Garage Door Spring

Background

Residential garage doors are a type of overhead door that is lifted and lowered using the assistance of two coiled springs which are wound to balance the weight of the door. The springs allow less expensive lifting mechanisms to be in the garage door design.

There have been numerous cases in which overhead doors have fallen and injured people in the vicinity.

Fortunately, residential garage door springs often fail with the door in the down position because that is the position in which the spring bears the most stress. 

When a metallic component fractures, two matching fracture surfaces are exposed and can be examined for features indicating the failure mode. Additionally, the microstructure of the metal can be examined for indications that manufacturing process played a role in the failure.

This failure is a great example showing features identified using typical metallurgical evaluation techniques.

Additionally, the sound created by these springs can be described as that of a metal snake approaching on the attack and, in this case, scared the bajesus (proximal damages??) out of the author when it occurred.


Process

Explico’s Eugene Lopez-Ona investigated the failure to determine the cause of crack initiation and failure of the spring. A thorough metallurgical analysis was conducted using technical services provided by Titan Metallurgy. The analysis consisted of visual examination, magnetic particle non-destructive examination, scanning electron microscopy, and metallographic preparation and examination. 

Magnetic particle non-destructive examination is a technique for detecting cracks at or near the surface of a part. When performing a metallurgical analysis of a failed part, it can be advantageous to identify cracks or features that are not part of the specific failure event. These features often contain details that could have been damaged during or after the failure. In this case, a crack was located 2-3 inches from the fracture site and was manually broken open in the lab for examination.

The opened crack was examined using a scanning electron microscope (SEM) and was mounted and prepared for metallographic examination. The fracture surfaces were generally in good condition and did not appear to contain corrosion products. The SEM image shows a ratchet mark at the initiation site with a region of Stage II crack growth across approximately 40% of the fracture surface. Metallographic examination revealed a tempered martensitic structure with partial decarburization at the OD surface and at the crack initiation site.


Results

The fatigue crack initiated on the OD of the spring at the extrados of the coil and propagated through the cross-section of the material until the applied stresses were sufficient to begin fast fracture. The fatigue crack most likely initiated due to differences in mechanical properties at the surface of the spring. Partial decarburization is a product of the manufacturing process and results in a local increased concentration of ferrite grains which can result in a reduction in bending strength. 


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