Characterization and Quantification of In-Service Windshield Fracture Mechanisms and Evaluation of Laminate Sharp Impact Resistance as a Function of Construction 2020-01-0607

Examination of field-fractured windshields was conducted for purposes of determining the principle fracture mechanisms experienced in-use. Samples for the study were gathered both in the United States (New York) and in Europe (France) to explore whether the primary causes of failure were similar for the different geographic regions. In total, over two hundred individual field-fractures were obtained and examined for the study. Detailed fracture analysis of the parts was performed, and multiple fracture mechanisms were identified and quantified. It was found that the two most frequently observed failure modes were common for both regions with the most frequent cause (~70%) of fractures being due to sharp contact of the exterior ply, while Hertzian cone cracking of the outer ply was the second leading cause (~20%). Several other modes were also identified.

Given that sharp impact fracture was the dominant observed failure mode, a high-speed, sharp impact test method was developed. The method consisted of using compressed gas to accelerate a ~2g diamond tipped dart into test samples and the method was thus named “Blow Dart”. The test method created fractures that closely replicated the sharp impact fracture mechanisms observed in field parts.

Blow Dart impact testing of numerous laminate constructions was performed, and results showed that sharp impact resistance of laminates was directly related to the square of the outer ply thickness of the laminate. Inner ply thickness also played a role, with impact resistance improving with reduced inner ply thickness. It was therefore concluded that highly asymmetric constructions utilizing a thick outer ply and a thin inner ply are preferred to improve durability against the most prevalent field failure mode for windshields. Furthermore, it was found that substantial light-weighting vs. today’s conventional windshields could be achieved while concurrently improving sharp impact resistance.