Author: britrifles
Subject: Reloading Info for 174 gr Bullet
Posted: May 13 2020 at 12:21pm
Zed,
Subject: Reloading Info for 174 gr Bullet
Posted: May 13 2020 at 12:21pm
Zed,
There is certainly bi-axial stress in the case from the internal pressure. When the case yields in the radial direction (which occurs first since the hoop stress On the case is 2x the axial stress), it will first pull the head forward and take up any head clearance. But as pressure builds, and radial expansion is restrained by the chamber, the case head will then get blown back firm against the bolt. It is this plastic axial overload strain on the brass that causes the thinning (“necking” is the technical term). You will see in the photo above that it occurs above the juncture of the web and solid head area, not at it. The exact location of the necking will likely vary somewhat depending on specific case dimensions.
This is not a fatigue crack, the thinning (necking) is an overload rupture failure from exceeding the materials yield (and ultimate) strength.
In reality, it is probably more complex than this involving localized bending stresses at the juncture of the case head, as you pointed out, the chamber diameter is considerably larger than the case and the case head restrains radial expansion at this point. The length of the case that has not firmly gripped the chamber also plays a role, so another effect of chamber geometry.
I’m no expert, not trying to insinuate I am, just applying some basic engineering principles to the problem.
How much of this is also influenced by the rear locking lug design would take some calculation (how much the bolt compresses under load) but that’s just more than we need to know. Reload until you see the white line and then toss the case. If your fortunate to have some spare bolt heads, try to minimize the head clearance based on the brass you are using. But, I would be very careful in reducing headspace below minimum (0.064), this could result in not being able to lock the bolt on a thick rim cartridge, that would be potentially dangerous.