A measurement-based methodology for determining field equivalent temperature ranges for designing thermal fatigue tests through year-round in-vehicle measurements

In designing accelerated tests to induce thermal fatigue, the field equivalent temperature range — a key test design parameter reflecting the actual environment of vehicle usage — is difficult to obtain from the existing literature. Most relevant information is proprietary to vehicle manufacturers, and publicly available data is limited to the brief reference provided in the Appendix of ISO 16750–1. The purpose of this study is to present a methodology for determining the field equivalent temperature range based on actual measurement data. To ensure data robustness, in-vehicle temperature measurements were conducted over the course of one year. The thermal characteristics of major engine compartment components were analyzed with respect to seasonal environmental variations, and the field equivalent temperature ranges were calculated by applying the Palmgren–Miner linear cumulative fatigue damage model. Unlike conventional approaches that rely on climate or weather data, the proposed method is grounded in directly measured temperature histories, thereby minimizing dependence on statistical and thermodynamic assumptions. Consequently, it was empirically demonstrated that the field equivalent temperature range varies depending on the mounting location of the component — a finding that substantiates and refines the existing understanding of location-dependent thermal exposure. The field equivalent temperature ranges obtained in this study for major automotive engine compartment components can serve as a practical reference for designing thermal fatigue acceleration tests and defining appropriate test conditions.