Fracture of glass/poly(vinyl butyral) (Butacite®) laminates in biaxial flexure

Glass-polymer laminates designed as safety glazing for automotive and architectural applications demonstrate a rich variety of deformation and failure modes due to the complex stress fields developed on loading and the statistical nature of glass fracture. This complexity in stress development results from the large modulus mismatch between float glass and typical polymers used in safety glazing (E-glass/E-polymer approximate to 10(3)-10(5)). We investigate stress development and the sequence of glass-ply fracture in model two-ply glass-poly(vinyl butyral) (PVB; Butacite(R)) laminates during loading in biaxial flexure using a circular (upper) punch on three-point (lower) support. The experiment is analyzed using a three-dimensional finite-element model with a viscoelastic constitutive model of plasticized PVB deformation. Our stress analysis shows that the maximum biaxial stress shifts location from one glass ply to the other as a function of loading rate and/or temperature and the loading-support dimensions. We identify two primary modes for the initiation of failure associated with changes in maximum stress location: (1) first crack initiated in upper, ring-loaded, glass ply tat the internal glass-polymer interface) and (2) first crack initiated in lower, supported, glass ply (outer glass surface). The sequence of glass ply fracture is seen to depend strongly on loading rate and temperature: high temperatures, relative to the polymer-glass transition temperature, and/or slow loading rates bias first cracking to the upper ply; low temperatures and/or high loading rates promote lower ply first cracking. We present a method to compute the probability of first cracking by combining our finite-element-based stress analysis with a Weibull statistical description of glass fracture. The test protocol and stress analysis presented can form the basis of a laboratory-scale test for laminates and can be readily extended to describe load-bearing capacity of laminate plates used in large-scale commercial applications.