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The direction of microcrack propagation at the crack tip of material ZG1 is the same as that of the major crack propagation, according to Figure 8c. The micro-cracks mainly propagate along the austenite grain boundary and the corresponding grains are oxidized, corroded, and even peel off; the propagation path is relatively straightforward after crack growth. Similarly, Figure 8d shows the crack propagation of the material ZG2 is restricted by martensite laths and precipitates. There is a negligible expansion along the grain boundary and a certain angle with the direction of crack propagation is observed. The analysis shows that martensite laths are helpful to reduce both the crack propagation speed and energy. Moreover, Figure 8e shows that the distribution of precipitates is relatively sparse and has little effect on the direction of crack expansion even in the vicinity of the crack. Although the rupture of the precipitate absorbs the crack expansion energy, the subsequent crack tip is still very sharp (Figure 8e). A large number of cracked precipitates are seen along the crack propagation direction. It can be seen that the rupture of the precipitate here not only absorbs the crack propagation energy but also smooths the crack tip, as illustrated in Figure 8f. The precipitates have a certain influence on the crack propagation direction and the subsequent precipitated phase consumes the crack propagation energy on the crack propagation path.
One of the reasonable places of crack formation between two plies could be the abrupt changes of section, such as ply drop-offs, unions between stiffeners and thin plates . Stress concentration formation in this area can be cause for damage and delamination in a composite layup. Such zones are common for structural parts of gliders. After full-scale lab fatigue testing of glider wing spar, the white spots noticed in the tapered wing spar end areas is example of delamination initiated of ply drop-offs zones where bonded carbon fiber rods represent the tapered end of glider's wing spar.
The Finite element (FE) method is very effective tool to analyse the phenomena of delamination mechanisms. There are several ways to model delamination: cohesive zone elements, tiebreak contacts, virtual crack closure technique. For most of those methods it is needed special material properties and simulation parameters as energy release rate, contact and fracture parameters, shear, and normal stresses as well as element size. Highly harmful interlaminar shear (ILS) stresses develop at local discontinuities such ply-drops, bonded and bolted joints, or during handling, assembly, or foreign object impact . These stresses need to be evaluated for structural applications, and delamination growth is the fundamental issue in the evaluation of laminated composite systems for durability and damage tolerance. 2b1af7f3a8