How did I void my iPhone 5 warrenty

so i bought a iphone 5 off of ebay and with the iphone came a warranty card so after a month or so my iphone died and would not come back on so i suspected that my iphone battery had died out i called the place where i had recived my phone from and they sent me a battery and the tools upon request never once did they say that i could void the warranty by lifting the screen up and and after replacing the battery they they phone still would not power on so that when i went ahead and sent the phone in so they could diagnose and fix the problem they said that im having logic board issues with my device so basically what im asking is can they say i voided my warrenty and if so how because there wasn't a disclaimer on the company's website or on the warranty card itself telling me i could void the warranty at all....and do u think this will stand up in court ?