Fracture of Stiffened Shell Plating caused by Multiple Site Damage

研究了加筋板中在加劲肋处产生的单个或周期性裂纹阵列的裂纹扩展。采用加筋板试件进行了疲劳裂纹扩展试验。数值模拟结果表明，加筋板存在多重裂纹时，裂纹扩展速率比存在单一裂纹时要高，这是由于多个加劲筋损伤增加了应力强度因子，并且在多重裂纹情况下相邻裂纹之间存在相互作用。以实际船舶甲板结构为例，研究了多部位损伤（MSD）裂纹与单部位损伤（SSD）裂纹扩展寿命的比值。结果表明，MSD的裂纹扩展速率是SSD的数倍。在研究的第二部分，设计了一种测试装置，以检查在过大侧压力下带有MSD的加筋板的最终不稳定破坏。采用带单裂纹的铝板试样进行了初步试验。成功地测量了试样的侧压力、应变和裂纹速度。在研究的第三部分，针对单调加载条件下的准静态脆性断裂和复合应力条件下的疲劳裂纹扩展，提出了非均质材料中相互作用裂纹系统裂纹形成的一般理论。在现有理论的基础上，提出了一种预测相互作用裂纹系统裂纹形成的计算方法，该方法采用考虑所有活动裂纹（扩展裂纹）相互作用的有限元方法来预测裂纹模式的逐步形成。给出了数值结果，验证了该方法的有效性。

Crack growth of a single or periodical array of cracks initiated at the stiffeners in a stiffened panel hasbeen investigated. Using stiffened panel specimens, fatigue crack propagation tests have been carried out. The corresponding numerical simulation has also been performed, and the results have shown that in case of stiffened panel with multiple cracks, the crack growth rate is higher compared with that witha single crack, because the multiple damaged stiffeners increase the stress intensity factors ahd also the adjacent cracks interact with each other for the case of multiple cracks. For a deck structure of an actual ship, the ratio of the crack growth lives for multiple site damage (MSD) cracks and for asingle site damage (SSD) crack have been studied. It is found that the crack propagation rate of MSD is several times higher than that of SSD .In the second part of the study, a testing apparatus has been designedand produced in order to examine the final unstable failure of a stiffened panel with MSD under excessive lateral pressure. A preliminary test has beencarried out by using a aluminum plate specimen with a single crack. The lateral pressure, strain, and crack velocity of the specimen were measured successfully.In the third part of the study, a general theory of cracking formation of a system of interacting cracks in heterogeneous materials is proposed for quasi-static brittle fracture under a monotonic loading condition and for fatigue crack propagation under a combined stress condition. Based on the present theory, a computational method for the prediction of the cracking formation of a system of interacting cracks is developed, in which a step-by-step formation of a cracking pattern is predicted by using the finite element method taking account of the interaction of all active (extending) cracks. Numerical results are given, and the validity of the method are confirmed.