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

An examination of field-fractured windshields was conducted for the purpose of determining their principle fracture mechanisms experienced in-use. Parts for the study were gathered both in the United States (state of NY) and in Europe (France) to explore whether the primary causes of failure were similar or different for the different 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 several different fracture mechanisms were identified and quantified. It was found that the two most dominant failure modes were common for both geographic regions. The most frequent cause (~70%) of windshield fracture was due to sharp particle impact of the exterior ply, while Hertzian cone cracking of the outer ply was the second leading cause (~20%). These and other observed failure modes are detailed below. Given that sharp impact fracture was the dominant failure mode observed, a new high-speed, sharp impact test method was developed and deployed to evaluate numerous laminate constructions for their resistance to this type of event. Testing consisted of using compressed gas to accelerate a 2g diamond tipped dart into the test samples and the test was thus named “Blow Dart”. The test method well-replicated the sharp impact fracture mechanisms observed in the field-failed parts. Blow Dart impact testing of the laminate constructions demonstrated that outer ply thickness of the laminate was the most significant contributor to laminate sharp impact resistance. Additionally, it was found that reduced thickness of the inner ply also increased durability to this failure mode. It was therefore concluded that highly asymmetric constructions that utilize a thick outer ply and a thin inner ply are optimal to improve durability against the most prevalent field failure mode of windshield. Details follow.