What Is Fatigue Analysis? Fatigue Stress, S-N Curves & Design Basics

What is fatigue stress in engineering?

So, what exactly is fatigue stress? Basically, it’s when a component or structure starts to weaken and break down over time due to cyclical stress or loading. Even if the stresses put on the component are within acceptable limits, fatigue failure can still occur due to the frequency of the load. This makes fatigue different from other types of mechanical stresses because it happens gradually over time, rather than all at once.

Mechanical fatigue can lead to cracks or other types of damage, and if it’s not detected and addressed, it can even lead to the failure of components or structures. Knowledge of material properties, loading conditions, and part geometry is critical to conduct fatigue analysis to be able to predict the fatigue life of a component.

Stages of Fatigue Failure

Fatigue failure develops in three distinct stages:

1. Crack initiation – a microscopic crack begins to form, usually at a stress concentration such as a surface flaw, notch, or material inclusion.
2. Crack propagation – with each subsequent load cycle, the crack grows incrementally, often leaving visible “beach marks” on the fracture surface.
3. Final fracture – once the remaining cross-section can no longer support the load, the component fractures suddenly, even though the applied stress may be well below the material’s static strength.

High-Cycle vs. Low-Cycle Fatigue

Engineers classify fatigue failures into two broad categories based on the stress level and number of cycles involved:

• High-cycle fatigue (HCF) – occurs when a component is subjected to a large number of cycles (typically more than 10,000) at stress levels below the material’s yield strength. Most everyday fatigue failures, such as a suspension component cracking after years of driving, fall into this category.
• Low-cycle fatigue (LCF) – occurs when stress levels exceed the material’s yield strength, causing plastic (permanent) deformation with each cycle. Because the material is damaged more severely per cycle, failure happens after a much smaller number of cycles, often fewer than 10,000.

The stress level and expected number of cycles determine which analysis method — stress-life or strain-life — is appropriate for a given component.

Why is fatigue important in structural engineering?

Material fatigue is experienced when mechanical structures or systems start to weaken even when the loads being applied to the system are within design parameters. Fatigue stress is important in designing all structures, such as buildings, bridges, and automobiles. For example, vehicles are constantly subjected to loads and stresses while driving, and if the chassis is not designed and tested to withstand fatigue stress, any of the components in the suspension system could be at risk of failure. Fatigue is the most common source of failures for mechanical structures and systems. Some of the ways that engineers test for fatigue stress is by conducting:

• proving grounds testing – these are basically test facilities that simulate real-world conditions and environments, so engineers can see how their designs will hold up over time.
• accelerated testing – is when engineers subject materials and structures to much higher levels of stress than they would experience in the real world, in order to see how quickly they will break down.

Some external factors which could affect and accelerate the fatigue stress in a component or system are corrosion, residual stresses, and temperature. If there is a significant impact due to these variables then they should be accounted for in any modeling/testing conducted otherwise they can be ignored.

What is the purpose of proving ground?

Proving grounds are facilities, installation or reservation engineers can evaluate the performance of new technologies, such as automobile prototypes, industrial equipment, or military weapons in a controlled setting that mimics real-world conditions.

In the automotive industry, proving grounds often include a variety of road surfaces, curves, and grades that simulate real-world driving conditions and are intended to fatigue-stress vehicles. By conducting testing in a controlled environment, engineers can gather more accurate and precise data on the performance of the vehicle, and identify any areas that may need improvement.

What is the purpose of accelerated testing?

Engineers can also use accelerated testing to simulate the wear and tear a system will have over years of use in a matter of days or weeks. The goal of accelerated testing is to uncover faults and potential modes of failure in a short amount of time. For example in the automotive industry, accelerated testing involves subjecting a vehicle or component to extreme temperatures, voltage, vibration, or other stressors that it may encounter during its lifespan. Identifying any weaknesses or failure points in the design so that they can be addressed before the product is released to the market.

An example of accelerated testing is when automakers cycle the ignition system of a vehicle 20,000 times over the course of several days to simulate a lifetime of stress and wear of the ignition system.

How are vehicles tested for durability (Analysis of Vehicle Structures)?

To evaluate the durability and reliability of a vehicle, automotive engineers use both virtual and physical models to understand the behavior of the system.

• Virtual Models include applying load histories to FEA models
• physical models include using loading histories to either conduct accelerated testing

This kind of fatigue analysis is exactly what separates a vehicle that holds up for 200,000 miles from one that starts developing rattles and failures early. See how durability testing like this factors into why some vehicles are engineered to last so much longer than others.

What is fatigue analysis in FEA?

Another way that engineers analyze fatigue stress is through something called “finite element analysis” or FEA. This is a computer-based method of analyzing stress in engineering materials and structures, and it’s often used in conjunction with physical testing to get a more complete picture of how a structure will behave under different conditions. FEA is utilized to evaluate if a structure is capable of enduring multiple loading and unloading cycles, rather than just one, as simulated in a static analysis. Many companies are pushing for increased used of FEA due to the significant cost reduction compared to physical testing.

What Is an S-N Curve?

An S-N curve (stress vs. number of cycles) is one of the most fundamental tools in fatigue analysis. It plots the applied stress amplitude (S) against the number of cycles to failure (N) for a given material, typically on a logarithmic scale. Engineers use S-N curves to estimate how many load cycles a component can withstand at a given stress level before fatigue failure occurs.

Some materials, like many steels, exhibit an endurance limit — a stress level below which the material can theoretically withstand an infinite number of cycles without failing. Other materials, including most aluminum alloys, have no true endurance limit, and their fatigue strength continues to decrease as the number of cycles increases.

What is loading history?

When it comes to testing systems for durability, engineers use a variety of methods to simulate different types of loading conditions. The two common types of loading conditions are uniaxial and multiaxial (vertical, F/A, lateral). There are also different types of loading complexities for fatigue tests, including constant amplitude and random variable amplitude loading.

What is constant amplitude loading?

A constant amplitude fatigue loading is a fatigue loading scenario in which all the load (fatigue) cycles are constant over time. In the automotive industry, this type of loading can be caused by things like the weight of the vehicle or consistent road conditions.

Example of constant amplitude loading in the automotive industry:

• Body-in-white (vehicle frames) – The frame of a vehicle is subject to constant load for an extended period to simulate the vehicle’s weight over its lifespan. For example, the front and rear header of the roof system is usually the attachment points of a sunshade or sunroof module. The headers must be designed to hold up this module for the whole life of the vehicle. By conducting fatigue testing engineers are able to identify any areas that may be prone to failure due to fatigue stress.
• Brakes system – Brakes experience a constant load during normal operation (assuming steady-state braking), and it’s crucial to ensure that they will hold up over time. By subjecting the brakes to constant amplitude loading during testing, engineers can identify any weaknesses and make improvements to the design.

What is variable amplitude loading?

Variable amplitude loading is a type of stress that fluctuates over time. In the automotive industry, this type of loading can be caused by road conditions such as potholes, and bumps, or aggressive driving such as sudden changes in vehicle speed. By understanding how engineering materials and structures respond to this type of stress, engineers can design safer and more durable vehicles that will hold up over time.

Example of variable amplitude loading in the automotive industry:

• Suspension systems – During testing, engineers will subject the suspension system to a variety of different road conditions, including potholes and bumps. By doing so, they can see how the suspension system will hold up over time and identify any areas that may be prone to failure due to fatigue stress.
• Internal combustion engine – Engines experience a lot of vibration during normal operation, and this can cause fatigue stress in the various components. By subjecting the components to variable amplitude loading during testing, engineers can identify any weaknesses and make improvements to the design.

What equipment is used in fatigue testing?

Fatigue testing is a crucial component of materials science and engineering. In order to accurately evaluate the durability and lifespan of materials and components, specialized equipment is required. Some of the equipment commonly used in fatigue testing includes all-electric dynamic test machines or higher frequency servo-hydraulic test machines, electro-mechanical test machines, and resonant fatigue test machines.

These machines are capable of applying tensile, compression, and alternating cyclical loading to material, component, or product. Simulating real-world use and allowing engineers to determine how the material will perform over time.

Stress-Life vs. Strain-Life Methods

Fatigue analysis is typically performed using one of two methods, depending on whether the component is expected to experience high-cycle or low-cycle fatigue:

• Stress-Life (S-N) method – best suited for high-cycle fatigue, where stresses remain within the material’s elastic range. Testing is typically run under load control at a stress ratio of R = 0.1.
• Strain-Life (ε-N) method – best suited for low-cycle fatigue, where stresses exceed the yield strength and cause plastic strain. Testing is typically run under strain control at a stress ratio of R = -1.

Choosing the right method matters: applying the stress-life method to a low-cycle fatigue problem can significantly overestimate a component’s fatigue life.

Frequently Asked Questions

What is fatigue in engineering?
Fatigue is the progressive weakening of a material or component caused by repeated cyclic loading, even when the applied stress stays within the material’s normal design limits.

What is fatigue analysis?
Fatigue analysis is the engineering process of predicting how many load cycles a component can withstand before fatigue failure, using methods such as the S-N (stress-life) or strain-life approach along with material properties, loading conditions, and part geometry.

What’s the difference between fatigue failure and static failure?
Static failure occurs when a single load event exceeds a material’s strength. Fatigue failure occurs after many cycles of loading, even when no individual cycle comes close to exceeding the material’s static strength.

What is the difference between high-cycle and low-cycle fatigue?
High-cycle fatigue occurs over many cycles (typically 10,000+) at stresses below the material’s yield strength. Low-cycle fatigue occurs over far fewer cycles at stresses that exceed the yield strength, causing plastic deformation with each cycle.

What is an S-N curve used for?
An S-N curve plots stress amplitude against the number of cycles to failure for a material, letting engineers estimate how many load cycles a component can withstand at a given stress level.

How do engineers test for fatigue?
Engineers test for fatigue using proving ground testing (real-world road and environmental conditions), accelerated testing (elevated stress levels to speed up failure), and finite element analysis (FEA) to simulate cyclic loading virtually.