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Failed Shock Absorber Bracket

Metallurgical failure analysis was requested of a failed four wheeler ATV rear shock absorber bracket which was involved in a fatal accident.

The failed shock absorber bracket was manufactured from a cast aluminum alloy. All sections of the failed rear shock absorber bracket were not located. An overall view of a portion of one of the failed shock absorber bracket pieces is shown in Photograph A.

Overall view of portion of failed shock absorber bracket.
Photograph A  Overall view of portion of failed shock absorber bracket.

The failed shock absorber bracket fracture surface shown in Photograph B contained a region of flat fracture, denoted with arrows. A profile view of the fractured shock absorber bracket, shown in Photograph C, reveals that the casting fracture is very straight and flat. The flatness of the fracture surface is denoted in Photograph C with arrows

Photo of flat fractured area of shock absorber bracket.
Photograph B  Shock absorber bracket. Arrows denote area of flat fracture.
Profile view of flat fracture
Photograph C  View of flat fracture. Arrows denote area of flat fracture.

Scanning electron microscopy (SEM) of the failed casting fracture surface was conducted in the areas denoted in Photograph D. The surface of the casting, at a magnification of 14x, revealed considerable casting shrinkage porosity. The casting porosity is shown in the scanning electron micrograph in Photograph E.

Denoted areas examined by Scanning Electron Microscope
Photograph D  Areas examined using the Scanning Electron Microscope (SEM).
SEM 14x showing casting porosity.
Photograph E  Micrographs at magnification (14x) magnification showing casting porosity.

A close-up view of the areas of casting porosity is shown in Photograph F. The characteristic appearance of shrinkage casting dendrites is clearly evident.

SEM 200x of aluminum casting porosity.
Photograph F  Micrograph at magnification (200x) magnification of aluminum casting porosity.

The fracture surface between the shrinkage porosity exhibited a dimple rupture fracture surface, and is shown in Photograph G. No evidence of metal fatigue (as preliminarily indicated by the flat fracture) was observed in the subject failed shock absorber bracket.

SEM 500x of dimple rupture fracture surface.
Photograph G  Micrograph exhibiting a dimple rupture fracture surface at (500x) magnification.

From the above studies, it was evident that this portion of the failed ATV shock absorber bracket was not the cause of the subject accident. The dimple rupture fracture surface was the result of sudden impact, overload failure that was the result of the accident.

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J.E.I. Metallurgical, Inc.

5514 Harbor Town
Dallas, Texas 75287

Phone: (972) 934-0493
Fax: (469) 737-3938
Email: r.c.jerner@metallurgist.com

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