Metal fatigue is a major cause of failure in engineering structures, representing up to 90% of all mechanical service failures in metals that undergo movement. This progressive form of damage occurs when a material is subjected to repeated loading and unloading, known as cyclic stress. Failure can happen at stress levels well below the material’s maximum strength, making it difficult to predict without proper analysis. This article explains how metal fatigue occurs and how engineers work to prevent it.
Defining Metal Fatigue
Metal fatigue is localized, permanent structural damage resulting from fluctuating stresses and strains over time. Unlike static failure, which involves a single, excessive load, fatigue requires the continuous application of force. A component under fatigue loading gradually weakens until a sudden, catastrophic fracture occurs, often without visible prior deformation or warning signs.
The concept of “cyclic loading” is central to metal fatigue, meaning the stress applied fluctuates between maximum and minimum values. This repeated application of force can be in the form of bending, tension, compression, or torsion, such as stresses experienced by an airplane wing or a rotating axle. The resulting failure occurs at a nominal stress level significantly lower than the material’s yield strength, the point where it would permanently deform under a single load.
The Three Stages of Fatigue Progression
Fatigue failure is a sequential process separated into three phases: crack initiation, crack propagation, and final fracture. The process begins at the microscopic level, often where local stress is higher than the average stress across the component. This high stress concentration can be caused by a surface scratch, a manufacturing defect, or an internal impurity.
The first stage, crack initiation, involves the formation of microcracks, typically less than 0.001 millimeters long. Cyclic loading causes localized plastic deformation within the crystalline structure, leading to the formation of intrusions and extrusions on the surface. These microscopic irregularities act as nucleation sites where the initial fatigue crack forms, usually at the component’s surface.
The second and longest stage is crack propagation, where the microcrack grows with each successive stress cycle. During this phase, the crack advances perpendicular to the main tensile stress, slowly extending deeper into the material. The rate of growth is directly related to the magnitude of the cyclic stress applied. As the crack lengthens, it creates characteristic microscopic markings on the fracture surface known as striations or, on a larger scale, “beach marks.”
The final stage is the rapid fracture, which occurs when the growing crack reaches a critical size. At this point, the remaining cross-sectional area is insufficient to support the applied load. The remaining section fractures instantly by static overload, leading to a sudden mechanical failure. The fracture surface visually displays two distinct zones: a relatively smooth area from the slow crack propagation and a rough, granular area from the final, rapid break.
Environmental and Design Factors
Several external conditions and design choices can accelerate the fatigue process, drastically reducing a component’s service life. The stress amplitude is an important influencing factor, defined as the magnitude of the variation between the maximum and minimum stress experienced during the cycle. A greater difference in stress fluctuation causes cracks to initiate and grow faster, even if the maximum stress remains the same.
The surrounding environment also plays a large role in corrosion fatigue. A corrosive medium, such as saltwater, humid air, or acidic conditions, chemically attacks the metal surface and promotes crack initiation. This combined action of chemical degradation and mechanical stress causes the material to fail sooner than it would under either factor alone.
Temperature fluctuations can also induce stress, leading to thermal fatigue. Repeated cycles of heating and cooling cause the material to expand and contract, generating internal stresses even without external mechanical force. This effect is commonly seen in components exposed to extreme thermal cycling, such such as engine parts or nuclear reactor components.
Design features that create stress concentrators are a major factor that must be addressed. Sharp internal corners, holes, notches, and poor surface finishes locally magnify the applied stress, making them prime locations for crack initiation. Conversely, a smooth surface finish and the use of rounded edges (fillets) help distribute the stress more evenly and significantly delay the onset of fatigue failure.
Testing and Mitigation Strategies
Engineers employ various methods to predict and prevent fatigue failure. A foundational tool is the creation of S-N curves (Stress-Life diagrams), which plot the magnitude of cyclic stress against the number of cycles a material can withstand before failure. For some materials, such as many steels, this curve flattens out at a certain stress level, defining the endurance limit. Below this limit, fatigue failure is theoretically unlikely to occur regardless of the number of cycles.
Non-destructive testing (NDT) is regularly performed on components in service to detect fatigue cracks before they reach a dangerous size. Techniques like ultrasonic testing or dye penetrant inspection allow engineers to find cracks without damaging the component. This preventative maintenance is essential in industries like aerospace, where undetected fatigue can have catastrophic consequences.
A common mitigation strategy involves altering the metal surface to enhance resistance to crack initiation. Shot peening is a mechanical process where small, hard spheres are blasted at the component’s surface, inducing a layer of compressive residual stress. Since fatigue cracks are driven by tensile stress, this surface compression effectively works to close microcracks that attempt to form, significantly increasing the component’s fatigue life. Other strategies focus on avoiding geometric stress concentrators by ensuring gradual transitions and rounded corners in load-bearing parts.