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

Glass-polymer laminates designed as safety glazing for automotive and archi tectural applications demonstrate a rich variety of deformation and failure modes due to the complex stress fields developed on loading and the statis tical nature of glass fracture. This complexity in stress development resul ts 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 mo del two-ply glass-poly(vinyl butyral) (PVB; Butacite(R)) laminates during l oading in biaxial flexure using a circular (upper) punch on three-point (lo wer) support. The experiment is analyzed using a three-dimensional finite-e lement model with a viscoelastic constitutive model of plasticized PVB defo rmation. Our stress analysis shows that the maximum biaxial stress shifts l ocation from one glass ply to the other as a function of loading rate and/o r temperature and the loading-support dimensions. We identify two primary m odes for the initiation of failure associated with changes in maximum stres s location: (1) first crack initiated in upper, ring-loaded, glass ply tat the internal glass-polymer interface) and (2) first crack initiated in lowe r, supported, glass ply (outer glass surface). The sequence of glass ply fr acture is seen to depend strongly on loading rate and temperature: high tem peratures, relative to the polymer-glass transition temperature, and/or slo w 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-eleme nt-based stress analysis with a Weibull statistical description of glass fr acture. The test protocol and stress analysis presented can form the basis of a laboratory-scale test for laminates and can be readily extended to des cribe load-bearing capacity of laminate plates used in large-scale commerci al applications.