Fatigue Failure in Engineering: Cyclic Stress and Crack Growth

What Fatigue Failure Is and Why It Happens Below Yield

One commonly overlooked factor is the fatigue strength of parts. People design and test parts under static loading conditions, obtain high safety factors and think they are safe. Later, after the part has been in service for perhaps six months, the worst occurs: a sudden crack appears. Fatigue strength accounts for the effects of cyclic loading, i.e., loading which is repeated many times. Even at stress levels considered safe for a single application, components of a machine can weaken and fail after a sufficient number of loading cycles. Fatigue failure often comes as a complete surprise, as in the example shown, a shaft tested safely for 10,000 pounds of static load failed after being subjected to loads of only 2000 pounds many times.

There is no indication of distress internally as cracks propagate slowly. The failure mode is time dependent, stress applied cyclically (on or off cycle) leading to failure over a period of time. Even low stress levels can be detrimental if sufficient cycles are applied. While calculations indicate adequate strength, failure occurs due to the simple fact that rotating parts are cycled on and off. Field failures of this type are not uncommon.

Fatigue is responsible for over 80% of the failures in rotating machinery, vehicles, engines, aircraft and anything else which rotates, vibrates or undergoes cyclic loading. It is not overload failures, nor material defects - it is fatigue. Failure to consider cyclic loading during design is design by guesswork.

Fatigue Crack Initiation, Propagation, and Final Fracture

This volume continues the thematic approach established in previous publications, presenting both theoretical models and empirical data that address fatigue crack initiation, growth, and the underlying mechanisms leading to material failure.

The fatiguing process can be described in three simple steps where failure occurs: 1. Crack Initiation - Fatigue cracks initiate at stress concentrations such as holes, corners, notches and rough machining. These stress concentrations can be internal flaws caused during fabrication such as surface cracks or slip streams emanating from surface scratches. Crack initiation life can vary from thousands of cycles to millions of cycles. It all depends on stress level and the quality of part geometry.

The vast majority of failures occur in three stages: the initiation of a crack, rapid crack growth, and slow crack growth. Second, the crack propagates slowly with each additional flight cycle. The rate of propagation is generally a function of stress range and the fracture toughness of the material. Often cracks can propagate a significant amount of life before discovery during routine inspection. Typically, most people do not have a good handle on the inspection process and utilize methods that are not effective for uncovering hidden cracks. As a result, cracks continue to propagate as the pilot has no idea the flaw exists.

Third: final fracture. The crack attains its critical length and the remaining cross section fails catastrophically. This event is typically very rapid. The life of the component is mainly divided between the first and second stages. Crack initiation and crack growth occur over a significant number of cyclic loading events. The third stage, sudden catastrophic failure, may occur within a single loading cycle.

The S—N Curve: Predicting Fatigue Life From Cyclic Stress

The S—N curve is often used to predict the fatigue life of metals subjected to cyclic loading. A typical S—N curve is a plot of stress vs. number of cycles and is frequently used in engineering design for component life prediction.

Predicting fatigue life requires the S-N curve. Commonly referred to as the Wöhler curve, the S-N curve is a graphical representation of stress amplitude versus number of cycles to failure. It's the primary means of answering the most critical fatigue-related question: How long will your part last under cyclic loading?

Under a lot of stress? This part failed after only a few cycles of loading/unloading, really a low cycle fatigue failure. Reducing the stress makes it good for many more cycles- a high cycle fatigue failure. Some materials, particularly steels, have an "endurance limit" where there is a stress level below which the material will not fail from fatigue, even after an infinite number of cycles.

One important difference is that aluminum and most nonferrous metals are said not to have an endurance limit. This means that they will actually fail at any stress level, if cycled enough. So, your design approaches must differ significantly in this respect. A simple steel shaft designed for low stress may be good forever. Not so with aluminum. Cycle count is always a consideration.

Design and Material Factors That Control Fatigue Life

Fatigue life is very detail sensitive. Stress concentrations such as sharp corners, holes, keyways, thread roots etc. are locations where cracks initiate. A smooth radius will cost much fewer cycles to failure than a sharp corner. Fatigue life is also sensitive to surface finish, rough machining marks providing locations for crack initiation while a highly polished surface can dramatically increase life.

Residual stress is often considered as a secondary concern. In reality, it is a primary design variable for fatigue reliability. Compressive residual stress can almost double or triple fatigue life, while tensile residual stress can greatly reduce fatigue life, due to processes like welding or machining. Mean stress (the "average" stress level) is important as well: stress in tension reduces fatigue life, stress in compression increases fatigue life. A part cyclically loaded in pure tension-compression will have a shorter life than a part cyclically loaded with mean compressive stress.

Components, products and entire systems can die quickly in the most hostile of environments. As well as the obvious affects of corrosion, the presence of an environment can also increase the rate at which cracks grow through a mechanism called corrosion fatigue, which can be worse than either corrosion or fatigue effects on their own. Furthermore, the effect of high temperature on fatigue life is well documented and universally worse than room temperature. Fatigue is not just a property of the material, it is a property of the system as a whole: being heavily dependent on design, manufacturing and operational factors. The life of a component can be measured in weeks or decades. It is often less than a year.

Low Cycle vs High Cycle Fatigue Regimes

Fatigue can be split into two categories based on the number of cycles to failure. High cycle fatigue (HCF) is typical of rotating machinery such as engines, turbines and bearings. Here stress levels are typically less than the yield strength of the material, so individual cycles do not plastically deform the part. Yet after millions of cycles, micro-damage occurs that eventually leads to crack formation. HCF is generally considered to be greater than 10,000 to 100,000 cycles to failure.

The low cycle fatigue (LCF) regime is distinct from the higher cycle to failure range where components are subjected to stress levels below those required to cause plastic deformation. This is manifest in pressure vessels where a commodity article may undergo failure after a relatively small number of pressurising / depressurising cycles or in equipment subjected to thermal cycling where large stress range is developed as component temperature is increased or decreased. Characteristic of LCF is the yield of the material on each cycle of loading with the cumulative effect being a critical factor leading to failure.

Predictions made using different models and methods for testing and prediction in the HCF and LCF regimes are worthless if incorrectly applied. Understanding in which regime a particular application exists, HCF or LCF, is critical because the underlying physics, failure mechanisms and design approaches are significantly different.

Design Strategies to Prevent Fatigue Failure

Fatigue resistance is not a property of a strong material; it is greatly affected by geometric details and the quality of the manufacturing process. Minimizing stress concentrations by incorporating large radius fillets and removing sharp corners or other features that cause stress concentrations improves the fatigue resistance of components. Severe cracks initiate at sharp features such as corners and edges, so take every opportunity to chamfer these areas. Additionally, improving the surface finish on critical areas, such as bearing surfaces, can increase the fatigue life by as much as 50% by preventing unwanted fatigue cracking caused by rough surfaces. Ensure that all machining marks are removed from critical surfaces.

Generate compressive residual stress into the surface of critical components through techniques such as shot peening or cold rolling. This provides a 'safety factor' where cracks cannot propagate until the stress has been reduced to a level that is greater than the applied load. Design for damage tolerance. Recognise that cracks will eventually form, but design the component such that cracks can be detected prior to failure. This includes designing accessible areas for inspection, preferred paths of crack propagation, and redundant load paths to ensure failure is not catastrophic.

Choose materials with high fatigue resistance and thoroughly evaluate their fatigue endurance limits and fracture toughness. Good engineers go beyond selecting durable materials though; they plan inspection intervals on the basis of predicted crack growth rate. Knowing the rate at which cracks will grow under the specific operating conditions allows the engineer to plan for inspections prior to the part reaching a critical crack size. Service time in hours or years means nothing to the engineer who cannot apply the fundamental physics of crack growth.

Why Fatigue Controls the Real Service Life of Components

Fatigue is a secret killer in the world of mechanical engineering. While individual parts might be stronger than the ultimate static load case (e.g. the worst case drop or single pull), components can catastrophically fail after repeated cyclic loads (e.g. cyclic vibration loads). People can generate really beautiful stress results with FEA software that clearly show that a part is in the "safety factor rich" green zone under static load case conditions. But when loads are cycled even once, that entire stress distribution can change – sometimes by a lot. The typical static stress analysis is just the tip of the fatigue engineering iceberg, and the vast majority of professional engineers haven't even seen the base of that towering mountain.

Learn fatigue risk from repeated loading due to rotating, vibrating or oscillating equipment and structures. Learn when fatigue damage occurs, even at stress levels deemed safe in design. Learn how to use S-N curves to make life predictions for structures of different loading conditions. Learn the design details that increase fatigue life and the details that are wishful thinking. Surface finish, fillet radius, residual stress - all critical parameters affecting fatigue risk.

Understanding fatigue is crucial for part design of rotating machinery, vehicles, aerial parts and structures subjected to vibration or cyclic loading. Learn the secrets of fatigue and you will know the secret life of your parts — how long they will last unseen until they reach their expiration date. Then it's on to the next level of part failure mechanisms, including creep, buckling and thermal failure, time-temperature and structural instability effects.