1. Ultrasonic Bolt Tension Measurement
Archetype Joint, LLC
140 Engelwood Dr, Suite D
Orion, MI 48359

2. Torque vs. Tension
Perhaps the most critical element of success in bolted joint design is recognizing the limitations of using torque as a indication of a reliable joint. In most cases, particularly in structural joints, tension is the parameter that in fact should be evaluated. Torque is simply easier to measure.
Archetype Joint has the capability of measuring real-time bolt tension of your production bolts in your assembly using the latest generation of ultrasonic sensor technology utilizing bonded sensors. By establishing a fixed acoustic path between the sensor and the fastener there is no measurement variation due to coupling between sensor and bolt.
A significant advantage of using ultrasonic s for bolt tension measurement is that it is the only measurement method that doesn't require and functional changes to the joint or the fastener. Therefor there is no risk that the measurement method altered the test results.
Ultrasonic sensor and pickup
Audit of fastener tension in assembled product
Archetype Joint, LLC

3. Ultrasonic Calibration Setup
Rather than rely on typical published material properties, calibration is performed by tensioning a subset of test bolts in a manner that simulates its stiffness in the actual joint.
SETUP – M8 bolt w/ 2.5 mm (0.10”) grip length
SETUP - 1” bolt w/ 13” (330 mm) grip length
Archetype Joint, LLC

4. Typical Calibration File
Ultrasonic Calibration
As an example of the high repeatability capable of the latest ultrasonic test technology, following is a typical calibration file relating load to elongation (measured as the time delay in the returning signal).
Typical Calibration File
Archetype Joint, LLC

5. Analysis of Torque-Angle-Tension Test Results
Torque-Angle is the most common method of displaying bolt behavior because it does not require that bolt tension is known. The slope of a Torque-Angle trace represents “presumed” stiffness. The presumption that it represents actual stiffness behavior is dependent on the Torque-Tension relationship being constant over the tightening range.
A Tension-Angle trace represents true stiffness as it is not effected by friction. A typical joint is expected to have constant stiffness (be linear) between the point the joint is fully aligned at the start of tightening until some element of the joint begins to yield. The trace at right shows that at approximately 80 kN joint stiffness falls off. As this is less than half the proof load of the bolt, the cause is likely within the joint members. Note that the a1-a2 line on the Tension-Angle trace is the same as the Torque-Tension trace above. The fact that the line lays over both traces in the same manner shows that the presumption that Torque-Angle represents true stiffness is a good one in this case.
Tension-Angle
Torque-Tension
An in-joint Torque-Tension trace is very useful in optimizing installation torque because it allows understanding of the true torque-tension relationship, as opposed to estimating it based on reference tables or bench-top torque tension measurements. In this case the relationship is linear (friction is constant). This is the reason that the Torque-Angle trace could be used as a good substitute for the Tension-Angle trace.
Archetype Joint, LLC

6. Case Study: In Joint Torque-Tension
An example of the effect on bolt tension of tightening then loosening and re-tightening the same fastener in-joint multiple times.
In this case the bolt was retightened eight times at the same installation torque of 960 ft-lb. The first time the bolt was installed the bolt tension achieved was approximately 45,000 lb. By the eighth installation that tension had fallen to less than 25,000 lb, a loss of nearly 50%.
Archetype Joint, LLC

7. Case Study: In Joint Torque-Tension
The following sets of torque-tension traces were captured from two wheels of different design. The six traces are the results of tightening each of the six studs sequentially in one stage in a crossing pattern.
Wheel A
Wheel A has a linear torque-tension relationship as expected, however the stud-to-stud variability is high
Wheel B
Wheel B has a non-linear torque-tension relationship, however the stud-to-stud variability is lower than Wheel A
Archetype Joint, LLC

8. Case Study: In Joint Tension Monitoring
Ultrasonic technology also allows long-term monitoring of joint tension to asses stability in use or after dynamic testing.
As in this case, tension can increase due to
thermal load when materials possessing different CTE are contained in the joint stack.
Archetype Joint, LLC