Experimental Investigation of Crack Development in Full-Scale Dou Components Under Vertical Compression: From Visual Inspection to Digital Image Correlation (DIC)

As key load-bearing components in the Dougong system, Dou components are critical to the stability of ancient Chinese timber buildings. Field investigations reveal widespread cracking, posing risks to structural safety. This study examines crack behavior under vertical compression by combining visual inspection with Digital Image Correlation (DIC). Based on Dou geometries from four historic buildings, 61 full-scale Douglas fir specimens were fabricated. Visual observation of 45 specimens identified three typical crack types: extension of pre-existing cracks, X-shaped cracks, and ring cracks. X-shaped and ring cracks frequently co-occurred, forming the dominant failure mode (75%). Sixteen additional specimens were monitored with DIC, enabling crack tracking across elastic, elasto-plastic, and plastic stages. Three development paths were classified: Path 1, with X-shaped and ring cracks arising in the elasto-plastic stage and associated with softening force–displacement curves (50% occurrence); Path 2, where ring cracks appeared later in the plastic stage; and Path 3, involving only pre-existing X-shaped cracks. These results characterize typical crack types and their development paths in full-scale Dou components and provide experimentally grounded observations that may assist in the interpretation and assessment of cracking phenomena in heritage timber structures.