Investigation of mechanical strength of open hole composite plates with finite element method

Composite materials have a growing appeal in many different fields, especially in aerospace and aeronautical due to their superior material properties such as high specific strength and stiffness. However, the orthotropic composite materials exhibit a more complex mechanical behavior than the isotropic ones. In particular, a discontinuity that can be created by drilling a hole in the structure makes this behavior even more complicated. Therefore, a large number of tests are needed to make safe designs. However, by using appropriate methods and finite element analysis, it is possible to decrease the number of these tests. Within the scope of the thesis, firstly general information about composite materials is given and the literature research about the stress concentration and strength in composite materials is summarized. Then, basic mechanics of composites, lamina level failure theories and strength prediction methods for layered composites with circular hole are mentioned in order to establish a theoretical background. Stress concentration and mechanical strength of layered composite specimens with one open circular hole are investigated by numerically and experimentally. For specimens manufactured in different width and hole diameter, the effect of hole diameter size change on stress concentration and strength is examined. By examining the hole diameter to specimen width ratio, the finite-infinite specimen width conditions are discussed. In addition, the size effect for the notched composite specimens is investigated by increasing the hole diameter and specimen width at the same rate. The mechanical strength estimation for the whole specimen configuration is performed using the Point Stress Criteria (PSC) method and validation was performed by experimental studies. As the second step, the situation of two-hole interaction is investigated numerically and experimentally. Interacting two holes are created on the multidirectional layered composite speicmens. For different types of specimens formed by positioning the holes in different positions according to loading direction, stress concentration and strength are investigated. The effects of positioning and interaction of two holes on stress concentration and strength are discussed. The stress concentration situation in each lamina of the composite specimens with interacting holes located at different positions is examined numerically. For all specimen types, it is discussed where the critical section is likely to cause damage. It is discussed under which conditions PSC method used for strength prediction could be applied for double-hole specimens. For specimens that do not comply with these conditions, the PSC method used in the prediction of single-hole specimen strength is modified and a method (EPSC) capable of estimating mechanical strength of two interacted hole situation is developed and validated experimentally.