brüchig
brittle
Describes a material that is brittle and breaks or falls apart easily.
Altes Plastik wird mit der Zeit oft brüchig.
Old plastic often becomes brittle over time.

brittle / shaky
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brittle
Describes a material that is brittle and breaks or falls apart easily.
Altes Plastik wird mit der Zeit oft brüchig.
Old plastic often becomes brittle over time.

The adjective 'brüchig' (brittle) is derived from the verb 'brechen' (to break). It describes the quality of something that breaks or shatters easily.
Vorsicht, das alte Holzbrett ist sehr brüchig.
Careful, the old wooden board is very brittle.
The noun that 'brüchig' describes reveals the meaning. Is it a material ('Holz' — wood, 'Knochen' — bones), a voice ('Stimme' — voice), or something abstract ('Frieden' — peace, 'Beziehung' — relationship)?
Die alten Knochen des Skeletts waren porös und brüchig.
The old bones of the skeleton were porous and brittle.
In its literal meaning, 'brüchig' often describes materials, as in 'brüchiges Eis' (brittle ice), 'brüchiges Gestein' (brittle rock), 'brüchige Knochen' (brittle bones), or also 'brüchige Fingernägel' (brittle fingernails).
Im Winter warnen Schilder vor dem brüchigen Eis auf dem See.
In winter, signs warn about the brittle ice on the lake.
The combination 'brüchige Stimme' (shaky voice) is a very fixed expression. It is almost always used to show that someone is moved by emotions such as sadness, fear, or being touched.
Mit brüchiger Stimme gestand er seine Fehler ein.
With a shaky voice, he admitted his mistakes.
tired
'müde' and 'brüchig' can both refer to material that no longer holds up reliably. 'müde' describes the cause or the process: a material becomes weaker through long-term stress. 'brüchig' describes the state: something breaks easily or falls apart. So a material can first become 'müde' (fatigued) and then be 'brüchig' (brittle). Learners may confuse the words because both have to do with weakness and breaking. For people, you normally say 'müde', not 'brüchig'; for things that break easily, 'brüchig' is more natural.

Im Keller ist das Holz der alten Kiste brüchig.
In the basement, the wood of the old crate is brittle.
Der alte Draht ist durch die ständige Bewegung müde geworden.
The old wire has become fatigued from the constant movement.