Curve & Fatigue Life Estimator

The foundation of high-cycle fatigue analysis is the Basquin equation, which models the linear region of the S-N curve on a log-log scale.

Here, $\sigma_a$ is the stress amplitude (the alternating stress you input). $\sigma_f'$ is the fatigue strength coefficient , a material property. $N_f$ is the number of cycles to failure (the output you're estimating). $b$ is the fatigue strength exponent, a negative number that defines the slope of the S-N curve.

Since real loading often has a non-zero mean stress ($\sigma_m$), the Basquin equation is used with an equivalent stress. A common model is the Goodman correction, which reduces the allowable alternating stress as the mean stress increases.

Here, $\sigma_{a,eq}$ is the equivalent fully reversed stress amplitude used in the Basquin equation. $\sigma_m$ is the mean stress. $\sigma_{uts}$ is the material's ultimate tensile strength. This equation shows that as the mean stress approaches the ultimate strength, the allowable alternating stress drops to zero—meaning even a tiny vibration would cause immediate failure.

Aerospace Component Design: Every bolt, bracket, and panel in an aircraft undergoes millions of stress cycles from take-off, turbulence, and landing. Engineers use S-N curves with high safety factors to ensure wings and engine mounts won't develop fatigue cracks before the next major inspection, often after tens of thousands of flight hours.

Automotive Suspension & Axles: A car's suspension springs and axles experience cyclic loading from every bump in the road. Fatigue analysis determines the right material and thickness so these components last for the vehicle's target lifespan (e.g., 150,000 miles) without catastrophic failure, balancing performance with durability.

Wind Turbine Blade Engineering: Turbine blades flex with every rotation and wind gust, causing complex cyclic stresses. Predicting fatigue life is critical for these massive, expensive structures located in remote or offshore sites, ensuring they operate reliably for 20+ years with minimal maintenance.

Medical Implants (e.g., Hip Stems): A hip replacement stem is loaded with every step the patient takes—over a million cycles per year. The S-N curve, adjusted for the body's corrosive environment, is used to select titanium alloys and designs that will not fatigue and fracture inside the patient's body over decades of use.

First, understand that "the S-N curve is not an absolute performance chart for a material." You often see references stating "the S-N curve for SUP9 is this," but this is data from test specimens with ideal shape, surface, and environment. For actual components, the correction factors your senior colleague mentioned are what matter. For example, even at the same stress amplitude of 100 MPa, the fatigue life can differ by more than a factor of 10 between a mirror-polished part and one with an as-machined surface. You'll see the curve drop sharply if you change the surface finish factor in the tool to "rough machined surface." That's reality.

Next, the influence of mean stress is often overlooked. This tool assumes "fully reversed loading (mean stress = 0)," but in practice, fluctuating loading (e.g., repeated loading only in tension) is more common. When mean stress is present, the fatigue life can change significantly even at the same stress amplitude. For instance, the presence of a mean tensile stress accelerates failure. Be careful, as evaluating this requires different methods like the "Goodman diagram."

Finally, avoid overconfidence that "infinite life means it will never fail." While it's true that failure is unlikely under high-cycle fatigue below the fatigue limit, the story changes if there's a corrosive environment or even a single unexpected overload. Furthermore, it is now known that failure can occur in the "very high cycle fatigue regime" beyond 10^7 cycles. A safety factor of 1.5 is merely a starting point for design. Prototype durability testing under real operating conditions is an absolutely indispensable step.