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SCANNING ELECTRON MICROSCOPY – MASSIVE EVIDENCE FROM MINUTE BROKEN COMPONENTS

 

Serious injuries and substantial property loss can result from sudden "catastrophic" failures of large assemblies. Often, the origin of the failures are traced to fracture of small key or critical components which secure the larger assembly i.e. welds, pins, clips, fasteners, etc. Although the failed component is often quite visible to the naked eye, the origin and cause of the failure of the component may be obscured due to the small size of the fracture surface. Scanning Electron Microscopy is a cost-effective means to assess the failure of small components (or large components if sectioning is allowed).

 

The Scanning Electron Microscope (SEM) has the ability to enlarge the small area of the fracture surface, providing the investigator a high quality image of the fracture surface and fracture surface and its associated evidence. In addition, the environment in which the component in which the component is placed during Scanning Electron Microscopy is conducive for testing of the component’s surface chemistry.

 

An example of a case in which Scanning Electron Microscopy identified key issues regarding the cause and origin of a failure involved a failure of a garage door spring. The garage door spring was used to prevent the sudden and uncontrolled closure of a garage door, thereby reducing the potential for injury. The spring itself was 18 inches long by 2 inches in diameter, however, the actual wire from which the spring was formed was 3/16 of an inch in diameter. The spring failed resulting in the rapid and dangerous descent of the door. The remains of the spring were examined using the Scanning Electron Microscope in order to determine the origin and cause of the failure of the spring.

 

Observing the fracture surface with a low power magnifying glass revealed three areas of different reflectivity. The actual cause for the difference in reflectivity was unknown due to the small size of the fracture surface and the lack of surface texture. On a balance of probabilities, it was believed that the centre area of the fracture was formed during ductile overload and the remaining two areas were associated with fatigue cracks. Through Scanning Electron Microscopy, it was confirmed that the centre region was in fact a ductile overload of the spring and the two remaining regions were in fact fatigue cracks which had propagated.

 

The ductile overload region under the magnified view of the electron microscope appears rough and dimpled. This dimpled morphology (surface texture) confirms this localized area of material was overloaded. The two areas of fatigue cracks, under the magnified view of the SEM were void of dimples, but full of fatigue striations. These striations show that the spring failed at less than nominal loads due to the insidious effect of fatigue. The Scanning Electron Microscope, in this case, not only supported the conclusions reached by the balance of probability assessment, but provided clear and concrete evidence that the conclusions were valid.

 

In addition to magnifying the fracture surfaces, Scanning Electron Microscopy can be used to assess surface contamination and surface chemistry. Although the machine has some restrictions when performing surface chemistries, it is a reliable and relatively simple qualitative test. The test can determine whether or not a surface is contaminated, the general surface composition, the presence of corrosion by-products, and the absence of alloying elements. This technology can be used to determine if a substandard weld material or fastener caused a failure.

 
The information contained in this web site is intended for marketing purposes only. It is not all-inclusive, and does not fully describe the many and varied services that the company provides, nor does it completely describe the education, training, skills, or expertise of our staff.

 
 
 

Walters Forensic Engineering | 277 Wellington Street West, Suite 800 | Toronto, ON M5V 3H2
Information contact: engineering@waltersforensic.com