Surface stress measurements on automobile windshields

Measurements of surface residual stress at various locations were made for three front windshields from the Ford Contour automobile using a grazing angle surface polarimeter sensitive to the optical birefringence produced in stressed glass. The two in-plane principal stresses were determined from these measurements. Also, an uncertainty analysis was performed to evaluate the precision of these measurements. The principal stresses were shown to have a relatively high inherent uncertainty due to the nature of the measurement technique. On the windshield outside surface the stresses were apparently uniform and both principal stresses were equal in magnitude. For most of this surface the variation was within the uncertainty bounds. Only near the edges of the glass was the stress not uniform. On the inside surface the stresses were less uniform and the principal stresses were different in magnitude. On this surface the variation did exceed the uncertainty bounds. The variation of stress at a given location from one windshield to the next was within the uncertainty bounds.