Fracture Analysis Of Biological Composite Materials

It is well known that most of biological materials such as shells, teeth, skeletons are hierarchical nanocomposites, from nano to macro scale. At the nanoscale, the mineral crystals are arranged in a staggered manner in protein matrix that the material has excellent mechanical properties. The relationship between biological materials’ mechanical properties and its multi-scale micro-nanostructure is highly desirable in designing bio-mimetic materials with excellent properties. This thesis studied the fracture mechanism in such biological materials with nanoscale hierarchical structures.In the first part, the crack propagation is simulated in a one-level structure model with a central crack. In this model, the volume fraction of protein matrix is much higher than that of the mineral crystals. It was found that the crack tip’s stress intensity factor(SIF) decreases as the increasing of the length of mineral crystals. These results showed that increasing the length of the mineral crystals can effectively weaken the crack tip’s stress concentration.Crack always grows by passing the mineral crystals and the crystals are pulled out from the protein matrix when the mineral crystals’ strength is far greater than the protein matrixes’.However, when the mineral crystals’ strength decreases, the shorter crystals are pulled out while the longer ones can be snapped.In the second part, the crack propagation behavior of bone-like hierarchical structures with a boundary pre-crack is studied. In this model, the volume fraction of protein matrix is far lower than that of the mineral crystal. For the one-level structure model, when the aspect ratio of mineral crystal is comparatively small, crack propagates primarily in the protein matrix and the mineral crystals are pulled out. As the aspect ratio of mineral crystal increases,crack propagates through the mineral crystal. For the two-level structure model, when the aspect ratio of mineral crystal is comparatively small, crack propagates in the protein matrix.With the aspect ratio increasing, the crack propagates in a zigzag-like pathway. Furthermore,when the aspect ratio of the mineral crystal in one-level structure is much larger, some two-level structures can be pulled out with fracture of some mineral crystal. We have found out a set of optimal structure to obtain superior toughness and strength, which may provide guideline for bio-mimetic materials designing.