Materials can deform both elastically (returning to its original shape and structure) and plastically (permanently changed in shape, structure, or both). Every material under any sort of load (lets consider a fastener in tension) is deformed elastically, even if it is such a small amount that it’s impossible to notice. The strain (deformation, elongation in this case) in the sample is proportional to the stress on the sample (force over area) times Young’s Modulus which is a property of the particular material.
Stress vs strain: E, the slope of the curve in the elastic range, is Young’s Modulus. This is an indicator of the stiffness of the material.
When the stress on a test sample reaches the elastic limit (aka the yield point) of the material in question, the deformation becomes permanent. For example, a head bolt or stud will elongate slightly (and temporarily) as it’s tightened up to its yield point, then it will begin to elongate more per unit of stress applied to it, and the deformation will be permanent. The deformation at this point is generally uniform, that is to say it occurs evenly along the length of the bolt, until the ultimate tensile strength of the fastener is exceeded and it fails (no longer provides the correct tension). At this point, a noticeable “necked down” section appears If more stress is applied, and the bolt will eventually fracture (break). Some fasteners, such as BMW head, main bearing, and rod bolts, are designed to be torqued past their yield point and deformed plastically when installed. This leads to a joint that is very resistant to fatigue in the fastener, because the tension in the bolt cannot be released and reapplied. The bolts also strain harden as they are tightened, increasing their ability to hold further loads applied after they are installed.
What does this mean to me?
Never reuse a bolt that requires angle torque, or that BMW requires be replaced. BMW specifies torque angles for fasteners that are torqued to yield because they are a more consistent way to specify the installed strain in a fastener than a torque value that can vary with the specifics of the application, such as lubrication on the threads.
Also, whenever a torque value is specified with a particular lubricant on the threads, ensure that it is always used. Failing to do so can lead to different amounts of stress in the fastener (and thus different clamping loads) at the same applied torque.