(化學)材料科學的問題!很急!請幫我的忙!!

Briefly explain the difference between fatigue striations
and beachmarks, both in terms of (a) size and (b) origin.

(a) With regard to size, beachmarks are normally of
macroscopic dimensions and may be observed with the
naked eye; fatigue striations are of microscopic size and it
is necessary to observe them using electron microscopy.
(b) With regard to origin, beachmarks result from
interruptions in the stress cycles; each fatigue striation
corresponds to the advance of a fatigue crack during a
single load cycle.

NOTE: Not all fatigue failures will show beachmarks.
Beachmarks are the result of oxidation and the startingstopping
of the applied stress which is causing the crack
to grow.