Steel doesn’t break quietly. Energy is released when a crack starts to form inside a metal structure under stress, such as a bridge, pipeline, or roller coaster track. Tiny ultrasonic wave bursts leave the injury site and move through the surrounding material more quickly than sound does through the atmosphere. Anyone standing close can’t hear them at all. However, sensors are able to identify them. The engineering community, which has been taking acoustic emission monitoring seriously for decades, is beginning to argue that theme parks should take note of this issue.
Piezoelectric transducers, which are tiny sensors attached directly to the surface of a steel structure, are how the technique operates. Stress waves are created when a microcrack starts or spreads under load. These waves move through the metal until they get to a sensor. These mechanical vibrations are transformed by the sensor into electrical signals, which are then processed by linked software. A computer may triangulate the source with a high degree of accuracy by comparing the times at which the same wave reaches several sensors positioned at various points on the structure. The end product is a map showing the locations where something is occurring inside the steel before it is evident on the surface or big enough to be picked up by standard inspection equipment.
This is what sets acoustic emission monitoring apart from the usual techniques used in amusement parks. Periodic tests include ultrasonic testing, eddy current testing, and magnetic particle examination. They take place during prearranged maintenance windows, either before or after business hours, when the ride is motionless and engineers systematically search the structure for signs of damage. They are important and have served as the foundation for safety inspections of amusement rides for many years. However, they are only snapshots. They describe the condition of the steel while the inspector was present. In contrast, acoustic emission monitoring can operate continuously while the ride is operating, recording damage events as they occur under real-world load circumstances—the same conditions that produce the pressures that initially lead to cracks.
Acoustic emission monitoring fills this gap, which has been shown in the most uncomfortable way. The Ohio Department of Agriculture’s inquiry revealed that the failing fastener had been examined the previous evening and was secure when a metal plate broke off from a train on Cedar Point’s Top Thrill Dragster in August 2021 and struck a customer in line. An “instantaneous overload fracture” was the term used to characterize the failure, which occurred when a component hit its stress threshold and gave way in a way that traditional inspection had not anticipated and could not have detected at the time it happened. The instance does not demonstrate that it might have been avoided with acoustic emission monitoring. However, it exemplifies precisely the type of failure that continuous stress wave monitoring is intended to identify at an early stage.
The industries that have made the most use of acoustic emission technologies are instructive. It has long been used by aerospace to monitor aircraft structural components, where the economics of continuous monitoring are easily justified and the consequences of missing a crack are devastating. AE systems are used by the pipeline and pressure vessel industries to keep an eye out for active crack growth during hydrostatic testing and operational circumstances. Where periodic visual inspection was obviously inadequate, bridge inspection systems in a few of nations have integrated continuous AE monitoring into aging infrastructure. In each of these situations, the technology demonstrated its worth by preventing malfunctions before they happened; it wasn’t always inexpensive to set up or run, but it was significantly less expensive than the alternative.
There are legitimate reasons beyond simply inertia why theme parks haven’t embraced it in a uniform manner. Significant background noise is introduced by a roller coaster’s operational environment, including the ambient acoustic pandemonium of a crowded park, the mechanical sounds of wheels on track, and the vibration of trains launching. Sophisticated signal processing and knowledgeable analysts are needed to distinguish a real fracture signal from that noise. There are actual expenses associated with false positives, such as needless closures, costly follow-up inspections, and interruptions to park operations. Although the technology’s manufacturers have been working on AI-assisted signal filtering that can more accurately differentiate mechanical interference from crack-related AE events, the problem remains extremely challenging.
