Article Acoustic emission monitoring of repaired composite laminates R Asokan 1 , V Arumugam 2 , C Santulli 3 and A Joseph Stanley 1 Abstract This paper presents the results of an experimental investigation conducted to study the strength of different repair technique on impact damaged glass fibre reinforced plastics (GFRP) using the acoustic emission technique. ASTM D3039 tensile specimens (250 mm  18 mm  2 mm) fabricated using bi-directional glass fibre and epoxy resin are impacted at low energy (1.89 J) so as to produce a limited yet visible amount of damage. Subsequently, the damaged region is repaired using different methods. These include the bare removal of impacted region (‘‘dressed’’ sample), taper sanded (‘‘scarf’’) repair or the application of single lap technique. Subsequently, the impacted and differently repaired samples have been subjected to tensile loading to failure monitored using acoustic emission to evaluate the quality of repair obtained, in particular distinguishing between the different failure modes obtained during post-impact tensile tests, namely matrix cracking, delamination and fibre breakage, according to the classification information obtained in previous literature. This allows understanding which mode of failure is dominant in each case and possibly proposing which might be the most suitable repair method for these laminates. Keywords Repair, acoustic emission, impact damage Introduction Impact damage may be a frequent occurrence in the service life of composite materials, in particular in some fields of application. In the case of air vehicles, composite structures may be subject to impact loads by foreign objects, such as a dropped tool during mainten- ance, debris from runways during takeoff or landing or bird strikes or hailstones during flight. 1 The knowledge of the damage behaviour and the transition of damage from a subcritical stage to a critical stage is therefore of considerable interest in the case of composite mater- ials. 2 Whenever a critical stage is deemed having being reached, either by the presence of barely visible impact damage (BVID) or by comparison with impact hysteresis curves or models, which suggest that the energy applied produced a significant amount of damage, decisions are to be taken about the component survival in service. One possibility in this respect is offered by repair: in particular, in thin composite structures, while resin injection may be sufficient for a limited amount of damage, in contrast, when the degree of damage is large, patching techniques are needed. In other words, it may be necessary to apply a flush bonded patch in order to restore the component’s structural integrity. The repair process involves removing all damaged material followed by carefully chamfering (scarfing) the edges of the hole out. 3 An alternative patching tech- nique can be put in place by adding more composite layers in a uniform lap or stepped lap repair. 4 The pur- pose of repair is of course to restore as much as possible 1 Department of Aeronautical Engineering, Hindustan University, Chennai, India 2 Department of Aerospace Engineering, Anna University, Chennai, India 3 Department of Chemical Engineering, Materials and Environment, Sapienza Universita ` di Roma, Roma, Italy Corresponding author: C Santulli, Department of Chemical Engineering, Materials and Environment, Sapienza Universita ` di Roma, Dip. DICMA, via Eudossiana 18, 00184 Roma, Italy. Email: carlo.santulli@uniroma1.it Journal of Reinforced Plastics and Composites 31(18) 1226–1235 ! The Author(s) 2012 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0731684412455957 jrp.sagepub.com