What Is Fatigue Testing?
Written by: Quintin Bentley Crevling
Fatigue testing is the most important stage in the manufacturing process. This is what ensures the product will work as advertised throughout its intended lifespan. Nearly all products worth purchasing are subject to some level of fatigue testing – an extensive cyclical process that tests for unexpected progressive structural changes that can cause irreparable damage under repeated stress. Companies run high and low cycle tests to check for various forms of fatigue, including but not limited to: material fatigue, creep fatigue, thermal mechanical fatigue, and corrosion fatigue.
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There are two cycles of testing that can be used to assess the durability of a product during manufacturing. High cycle tests are typically used to determine the breaking point of a material, and low cycle tests are for analyzing the integrity of a material under stress. Low cycle testing is often used for products that have an applicable lifespan, such as: car parts, building materials, and electronics. A customer would not want their brand new Bentley test driven for hundreds of hours when a quick test for basic functions will suffice.
Material fatigue is a common cause of malfunction in many products. This is when the material is not strong enough to hold up under repeated stress and strain over time. Fatigue begins to appear in the material as a small crack that gradually grows and can cause catastrophic failure. Imagine bending a paperclip over and over until it snapped. To test for material fatigue, a manufacturer would use a high cycle test to find the useful lifespan of the product, as well as the breaking point of the material.
Creep fatigue is when a material malfunctions when it is exposed to fluctuating extreme temperatures. Creep fatigue can happen in several stages. Primary creep is when fatigue initially happens at a very quick rate and then slows with time. This could look like a hairline fracture in a coffee mug that happens instantly, and doesn’t appear to grow at a rapid pace. The second stage is called secondary creep, which is when fatigue continues to worsen at a constant rate. Now our hairline fracture begins to spider outwards each time we pour warm coffee into the mug. Tertiary creep has an extremely rapid fatigue rate that will not stop until the material breaks. Our delicious morning energizer has spilled all over our shirt as the fractured mug finally broke under stress.
Most moving parts on machinery undergo some combination of stress and heat which causes thermal mechanical fatigue. The combination of strain and heat over the course of a few thousand operating cycles can cause permanent failure. The most common malfunctions created by thermal mechanical fatigue occur in the startup and shut-down cycles of high temperature components.
Corrosive fatigue occurs when an object is weakened due to a dangerous environment. This type of fatigue can be rapidly accelerated when the corrosion eats away at the protective coating on the material. Once the material is weakened it will only be able to withstand a fraction of the stress as it could handle when it was new. Many products that are subjected to such conditions will have a protective coating, or a special sealant to avoid corrosion.
Fatigue testing is an incredibly important aspect of manufacturing. A company will want to ensure that their product will withstand the normal stresses that it will be subjected to throughout its lifespan. A manufacturer’s goal should be to find every possible way to break a product before it gets into the hands of a customer.