How can repeated heating and cooling cause a vacuum tube to fail?

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electron tube Elements of the simplest electron tube, the diode.

Repeated heating and cooling of a vacuum tube causes failure through a process known as thermal fatigue. As equipment is switched on and off, the internal components of the tube undergo cycles of expansion and contraction. Over time, this stress leads to a physical fracture within the tube's interior structure, rendering the device inoperable.1

While vacuum tubes were essential for early technologies like radio and the ENIAC computer, their fragility and tendency to wear out—either through thermal fatigue or the degradation of the cathode by residual gases—eventually motivated the development of more reliable transistor technology.1