Simple past tense and past participle of flatten.(verb)
Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License
Use "flattened" in a sentence
"Her expression flattened; she let go of my hand, got up from the table, and walked out into the hotel to look for another sponsor."
"It is shown that the rotation curves for the Milky Way, NGC 3031, NGC 3198 and NGC 7331 are consistent with the mass density distributions of the visible matter concentrated in flattened disks."
"It seemed such a little thing, so bright and small and still, faintly marked with transverse stripes, and slightly flattened from the perfect round."