Analysis Of Common Causes Of Bolt Breakage-Fatigue

In the previous article, we shared the Analysis Of Common Causes Of Bolt Breakage-Stress Factors. As a professional fastener manufacturer, Xinchi Fastener Company will continue to introduce another cause of bolt breakage – fatigue.

In the current regulations on industrial fasteners, there is no special legislation to instruct suppliers to purchase key components in line with industrial standards. In particular, there is no mention of fatigue, the main cause of fastener failure. According to the data, the damage caused by fatigue is estimated to account for 85% of the total fastener failures.

Fatigue in bolts is due to the constant action of cyclic tensile loads, so bolts are subjected to relatively small preload and alternating working loads. In the case of prolonged exposure to this type of double load, bolts at less than their rated tensile strength will fail. The fatigue life is determined by the number and amplitude of loading stress cycles. Some joints under pressure, such as press, stamping equipment, and molding machinery, may also have fatigue fractures. A variety of compound stresses occur between the dynamic force and the preload force during operation. For example, the frequency and amplitude of stress changes are affected by the degree of fatigue and damage during repeated stretching.

Typical industrial fasteners, such as hex socket screws, extend and return to their original shape consistently within a certain range of elasticity. If they are subjected to more than normal stress, beyond their elastic range, they will be permanently deformed until they finally break. Elongate-recover-elongate, such behavior is called a loop. A hex socket screw can withstand approximately 240 to 10° cycles per day (maximum).

Modified Goodman relation

The virtual dot diagonal predicts a continuous 10 million cycles, alternating the average screw load with a 90% probability. The real diagonal shows that the maximum deviation between dynamic load and mean stress is plus or minus 12ksi when screw preload reaches 100ksi.

Fasteners will eventually crack due to repeated peak-to-peak stress cycles. Breakage usually occurs at the point where a fastener is most vulnerable, which engineers call the “zone of maximum stress concentration.” Once a microcrack occurs at the stress concentration of the fastener and continues to be stressed, the crack will expand rapidly and the fastener will suffer fatigue damage. Manufacturers of industrial fasteners are constantly exploring new forming processes, and constantly designing and developing new manufacturing methods that can overcome these fatal weaknesses.

The most common locations for fatigue failure include joint (i.e. first load thread), root fillet, thread, and thread termination. As manufacturing has improved fatigue strength through the development of better materials and production methods, the thread has become the weakest point of fasteners and is currently the cause of the highest proportion of fatigue failures.

The interrelationship between designed stress variables and fastener performance characteristics makes it a difficult task to set a standard for fatigue strength. At present, the number of “cycles to break” is determined and the relative strength of a series of fasteners is measured. It’s a complicated process.

Xinchi Fastener Company is a trustworthy fastener company, especially in furniture nuts. And we are the designated fastener supplier to“STAPLE”. As a trusted fastener supplier,  Xinchi Fastener Company can offer a wide range of high-quality fasteners, including tee nuts, screws, bolts, rivets, washers, etc. If you want to know more about fasteners or want to wholesale fasteners, please feel free to contact us. And then our experienced and expert service team will reply to you as soon as possible.