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Metal Fatigue: The basics
Published on 21st September 2020
Metal fatigue is a real and present danger in all parts or components subjected to repeated stresses and/or exposure. This makes the testing of the tensile strength of metal assets crucial, not only to ensure the safety of users but to take advantage of the full lifespan of critical and expensive equipment.
Metal fatigue testing is a vital stage during the design and manufacturing process, allowing engineers to learn the limits of materials and parts, as well as determining the viability of planned designs.
The three stages of metal fatigue.
While the subject of metal fatigue is complex, what can easily be determined are the three stages of fatigue failure*.
- Stage one – Initiation: During this stage, irreversible changes are caused to the integrity of the metal. This cumulative damage is due to repetitive sheer stresses, and the site from where this is initiated is very small. Many different aspects have a bearing, including environmental, metallurgical, and the geometric structure of the part in question.
- Stage two – Propagation: Here the microcrack that’s appeared in stage one changes direction and grows in a perpendicular trajectory to the tensile stress.
- Stage three - Final Rupture: The crack continues to propagate until a complete fracture occurs from a single load application.
The importance of metal fatigue testing.
The ability to determine the early stages of metal fatigue, and monitor them, is vital. At the manufacturing level, components can be reliably tested before application, allowing engineers to discover flaws in designs.
In the real world, the discovery of metal fatigue allows users to monitor metal assets, ensuring the full advantage of maximum longevity and an effective repair and/or replacement program to be put into place.
Un-monitored or untested components in the workplace can lead to catastrophic situations, posing serious risks for the safety of worker and damage of expensive equipment.
Testing options
During the manufacturing, process engineers will reproduce the stresses an application will experience during use. This is done using a method called cyclic loading. Repeated stresses are applied to discover the point at which the structural integrity of a component is compromised.
There are different types of stresses, including:
- Vibration
- Tension
- Bending
- Torsion
- Axial
- Weight
- Compression
Engineers will use different metal fatigue tests to discover loading limits. The following are the most common:
- Mechanical: These evaluate the physical properties of the component, and apply the maximum level of stress until the material fails. Such tests include high and low cycle fatigue tests, bend tests, fracture testing and cyclical displacement testing.
- Temperature: Components will have different limits depending on environmental factors, including that of temperature. Heat and cold causes expansion and contraction of the metal, so it’s necessary for such stress tests to be carried out under these extremes.
- Metallurgical: Raw materials have inherent weaknesses and this is therefore replicated in the finished product. Metallurgical analyses are carried out in conjunction with cyclical stress tests to identify such flaws at a microstructure level.
Metal fatigue and non-destructive testing
Non-destructive testing (NDT) not only determines points of metal fatigue, but allows an estimation of the growth rate of a crack in any particular situation. Because the time span between stages one and two of metal fatigue are relatively slow, users are able to calculate an appropriate inspection interval (with a built-in margin of safety) to ensure that any cracks discovered cannot grow to a size that has any danger of catastrophic failure.
As technology advances, NDT testing becomes more accurate, as well as the equipment that allows for the analysis of metal in hard to reach areas. In industries such as the oil and gas, the provision of such kit can have a major positive impact on issues such as planned maintenance, repair and replacement, and provide a crucial increase in worker and equipment safety.
Leading equipment supplier, Nexxis, are at the forefront of the latest NDT, robotics and inspection cameras, working with their customers to determine the most accurate, affordable, and reliable solutions. Run by a team with decades of industry experience, Nexxis understand the real-world pressures necessary to overcome the ongoing challenges and partner with their customers to provide a truly flexible solution.
Find out more at nexxis.com.au or give them a call on 08 9418 4952 for a no-obligation chat.
