These words are interchangeable in their meaning of "weak" and "delicate," but they do have distinct uses. Fragile is the opposite of sturdy and suggests that something must be handled carefully to avoid breakage or damage: "This beautiful china is fragile." Brittle also refers to something that snaps or breaks into pieces but is usually applied to objects that have a hard surface or finish: "This material is as brittle as glass." Frail applies particularly to persons, rather than things, and usually refers to such matters as health, mental qualities, and temperament: "After a wasting illness, his body seemed more frail than ever." "His bold hopes for the future have a frail foundation." Bric-a-brac is fragile, some conversation is brittle, and an invalid is frail.
Dictionary of problem words and expressions. Harry Shaw. 1975.