Measuring Fastener Tension with Ultrasonics

As most practitioners are aware, installation torque is often a poor predictor of the clamp load that will be achieved in a bolted joint. Variation in underhead and thread friction translates to variation in clamp load, and sufficient clamp load is the usually the key to a reliable joint. In addition to friction, joint relaxation can cause joint clamp loads to be significantly lower than would be predicted by recommended torque tables. Because clamp load is the direct result of bolt tension generated by elongation during tightening, ultrasonics provide a means of measuring tension directly, rather than estimating it through applied torque. Other methods of directly measuring bolt tension, such as placing a load cell or force washer within the bolt’s grip range change the characteristics (stiffness, grip length, etc) of the joint.

Ultrasonics are used in a wide range of commercial applications, from cleaning, to medical testing, to measurement of length, flow, or changes in density. Use in fastening is a relatively recent and small part of the total ultrasonic market, however this usage is growing due to improved testing hardware and software and an increased recognition of the risks inherent with relying on torque as an indicator of joint quality. One of the most important developments is the evolution of smaller transducers and more usable and more repeatable interfaces between bolt and transducer.

While ultrasonics has significant benefits in joint development, there are also downsides. These include:

  • The transducer must be intimately coupled to the fastener. This means a flat surface with a finish of 32-64 µin that is large enough to contain the transducer/pickup must be created on one end of the fastener. The minimum required diameter is about 5/16″. The opposite end of the fastener must also be finished flat and parallel to the first so the echo can be read cleanly.
  • While a transducer may be coupled to a fastener and readings taken without any preparation, the accuracy of those readings will be compromised. If the bolts (or a representative sample) are not calibrated before installation, calculations will be based on theoretical values of sound velocity and correction factors for temperature and stress. While bolt calibration will always lead to greater accuracy, it is an additional step that adds time and cost to the joint development process.
  • Ultrasonic measuring equipment is more expensive than most other tools used to assess joint quality and determine desired installation torque. The skill level required to take readings is also higher than most other options.

In summary, while the restrictions mentioned prevent ultrasonic measurement from being a common production practice, it is still the best means of monitoring clamp load in an actual joint without altering the characteristics of that joint.

To illustrate the use of ultrasonics in joint development, we have put together the following pictorial. It traces the steps required in calibration and in determining the in-service tension and elongation of 1/2 -13 Grade 6 hex head bolts.

STEP 1 – Fastener Preparation

To apply ultrasonic measurement to threaded fasteners, the ends of the fastener must be prepared to be parallel to one another and normal to the axis of the fastener. The surface finish of the area under the transducer and on the opposite end of the fastener should be 32 µin to 64 µin.

For the purposes of this illustration the bolt was prepped manually with a pedestal disc grinder.

Surface Preparation

STEP 2 – Transducer Bonding

Archetype Joint uses the Micro Controls MC900 transient recorder, software and transducers for our ultrasonic work. One of the key advantages of this system is that the active piezoelectric element is directly bonded to the fastener. This creates the desired condition of the closest and most stable coupling to the fastener. In contrast, significant interface variation is often present with removable transducers and will ensure minimum interface loss. The chip is bonded to the fastener using an anaerobic retaining compound. The adhesive is dispensed with a precision pneumatic dispenser and the chip is placed with a vacuum pickup. The chip can be bonded to either end of the fastener if both have enough available area to seat the chip and pickup used to read the chip’s output.

Transducer Bonding

STEP 3 – Calibration

A sample group of the fasteners must be calibrated so the relationship between axial load and resultant elongation can be correlated more precisely than approximations from calculations based on geometry and material properties. The fastener is secured into custom tooling on a UTM (tensile tester) in a manner that simulates the grip length of the fastener in the production joint. The UTM loads the bolt to a level a bit above the expected in-service load but comfortably below the elastic limit. Both the output of the UTM load cell and the ultrasonic pick-up are read by the MC900 and regression analysis is performed, developing a load vs. elongation relationship. Calibration of temperature compensation can also be performed with use of an environmental chamber.

Calibration of Temperature Compensation

Results of the calibration of a series of 1/2 -13 test bolts are shown below. The load/elongation relationship can be expressed as a polynomial up to fifth order, although the fit of a first order linear equation is usually essentially equivalent. The correlation coefficient in this case is 0.99972.

Time Delay vs. Load

STEP 4 – Reading Tension of an In-Service Bolt

Tension Being Read From an In-Service Bolt

When it is desired to measure clamp load in an actual joint, the magnetic pick-up is applied to the piezoelectric chip, the echo measured by the transient recorder is compared to the reference signal and the calculation stretch and load is displayed. Figure 5. shows the comparative results of overlaying simultaneous readings of bolt tension generated by ultrasonics and by a load cell. As the difference in the two traces is difficult to discern, a portion of the graph has been magnified.

Simultaneous Tension Measure

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