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和断裂按ASTM标准设计的紧凑拉伸试件承受垂直于裂纹表面的拉应力。使用了Sugi和铁杉。蠕变试验是在最大载荷的45%到85%的应力水平下进行的。在试验过程中同时测量了裂纹张开位移、声发射、裂纹长度、湿度和温度。在蠕变失效过程中，试件的寿命可以通过稳定蠕变阶段的最小蠕变速度来估算。在60%以上的应力水平上，在蠕变初期和灾难性断裂之前，经常检测到声发射。在稳定蠕变阶段也检测到少量的声发射。蠕变极限估计低于50%的应力水平，因为在这个应力水平上没有检测到声发射。含裂纹木材在潮湿非稳态条件下的蠕变和断裂研究在自然环境条件下进行了木材蠕变试验。骨折发生的应力水平相对较低，低于40%。当高湿度条件持续时，如雨天，裂纹扩展。鳕鱼蠕变曲线呈日循环变化，但未观察到透明木材试件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。加载速度和应力松弛试验模型化的影响加载速度对有裂纹木材断裂的影响。通过应力松弛试验和Maxwell模型的模拟，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判据来表征临界状态。

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.