Failure in automobile headlight lenses

Abstract

One of the documented failures in the automotive industry is the presence of cracks in polycarbonate (PC) headlight lenses after a short period of use (low mileage kilometers). The objective of this work is to identify and discuss the failure mechanism. Characterization of headlights were performed using the Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (ATR-FTIR), Scanning Electron Microscopy (SEM) and VICAT softening point measurement. The Environmental Stress Cracking (ESC) test is a determinant test for evaluation of the causal mechanism of failure, which is based on the stress of the component. This test consists of PC part immersion in a solution of toluene and n-propyl alcohol, and the stressed part cracks. Absence or inefficiency in the annealing lens process, stress after attachment lens on headlight housing, headlight stress after assembly on the vehicle body, and thermal fatigue inherent to part uses can cause stress on lens. The loss of acrylic coating's integrity reduces its protective action and it exposes the PC lens to ESC agents, which can cause fracture. The degradation of the acrylic coating was identified and indicates the possible occurrence of lenses' fracture caused by ESC, however, it does not exclude the possibility that the lenses have been fractured by creep before it had the chance to be fractured by ESC.