Two dimensional infrared resolution of shear bands formed during low velocity impacts of NaCl crystals using fast infrared detectors

Abstract

NaCl crystals were subjected to low velocity impacts provided by a drop weight impact machine. The shear bands were imaged with the use of arrays of fast responding infrared detectors. The detectors monitored the amount of energy given off as infrared photons by the impacted crystalline solid. The main emphasis of this study was the real time observance of the shear band formation during the impact. The signal was recorded over the duration of the impact, heating I of the solid during its crushing, and the relaxation of the crystal. These crushing processes occurred over a 40-100 ,us timescale. Rapid infrared photons corresponding to shear banding were recorded on a 30-90 ns timescale.

Hot spots in energetic materials generated by infrared and ultrasound, detected by thermal imaging microscopy

“…1 The MWIR camera was specified by the manufacturer to produce single snapshots with a time gate of 0.48 μs, but in practical use 3 μs appeared to be needed to provide the full signal-capture capability. Thus, this MWIR camera could obtain an image, in single-image capture mode, as in the Woody studies, 13,14 with 3 μs time resolution, given proper synchronization and provided the hot spot was hot enough to produce significant thermal emission during that time window. We estimate, based on the results obtained here, that such highspeed single-image capture requires a minimum temperature of ∼460 K.…”

“…6,7 As a benchmark, it has been shown by calorimetry that RDX decomposes with a strong exotherm when heated to 530 K. 8 Almost all previous observations of hot spots in EM relied on visible emission techniques such as high-speed photography. 1,2,[9][10][11][12] Notable exceptions were two 1992 studies by Woody, 13,14 who used a fast IR detector array to obtain single time-gated images of shear bands created in salt crystals subjected to low-velocity impacts, and more recent works by Dickson and co-workers 15 and Perry and co-workers, 16 who studied impacted EM with a combination of visible and IR imaging.…”

Ultrasonic hammer produces hot spots in solids

“…The process has been widely presumed to involve hot spot formation within the EM by thermo-mechanical energy deposition at specific microstructures [11][12][13][14][15] . Direct experimental evidence for such hot spots, however, is so far exceptionally limited, the mechanisms for their formation are poorly understood and one's ability to image and control hot spot locations remains elusive 16,17 . Typical solid explosives are composites of EM crystals with polymer binders 18 .…”