Low computation acoustic emissions structural health monitoring through analog signal pre-processing

Abstract

In this research an innovative acoustic emission sensing systemhas been developed intended for structural fatigue crackmonitoring. The innovation lies in analog pre-processing ofthe detected acoustic emissions for signal enveloping, thus relyingon cheap high bandwidth components. The technique isbased on heterodyning the amplified acoustic emission signalwith a carrier signal of a chosen frequency. Next, the signalis filtered and the signal envelope is obtained by phaseshifting the signal by 2 creating an analytic signal that isdigitally sampled. This results in need of low sampling ratedigital acquisition equipment giving relatively small amountsof data to be processed and stored, considerably reducing thesystem cost. This is particularly suitable for applications involvinglarge or complex structures to be monitored, wherea multitude of sensors are needed. The system was built andtested on aluminum test coupons during tension-tension fatigue.The envelope signal is filtered for background noisethrough thresholding based on statistical knowledge of thenoise distribution. The accumulation of acoustic activityshows promising results with an early period of high acousticactivity during settling of the material that asymptoticallyconverges at a certain level. At the next stage, there is no activityuntil a certain point is reached where a sudden ramp upof AE is detected close to the end of the experiment. Throughextensometer measurement, the change in coupon length atthis point in time strongly indicates that the ramp up of AEactivity is due to crack initiation and propagation.