Element Materials Technology
Fatigue and Fracture Testing For the Rail Industry
Introduction
Fatigue of metals was first noted in the 19th century when Jean-Victor Poncelet, a French engineer, and mathematician served most notably as the Commanding General of the prestigious École Polytechnique, described parts as becoming “tired” in his lectures at the military school at Metz. In the wake of the Versailles rail accident in May 1842, the cause of which was the failure of an axle in the leading locomotive, the first scientific investigations into the little-understood phenomenon was launched. In 1870, In 1870, German railway engineer August Wöhler published his work on the relationship between the stress applied to a part and its fatigue life. The S-N curve, often referred to as a Wöhler Curve, allowed the relationship to be quantified for the first time, leading to better engineering design by limiting the stress at critical areas.
Fatigue in metals caused by cyclic loading progressively damages a localised area of a structure, eventually leading to the formation of cracks. Once a crack is formed, it will grow with each application of load. The growth rate will depend on a variety of factors; the magnitude of the load, the length of the crack and the remaining portion of un-cracked material, and the geometry of the crack. The crack will continue to grow until it reaches a critical size, which occurs when the stress-intensity factor exceeds the material’s fracture toughness. At this point, the crack growth will very rapidly, and the structure may fracture completely.
Today, fatigue mechanisms are generally considered to be well understood, at least for materials that are widely used, such as steel, aluminum, and nickel super-alloys. The stages of fatigue can be broadly broken down in the following:
- Crack Initiation
- Crack Propagation
- Fracture
Crack Initiation
Stress or strain-controlled fatigue tests allow an investigation into how long a material may remain undamaged when subjected to a certain amount of cyclic deformation. That is to say, the repeated application and removal of said deformation. Typically, this is either defined as fixed stress (load) or strain (extension). The deformation level for each cycle is the same, and the test is continued until either the specimen fractures or a specified number of cycles have been completed (runout). The majority of the cycles performed during the fatigue test are spent forming a crack or cracks, and only the very final stages of the test are spent in propagating the crack. This is why many fatigue failures of in-service parts can appear to happen so quickly and why it is so essential to understand the behaviour.
Under test conditions, the material can be very closely monitored for changes in response signals. For example, during a stress-controlled test, the change in the test piece’s extension can be monitored. As a crack forms, the sample’s total extension will increase even though the same force is applied. These changes in the material are not easily detectable outside of the laboratory, but understanding the effects of deformation on the material can help engineers make well-informed design and service decisions.
