Bolt Tension

Bolts are one of the most common joining techniques and are used in most critical and demanding industries, including infrastructure, power generation, oil & gas, mining, construction. They do however represent stress concentrators that can bring down a structure if not properly installed and maintained. The US Bureau of Safety and Environment Enforcement has stated that prescribed inspection practices are no longer sufficient .

Innerspec´s existing range of products incorporate advanced applications tailored to specific markets. Innerspec is aiming at providing a universal solution that can be applied to any installed bolt and does not rely on previous knowledge of each specific bolt. This method will be based upon our novel dual wave (Shear and Longitudinal) technique which combines two physical phenomena, bolt elongation (Hooke's Law) and acousto-elasticity.

The ratio between the Time of flight (TOF) of the two waves and applied load has been shown to be linear.

However in recent trials on batches of bolts from different manufacturers, we have found that in some circumstances, whilst the slope of the graphs of TOF against varying load are identical ,the initiation points are quite different leading to variations in accuracy of +/-100kN. Previous research work on this topic have identified the cause to be the variable nature of the bolts crystalline structure, even within bolts from the same batch.

We are therefore seeking to undertake a study supported by NPL and NMG to better understand how the differences in the crystalline structure of bolts affect the measurements of load with ultrasonic transducers and to identify a cost effective , site deployable method to overcome this.