Any tightening method applied will result in stretching of the bolt. The bolt behaves like a spring, in that it has a certain elastic limit that it can reach and still return to its original shape and size when the force is removed without incurring any distortion. If a bolt is in tension, the stress it experiences is a tensile stress, representing the amount of force applied. Tensile strength is the maximum tension applied stress a fastener can support before it fractures. The amount of distortion occurring in a part, as long as it is kept within the material’s elastic limits, will be directly proportional to the applied force. If the force exceeds these limits, the result is permanent set. Permanent set of a bolt refers to the elongation that remains in a fastener after it is unloaded, and is also commonly known as necking or yielding.
Simply put, necking occurs when a bolt is stretched past the spring-like state and the bolt becomes longer than it originally was. In this situation, the bolt is overstretched and the material to create this length has been displaced from other areas of the bolt. Typically, bolt material is transferred from the diameter when this elongation occurs. It is not visibly apparent when the stud bolt is engaged, but this material migration can cause major problems if ignored or unnoticed. Necking decreases a part’s strength, which therefore reduces the integrity of the bolted joint as a whole. When the bolt needs to be tightened again, there will be a new stress-strain curve and the thread pitch (how closely the threads in a screw or bolt are spaced) will change, even on the threads that were not engaged during tightening. Hence, when comparing a new or properly used bolt to a bolt of the same makeup that has previously exceeded its capacity, the threads will not match.
Yield strength is another important parameter in bolt tensioning. It is defined as the tension-applied stress at which the fastener experiences a specified amount of permanent deformation. When tightening a bolt, it is crucial to remain below the yield strength up to the proof load. The proof load is typically defined as the tension applied load that the fastener must support without evidence of any deformation.
Unfortunately, it is a common misconception that simply tightening a joint is something of a cure-all for leaking connections. Of course, there is the possibility that tightening the joint will correct the issue. However, the likelihood also exists that the joint was previously overtightened and therefore has compounded problem areas, including necking issues or distorted flange faces. This brings up a key point that illustrates how essential it is to maintain accurate records of the work performed on each application. Detailed documentation saves time and resources regarding the continued maintenance and performance of any asset.