Inside the failed sockets, the zinc casting that held the cable wires in place experienced significant material flow, or creep, allowing some of the wires to slip.
The neutron imaging revealed that the zinc flowed slowly under stress when the splayed-out cable wires alone could no longer resist the tension on the cables. The sockets failed because the zinc continued to flow until the cables entirely pulled out of the sockets.
“Our analysis detected significant slippage of wires in the region of interest, on the order of multiple wire diameters,” Brügger said. “This indicates that the wires slipped at some point, but it cannot be surmised whether this slipping occurred during the structural failure event or before.”
Although the final report by the forensic engineers concluded that other natural forces had only a negligible effect on the collapse, the earthquakes and hurricanes that impacted Puerto Rico over the years do generally add dynamic loading on such structures. “I suppose you could say the telescope at Arecibo had been shaken and stirred by those events,” Brügger quipped, alluding to James Bond’s favorite drink.
HFIR’s MARS CG-1D instrument, formerly named IMAGING, was used to perform high-penetration neutron imaging of the samples.
Like X-ray imaging, neutron imaging is very useful in the field of nondestructive testing. Neutrons, however, are ideal for tasks that are difficult or impossible for conventional X-ray imaging, including studying light elements, such as hydrogen and carbon. Neutrons can also penetrate heavy metals, such as lead and titanium, facilitating the study of complex engineering materials in a wide range of sample environments.
The Thornton Tomasetti report, which includes the neutron analyses, states the two cable failures that occurred before the collapse and the third cable failure that triggered the collapse all happened near or inside zinc-filled sockets at cable ends. Each failure involved both the rupture of some of the cable’s wires and a deformation of the socket’s zinc, and was therefore a failure of the cable/socket assemblies.
HFIR is a Department of Energy Office of Science user facility.
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