"relentless work" is correct and usable in written English. You can use it when referring to someone who is continually working hard at something without giving up or taking a break. For example: "Through his relentless work, John was able to finish the project ahead of schedule.".
Time and time again Mexico has been ranked within the top 10 hardest-working countries in the world, testimony to the relentless work ethic that governs the nation.
It's an accomplished, affecting, relentless work.
The relentless work left little opportunity for education.
I'm so lucky I can support myself, though it is sort of relentless work".
I just want people with a really relentless work ethic, and a pursuit of excellence.
"He has a relentless work ethic and wants to kill everything in e-commerce.
This obsession with detail was accompanied by a relentless work ethic.
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Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia