Fastener Clamp Load/Preload

Direct measurement of fastener clamping force

Torque and extensometry are the most common methods of measuring fastener preload which is an indirect and often erroneous assumption of actual clamping force. According to industry experts 95% of threaded fastener related failures are due to improper installation and maintenance. In some cases where automated or semiautomatic tooling is used to fabricate assemblies, torque measured on the assembly tool can actually hide a completely unclamped assembly. In particular, this fastener assembly fault was first observed in our joint development efforts with Howmet Aerospace focused on developing a method to ascertain fastener preload on small blind fasteners. Here this issue arose where a fully deformed fastener expansion sleeve can actually have zero clamping force; as evidenced by the fastener rotatating freely in its pre-drilled hole. Due to the unique geometry of these fasteners the direct measurement of the applied stresses in the area immediately surrounding the fastener was developed. Similar types of load fault condition may occur in other assemblies were the actual load introduced into the clamped substrates are not reflected accurately through conventional load measurement methods. The physics of Electromagnetic Acoustic Spectroscopy was investigated as presented herein. We developed a new technique using the advanced methods of Electromagnetic Acoustic Spectroscopy to measure the true clamping load exerted by the fastener on the conjoined substrates.

The above chart is data from tests on a small fastener where the fastener was loaded (tightened) and unloaded (loosened) multiple times. The amplitude data repeats within 1%, note bolt stretch occurs at 2000 lbs. load for a ¼ inch bolt.

The following photograph and graph depict a similar load and unload ultrasonic amplitude test against a torque wrench. Note the linearity of the amplitude response to the applied load on the stainless steel/CFRP assembly.