Traditionally, a torque specification is a calculated number. This can be overcome by designing and offering better fasteners to address these conditions. However, conventional bolts can be an issue when they are used on aluminum heads, given that the expansion rate of this type of head can stretch typical bolts past their yield point.
As long as the bolt is not stressed beyond its proof load (the maximum load a bolt can withstand and still behave in an elastic manner), it will return to its original length once the torque is relieved.
Bolts are elastic by nature, meaning that within their elastic range, they'll stretch as the load on the bolt increases.
