The word "rend" (Verb: "to tear (something) into pieces with force or violence") is such an effective word. Descriptive and visceral. Yet it seems to me it's fading from literature and becoming an antiquated term used mainly in pre-20th century literature. Is this true? It would be a shame to let such an effective word fall into disuse.
This is a case where Ngram has to be used very judiciously. Looking at the results for "rend", there appears to be a very steep drop in usage beginning right around 1959. However, the vast majority of actual results for the period immediately preceding this drop are not for the verb but for the abbreviation "rend." which is part of the abbreviation for Comptes rendus de l'Académie des sciences. Per Wikipedia, this is a French scientific journal that has been published since 1666. Abbreviations found by Ngram include Compte. rend., C. rend., and occasionally just Rend.
Also relevant is the fact that although Ngram claims to be case-sensitive, some of the results found for "rend" are actually "Rend." Searching for "Rend" shows a very sharp up-tick in usage corresponding to the drop in "rend" which I take to reflect either a change in capitalization of the journal's abbreviation or a change in how well Ngram identifies case.
In any case, "rend/Rend" drops like a stone beginning in 1966. This corresponds quite nicely to the splitting of Comptes Rendus into multiple sections, now abbreviated "C.R.[appropriate abbreviation of sub-title]."
Additionally, in earlier eras the instances of "rend" were at least equally likely to be the French for "makes" (preserved in our English "rend it useless" etc.) as the English word meaning rip; in recent decades, the English corpus includes far fewer instances of extended passages in French.
None of which means that the word isn't declining in usage. Looking at phrases like "rent asunder" and "rend in pieces" shows a peak of usage in the 1840s and then a smooth, gradual decline in usage through the 1980s. I suspect this was because we have both rip and tear which can mean the same thing. The fact that the past tense is the irregular "rent" also doesn't help--in these phrases, rent is anywhere from five to ten times more common. But if I say "I rent the dress" do I mean that I ripped the dress, or that I loan it out for pay? Much easier to say "I ripped the dress."
However, from the 1990s to 2008 (the latest year for which Ngram shows data) usage of these phrases picked back up--starting with "rend asunder" in 1990 with most of the other variations starting to rise again around 2000. I'm not sure what that's about.