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

1. 具有裂纹的木材在湿度稳定状态下的蠕变和断裂按照ASTM标准设计的紧绷试样承受垂直于裂纹表面的拉应力。使用杉木和铁杉。蠕变试验在最大载荷的45%至85%的应力水平下进行。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。在蠕变破坏期间，试样的寿命可以通过稳定蠕变阶段COD的最小蠕变速度来估计。在应力水平为60%以上时，声发射在蠕变初期和突变断裂前频繁出现。稳态蠕变阶段也有少量声发射。估计蠕变极限低于50%的应力水平，因为在该应力水平下没有检测到声发射。对含裂纹木材在潮湿非稳态下的蠕变与断裂进行了自然环境条件下的蠕变试验。发生断裂的应力水平相对较低，低于40%。在雨天等高湿条件下，裂纹会持续扩展。COD蠕变曲线表现为日循环变化，但未观察到在透明木材试样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。通过应力松弛试验研究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的建模解释了速度对载荷- cod图的影响。提出了一种基于声发射弹性分量的临界断裂准则。

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.