Ultrasound is commonly used to non-destructively examine cracks in structures. When a cracked structure is loaded (either by residual stress or normal duty) the faces of the crack may be pushed into contact. Ultrasonic waves are reflected by discontinuities within the structure. If the crack is sufficiently compressed, i.e. a high percentage of the crack faces are in contact, a high percentage of the ultrasound is transmitted through the crack and it may remain undetected. The detectability of closed or 'tight' cracks is important for safety assessments.
Work is being conducted to develop criteria for the detectability of closed cracks. Apparatus has been developed that enables the simultaneous loading and normal incidence scanning of the cracks, shown schematically. Several cracks have been grown in compact test specimens under controlled loading conditions. The specimens have been ultrasonically investigated under increasing compressive force.
The interaction of ultrasound with a loaded interface can be modelled theoretically. An interface between two separate bodies will deform more than the bulk for a given load. This is due to the presence of air gaps between the mating surfaces. The additional compliance can be modelled as a spring of appropriate stiffness, K. The stiffness of a spring is the rate of change of deflection, du, for a given increment of load, dP.
To predict the interfacial stiffness a numerical contact model is used with real digitised crack surface profiles. This is used to predict the geometry of 'air gaps' along the interface under given loads and hence the stiffness of the interface. Tattershall (1973) related the stiffness of an interface to the reflected spectra of an ultrasonic wave from it. This technique therefore can then be used to determine the detectability of cracks under compression.
We are grateful to Advantica Technologies Ltd and the EPSRC for the sponsorship of this work.
- Hankinson, N., Dwyer-Joyce, R.S., Drinkwater, B.W. & Morgan, L., (2001), "Observations of Rough Crack Face Contact Under Load", Proceedings of the 27th Leeds-Lyon Symposium on Tribology, Elsevier Tribology Series No. 39, pp819-827.