Fastener failure on a product can have potentially disa […]
Fastener failure on a product can have potentially disastrous consequences. In an attempt to ensure that such consequences do not occur, rigorous and extensive testing of a product is frequently completed. However in many applications, extensive testing is neither practical nor economic. In such instances, the Engineer usually relies upon analytical analysis together with his experience and judgment to ensure that failure does not occur.
Failure of a threaded fastener generally occurs in one of three modes. Failure through the shank or threaded section of the fastener, thread stripping of the external thread, or thirdly, thread stripping of the internally threaded member. Considering each in turn:
Failure through the male thread or thread shank
The majority of fastener failures occur with fracture through the male thread. Under static loads, the strength of the thread is determined by the stress area. This is based upon the mean of the minor and pitch thread diameters. Engineering handbooks have, typically, tables of stress areas for various thread sizes.
In tapped holes, the thread height is dictated by the diameter of the tapping drill. To reduce the risk of failure, the Design Engineer is often cautious and specifies high percentages of thread height in tapped holes. From a production standpoint these higher percentages of thread height result in higher tapping torques, increased tap breakages and, as such, are not favored. For short lengths of thread engagement, the minor diameter size - resulting from the tapping drill - has a significant effect on assembly strength. Studies have shown that for threaded assemblies of usual proportions, tap-drill size is relatively unimportant so long as the percentage of thread height is greater than 60%. Tapping costs are likely to be lower if the lowest possible thread height is used.
When a bolt is tightened the shank sustains a direct stress, due to the elongation strain, together with a torsional stress, due to the torque acting on the threads. Most tables of bolt tightening torques ignore the torsional stress and assume a direct stress in the threads of some proportion of the bolts yield stress, usually 75%. For high frictional conditions the magnitude of the torsional stress can be such that when combined with the direct stress, an equivalent stress over yield can result, leading to failure. A more consistent approach is to determine the magnitude of the direct stress which, when combined with the torsional, this will give an equivalent stress of some proportion of yield. The proportion commonly used with this approach is 90%. The computer program TORQUE provides state of the art analysis for the torque tightening of threaded fasteners.
Thread stripping can be a problem in many designs where tapped holes are required in low tensile material. In general terms, thread stripping of both the internal and external threads must be avoided if a reliable design is to be achieved. If the bolt breaks on tightening, it is obvious that a replacement is required. Thread stripping tends to be gradual in nature. If the thread stripping mode can occur, assemblies may enter into service which are partially failed, this may have disastrous consequences.