Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Bond characterization of monolithic and layered glass panels and ultrasonic tests to control glued surfaces Giuseppe Campione a, ⁎ , Francesco Orlando a , Giuseppe Fileccia a , Margherita Pauletta b a Department of Civil, Environmental, Aerospace and Material Engineering (DICAM), University of Palermo, Viale delle Scienze, 90128 Palermo, Italy b Polytechnic Department of Engineering and Architecture, Viale delle Scienze 206, Udine, Italy ARTICLE INFO Keywords: Glass Adhesion Mechanical testing Ultrasonic testing Glued joints ABSTRACT An experimental investigation is presented regarding the compressive and shear strength of monolithic and PVB laminated glass elements connected by acrylic glue. Ultrasonic tests were also used to control the efficiency of glued surfaces of glass panels. Twenty-four triplets composed of three float glass elements glued with acrylic adhesive were prepared to perform bond tests. Of these twelve triplets were made with monolithic glass elements with a nominal thickness of 20 mm, while twelve were made with layered glass elements 20 mm thick. Three single elements of monolithic glass and three of layered glass were tested for compressive strength. Ultrasonic tests were performed on a sample made by gluing two layered glass panels 200 × 300 × 20 mm in thickness in which defects in glued surfaces were generated artificially. Experimental stress-strain curves in compression for glass elements and shear stress-slippage curves from bond tests were also derived with crack patterns at rupture. 1. Introduction For a safe design, glass structural elements (plates, columns, beams) have to be assembled in such a way as to ensure that any failure of a glass plate is not a catastrophic event, but simply activates secondary, redundant glass elements. Utilizing the design principle of redundancy of glass elements, it is possible to ensure safe design of glass members. Another important aspect to take into account in the design of glass elements is the connections between two or more elements and the choice of adequate thickness and strength of each glass sheet to form a composite section [1]. For structural glass members (beams and columns), it is important to choose the best shape of the transverse cross-section to resist flexural and shear forces. To increase the load-carrying capacity of glass beams and reduce their deformability, T or H shapes are preferred over rec- tangular ones, and steel reinforcements (or others such as timber, plastics, etc.) can be added at the bottom of rectangular cross-sections [2–5]. In addition, by using external pre-stressed steel the load-carrying capacity increases and behavior can be ductile [6]. To form composite glass members with T or H cross-sections, it is necessary to assemble glass flanges and webs using glued or bolted joints. Glued joints, unlike bolted ones, provide many important ad- vantages for brittle glass, such as uniform stress distribution along the larger connection area, the absence of drilling holes in the glass or the possibility of putting together materials with different mechanical and thermal properties. The shear strength of the connection due to bond effects is influ- enced by many factors [7–11] such as: adhesion of glue to the substrate; glue cohesion; joint dimensions and geometry; environmental factors (relative humidity, UV radiation, and temperature); thermal expansion coefficients of joining elements; duration of load; rate of load applica- tion; surface preparation and surface roughness; workmanship and curing. Time-dependent effects on the polyvinyl butyral (PVB) inter- layer and the adhesion between two glass members [6] are also im- portant aspects to take into account. The requirements of the adhesive layers are mainly strength and stiffness. Common adhesives can be divided, according to their modulus of elasticity and shear modulus, into flexible-elastic (i.e., silicones, mod- ified silicones and polyurethanes) and rigid (i.e., epoxy resins, acrylics). Rigid adhesives offer extremely high strength but very low elongation in comparison with elastic adhesives, which show elongation at failure even exceeding 250%. Practical application of different types of polymer adhesives depends on their behavior under loading [9–11]. During selection, special emphasis has to be placed on UV (ultraviolet) stability and long-time behavior. The maximum strength of the ad- hesive depends on several factors such as thickness and type, with va- lues ranging from 0.5 to 20 MPa for silicone, polyurethane, epoxy, 2P- acrylic and UV-acrylic, respectively. https://doi.org/10.1016/j.engstruct.2019.109545 Received 25 September 2018; Received in revised form 11 August 2019; Accepted 12 August 2019 ⁎ Corresponding author. E-mail addresses: giuseppe.campione@unipa.it (G. Campione), margherita.pauletta@uniud.it (M. Pauletta). Engineering Structures 198 (2019) 109545 Available online 20 August 2019 0141-0296/ © 2019 Elsevier Ltd. All rights reserved. T