An ARP bolt will stretch and it is how many pro engine builders measure rod bolts during installation. Lastly the bolt will stretch or pull the threads right out of the crank snout if continued to re torque with out loosening first. Every bolt will stretch a little and relax after heat cycles it may appear to be at a different torque or appear to be loose. The only accurate way to know is to measure its length just like they do on rod bolts. Ever wonder why there is a dimple in the head of an ARP bolt. That's for measuring.
Here's a link to ARP read up there's a lot of good info.
http://arp-bolts.com/p/technical.php