Based on fracture extreme theory (FET), the size effect on initial fracture toughness \(K_{\mathrm{I}}^{\mathrm{ini}}\) and unstable fracture toughness \(K_{\mathrm{I}}^{\mathrm{un}}\) of concrete for three-point bending beam was investigated. Nine groups of geometrically similar specimen were simulated to obtain peak load and critical crack mouth opening displacement, of which specimen depth was from 200 to 1000 mm and initial crack length-to-depth ratios were from 0.1 to 0.6. The \(K_{\mathrm{I}}^{\mathrm{ini}}\) and \(K_{\mathrm{I}}^{\mathrm{un}}\) were calculated by FET and double-K method, in which FET adopted the linear, bilinear, and trilinear cohesive stress distribution assumptions and double-K method only used the linear cohesive stress distribution assumption. With linear cohesive stress distribution assumption, \(K_{\mathrm{I}}^{\mathrm{ini}}\) and \(K_{\mathrm{I}}^{\mathrm{un}}\) determined by FET and double- K method were compared. Then, the influence of specimen depth on \(K_{\mathrm{I}}^{\mathrm{ini}}\) and \(K_{\mathrm{I}}^{\mathrm{un}}\) was discussed. In addition, \(K_{\mathrm{I}}^{\mathrm{ini}}/K_{\mathrm{I}}^{\mathrm{un}}\) calculated via FET using different cohesive stress distribution assumptions were analyzed.

根据断裂极限理论（FET），尺寸对初始断裂韧性\（K _ {\ mathrm {I}} ^ {\ mathrm {ini}} \）和不稳定断裂韧性\（K _ {\ mathrm {I}}的影响研究了用于三点弯曲梁的混凝土的^ {\ mathrm {un}} \）。模拟了九组几何相似的试样，以获得峰值载荷和临界裂纹张口位移，其中试样深度为200至1000 mm，初始裂纹长度与深度之比为0.1至0.6。的\（K _ {\ mathrm {I}} ^ {\ mathrm {INI}} \）和\（K _ {\ mathrm {I}} ^ {\ mathrm {未}} \）通过FET和双计算ķ FET采用线性，双线性和三线性内聚应力分布假设并采用双K方法仅使用线性内聚应力分布假设。与线性内聚应力分布的假设，\（K _ {\ mathrm {I}} ^ {\ mathrm {INI}} \）和\（K _ {\ mathrm {I}} ^ {\ mathrm {未}} \）由下式确定比较了FET和double-K方法。然后，标本深度对\（K _ {\ mathrm {I}} ^ {\ mathrm {ini}} \）和\（K _ {\ mathrm {I}} ^ {\ mathrm {un}} \\）的影响为讨论过。此外，使用不同的内聚应力分布假设通过FET计算的\（K _ {\ mathrm {I}} ^ {\ mathrm {ini}} / K _ {\ mathrm {I}} ^ {\ mathrm {un}} \}分析。