Ultrasonic measurement of self-loosening in bolted joints

Abstract

Self-loosening of bolted joints in response to vibration can lead to the catastrophic failure of a range of engineering components and structures. Many techniques employed to study this phenomenon focus on directly measuring the preload remaining in the bolt itself, and offer little insight into the behaviour of the clamped interface. In this study, a non-intrusive ultrasonic reflection-based technique is used to first characterize interface pressure in the joint, and then determine the rate at which relaxation occurs. A key advantage of the technique is that it does not require the modification of the contact conditions, and rather utilizes the spring-like behaviour of a rough surface interface when subject to ultrasonic excitation. A series of different bolt torques were investigated, along with the inclusion of both plain and spring washers under the bolt head. All test samples were subjected to an oscillating fixed-displacement vibration cycle. An initial rapid reduction in interface clamping pressure was observed, followed by a more steady-state period. Increasing bolt torque was seen to enhance joint integrity, whereas both the plain and spring washers showed little improvement. The spring washer was observed to extend the secondary steady-state phase of loosening, though as the majority of pre-load was removed prior to this period any change was largely unbeneficial.

Estimating bolt tightness using transverse natural frequencies

“…Ultrasonic techniques have also been used to study contact pressure distribution at the clamped interface of bolted joints [11,12]. It was further shown that ultrasonic reflections from the clamped interface could be used to estimate the progress of relaxation in bolted joints [13]. The ultrasonic method provides a better accuracy compared to the torque wrench, and also determines bolt tension without any impact on the joint stiffness, as opposed to strain gauges.…”

Review of Bolted Connection Monitoring

“…When the ultrasonic wavelength is long compared to the scale of the air gaps in the interface, the interface as a whole behaves like a reflector. Marshall et al use the reflection coefficient of the ultrasonic for measuring the contact pressure relaxation in bolted joints [25].…”

An evaluation of ultrasonic arrays for the static and dynamic measurement of wheel–rail contact pressure and area

“…Additional work was carried out by Dwyer-Joyce to further understand the relationship between surface roughness and stiffness [15], [16]. This work has been continued and numerous static ultrasonic contact measurements have been carried out using a scanning system to investigate machine element interfaces, such as Marshall et al [17], [18] investigating the contact between bolted plates, interference fits [19] and the static contact pressure and area between a sectioned railroad vehicle wheel and rail [20]. The technique involves pulsing an ultrasonic pressure wave towards the contact interface and deriving information from reflected signals.…”

“…By monitoring the phase and amplitude of the reflected wave, it is possible to understand a great deal about the host material and the interface. This technique has been successfully employed to measure contact pressure [3], [16], [17], [18], [19], [20], [21], [22], [23], but also lubricant film thickness [25], viscosity [26], and wear [27].…”