Metallurgical Analysis - Part 4: Using SEM & EDX to Uncover Spring Failures
Written by Matthew Bridson, Metallurgist, IST
Once the initial inspection and sectioning process is complete, the next stage of failure analysis involves examining the fracture surfaces in far greater detail. This is where two advanced techniques — Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) — become invaluable.
Together, these methods allow engineers to investigate microscopic fracture features and identify the elemental composition of materials and contaminants, helping pinpoint the true root cause of a spring failure.
SEM
The next stage of a failure analysis is to image the fracture faces using our scanning electron microscope, or SEM for short.
We mount the sectioned fracture faces on a stand using putty. This is to ensure we achieve the best imaging angle for the fracture faces and they don’t fall while being examined. The samples are also painted in a conductive, carbon based compound as good electrical conductivity is required.
An SEM examines fracture faces by scanning them with a focused beam of electrons. Using electrons rather than light gives a significantly higher resolution image and gives a better depth of view. This is invaluable when looking for small details on uneven surfaces.
When the electrons hit the sample it will emit its own electrons from the surface, known as secondary electrons. The number of electrons emitted depends on the surface shape (rough, smooth, cracks), presented as varying contrasts, revealing features such as crack paths, pitting and brittle vs ductile structures amongst others.
As the beam scans across, a detailed, high-magnification image is formed. We then save these images and use them within the reports. The stage that the fracture faces are sitting on can rotate; allowing us to view the wire surface near the fracture face, gaining more evidence.
EDX
Next, we can perform an energy dispersive x-ray spectroscopy also known as EDX analysis. In our line of work, elements found during this process can give us some big clues into how the spring has failed. We could find unwanted corrosive elements, elements which don’t match the material certificates or elements used for surface plating within cracks. All of which would help direct us to a root cause.
The EDX uses the electron beam within an SEM. As the electrons hit the sample, atoms in the sample emit x-rays. Each one of the elements at the surface emits an X-ray with a unique energy signature. This is picked up by a detector and presented on a graph as a spectrum of energies. This is compared with standard spectra for each element, allowing the makeup of the region under investigation to be known.
This is particularly useful for suspected corrosion. Corrosion is one of the most common forms of failure accelerant, and if corrosion has occurred EDX will display elements associated with corrosion, e.g. sodium, chlorine and elevated levels of oxygen. These labelled graphs can then be used in the report to highlight regions of, for example, corrosion.