% fatigue

Measurement of Percentage Fatigue Life Using Non Destructive Techniques

The aim of this project is to develop a non-destructive ultrasonic technique for determining the remaining fatigue reserve of engineering components.

Ultrasound has proven to be a powerful and effective technique for nondestructive testing. Traditional linear techniques are based on the detection of an ultrasonic signal that contains information about possible defects and their position, followed by appropriate repair where necessary. The checks are performed periodically and therefore need to be reliable, time efficient and cost efficient. The detection of defects is based on linear methods such as signal reflected from a defect or obstruction of ultrasonic transmission altogether. Although most traditional techniques are competent in detecting gross cracks, they are insensitive to the presence of clusters of microcracks, contacting defects, diffusion bonds, or disbonded delaminations. General degradation of a component can be very well hidden and degraded materials can pass for flawless under standard ultrasonic tests.

As engineering components go through their life cycle, they undergo microscale plastic deformation in response to stress from which they do not quite recover fully (fatigue). In metals, fatigue progressively leads to formation of microcracks which will eventually develop into critical cracks and failure. The question is to develop a method able to monitor the progress of fatigue before the initiation of critical cracks.

The accumulation of microcracks due to fatigue produces subtle changes in the material's elastic constants. These changes can be observed in the nonlinear response of the material. The method we use to observe the nonlinear response of the material is based on monitoring of the phase modulation of a high frequency (HF) surface acoustic wave interacting with a low frequency (LF) high amplitude stress inducing surface acoustic wave. The HF is generated by a laser and the LF by a contact transducer. The detection is done optically by means of a knife edge detector. The use of laser based ultrasonics in this experiments answers several problems encountered using contact transducers: It is a non contacting method for generation and detection of ultrasound meaning that it can be used remotely (e.g. hostile environments) and it is couplant free, hence free from nonlinearities introduced by the couplant medium. It is also suitable for complex geometries.

The project is funded by DTI and the principal investigators are Mike Somekh and Matt Clark. The lead researcher is Teti Stratoudaki. It is a follow up of research performed by Ian Collison during his PhD thesis, an effort that is continued by Rob Ellwood in his current PhD study.

Publications and Conferences

• Stratoudaki T., Ellwood R., Sharples S., Clark M., Somekh M.G., Collison I.J. (2011) Measurement of material nonlinearity using surface acoustic wave parametric interaction and laser ultrasonics. J. Acoust. Soc. Am. 129(4), 1721-1728.

• Stratoudaki T., Collison I.J., Ellwood R., Sharples S., Clark M., Somekh M.G. (2010) Measuring material nonlinearity using frequency mixing of surface acoustic waves. 10th Anglo-French Physical Acoustics Conference (Lake District, UK).
• Sharples S., Stratoudaki T., Ellwood R., Collison I.J., Clark M., Somekh M.G. (2010) Laser ultrasonics for detection of elastic nonlinearity using collinear mixing of surface acoustic waves. Review of Progress in Quantitative Nondestructive Evaluation, Ed. Thompson D.O. and Chimenti D.E. 1211, 287-294.