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Many automobile accidents are complex cases with multiple events during a single accident. The aim of forensic investigations of such (and all other) accidents is to reconstruct the sequence and the severity of events during the accident and then to establish cause and- effect relationships between the injuries (or damages) and the probable factors - vehicle design (e.g. brakes, structure, airbags); vehicle operator (e.g. distracted driver, alcohol impairment, reduced vision); and operating conditions (e.g. fog, failed traffic sign, excessive speed, icy road).

Most often, the traditional practice of accident reconstruction involves using empirical algorithms (generally 'lumped mass-andspring models') with some measurements from the accident site and from the car's exterior damage to estimate the crash parameters. In accidents involving multiple impacts, such methods may not be adequate and it becomes necessary to develop new techniques for obtaining reliable information regarding the accident.

This article describes such a technique developed by the author, using the 'stored information' in a windshield when it is damaged from impacts in an accident. As illustrated below, analysis of highresolution and properly-detailed photographs of the windshield can provide significant additional information for accident reconstruction and for injury causation evaluation.

Laminated Windshield Construction enables vehicles to meet the requirements of the Federal Motor Vehicle Safety Standards for protection of vehicles' occupants in certain types of crashes. The 'laminated safety glass' in these windshields consists of three layers - (i) an outermost glass layer, (ii) a thin laminate of plastic material known as 'Polyvinyl Butyral' (or 'PVB'), and (iii) an innermost glass layer. Typically, each glass layer may be 2.5 millimeters thick and the intermediate PVB laminate may be 0.75 millimeter in thickness. All three layers are bonded together and the entire assembly is encased in the vehicle's windshield frame which is a part of the vehicle's structure. The above thicknesses may vary from one manufacturer to another and from one model to another. When subjected to severe impacts, the inner and the outer glass layers will fracture at relatively low values of stress. The intermediate PVB layer is a 'hyperelastic' material and it can withstand large amounts of deformation and strain without rupture.

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Dr. Mukul Verma, is a well-known expert in Automobile Safety and Crashworthiness, Vehicle Structures, Product Design, and Statistical Analyses of Traffic Trends and Regulations . He has worked in many engineering and management positions at a major automobile manufacturer including assignments in R&D, vehicle design, analysis and testing and engineering program management.

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