# Review of the Main Mechanical Testing Methods for Interlayer Characterization in Laminated Glass
## 1 Introduction  
Over the past decades, the demand for complex transparent architecture considerably increased due to the aesthetic requirements and the improvement of the safety level owing to the increased knowledge of the mechanical properties of the materials [1]. Structural glass is a composite material obtained through the process of lamination by joining, at high pressure and temperature, glass plies and polymer films. Laminated glass (LG) combines the high stiffness, compressive strength, and environmental durability of the glass with the ability of the interlayer to bond the glass plies. LG has been proven to be a valuable solution to fulfill the fail-safe approach according to the requirements of standards and codes [2; 3; 4; 5]. Indeed, the interlayer of LG has the ability to retain glass shards in case of failure reducing the risk of injury, and depending on the type of glass/interlayer, a noticeable post-failure strength can be achieved [6; 7]. The design of structural elements is challenging and involves several problems [8; 9] related to the structural response of the composite material \"laminated glass\". The experimental evidence can be the basis of theoretical validations to define constitutive bonds to be used in a model calibration on the base of simple relaxation tests on laminated glass specimens [10; 11]. Owing to the spread of glass structures and non-structural elements [7; 8; 9; 10; 11; 12; 13; 14; 15; 16], many new interlayer materials have been developed and patented to improve the overall behavior, since the interlayer properties affect the performances at different limit states and under different temperature and load duration. Starting from the first polyvinyl butyral (PVB) interlayer (which is currently still the most popular) [17; 18; 19], new formulations such as ethylene vinyl acetate (EVA), SentryGlas (SG), and a plasticized PVB (known as DG41) have been developed. The mechanical properties of the interlayers have been improved over the years both in terms of stiffness and long-term properties at different temperatures. Indeed, these polymers are viscoelastic materials characterized by a relatively low stiffness which is strongly affected by the service temperature and by the load duration [1; 18; 19; 20]. These properties strongly affect the behavior of the LG, which can behave as a monolithic element or as a layered element, in case of lack of collaboration between glass plies. Therefore, the definition of the interlayer properties is of primary concern to assess the displacement at the serviceability state, the resistance at the ultimate limit state, and the residual capacity at the collapse limit state. However, the experimental assessment of the mechanical properties is not straightforward because it requires the evaluation of the constitutive law of the interlayer, which, in principle, should be evaluated on the pure material. However, the lamination process (at high temperatures and pressure) alters the properties of the material itself; therefore, the properties of the interlayer must be evaluated under its conditions of use (in laminated glass). Therefore, several test procedures have been developed over the years considering either only the interlayer or the LG [21]. At the standard level, the most dated documents refer to interlayer tests (ASTM D638 [22] and ISO 6721-1 [23]), while recently the standard EN16613:2019 [24] has been adopted at the European level and presents three different test methods to be adopted depending on the type of the interlayer. The ASTM D638 [22] and the ISO 6721-1 [23] are standards developed for plastics, and thus do not account for the lamination process. Due to this reason, several researchers [25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39] have proposed different test methods to assess the mechanical properties of the interlayer in the laminated glass. However, these different procedures are still at the research level and have not been standardized.