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双K断裂准则能够定量描述混凝土裂缝的起裂、稳定扩展和失稳断裂。该文采用混凝土三点弯曲梁试件,通过在试验中测得的起裂荷载Pini、最大荷载Pmax及临界裂缝口张开位移CMODC计算了初始缝高比为0.3～0.9共7组试件的起裂断裂韧度K IiCni和失稳断裂韧度K IuCn。结果表明,当初始缝高比为0.3～0.7时,混凝土裂缝扩展经历起裂、稳定扩展和失稳破坏3个阶段,双K参数均是与初始缝高比无关的材料参数;当初始缝高比大于或等于0.8时,混凝土裂缝起裂后便进入失稳扩展阶段,起裂荷载即为最大荷载,且计算得到的K IiCni仍与初始缝高比无关。因此,在确定K IiCni时,仅需测得初始缝高比大于或等于0.8试件的Pmax,将Pmax作为Pini直接计算得到K IiCni。同以往的试验方法相比,其结果更为准确且试验方法简单。
Double-K fracture criterion can quantitatively describe the initiation, steady expansion and instability of concrete cracks. In this paper, concrete specimens with three-point bending beam were used to calculate the initial fracture-height ratio of 0.3-0.9 (Pini, maximum load Pmax and critical fracture opening CMODC) Fracture initiation toughness K IiCni and instability fracture toughness K IuCn. The results show that when the initial fracture height ratio is 0.3-0.7, the crack propagation in concrete undergoes three stages of initiation, steady expansion and failure. Both K parameters are material parameters unrelated to the initial fracture height ratio. When initial fracture height When the ratio is greater than or equal to 0.8, the concrete crack enters the stage of instability propagation after the initiation of cracking, and the crack initiation load is the maximum load, and the calculated K IiCni is still independent of the initial fracture height ratio. Therefore, when determining K IiCni, only the Pmax of the specimen with the initial seam height greater than or equal to 0.8 should be measured, and K IiCni should be calculated directly with Pmax as Pini. Compared with the previous test method, the result is more accurate and the test method is simple.