高温后混凝土断裂全过程的裂缝扩展阻力曲线

采用楔入劈拉法试验,设置20～600 ℃共10组温度,试件尺寸统一采用230 mm×200 mm×200 mm,初始缝高比为0.4.根据试验得到的荷载-开口位移曲线（P-CMOD）求得应力强度因子曲线（K曲线）.基于虚拟裂缝黏聚应力,对高温后混凝土断裂全过程的裂缝扩展阻力曲线进行求解.将混凝土断裂全过程划分为4个部分,裂缝黏聚应力呈Petersson双线性软化曲线分布.计算表明,在某一特定温度下,裂缝扩展阻力曲线随裂缝扩展长度的增加而增长；阻力曲线随温度的上升呈下降趋势.对应力强度因子曲线与裂缝扩展阻力曲线进行裂缝扩展的稳定性分析.结果表明,当裂缝扩展阻力曲线位于应力强度因子曲线上方时,裂缝稳定发展；反之,裂缝将失稳发展.该判据可以等同于双K断裂准则,起裂韧度、失稳韧度是裂缝扩展阻力曲线上的2个关键点.

Residual crack extension resistance during complete fracture process of post-fire concrete

Wedge-splitting tests with totally ten temperatures varying from 20 ℃ to 600 ℃ were conducted, and the specimen size of 230 mm×200 mm×200 mm with initial-notch depth ratios 0.4 were considered. The load-crack mouth opening displacement curves (P-CMOD) of post-fire specimens were obtained from which the residual stress intensity factor curves (K-curves) were calculated. The residual crack extension resistance curves (KR-curves) of post-fire concrete, associated with cohesive force distribution on fictitious crack zone of complete fracture process, were obtained. The cohesive force distribution corresponding to four different crack propagation stages, was determined according to Petersson bilinear softening traction-separation law. In each temperature, with the distribution of cohesive force along the fracture process zone, the residual fracture toughness increases with the increasing crack length Δa. The KR-curves decrease with increasing temperatures for the thermal damage induced. The stability analysis on crack propagation demonstrates that when the KR-curve is higher than K-curve, the crack steadily propagates|otherwise, the crack unsteadily propagates. This criterion can be reduced to dual-K criterion in which the initial fracture toughness and the unstable fracture toughness are two key points on the KR-curve.