Using Electron Microscopy for Metallurgical Failure Analysis
- Written by Derek Snider
Understanding why things fail is critical to preventing the failure in the future. Whether it is a single catastrophic failure whose root cause needs to be understood to prevent future critical failures or a test run of a prototype that is about to go to production understanding the root causes of a failure are essential.
Mechanical failures in particular can be complex and difficult to understand. When there is a mechanical failure of a material, several tests and images must be taken in order to understand the cause of the failure. Taking your sample to a lab with electron microscopy services can help you dig down further to find out where your failure might have occurred.
Advantages of Electron Microscopy in Mechanical Tests
Electron Microscopy has several advantages which can be leveraged for performing mechanical analysis. First and foremost is the resolution. Scanning Electron Microscopes (SEMs) are an invaluable tool in the hands of an experienced technician. Not only are they capable of producing much higher magnification than an optical microscope, they have an array of analytical tools that can be used to enhance investigations.
Enhanced resolution allows an investigator to be able to look down to the atomic scale and look at developing grain boundaries, crystalline formations and obtain element analysis. This means that looking at the right set of images could point directly to the root cause of a failure at the molecular level.
Failures can happen quite a few different ways. When looking at a failed component, detailed images can show materials experts clues to what might have been the cause. For example, a material that fails due to one powerful tensile force is going to look very different than something that experienced high cycles of very low forces, or damage due to low frequency vibration. A skilled lab with an SEM can take detailed images that will determine the differences between these two events.
SEMs are capable of taking images using much lower voltages. This allows the microscope to take images showing much more detail along the surface of the failure. The process is referred to as surface topography.
When a part fails due to fatigue, be it high tensile force or low force repetitive failure, the surface of the material will stretch and have formations called striations. These formations will indicate the cause of the failure. Being able to see as much detail as possible when looking at fatigue striations will allow engineers and materials experts to determine what mode or modes might have caused the part to fail. All of this is made possible with images taken by skilled SEM lab operators.
Structural Analysis and Crack Propagation
Another type of failure that can occur to a material is cracking. Cracks generally form at the micro-level along formations in a material called a grain boundary.
When one material is combined with another (in metallurgy this is called alloying), the two dissimilar materials will bond together in groups. The easiest way to think of this would be to picture a rice cereal treat. The full bar is made up of small pieces of cereal, you can see the boundaries between the rice granules. A similar thing happens in metals.
If materials are combined properly then the individual “grains” will be small. There will be short boundaries between the two grains. In some cases, the grains are large and lead to long boundary lines between them.
When a material is subjected to stresses, cracks can form along these boundaries. Over time, a crack will follow this grain boundary, becoming larger and larger until the part fails.
Well taken images from an electron microscope will allow materials experts to view the granules and the grain boundaries of a material. This can clue them in to where cracks in the material can form, how a crack propagated through a material and was the material at fault for a failure.
Consider Your Sample Size
One thing to consider when thinking about using a lab for electron microscopy is how your sample is. Sample size can determine if testing is able to be done in a way that will not damage the sample, allowing other testing techniques to be used on it in the future.
Samples that are too large will often need to be cut to the proper size for the electron microscope. If this is done, it could compromise the sample for future use. It is very important to know your lab’s capabilities and what sample sizes they can handle before testing begins.
Using an experienced and skilled lab for your SEM failure analysis could mean the difference between root cause determination of the failure or waiting around for further failures to conduct more testing. Contact us today for about Electron Microscopy services!