Measurement of biaxial strength of new vs. used windshields

This paper presents the strength data for conventional automotive windshields in both the new and used conditions. More specifically, the biaxial strength of outer surface of curved and symmetrically laminated windshield, measured in biaxial flexure, is reported. The relative contributions of inplane membrane stress, which can be significant for new windshields, and bending stress are quantified with the aid of strain gauge rosettes mounted on both the outer and inner surfaces of windshield. The strength distribution for new and used windshields, based on Weibull distribution function, is found to be multimodal indicating more than one family of surface flaws. Depending on handling damage during manufacturing, assembly and installation processes, the low strength region of new windshields can approach that of used windshields with 50,000+ road miles! These data demonstrate the need for proper training and careful handling of new windshields to preserve their high initial strength thereby delaying the onset of fracture and ensuring occupants' safety in service. The edge strength of automotive windshields, measured by vertical bending of uniaxial edge specimens prepared from individual glass plies prior to lamination, is also reported. In view of uniform flaws induced by the edge finishing process the strength distribution is found to be unimodal. The strength data for both the surface and edge regions are then used to estimate the flaw depth which agrees reasonably well with that given by fractographic analysis. The paper concludes with an assessment of slow crack growth potential in service.