2 September 2002 Physics Letters A 301 (2002) 389–397 www.elsevier.com/locate/pla Monitoring the acoustic emission of the blades of the mirror suspension for a gravitational wave interferometer S. Braccini d , C. Casciano d , F. Cordero c , F. Frasconi d , G.P. Gregori c , E. Majorana d , G. Paparo c , R. Passaquieti d , P. Puppo a , P. Rapagnani a,b , F. Ricci a,b,∗ , R. Valentini e a INFN, Sezione di Roma 1, Roma, Italy b Dipartimento di Fisica, Università di Roma “La Sapienza”, Roma, Italy c Istituto di Acustica “O.M. Corbino”, C.N.R., Roma, Italy d INFN, Sezione di Pisa, San Piero a Grado (Pisa), Italy e Dipartimento Ingegneria Chimica e Scienza dei Materiali, Università di Pisa, Pisa, Italy Received 3 July 2002; received in revised form 3 July 2002; accepted 23 July 2002 Communicated by P.R. Holland Abstract We monitored the acoustic emission activity of the steel blades to be used for the mirror suspension system of a gravitational wave interferometer. We have collected several sets of events getting evidence of a material memory effect (Kaiser effect) associated to the dislocation motion in the steel. This result is more evident when we apply a standard fractal analysis procedure (box counting method) to the timing series of acoustic emission bursts. We conclude that a significant reduction of the emission rate is obtained by applying a few stress cycles to the elastic blades.  2002 Elsevier Science B.V. All rights reserved. PACS: 62.20.-x; 62.40.+i 1. Introduction An acoustic emission (AE) signal is a stress wave, propagating through a material as a consequence of a sudden strain energy release from a localised source. AE is a typical event detectable in metals during deformation. In the case of loaded metallic samples the main sources of detectable AE signals are dislocation motion, twinning and fracture often originating where inclusions and precipitates are located. * Corresponding author. E-mail address: fulvio.ricci@roma1.infn.it (F. Ricci). James and Carpenter [1] have proposed that as soon as the stress increases on a set of pinned disloca- tions, one or a small number of them break away from their pinning points. In a short time interval the small stress applied can cause unpinning of additional dis- locations triggering their avalanche motion within a small bulk. The material microstructure influences the probability of the nearly simultaneous motion of a dis- location group: in the steel the precipitation in age- hardening systems is another phenomenon influencing the avalanche formation, the glide distance and veloc- ity. If one or more dislocation avalanches are formed, there is not a guarantee on AE signals observation. 0375-9601/02/$ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII:S0375-9601(02)00991-X [11]