How Direct Tension Indicators Help Minimize Fatigue Failures in Fasteners

Direct Tension Indicators or DTIs are a simple, useful way to identify the tension present in threaded fasteners. Although the correct torque may be applied, as friction in the system increases, the actual amount of tension in the fastener will decrease. This can allow the joint to be less restrained than intended and enhance the possibility of fatigue failures due to lower preload than expected. This presentation is a case study of reverse bending fatigue in steel fasteners used on over the road trucks and how these fatigue failures were overcome by proper application of tension to the fasteners and the use of DTIs to measure the tension.

Lack of appropriate fastener preload was suggested as a potential contributor to the fatigue failures and the use of DTIs was suggested to the client. When the client used the DTIs, they indicated they were not functioning properly and requested ESI perform similar verification testing with the required torque. ESI also experienced poor performance of the DTIs until the recommendations for their use were followed to the letter. These included using a molybdenum disulfide (MoS₂) grease containing at least 70% solids. When the grease was used in appropriate locations, the DTI’s performed properly. This information was presented to the customer. When the appropriate torque was applied, the fatigue failures in these bolts were eliminated.