Development of a sensor for monitoring mechanically stressed adhesive joints F. Lehmann a,⇑ , S. Chataigner b , V. Birtel a , J. Wang a , O. Konrad a a MPA University of Stuttgart, Pfaffenwaldring 2b, 70569 Stuttgart, Germany b IFSTTAR, MAST Department, SMC Laboratory, Bouguenais, France highlights  Delamination sensor reliably detects debonding of retrofitted CFRP from steel.  The sensor materials are selected to optimally peel within the capacitive plane.  A model was developed to describe the three phases of sensor delamination.  Allows qualitative monitoring of the effectiveness of rehabilitation measures with CFRP. graphical abstract article info Article history: Received 15 March 2019 Received in revised form 3 July 2019 Accepted 1 August 2019 Keywords: Strengthening in fatigue Steel bridges Adhesively bonded joints NDT Monitoring Quality assurance Delamination abstract A strengthening system with adhesively bonded CFRP reinforcement was developed in the Infravation project FASSTbridge to extend the remaining fatigue lifetime of existing steel bridge constructions. The premature debonding of CFRP and steel is seen as a major risk to lose the contribution of the retrofitting system. Consequently, an ultrathin debonding sensor to be integrated in the adhesive layer between steel and CFRP was developed as part of the strengthening system. It is suited for continuous monitoring of the integrity of the adhesive joint. The paper explains the underlying physical principles of the sensor and gives insight into the per- formed tests for the calibration and verification of its functionality. The sensor conception takes the change of a capacitor’s electrical impedance as a basis, which occurs when a change in distance between the adjacent conductive faces takes place. It is well assumed that debonding yields separation, which leads to an increase of the monitored impedance. The relationship is explored through single lap sheer experiments and vertical tension bond tests. The sensitivity and the accuracy of the impedance with respect to deformations are revealed by comparing several groups of prototypes with different dielectric layers and viscous materials. The influences of different temperature effects and fatigue loadings are also presented. Ó 2019 Elsevier Ltd. All rights reserved. 1. Introduction A significant share of road and railway bridges worldwide are built as steel constructions, judging from the figures for Europe and the United States alone. They are about 15 and 34%, https://doi.org/10.1016/j.conbuildmat.2019.08.008 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: frank.lehmann@mpa.uni-stuttgart.de (F. Lehmann). Construction and Building Materials 227 (2019) 116627 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat