Experimental studies on the influence of blisters on the load bearing capacity of sandwich panels

Lightweight sandwich panels are an economical solution for wall and roof claddings. The panels consist of two thin faces of steel and a core with thermal insulating properties. The shear-resistant bond between faces and core results in a high load-bearing capacity combined with a low dead weight of the panels. Recently, polyisocyanurates (PIR) have been increasingly used as a core material instead of polyurethane (PUR). During service life, the panels may be damaged by blisters, when the outside of the façade is exposed to high temperatures. Thereby, the outer face separates from the core over a large area and the integrity of the load-transmitting components may be negatively affected as a result. The consideration of blistering in literature and in the standard (EN 14509) is limited to the durability of the panels. However, the load-bearing capacity of the panels may also be impaired depending on the size and position of a blister. Still, in practice, no design rules or recommendations are available for assessing the load-bearing capacity of blister-damaged panels. This article presents the results of experimental studies on the effect of large blisters on the load-bearing capacity and the concerning failure pattern of sandwich panels with a PIR-foam core. Carrying out six-point-bending-tests with artificially inserted blisters, the maximum failure loads and failure patterns are recorded for this purpose. The aim is to analyse the effects of blisters in dependence of their position and to gain an understanding of the resulting type of failure. First results reveal a possibly significant influence for blisters located centrically at midspan. It also became apparent that the direction and velocity of blister propagation determine whether a blister affects the component failure or not. Thus, the load capacity of the tested panels was in some cases significantly reduced compared to undamaged reference panels.