Fracture Strength Measurement of Miniature Internal Thread in Glass
Shigeki Matsuo
Department of Mechanical Engineering, Shibaura Institute of Technology
1.

Outlinewater
In tour laboratory, we have been studying thefabrication of miniature internal threads on glass
glass
slides. Glass is a typical brittle material, thus it isdifficult to machine. Laser processing is one of themethods to process glass, and it is used for cutting
and drilling through holes. However, it is difficult
lens
to fabricate holes other than simple straight holes,Laser pulse
such as internal threads.
Thus, we used a technique called "in-liquid Figure 1: Sheme of fabricating
miniature internal thread in glass
ablation" or "liquid-assisted laser processing". In using
water-assistedlaser
this technique, the solid material to be processed processing.
is brought into contact with a liquid, water inthis case, and laser pulses are focused near theinterface between the solid and liquid (Fig. 1).
By this method, we were able to fabricate an
S0.5 internal thread. [1]However, when the fabricated screw was
broken and its cross section was examined,cracks were observed as shown in Fig. 2. We
expected that the strength of the internal thread
100 μmwould be degraded because of the cracks.
Therefore, in this study, we inserted a male Figure 2: Cross section of an internalscrew and pulled on it to measure the strength thread fabricated in a glass slide.
of the fracture around the internal thread andthe removal of the male screw. In addition, weload
attempted to improve the strength by heat treatment.
2.

Results
We inserted a commercially available S0.5 malescrew into the S0.5 internal thread on glass slide.
Then, we attached a hook to the head of the malescrew, and pulled the male screw to measure thestrength at which the glass slide was fractured (Fig.
3). The hook was attached to the eDPU-50N load

-1-glass

Figure 3: Scheme of measurementof fracture strength.

cell and pulled up using the EMX-1000 N test stand (Fig. 4). The tensile speed was set at
10 mm/min.
Example of the results is shown in Fig. 5. This
figure shows the data for three samples (three glassslides) without heat treatment. Three data areoverlaid. As seen, the load increased gradually withtime and then decreased rapidly after the maximum.
There were also cases where smaller local maxima
were observed around the maximum value (red andgreen lines). We interpreted this data as follows:
sometimes growth and fracture of small cracks dueto the load did not result in the fracture of wholethread structure; fracture of whole thread structure
occurred when larger load was applied. The averagevalue of the maximum load in these three trials was Figure 4: Photograph of fracture
strength measurement device.
about 17 N.

Figure 5: Example of results of fracture test (sample without heat treatment).

Samples after heat treatment were measured in the same way. Heat treatment wascarried out in an electric furnace with an air atmosphere. The sample was kept at 570 °C
or 620 °C for about 30 minutes. When the sample was treated at 630°C, the internal
thread was deformed and no longer functioned as thread, thus 620°C seems to be theupper limit temperature. After heat treatment at 570 °C, the strength was almost the same-2-as that without heat treatment. After heat treatment at 620 °C, the average value of themaximum load became about 27 N, indicating an improvement in strength. In addition,a decrease in cracks was observed in the cross section. However, a calculation with asimple model indicated that the value of 27 N corresponds to the maximum tensile stressin the glass about 1.3 MPa,[2] which is more than an order of magnitude smaller than thegeneral strength of glass (50 to 100 MPa), so there is still room for improvement.
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 forfracture. In the past, the load was applied using aprimitive method of hanging a bucket of water (Fig. 6).
By introducing the force test stand, the load can now bevaried continuously and stably. In addition, thephenomenon of multi-step fracture, which was notexpected before, was found for the first time by using
this device.
4.

Future development
We are currently working on the development of anew processing method to prevent cracks, rather thanreducing cracks by heat treatment. This has not yet beenrealized, but when it is, we hope to measure the fracture
strength.

Figure 6: load application
method before introducing
force test stand.

5.

Acknowledgements
We would like to appreciate IMADA Co., Ltd. for providing us with equipment suchas Force Test Stand, Force Gauge, and Load Cell.
Many students contributed to this research are also acknowledged. The results shownin this report are mainly obtained by Mr. Ryosuke Yaekashiwa.
References
[1]

[2]

H. Degawa, N. Urano, and S. Matsuo, “Laser Fabrication of Miniature Internal
Thread in Glass Substrate,” Micromachines, vol. 8, no. 2, p. 48, Feb. 2017, doi:
10.3390/mi8020048.
R. Murakami, H. Nakagawa, and S. Matsuo, “Water-Assisted Laser Drilling for
Miniature Internal Thread in Glass and Evaluation of its Strength,” J. Laser
Micro Nanoeng., vol. 12, no. 3, pp. 203–206, 2017, doi:
10.2961/jlmn.2017.03.0005.
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