Can a piece of glass be fragile if it never breaks?

To classify something as 'fragile' is just to identify it as the sort of thing that is relatively easy to break (i.e., that by its nature it will break if stressed in certain way(s) beyond certain extent(s) that aren't uncommon for the relevant context(s)). We classify wine glasses as fragile because their material and shape makes them very easy to break in fairly common interactions, like being dropped on a hard floor.

Setting aside the details of the above definition, the bottom line is that classifying something as 'fragile' is identifying an aspect of it's causal nature. That is, one is identifying a thing's potential for certain (inter)action(s). This is quite different from identifying an actual effect which that causal nature has in fact brought about, and we have a different concept for precisely that: 'broken'.

Potentials aren't actuals -- and importantly, both are quite common in our everyday efforts to navigate reality. So if we aim to live on earth we have an urgent cognitive need to be able to distinguish the two conceptually -- hence that handy conceptual pair of 'potential' vs. 'actual', and more specific pairs like 'fragile' vs. 'broken'.

I wonder if the questioner is familiar with the potential-actual distinction, and with the principle that a potential can exist even if the potential has not yet been actualized.

(This question also sounds similar to the claims of Quantum Physics that quantum things might not actually exist in a given form and place until an experiment is set up to detect them in that form and place.)

Update: Primary vs. Secondary Qualities

There is undoubtedly a question somewhere in the questioner's material, but I find that I need considerable effort to infer what it might be, and I remain unsure if my inferences are correct. I noted several specific issues, however, which I can address from the literature of Objectivism, including the ITOE material noted by the questioner.

1) Actual vs. Potential. See ITOE 2nd Ed., p. 285:

Prof. F: ... I thought that you rejected the concept of "potentiality."

AR: No. What made you think that? I have referred to actual and potential in any number of ways in any number of articles. Even if I didn't write on this subject directly, what would make you think that we reject the Aristotelian view on this? ... Unless it was in some context of what Aristotle makes of it, as in regard to his form-matter dichotomy.

This is followed by a discussion of some specific applications, such as Aristotle's views on "prime matter" and "motion" -- followed further by a discussion of "dispositional properties" such as fragility, combustibility, etc.

2) Dispositional vs. Constitutive Properties

This discussion begins on p. 282 in ITOE 2nd Ed. "Dispositional" refers to properties that pertain to the way an entity responds to the way it is "disposed" of, such as burning when heated, as against floating or melting or boiling, or producing a musical pitch like a boiling tea pot, etc. "Constitutive" refers to properties that pertain to the way an entity is "constituted" or constructed. Ayn Rand's own view on this distinction is stated very simply on p. 283:

You are making an artificial dichotomy. Why divide properties into two categories, first of all?

In other words, one can't always "force" the properties of an object into one category or the other.

3) Metaphysical Status of Sensory Qualities

This is a related topic discussed in OPAR, Chap. 2 ("Sense Perception and Volition"), in the subsection titled, "Sensory Qualities as Real" (pp. 44-48). Here are some key highlights:

...the objects we perceive have a nature independent of us.... [Are] colors, sounds, smells [etc.] ... "in the object" ... [or] merely "in the mind" and therefore are subjective and unreal? [If the latter] ... the world of colored, sounding, odoriferous objects they reveal is utterly unlike actual reality.

[Ayn Rand answers:] A thing may not be condemned as unreal on the grounds that it is "only an effect," which can be given a deeper explanation.... One does not detach the material world as we perceive it from reality when one shows that certain elements in reality produced it.... If the elements of reality themselves combine inevitably to produce [objects as we perceive them], then by the nature of their genesis, they are inherent in and expressive of the essence of existence.

That section of OPAR concludes with a discussion of "naive realism" and why it does not apply to Ayn Rand's view.

Once again, the only accurate name for the Objectivist viewpoint is "Objecivism."

For a briefer overview of the Objectivist view of sensation, refer to the topic of "Sensations" in The Ayn Rand Lexicon. OPAR further explains (pp. 43-44):

No type of sense perception can register everything. A is A -- and any perceptual apparatus is limited. By virtue of being able directly to discriminate one aspect of reality, a consciousness cannot discriminate some other aspect that would require a different kind of sense organs. Whatever facts the senses do register, however are facts. And these facts are what lead a mind eventually to the rest of its knowledge.

The answer to the short form of the question, then, is yes. ("Can a piece of glass be fragile if it never breaks?") Is "fragility" dispositional or constitutive? The answer is yes. I.e., it's both; or, rather, it is an objective result of the nature, the identity, of the object (including its structure and material composition). We can't know this by mirror-like sensation or perception alone, but that doesn't mean that our minds create it independently of anything metaphysically existing, or by arbitrarily mixing different metphysical elements together in an unintegrated, haphazard, ultimately subjective, association. (The specifics of what man's mind does on the conceptual level comprise an entire topic in itself.)