## 1 Introduction
### Detection and Annual Statistical Analysis on Weathering Appearance Defect Inconsistency  
To study the relevance between weathering external factors distribution and appearance defects in non-uniform thermal environment, the weathering appearance defects of the automobile were simultaneously detected and statistically analyzed during the natural exposure test. Before the test, the existing weathering defects of the testing car were detected and recorded firstly. According to the measurement standards and methods related to appearance defects of weathering [29; 30; 31], the appearance inspection of up to 320 important parts was implemented every two months, while color difference measurement was also performed on 65 typical parts, including bumpers, doors, dashboards, seats, interior decoration of roof, etc. The number and distribution of newly added appearance defects were recorded in detail in each test of appearance inspection. During the study, the test car was cleaned regularly, and the weathering products on the surface of the car were ensured not to be removed in the cleaning process. As the car was directly exposed to weathering external factors (mainly solar radiation and temperature), newly-added weathering appearance defects in the surface of car were gradually increasing, including chalking, seam separation, cracking, blistering, sticking and whitening. These defects not only damaged the appearance of the car, but might also bring potential safety hazards, such as the fading of the safety warning button, the cracking of the safety belt buckle, the crackle appearing in the lampshade, etc. As shown in Table 3, the number of new defects in the test car reached up to 80 throughout the year, and the defects distribution in time and space was highly non-uniform. In term of time distribution, the weathering processes among auto-parts were obviously inconsistent. As early as in the second month, there were eight new defects on the interior and exterior trims of the car body, all of which appeared much earlier than the shielded parts in engine case and trunk. Moreover, there were 28 newly-added defects in the whole car in the sixth month, which accounted for more than one-third of the total in the whole year. In terms of spatial distribution, the numbers of new defects in the exterior and interior parts of the car body reached up to 45 and 31, respectively, while those of new defects in engine case and trunk were only 3 and 1, respectively. As far as the cabin is concerned, new defects were concentrated in the regions of high temperature and radiation, among which there were 7 new defects in the dashboard assembly throughout the year, accounting for nearly a quarter of the total number of defects in the cabin, mainly including sticking and whitening of the dashboard, cracking and yellowing of the trim panel on the defrosting outlet, whitening of the warning light switch, seam separation between the combined instrument and the dashboard and loosening of the dashboard trim. However, there were no newly-added defects in the sheltered parts such as the lower dashboard trim and the floor. Figure 4: Dynamic variation on non-uniform measured temperature of key parts during day and night. Comparing comprehensively the distributions of weathering external factors and newly-added defects in the above two tests, it can be seen that the interior trim parts near the window and exterior trim parts were exposed to intense radiation and high temperature, while there was no solar radiation in engine case and trunk, thus the new defects on the interior and exterior trim parts were formed much earlier, and their number was greater than those in engine case and trunk. In the cabin, the radiation and temperature of the interior trim parts near the window were significantly higher than those of the obscured interior trim parts, which is highly consistent with the defect test results that the number of new defects on the dashboard assembly was obviously more than that of defects on the obscured interior trim parts.