Study on Stress Luminescence Phenomena
Shigeki Matsuo
Department of Mechanical Engineering, Shibaura Institute of Technology
1.

OutlineSolid materials that emit luminescence upon fracture (e.g., sugar ice) have been known
for a long time. In 1999, a material that emits strong luminescence (observable with thenaked eye) under stress in elastic deformation region was reported for the first time. [1]In the following, the latter will be referred to as “stress luminescent material”. A stressluminescent material stores energy by absorbing short wavelength light, and releases thestored energy as luminescence when stress was loaded. We have conducted basic researchon the luminescence properties of a stress luminescent material.
2.

Results
We used strontium aluminate (SrAl2O4:Eu2+) in the experiments. This is the firstreported and most representative stress luminescent material. Powder of strontium
aluminate was mixed with liquid epoxy resin and poured into a 6 mm square mold, thenthe resin was cured. In this process, it was difficult to distribute the powder evenly; thepowder was distributed mostly along one of six planes of the epoxy cube.
Next, we developed a device to irradiate with blue laser light (wavelength of 405 nm),apply a load, and measure intensity of luminescence due to stress. Figure 1 shows thephotograph of this device.

Figure 1: Device for measurement of stress luminescence.
LPF: Low Pass Filter.
-1-BPF: Band Pass Filter,A load is applied from the top of the sample, a laser beam is irradiated from the side, andluminescence is detected in the orthogonal direction. The load cell is eDPU-1000N.
Two filters are placed between the sample and the optical sensor (photodiode) to cut off
the laser light and detect only the luminescence. The intensity of luminescence ismeasured as voltage. In the measurement, the following cycle was repeated three times:
The contact section is lowered at 70 mm/min to contact the sample, then holds thespecified load for 0.3 s, and returns to the initial position at 300 mm/min. A dark box was
also fabricated, which covers the entire apparatus including the force test stand.
First, basic measurements were carried out, and it was found that the intensity ofluminescence under load was stronger when the powder-rich plane was placed on the topor bottom. This is consistent with the numerical analysis of the stress distribution bysimulation. It was also found that 10 seconds of laser irradiation before the application ofload was sufficient, and that the intensity of luminescence did not increase even after
longer irradiation.
Next, we investigated the relationship between the retained stress and the luminescenceintensity. In this experiment, the above cycle of sapplication of load was started 10seconds after the laser beam irradiation. Figure 2 shows a graph of the time versus
luminescence intensity; the peak around 2 seconds indicates the luminescence caused by
the first application of load.

Voltage [V]

54.5

50N

100N

300N

4500N

700N

900N

3.5
3
2.5
2
1.5
1
0.5
00

12

3

4567

Time [s]Figure 2: Time variation of intensity of stress-luminescence-2-8910

1.2

Voltage [V]

10.8
0.6
0.4
0.2
010100load [N]Figure 3: Relationship between luminescence intensity (difference between voltagevalues of the first peak and valley just before the first peak) and load

The relationship between the retained load and the luminescence intensity (first cycle)
obtained from this experiment is shown in Figure 3. As shown in Figure 3, theluminescence intensity increased linearly with the logarithm of the load. Here,logarithm was taken just to fit the data; there is no theoretical background.
3.

Progress with the Introduction of Force Test StandIn this study, the EMX-1000N force test stand(IMADA Co., Ltd.) was used to apply the load. Inthe past, the load was applied by a self-made device
using a lever (Fig. 4). However, it was difficult toobtain a sufficient load for luminescence. Thus we
attached a weight to the furthest point from thefulcrum of the lever and let it fall, so that
instantaneous impact force was applied, then thesample emitted luminescence. Although we could Figure 4: load application
device before introducing force
observe the luminescence with this device, there
test stand.
The horizontal
were several drawbacks, such as the inability to bar is the leverage.
apply the load for a long time (control of the load intime axis was impossible), variation occurred in each trial, the load applied to the leverrod was so large that the lever wad deformed, and this device was not suitable forautomation. By introducing the force test stand, it is possible to apply stress quantitatively,-3-1000

and control the stress in time accurately.
4.

Future development
In this experiment, it is necessary to control the illumination (light irradiation to thestress luminescent material) and measure the luminescence intensity, in addition to theapplication of load. Our current task is to automate all of these processes so that they canbe controlled from a personal computer.
Moreover, the spectrum width of luminescence becomes narrower at low temperatures,
then the wavelength of luminescence can be measured more precisely. For this reason,
we would like to build a device that can apply a load at low temperatures.
5.

Acknowledgements
We would like to appreciate IMADA Co., Ltd. for providing us with equipment suchas Force Test Stand, Force Gauge, and Load Cell.
The powder of strontium aluminate used in this study was prepared by MorigaLaboratory, Department of Chemical Science and Technology, Tokushima University. We
would like to express our gratitude to them.
Many students contributed to this research are also acknowledged. The results shownin this report are mainly obtained by Mr. Yo Shitara.
References[1]

C.-N. Xu, T. Watanabe, M. Akiyama, and X.-G. Zheng, “Direct view of stress
distribution in solid by mechanoluminescence,” Appl. Phys. Lett., vol. 74, no. 17,p. 2414, Apr. 1999, doi: 10.1063/1.123865.

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