Failure analysis of a 41-m long neutron beam line guide

A 41-m long glass tube structure, used to guide low energy neutrons to a backscattering spectrometer, fractured in five places in mid-September 2011. The structure was made of hundreds of 1.5 cm or 2.5 cm thick bonded borosilicate glass plates. Fractographic examination identified the fracture origins and the stress states when fracture occurred. The cause of fracture was traced to damage introduced during the magnitude 5.8 earthquake that occurred 150 km away in Virginia on August 23, 2011. Lateral displacements of the earth created bending and torsional displacements in the beam line of as much as 2 mm caused cracks to form in the glass plates. Finite element modeling verified the displacements and stresses that caused damage.