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Case Study

Horse Trailer Metallurgical Failure Analysis September 15th, 2003

Background

In the summer of 2002 three family members were driving north after dark on US 101 near the central coast of California. In an instant their car was struck by a large object, removing the roof of their compact car and killing all three passengers. The object of their demise was a 1,000 lb stallion that had fallen out of a trailer being driven 500- 800 hundred feet ahead of them. Based on accident scene evidence and witness testimony a horse was ejected from the trailer when the back door swung open abruptly and the unbridled animal was unable to keep its footing. Sparks emanated from the hind hoofs as the horse tried desperately to climb back into the trailer (which was traveling at 70+ MPH) seconds before it fell out the back, rolled several times and then stood up in the middle of the road.

Approximately 3-5 seconds later the victims struck the standing animal, killing them and the horse. A second horse (mare) remained in the trailer and was unharmed. The driver of the truck pulled over, saw the dead animal and promptly left the scene. At trial he claimed no knowledge of the collision as the victims’ car came to rest in a ditch. The driver contacted authorities several days later admitting to his involvement in the collision after seeing on the news that three people had been killed.

The collision resulted in the driver being charged with three counts of vehicular homicide with a gross negligence enhancement that could potentially put the driver behind bars for 23.5 years. The charges stemmed from the condition of the trailer and its road worthiness. The trailer was in an obvious state of disrepair, and the District attorney determined it was an act of gross negligence to tow such a trailer on a public highway carrying > 1000 kg of livestock. Criminal gross negligence is defined as “a negligent act which is reckless or flagrant and is such a departure from the conduct of an ordinary prudent, careful person as to be contrary for proper regard for human life…”.

There were several issues with the trailer in terms of construction, load capacity and maintenance. The main door was of particular interest because it was flimsy and poorly constructed. This door consisted of soft angle iron and 1/8” plywood and was easily deformed by hand. However, the most important attribute to proving the condition of the trailer was the hinge on the door. Initial inspection suggested the entire upper hinge was broken prior to the collision. If this was true, then the door would be barely be able to stay closed and was in no condition to haul anything, much less two horses on a public highway.

Our investigation primarily focused on the hinges and whether they broke on the night of the collision or had broken prior. So one could say the case hinged on the hinge…

Results of Investigation

The hinges were examined closely and one was removed to be tested in a laboratory. Testing consisted of optical microscopy and scanning electron microscopy (SEM) using energy dispersive spectroscopy to identify the atomic elements present on the surface of the hinge. The EDS results would be important in identifying the presence of paint. Based on our laboratory analysis we concluded the hinge was broken prior to the collision and had likely been broken for a significant amount of time. Our conclusion are supported by three main points.

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1. The hinge was seized in the open position, meaning the hinge if still connected to the frame would require the door be wide open.

Figure 1 illustrates the overall poor condition of the trailer. The door was attached to the trailer frame via two hinges, each hinge had six segments. Of those six segments, three were welded to the door and the other three welded to the frame in an alternating order. The welds were of extremely poor quality with significant undercut, lack of penetration and porosity. Figure 2 illustrates the broken hinges at the top of the door. Figure 3 illustrates the orientation and fracture surfaces of the upper hinge. In this image the door is in the closed position, yet the original contact points (now fracture surfaces) are 90º apart. Only when the door was open did the surfaces line up to their original positions. Had the hinge broken off the trailer on the night of the collision it would have frozen in the closed position (via a bent pin) or would have remained loose to freely rotate about the pin. The hinge pin was not bent and despite significant effort and very large tools we were unable to move the hinge independent of the pin. The hinge could not have frozen in the short period of time between the collision and the investigation. It is postulated the hinge fractured years prior to that fateful night and exposure to moisture caused corrosion to build up in the narrow clearance between the pin and hinge, effectively seizing it permanently.

2. The fracture surface had significant mechanical deformation and oxidation

If it is assumed the hinge fractured on the night of the collision then the resulting surfaces would appear as newly separated having characteristics typical of a fresh-fracture. The surfaces in question however had significant mechanical deformation, oxidation and contamination. This suggests that the hinge fractured, and then over the course of years vibrated against the trailer frame smoothing the surface morphology. A macro-view of the fracture surface is illustrated in Figure 4. Note: the top of the picture is the weld bead and the high point which would have received the most vibratory impact on the trailer frame. The amount of corrosion was excessive for the environment the trailer came from assuming the fracture was freshly created on the night of the collision. Corrosion growth rate is dependent on many factors and can be very difficult if not impossible to accurately date. However, using other ferrous alloys in the same storage location as the trailer a good estimate of corrosion was established. Being indoors the rate of corrosion was very low, even on unprotected plain carbon steel surfaces.

3. Significant amounts of paint were found on the fracture surfaces

The most significant evidence was the presence of paint on the fracture surface. Both the fractured hinges and mating surface on the trailer frame had significant paint overspray. It was established the trailer had been painted with metallic paint several years prior to the collision. Figure 5 illustrates the paint present on the fracture surfaces and is but one example of many observations. The paint being a silver metallic color may appear as a fresh fracture surface to the untrained eye. To prove that paint was present SEM analysis with energy dispersive spectroscopy (EDS) was performed to identify the paint via the elements present. The chemical signature of the fracture surface matched that of a paint sample taken from the front of the trailer. The rebuttal to this was that micro-cracks in the weld allowed paint

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to flow in during the painting process. Based on an interview with a paint expert, this would not happen as spray paint does not flow in this manner and even if some did it could not have covered the entire fracture surface as observed on the trailer.

Conclusion

Based on the evidence the hinge was broken prior to the collision in August 2002. The frozen position of the hinge, deformation and corrosion of the fracture surface and most importantly presence of paint on a fracture surface prove this fact beyond a reasonable doubt. The result of the trial was a hung jury based on other attributes of the case beyond the scope of this analysis. The defendant subsequently pleaded guilty to a lesser charge to avoid another trial and state prison.

The complete article can be found in the August 2006 issue of the Journal of Failure Analysis and Prevention, ASM International.

Figure 1. Overview photograph of horse trailer involved in collision. No bridle was used to hold horse in position so when the door opened the horse had nothing restraining him.

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Figure 2. Door and frame of trailer illustrating the upper broken hinge from trailer. Note each hinge had six segments, welded alternately to door and frame. Weld quality was very poor.

Figure 3. Closer image of broken hinge illustrating the separation and orientation of the fracture surfaces. In this image the door is closed but the hinges are oriented (and frozen) in the open position. Note the “silver” color of the fracture surface. The entire fracture on the frame is covered with silver spray paint.

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Figure 4. Macroscopic view of fracture surface. At the top of the photo (black arrows) is the weld bead fracture which was the high point of the surface. Its smooth morphology was the result of significant mechanical vibration against the trailer frame. The right lower area (above scale bar) had silver speckles on the surface that were tested and determined to be paint.

Figure 5. Broken hinge illustrating fracture surfaces coated with paint. The silver color can be misconstrued as fresh metal breakage. With both surfaces covered with paint and admission by the

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defendant that the trailer was painted prior to the collision, it stands to reason the hinge was broken before the night of the collision.