A notch is a change in the cross-section of a component that may result from a difference in bore, groove or section. The notch can cause uneven stress distribution and generate stress peaks, which is the "gap effect". The notch effect reduces the load capacity, so it is a key parameter in measuring the mechanical strength of a component or structure, especially for components subject to tensile, compressive, bending, shearing or torsional stresses. Gap effects (or stress peaks) can cause early failures and are therefore often seen as an adverse effect. The solution to this problem is to design the part to a size large enough.
But the gap is not necessarily useless. With a certain design, they can increase the strength of the part, for example strain hardening the root of the thread, or form a predetermined breaking point.
Due to the notch effect, the bolts subjected to stretching produce local stress peaks. Because the lateral shrinkage is prevented, the tensile fracture stress of the ductile material can be up to 20% higher than the unnotched round steel.
The notch effect of the free load threads of the bolts is not as large as the thread tail or the thread that is initially subjected to the load, because the threads that are connected together reduce the notch effect. Under tensile stress, the bolt will break at the free load thread, thus achieving the design principle that the bolt should break here. Normally, the thread will undergo toughness deformation in advance, so we can find the failure in time.
In addition to tensile stresses, bolts are often subjected to repeated loads, so their load capacity is also important. The notch creates a high notch pressure, so the bolt cannot conduct a vibration load. This can even lead to fatigue fracture of the bolt.
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