A garden trailer wheel rim exploded injuring a woman working in the garden. A metallurgical failure analysis of the failed wheel rim was requested.
The subject garden trailer was owned by a retired engineer and his wife. Being a good, careful and safety conscious engineer, he had written the tire pressures of all of the tires around the home in a small notebook. However, the tire pressure that was written on the side of the garden trailer tires were in very small letters that read 30 psi. Because of the small print on the sidewall the safe pressure looked like 80 psi which the owner recorded in his notebook. On the day of the accident, after losing some air pressure, the garden tractor wheels were inflated to 78 psi. The engineer's wife was leaning down near the tire when the wheel explosion occurred.
Photograph A Exploded wheel rim, tire and tube.
Photograph A shows that the subject wheel rim is a split rim design. The two halves of the wheel rim are formed and pressed from a steel sheet into the shape of the wheel half. The two wheel halves are bolted together forming the full wheel rim. An overall view of the outside half of the failed wheel rim is shown in Photograph B.
Photograph B Outside surface of the failed outer split rim.
A close-up visual examination of the failed outer split rim revealed several important features. A close-up view of the bolt hole at 1 o'clock in Photograph B is shown in Photograph C.
Photograph C Upper right corner of the subject split rim bolt hole shown in Photograph B.
It was noted, as shown in Photograph C, that the outer rim had literally been forced or extruded over the hexagonal bolt head which held the two split rims together. Stress cracking of the paint was observed around the bolt hole and in the sharp fillet corner of the rim. A schematic view of the split wheel rim is shown in Photograph D.
Photograph D Red arrows denote sharp fillet corner where paint cracking was noted.
Photograph E Hexagonalized bolt hole.
It should be noted that this hexagonal hole was originally circular. For comparative purposes a white circle has been superimposed in Photograph E to assist visualization of the relative size of the original bolt hole to that of the extruded, hexagonalized hole.
Conclusions and Opinions
Based on the results obtained, the following opinions and conclusions can be stated.
- As a result of the garden crate wagon incident/failure, the subject garden crate wagon split rim wheel has been grossly and permanently deformed.
- This deformation of the split rim wheel caused a curvature and deformation of the formerly flat contact surfaces between the two split rims.
- The subject split rim wheel was defectively designed and manufactured.