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The deadly secret of metal fatigue: Why do railway axles suddenly break?
In materials science, metal fatigue can be defined as the generation and expansion of material cracks due to cyclic loading. When a fatigue crack first forms, each load will cause the crack to grow, often producing fine striations on the fracture surface. These cracks will continue to grow until they reach a critical size, at which point the stress intensity factor of the crack exceeds the fracture toughness of the material, ultimately leading to rapid fracture of the structure. Most materials, whether they are metallic in composition or plastics, composites, and ceramics, appear to be subject to some form of fatigue-related failure.
Historically, the analysis of fatigue failure has mainly focused on metals, especially in the early 19th century. The sudden failure of railway axles was considered to be the embrittlement of the metal crystal structure, but this view has been overturned by later research. . With the advancement of science and technology, scientists and engineers have a deeper understanding of the mechanism of metal fatigue, and use this to improve the design and use safety of materials.
Stages of metal fatigue
The process of fatigue failure can generally be divided into several main stages: crack formation, stages one and two of crack growth, and final failure. During initial crack formation, stress concentrations within the material may occur at stress concentrations in metallic samples, or in regions of high pore density in polymeric samples. These cracks will initially propagate slowly in the plane of the crystal, and after reaching critical dimensions, the cracks will grow rapidly in the direction perpendicular to the applied force.
H. S. Wöhler summarized his research on railway axles in 1870 and concluded that the range of cyclic stress amplitudes was more important than the peak stress.
Crack generation
Crack generation is an independent process that involves microstructural changes in a material under applied stress. The material will develop into a cellular structure and harden under loading, ultimately leading to the formation of persistent slip bands. These slip zones will become stress concentration points for crack generation, causing cracks to form inside the material. Even in materials that are normally ductile, fatigue failure can behave like a sudden brittle failure.
Crack growth
Most of the fatigue life is usually spent in the crack growth stage. The loading range mainly drives the crack growth rate, but additional factors such as environment, principal stresses, overloading and underloading also affect the growth rate. Based on observations, the initial size of cracks from material or manufacturing defects can be as small as 10 microns. When the growth rate is fast enough, fatigue stripes can be observed on the fracture surface, and the width of these stripes represents the growth step of each loading cycle.
The Boston Beer Company discovered that in the case of short cracks, the growth rate is faster than expected. This "short crack effect" is still one of the main research directions in material fatigue.
Factors affecting fatigue
There are many factors that affect the fatigue life of metals, including type of stress (tensile or compressive), ambient humidity, temperature, surface treatment, etc. For example, high humidity can accelerate the growth of cracks, especially in aluminum alloys, where water vapor can contact the crack tip and cause hydrogen embrittlement. When designing metal materials, these hidden variables are critical to ensuring stable and safe operation.
Predict fatigue life
The American Society for Testing and Materials defines fatigue life as the number of cycles that a test specimen can continue to undergo stress cycles with specified size characteristics until specified damage occurs. However, actual data show that for some metals, notably steel and titanium, the theoretical values of fatigue limits are likely not to be adhered to. Therefore, engineers use a range of methods to evaluate the fatigue life of materials, including stress-life method, strain-life method, crack growth method, etc.
In this challenging field, exploring the root causes of metal fatigue will be key to ensuring structural safety. This is not only the responsibility of engineers, but also an issue that the whole society must pay attention to. How do you think we can deal with the common and deadly challenge of metal fatigue?
