Reliability Evaluation of Thin, Lightweight Laminates for Windshield Applications 2016-01-1401

The use of lightweight materials to produce automotive glazing is being pursued by vehicle manufacturers in an effort to improve fuel economy. As glazing’s become thinner, reduced rigidity means that the critical flaw size needed to create fracture becomes much smaller due to increased strain under load or impact. This paper documents experiments focused on the impact performance of several alternative thin laminate constructions under consideration for windshield applications (including conventional annealed soda-lime glass as well as laminates utilizing chemically strengthened glass), for the purpose of identifying new and unique failure modes that result from thickness reduction. Regulatory impact tests and experiments that focused on functional performance of laminates were conducted. Given the increased sensitivity to flaw size for thin laminates, controlled surface damage was introduced to parts prior to conducting the functional performance tests. Damage levels were selected based on teardown characterization of real world windshield parts that had been in-service.

Regulatory impact testing showed that even very thin (< 2.3 mm total glass thickness) laminates could comply with requirements, as long as parts were properly laminated. Functional performance tests were more discriminating.

External ball bearing impact experiments showed that as unstrengthened glass laminates became thinner, a fundamental shift in failure mode occurred, and there was an increased prevalence of fracture of the interior ply due to biaxial flexure. Laminates made with a thin interior ply of chemically strengthened glass did not experience fracturing of the interior ply. Ring-on-ring testing of annealed (unstrengthened) glass showed significant reduction in retained strength when minor use case surface damage was present; whereas compressive stress in chemically strengthened glass made it relatively unaffected by the use case damage. This difference between the two materials is the likely underlying cause of the shift in failure modes observed when impacted by the ball bearing.