Sergej Aman

Fast Modification of Microdischarge Emission Bands by Fracture of Sugar

Intensive light pulses with widths of about 10 ns were observed during the crack propagation by cleavage of crystalline sugar at atmospheric pressure. The observed light pulses were caused by a sequence of gas microdischarges (MDs). The pulses were detected in the UV/VIS and NIR wavelength ranges. First the light pulses appear in the UV/VIS wavelength range, and then after a delay of about 0.4–1.2 ns in the NIR wavelength range. This characteristic feature of MDs can be used for the characterization of crack propagation.

Microwave based method of monitoring crack formation

The formation of cracks in glass particles was monitored by application of linearly polarized microwaves. The breakage behavior of glass spheres coated with a thin gold layer of about 50?nm, i.e. a thickness that is lower than the microwave penetration depth, was tested. In this way the investigation of fracture behavior of electronic circuits was simulated. A shielding current was induced in the gold layer by the application of microwaves. During the crack formation the distribution of this current changed abruptly and a scattered microwave signal appeared at the frequency of the incident microwaves. The time behavior of the scattered signal reflects the microscopic processes occurring during the fracture of the specimen. The duration of the increasing signal corresponds to the crack formation time in the tested specimen. This time was estimated as particle size divided by crack development speed in glass. An intense emission of electrons occurs during the formation of cracks. Due to this, coherent Thomson scattering of microwaves by emitted electrons becomes significant with a delay of a few microseconds after the initial phase of crack formation. In this time the intensity of the microwave signal increases.

Light emission during impact stressing of a particle layer

The mechanical stress detection technique was developed based on light emission properties of ZnS:Mn particles. The light emission properties of ZnS:Mn particles were characterized by the use of the impact tester that includes a stressing tool, photomultiplier and a contact time measurement system. The mechanical stressing of particles was caused by the impact of a metallic ball, dropped from different heights. At impact, the metallic ball achieves direct contact with the upper surface of the metallic anvil. This allows the measurement of the contact time by means of the electrical current that flows between the anvil and the metallic ball during contact time. The stress, caused at the collision, is transmitted through a metallic anvil to the layer of particles and produces the deformation of particles. The applied stress was detected using a piezoelectric sensor. It was shown that the ZnS:Mn particles generate the light during the action of the loading force. After removal of the loading force the light emission from the particle layer disappears in a few microseconds. The measurement was carried out using different ranges of applied forces. In this way, it was shown that the particle layer exhibits a high damping factor and failure resistance. One of the possible applications of these sensor systems based on light emission properties of ZnS:Mn particles is structural health monitoring.