Interface-dominated environmental stress cracking in PMMA automotive lamps: From field analysis to component- and specimen-level evaluation

This study investigates the field cracking phenomenon of poly(methyl methacrylate) (PMMA) used in automotive rear lamps and establishes systematic evaluation methods to reproduce these failures. To clarify the crack formation mechanism, rear lamps that exhibited cracking during service were collected and analyzed. Surface analysis revealed that the cracks typically propagated along the dual-bonded interface. To reproduce these field cracks under laboratory conditions, actual rear lamps were immersed in isopropyl alcohol (IPA) at two temperatures (22 and 40 °C). The results confirmed that lamps exposed to IPA exhibited crack propagation along the dual-bonded interface, identical to the field failures, which was attributed to environmental stress cracking (ESC). Additionally, ESC tests were conducted on dual-bonded dog-bone specimens under varying temperatures and bending strains. Notably, quantitative analysis revealed that the crack initiation time for dual-bonded specimens was approximately 7 times faster than that for single materials (e.g., 2.33 s vs. 16.3 s at 22 °C), providing definitive evidence of the structural vulnerability of the interface. Surface analysis of these specimens showed crack morphology consistent with both component-level tests and field failures. By effectively correlating specimen-level quantification with actual component behavior, this study demonstrates that laboratory-scale evaluations can accurately replicate complex field phenomena. This systematic framework provides a robust scientific basis for predicting long-term durability and facilitates early-stage material selection and design optimization to prevent field failures in transparent or translucent polymer-based components. • Identified the ESC mechanism in automotive PMMA rear lamps through characteristic mixed-mode interfacial crack patterns distinct from mechanical failure. • Developed a dual-level evaluation system reproducing field failures at both component and specimen scales with temperature-accelerated quantitative assessment. • Experimentally verified that the bonded interface is the most vulnerable structural weakness to ESC, exhibiting crack initiation 7 times faster than single material under identical conditions.