Answered By: Beth Hylen
Last Updated: Sep 30, 2016     Views: 29

Have you seen the new video about Prince Rupert's Drops? It gives a good explanation of the internal tension found in the drops.

In the sources I checked, I did not find an experiment trying to scratch or crush the bulb.

I can only tell you what happens to tempered glass when it is abraded or scored - the glass quickly fails.

Shand and McLellan's Glass Engineering Handbook states that:

"Failure always results from a tensile component of stress, even when the load is applied in compression. Glass is much stronger under compressive loads than under tensile loads. The spread of breaking stresses obtained from testing a large group of glass specimens is much greater than that from a corresponding group of metal specimens, so that the breaking stress for any individual specimen cannot be predicted with accuracy. Fatigue, or the decrease of breaking stress with increased duration of load, is pronounced under conditions of static loading. There is little difference between the effects of static and cyclic loads on the fatigue of glass when considered over the same interval. The size effect is ordinarily pronounced for glass. Fine fibers may have many times the nominal strength of larger sections.

"It is now generally accepted that fracture of glass originates at small imperfections, or flaws, the large majority of which are found at the surface. A bruise or contact with any hard body will produce on the surface of glass very small cracks, or checks, that may be invisible to the eye but that, because of their extreme narrowness cause a concentration of stress that may be many times greater than the nominal stress at the section containing them. Metals, because of their ductility, yield at such points and equalize stress before failure occurs. Since glass cannot yield, the applied stress, when it is high enough, causes these flaws to propagate. The local stress required for the slow propagation of a flaw is probably of the order of 6900 MPa ... for fractures by impact, it may be several times this value....

"Figure 6.1 shows the frequency distribution of bursting pressure obtained from tests on 152 pieces of gage glass tubing and the estimated depth of the flaws that will produce this distribution.... Different methods of fabrication and treatment of glassware produce flaws of different characteristics and sizes. Table 6-1 provides a list of breaking stresses as correlated with these factors. These values can be no better than rough approximations and many exceptions to them can be cited....

Curves such as those of Fig. 6-1 and flaw distribution functions obtained by analysis are useful in the theoretical study of strength of glass. Table 6.1 shows clearly that strengths obtained are not basic characteristics of the material. Commercial ware fabricated under conditions seemingly identical, but at different periods, may differ significantly with respect to both strength level and flaw distribution." pp. 6-1 to 6.2.

                   Shand, Errol B.
                   Glass engineering handbook. George W. McLellan, technical
                   consultant, Errol B. Shand, technical consultant. 3rd ed.
                   New York : McGraw-Hill, c1984. [ca. 500] p. in various pagings.
                   Includes bibliographical references and index.  TP857 .S52 1984

I scanned and attached the chart for you.

The chapter continues with formulas for various types of stresses.

It seems probable that either scoring or pressure would eventually result in failure of the bulb. It would be an interesting experiment!

I added a bibliography for Prince Rupert Drops.

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Please let me know if you have further questions.