The effects of span-depth ratios on the energy release rate for three-point bending beams were examined. Expressions for the energy release rate for three-point bending specimens with various span-depth ratios were developed using linear interpolation method. The double-G fracture parameters were calculated through tests of three-point bending beams with the same span (800 mm) but various depths (100 mm, 150 mm, 200 mm, 250 mm and 300 mm) and the findings in literature. Results indicate that (1) with increasing the span-depth ratio, an almost constant initial fracture energy release rate was obtained, and slight decrease tendency of the unstable energy release rate was observed; (2) The empirical relationship between the initial fracture energy and total fracture energy suggested in size effect model was conformed using the unstable fracture energy release rate which was the same as the initial fracture energy; (3) Equivalence between double-G and double-K fracture model was verified; (4) Moreover, the analytical results of unstable fracture energy release rate exhibited a good consistency with the experimental ones, which were insensitive to the softening function.

研究了跨度-深度比对三点弯曲梁能量释放率的影响。利用线性插值法建立了不同跨度-深度比的三点弯曲试样的能量释放率表达式。双G断裂参数是通过对跨度相同（800毫米）但深度不同（100毫米，150毫米，200毫米，250毫米和300毫米）的三点弯曲梁进行测试得出的，并根据文献进行了计算。结果表明：（1）随着跨深比的增加，获得了几乎恒定的初始断裂能释放率，并且观察到不稳定能释放率略有下降的趋势；（2）尺寸效应模型中建议的初始断裂能与总断裂能之间的经验关系符合与初始断裂能相同的不稳定断裂能释放速率。（3）双G和双K之间的等价关系断裂模型得到验证；（4）断裂能释放速率不稳定的分析结果与实验结果具有较好的一致性，对软化功能不敏感。