Investigating what caused a ladder to break

When a window installer fell, MEA Forensic was called upon to investigate whether this was due to a fault in the ladder.

The Event:

A window installer ascending a trestle ladder, and carrying a window pane, fell to the ground and was injured. During the aftermath, it was observed that the ladder was broken. MEA was asked to determine if the ladder failed and led to the fall and, if so, what caused this failure.

Our Analysis:

First, our forensic experts conducted a detailed examination of the ladder. It was a trestle ladder, in which two symmetric halves were hinged together at the top. The two halves made an inverted “V” when the ladder was set up for use. It was observed that the hinge that held the two halves together had pulled out of one of the ladder halves. The hinge had been secured to the top of the ladder with six rivets. All three rivets had pulled out of their holes on one side, and there were cracks emanating from each rivet hole.

Based on the pulled-out hinge and cracked rivet holes, it could not be determined if the hinge pulled out because it was overloaded when the ladder fell and struck the ground or if the hinge let go first which then led to the fall. Our engineers used metallurgy and fracture mechanics to determine which of the two scenarios was more likely.

The ladder was made of aluminum but its properties, necessary for subsequent calculations, were not known. Using metallurgy, the aluminum’s properties such as alloy chemistry, heat treatment, strength, and fracture toughness were determined.

With the material properties known, we then considered the loads on the ladder hinge and rivets. From conventional stress analysis, we found that the ladder was strong enough to support a person and the window pane. However, from fracture mechanics, we determined that the ladder was not strong enough to resist cracking at the rivets when exposed to repetitive climbing. Repetitive climbing introduced cyclic stresses on the aluminum at the rivets. These cyclic stresses resulted in fatigue cracks at the rivet holes, which ultimately caused the hinge to let go and the ladder to fall.

The Results:

Our work helped our client show that the ladder failed and led to the fall. Furthermore, our results showed that the ladder failure was due to a design defect. The designers of the ladder had built it strong enough to withstand the weight of a user and a window but had not considered fatigue. Fatigue is the phenomenon where certain metal alloys, notably aluminum, begin to crack when loads are repeatedly applied and removed, as when a person ascends and descends the ladder. The ladder was not built to withstand repeated climbing and descending and developed fatigue cracks that eventually led to its failure.

Fracture mechanics and metallurgy formed the foundation for our conclusions. Our experts routinely conduct metallurgical analysis in our laboratories, and carefully examine cracks in a variety of materials to determine what caused them.

If you have questions about how a piece of equipment failed, our Failure Analysis group can provide answers.

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