Recent Investigations - Consumer Products

 

Garden Trailer Wheel Explosion

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: boat trailer - 45 psi, car - 32 psi, bicycle - 15 psi, etc. However, the tire pressure that was written on the side of the garden trailer tires were written in tight small letters that read 30 psi but, because of the small print, looked like and he recorded as 80 psi in the notebook. On the day of the accident, after losing some air pressure, the garden tractor wheels had been inflated to 78 psi. The engineer's wife was leaning down near the tire when the "explosion" occurred.

An overall view of the seven inch diameter failed wheel rim and tube/tire are shown in Photograph A.

Overall view of exploded wheel, tire and tube.
Photograph A: Overall view of subject exploded wheel rim, tire and tube.

As shown in Photograph A, the subject wheel rim is a "split rim" design. The two halves of the wheel rim are deep drawn to deform and press a steel sheet into the shape of the half wheel. 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.

Overall view of outer split rim. Photograph B: Overall view of the outside surface of the failed outer split rim.

A close-up visual examination of the failed outer split rim revealed several interesting features. A close-up view of the bolt hole at 1 o'clock in Photograph B is shown in Photograph C.

Photograph C
Photograph C: Enlargement of the 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 head of the hexagonal bolt head which held the two split rims together. Stress cracking of the paint was noted 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.

Schematic view of split rim.
Photograph D: Overall profile schematic view of the subject split wheel rim. Red arrows denote sharp fillet corner where paint cracking was noted.

A close-up view of  "hexagonalized" reshaped bolt hole is shown in Photograph E.

Photograph E
Photograph E: Close-up view of "hexagonalized" bolt hole. White circle of the same (approximate) diameter as the original bolt hole has been superimposed for comparative purposes.

It should be noted that this hexagonal hole was originally circular. For comparative purposes a white circle has been superimposed in Photograph E to facilitate visualization of the relative size of the original bolt hole to that of the extruded, "hexagonalized" hole.

The two split rims, shown in Photograph F, are bolted together with five, 7/16 inch hexagonal head, 5/8 inch long bolts. A profile view illustrating the deformation of the flat face of the subject outer rim when compared to an exemplar rim is shown in Photograph F.

Profile view of failed and exemplar split rims.
Photograph F: Profile view of failed outer subject and exemplar inner split rims. The bulged (outwardly deformed as seen in this photograph) surface of the subject (left) is in clear contrast to the flat surface of the new exemplar split wheel rim (right).

The bulging and permanent/plastic deformation of the subject outer split rim, shown on the left, is clearly evident and in stark contrast to the flat surface in the new exemplar split rim shown to the right in Photograph F.

The inside surface of the inner split rim half is shown in Photograph G.

Inside surface of inner half of split rim.
Photograph G: Inside surface of "inner" half of the subject split rim wheel assembly. The star shaped doubler plate is denoted with an arrow. Black arrows denote a fillet at the transition to the rim body.

Several features are evident in Photograph G. The center profile of the split rim is manufactured to be flat. A small sharp fillet transition from the flat surface to the rim body is clearly denoted with black arrows in Photograph G. It should be noted that this is the outside surface of the split rim which faces the garden trailer. This fillet is denoted with red arrows in Photograph D above. The fillet in Photograph G is observed to have a network of cracks around the entire fillet circumference. These paint cracks are the result of overstress and plastic deformation of the fillet on the inside surface of the inside split rim, as a result of air inflation of the garden tractor tire. The tips of the star doubler plate are observed to extend beyond and encompass the hexagonal attachment bolt holes. A close-up view of the top of the star doubler plate is shown in Photograph H.

Bottom Right of inside surface of inner half of split rim.
Photograph H: Bottom right corner of the inside surface of "inner" half of the subject split rim wheel assembly.

The cracks in the wheel paint around the circumference, are "fingerprint" evidence that this split rim fillet has experienced very high tensile stress. It should be noted that the fillet paint cracks are separate and distinct individual paint cracks. The paint cracked much like "brittle lacquer" or "stress coat" used in experimental stress analysis. It will also be noted that these paint cracks are more concentrated at the "tips" of the star doubler plate and near the rounded star points and that they have a curvature somewhat conforming to the shape (between star points) of the star doubler plate. The five hexagonal nuts are used to tighten the bolts attaching the two split rim halves together.

Careful examination of the outer wheel half of an exemplar wheel shown in Photograph F indicated a slight pucker in the flat face of the split rim. When received by the garden wagon manufacturer, their rims are fully assembled and are inflated to a pressure of 30 psi.

Because of the deformation in the face of the subject split rim wheels metallographic samples were cut from the subject split wheel rim. Metallographic samples and sub sized tensile test specimens were also cut from the exemplar wheel. Samples were also cut from an "alternate" wheel, designated "A". The alternate wheel was another wheel on the subject garden trailer, presumably made from identical metal at the same time as the subject, exploded, split rim

Microhardness
Microhardness testing and microhardness measurements were made on each sample. The Knoop microhardness values are tabulated in Table 1.

 

SAMPLE DESCRIPTOR

 KNOOP MICROHARDNESS (Testing Location) KHN (AVG)
Subject Outer

(S)

Middle 133
Quarter Thickness
(1/4 t)		 145
Alternate Outer

(A)

 Middle 120
Quarter Thickness
(1/4 t)		 134
Exemplar Outer

(N)

 Middle 164
Quarter Thickness
(1/4 t)		 157
Exemplar Inner

(N)

 Middle 128
Quarter Thickness
(1/4 t)		 128
Doubler Inner

(N)

 Middle 229
Quarter Thickness
(1/4 t)		 236

Table 1: Table of Knoop microhardness values for the subject, alternate and exemplar split rim segments. "S" refers to subject, "N" refers to new exemplar and "A" refers to alternate. KHN is Knoop microhardness.

As can be seen in Table 1, the Knoop Microhardness values for subject "S" and alternate "A" ( presumably from the same manufacturer ) vary from KHN 120-145 and average KHNavg 131. This Knoop microhardness corresponds to a tensile strength of 56-57 Ksi.

The exemplar split rim microhardness values present an interesting contrast to the subject (described above). The "inner" split rim has a Knoop microhardness, KHNavg 128, in the same Knoop hardness range with the "outer" split rim microhardness of the subject and alternate "outer" split rims. However, the "outer" exemplar split rim, the split rim that was positionally in the same location as the subject failed outer split rim, had a much higher Knoop microhardness, KHNavg 160. A KHN 160 corresponds to a tensile strength of 70 Ksi. Obviously, this tensile strength is 10 Ksi (or 16.7%) more than the tensile strength of the subject outer split rim.

The exemplar star doubler plate exhibited even higher KHN results. The KHNavg 232.5 was much higher in hardness and strength than any of the subject, alternate or exemplar split rims. This KHN corresponds to a tensile strength of 101 Ksi.

It appears as if the exemplar split rim manufacturer intentionally increased the steel tensile strength in the "outer" split rim. However, they apparently reduced the inner split rim tensile strength to about the same as the subject (and alternate) outer split rim, but strengthened the design by adding the much stronger doubler plate. Thus, the exemplar split rim designers acknowledged the importance of the star doubler plate to increase the overall strength of the split rim while using the lower strength 56 Ksi sheet steel to fabricate a deep drawn inner split rim.

The two tensile specimens cut from the exemplar split rim are shown in Photograph I.

Over all view of where tensile test specimens were extracted. Photograph I: Overall view of location where tensile test specimens were extracted. The machined tensile test specimens are shown in the locations from where they originated.

Sample Yield Load (lb) Ultimate Load (lb) Yield Strength (Ksi) Tensile Strength (Ksi)
Ten #1 528 701 46.3 61.5
Ten #2 511 707 44.8 62.0

Table 2: Table of results from tensile test on the exemplar split rim.

The tensile test results indicate that the outer exemplar split rim with an average KHN of 160 had a tensile strength, as determined by actual tensile testing, of 62 Ksi.


Finite Element Analysis

The split rim was modeled for linear elastic finite element analysis (FEA). Finite element analysis is a sophisticated computer based mathematical representation of the stress achieved at various levels of load application. The FEA was performed by Mr. Steve Roensch, Roensch Engineering, Grafton, Wisconsin.

In FEA, the object to be analyzed is broken up into tiny mathematical elements which, when added together with the aid of a powerful computer, form a repeatable portion of the device or in this case split rim. In developing the model, Mr. Roensch was instructed to assume a split rim sheet metal thickness of 0.048 inches.

The finite element analysis results for the outer wheel, without a star plate, are displayed in Figure 1.

FEA results. Figure 1: Results of FEA using a tire inflation pressure of 80 psi. The steel thickness in this model was 0.048 inches. The areas depicted in green, yellow, orange, burnt orange, and red are experiencing stress levels above the yield strength of the steel split rim.

The results displayed in Figure 1 show that the areas of green, yellow, orange, burnt orange, and red are above the yield strength of the subject steel.

This means that at 80 psi inflation pressure the yield strength of the metal is exceeded in the areas depicted with green, yellow, orange, burnt orange and red are depicted in Figure 1.

The star doubler plate was added to the basic FEA model. The model is shown in Figure 2.

FEA model of split rim. Figure 2: FEA model of split rim with "star doubler plate" added.

The stress field with an inflation pressure of 80 psi with the star doubler plate added, is shown in Figure 3. At this location, the total plate thickness, with and under the star doubler plate, is 0.097 inches.

The FEA model results, using an inflation pressure of 80 psi, are shown in Figure 3.

FEA model of split rim with different thickness. Figure 3: FEA model of split rim wheel with the "star doubler plate" thickness of 0.048 inches added. Inflation pressure in this model is 80 psi.

The areas in Figure 3 depicted in green, yellow, orange, burnt orange and red are all above the rim sheet steel yield strength of 45 psi.

The star doubler plate, superimposed on the FEA model for the outer split rim, moves the high stress region from the hole and out to the tip of the star doubler plate.


Conclusions and Opinions

Based on the results obtained, the following opinions and conclusions can be stated.

  1. As a result of the garden crate wagon incident/failure, the subject garden crate wagon split rim wheel has been grossly and permanently deformed.

  2. This deformation of the split rim wheel caused a curvature and deformation of the formerly flat contact surfaces between the two split rims.

  3. This deformation renders unreliable any hardness or strength determination made on the subject split rim without destructive testing.

  4. Comparison of metallographic microstructure, microhardness and tensile test results from an exemplar split rim wheel provided insight into the hardness and tensile strength of the original split rim when manufactured.

  5. The microstructure and microhardness of the exemplar split rim wheel and the subject split rim wheel are similar. However, slight variations in microstructure correlated with variations in microhardness and tensile strength.

  6. By comparison with exemplar split rim tensile test results and the microhardness of each of the metallographic samples, it was determined that the yield and tensile strength of the subject split rim sheet metal was approximately 45,000 and 60,000 psi, respectively.

  7. The safety factor of the subject split rim assembly could be increased by increasing the thickness of the sheet steel from which the split rim is fabricated. Doubling the split rim metal thickness should double the punching shear strength thereby dramatically reducing the probability of an accident of this type.

  8. Fabrication and production drawings for the subject split rim were not produced. This lack of drawings precludes making any firm statements about material thickness changes in the production/manufacturing process. Alternatively, since the only area needing further strengthening is around the split rim bolt holes a "five star reinforcement doubler plate" could be installed during original fabrication to the outside split rim half of the wheel.

  9. A five star doubler plate is already in use in the present split rim wheel design on the inside half of the split rim. Thus, this addition would only require fabricating the outer split rim in the same manner as is being presently used to fabricate the inner split rim.

  10. Microhardness test results from the exemplar split rim wheel revealed that the star doubler plate was APPRECIABLY stronger than the steel sheet used to fabricate/manufacture either of the exemplar spit rims.

  11. The finite element analysis (FEA) indicated that at an inflation pressure of 30 psi, the local stress around the bolt holes exceeded the yield strength of the metal from which the split rims were fabricated. Therefore, the design and fabrication of the subject outer split rim carried ABSOLUTELY no design safety factor,

  12. The incorporation of the star doubler plate into the outer split rim design would strengthen the area around the bolt holes.

  13. The Finite Element Analysis indicated incorporation of a star doubler plate moved the highest stress, the most probably area of failure, from around the bolt holes to beyond the rounded end of the star doubler plate into the body of the split rim.

  14. Moving the area of failure away from the bolt holes precluded the type failure that resulted in the subject accident.

  15. Moving the area of probable failure to the split rim beyond the end of the "star doubler plate', changed the failure mode from extrusion or "punching shear" to circumferential cracking and progressive failure of the split rim.

  16. If the subject rim, with a star doubler plate in place, were to crack, no violent, unexpected "explosion" or rupture of the split rim wheel would occur.

  17. Cracking of the split rim as explained in 16) above would give a user warning (by visibly seeing a crack in the split rim) before that crack (or cracking) would have grown circumferentially prior to final separation.

  18. The subject split rim wheel was defectively designed and manufactured.

Click the link below to read the full report.

Full Report

 

Updated 10/19/10