Mechanoluminescence (ML) induced by fracture
of solids is known as fracto-mechanoluminescence. Nearly one half percent of
all inorganic salts an organic molecular solids exhibit the phenomenon of
fracto ML. The fracto ML can be induced by slow compression, stretching,
bending, pressure steps, pressure pulses, impulsive deformation, cutting,
cleaving, grinding, rubbing, shaking, scratching, laser shocks, ultrasonic
shocks, drastic cooling or heating of solids. The fracto ML of certain organic
and inorganic solids such as ditriphenyl phosphine oxide manganese bromide,
Eu,Dy co-doped strontium aluminate, europium tetrakis ( dibenzoylmethide)
triethyl ammonium, impure saccharin etc. is so intense that it can be seen in
day light with naked eye. As fracto ML is produced during the fracture of
solids, there exists a systematic correlation between the signal and source,
that is, between the ML and fracture of solids. This inherent correlation has
been exploited in the recent past to design fracture sensors (Chandra and Zink
1980a, b, Xu et al. 1999a), damage sensors (Sage et al. 2001, Sage and Bourhill
2001), and the fuse system for army war head (Dante 1983). The ultimate aim of
the research in fracto ML is to get prior information of the occurrence of
earthquakes and mine failure as these events are followed by the light
emission. It has been reported that the earthquake light occurs before the
occurrence of earthquakes, and therefore, it may be used as earthquake sensor
(Freund 2003, Freund 2006, Freund 2010).

To understand
the process involved in ML would be the monitoring of the evolution of physical
states of the sample independent of the light emission. This type of attempts
have been made for a large number of samples, and it has been found that, in
most cases, the light excitation and emission themselves are so rapid that
their evolution serves as a monitor of the physical changes occurring during
the deformation and fracture of crystals. Only in a few cases, the luminescence
lifetime is longer than the material relaxation time. Generally, excitation and
emission of gas and solid occur as dislocations move through the material, as
fracture propagates or as excited surface states are formed and destroyed. The
ML emission as well as excitation occur over significant time duration from
many different excited states with different lifetimes, and give meaningful
information about the deformation, the crack propagation, and the relaxation of
the material after deformation and fracture. Moreover, the comparison of the
results related to relaxation in air and vacuum gives insight into the role of
air, and especially oxygen in the process of ML. In the earlier devices used
for measuring the time-dependence of ML, the response of detectors was slow,
however, modern photon counters are much faster and more sensitive and they can
be used for measuring the ML emission during deformation and fracture, as well
as for monitoring the erratic decay of ML after deformation and fracture with a
resolution of nanoseconds (Dickinson et al. 1984a, Dickinson et al. 1982a, and
Dickinson et al. 1982b).