Fracture mechanics approach on evaluation of residual strength of structural timbers having a crack which emanates in moisture non-steady state condition

1.含裂纹木材在湿度稳定状态下的蠕变与断裂按ASTM标准设计的紧凑拉伸试样在垂直于裂纹面的方向上受到拉应力作用。使用了杉和铁杉。蠕变试验是在最大载荷的45%至85%的应力水平下进行的。在试验过程中同时测量裂纹张开位移、声发射、瓣长、湿度和温度。蠕变破坏时试样的寿命可以通过稳态蠕变阶段COD的最小蠕变速度来估算。在60%以上的应力水平下，在蠕变初始阶段和灾难性断裂之前经常检测到AE。在稳态蠕变阶段也检测到少量AE。蠕变极限估计为低于50%的应力水平，因为在该应力水平下未检测到AE。含裂纹木材在湿度非稳态下的蠕变与断裂对杉木在自然环境条件下进行了蠕变试验。断裂发生的应力水平相对较低，低于40%。裂纹扩展发生在高湿度条件下，如下雨天。COD蠕变曲线呈日循环变化，但未观察到木材中常见的机械吸附现象。吸湿引起的内应力增大了裂纹尖端附近的应力强度因子，加速了裂纹的扩展.加载速度的影响及应力松弛试验模型化研究了加载速度对含裂纹木材断裂的影响。通过应力松弛试验和麦克斯韦模型解释了速度对负荷COD图的影响。提出了一种基于声发射弹性分量的断裂准则来表征临界条件。

1. Creep and fracture of wood having a crack under humidity steady stateCompact tension specimens designed by an ASTM standard were subjected to tensile stress perpendicular to a crack surface. Sugi and hemlock were used. Creep tests were done under stress levels from 45 % to 85% of maximum load. Crack opening displacement, acoustic emission(AE), clack length, humidity and temperature were measured simultaneously during tests. A life time of a specimen during a creep failure can be estimated by a minimum creep speed of COD during a steady creep stage. Above stress level of 60%, AE was detected frequently at initial creep stage and before catastrophic fracture. A few AE was also detected during steady creep stage. A creep limit was estimated to be below a stress level of 50%, because no AE was detected at this stress level.2. Creep and fracture of wood having a crack under humidity non-steady stateCreep tests of sugi under natural environmental condition were conducted. Fracture occurred relatively low stress level, below 40%. Crack propagation occurred when high humidity condition continued in such as a rainy day. COD creep curves showed daily cyclic change, however, a mechano-sorptive phenomenon which commonly occurres at a clear wood specimen was not observed. Inner stress induced by moisture absorption increased a stress intensity factor near a crack tip, and it accelerated a crack propagation.3. Effect of loading speed and modelization by a stress relaxation testEffect of loading speed on fracture of wood having a crack was tested. Speed effect on load-COD diagram was explained by stress relaxation tests and a modelization using a Maxwell model. A fracture criterion based on elastic component of AE was proposed to characterize a critical condition.