17 Cluster Analysis of Acoustic Emission …
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factors of C/SiC specimen damage. Therefore, the whole loading process is divided
into 4 stages according to the development law of main factors.
The first stage: it is generally considered that there is a damage initiation stage, but
through the acoustic emission research method, it is found that even in the damage
initiation stage, there are still obvious micro fracture events. At this stage, the main
AE signals are matrix microcrack growth, pyrolytic carbon interface damage and
filament fracture. During the preparation of C/SiC composite, thermal stress will be
generated, and the pores in the microstructure will become the crack source, and
expand under low stress. At the beginning of loading, the residual thermal stress of
the material is released. When the local stress exceeds the bearing capacity of the
SiC matrix, the pores of the SiC matrix as the crack source generate microcracks and
expand. At the same time, with the increase of stress, matrix cracks begin to appear
in the low strength SiC region. As the fiber blocks the growth of microcracks in the
matrix, the cracks deflect along the interface, resulting in interface debonding, as
shown in Fig. 17.3a. At the same time, a small number of weak fibers break in the
fiber bundle.
The second stage: the number of high energy AE signals is increased rapidly, while
the matrix cracking events and interface debonding events are very few, indicating
that the stage of crack saturation has been entered. In two-dimensional plain weave
composites, due to the extrusion between the fiber bundles, the braided joints are easy
to produce hole defects, which is easy to cause matrix crack convergence and stress
concentration of the fiber bundles in braided joints. In the crack saturation region,
fiber and fiber bundle are the main bearing objects of tensile load. Strain energy is
released more through fiber and fiber bundle fracture. Some fibers begin to break at
the same time, and the AE energy is higher than that in the first stage (Fig. 17.8),
and the AE events of fiber bundle breakage begins to appear (Fig. 17.7). It can be
seen from Fig. 17.3c that the longitudinal fiber bundle breaks at the braided node,
and there is obvious fiber pullout. In addition, during this period, due to the effect
of fibers and fiber bundles on the matrix crack growth, cracks are generated in the
cross area of longitudinal and transverse fiber bundles, and begin to extend along
the longitudinal direction of fibers and fiber bundles, resulting in more interlaminar
delamination. Meanwhile, It can be seen from Fig. 17.3a that most of the interlaminar
delamination occurs in the cross area of fiber bundles. The interfacial slippage, fiber
bundle fracture and interlaminar delamination in the process of fiber bundle pullout
cause the elastic modulus of the material to decrease gradually at this stage, which
is consistent with the existing understanding of the accelerated damage stage of the
material.
The third stage: the fiber bundle breakage almost stopped, and other damage types
continued to occur. This is due to the large number of fractures of fibers and fiber
bundles as bearing parts in the saturated area of microcracks in the second stage; with
the increase of stress, the matrix part begin to be the bearing object in the fracture
area of fibers in this stage, and at the same time, the matrix cracks begin to form in the
rich matrix area, as shown in Fig. 17.3b. With the growth of long cracks in the matrix,
the interfacial debonding also increases continuously. The energy dissipation mainly
195
factors of C/SiC specimen damage. Therefore, the whole loading process is divided
into 4 stages according to the development law of main factors.
The first stage: it is generally considered that there is a damage initiation stage, but
through the acoustic emission research method, it is found that even in the damage
initiation stage, there are still obvious micro fracture events. At this stage, the main
AE signals are matrix microcrack growth, pyrolytic carbon interface damage and
filament fracture. During the preparation of C/SiC composite, thermal stress will be
generated, and the pores in the microstructure will become the crack source, and
expand under low stress. At the beginning of loading, the residual thermal stress of
the material is released. When the local stress exceeds the bearing capacity of the
SiC matrix, the pores of the SiC matrix as the crack source generate microcracks and
expand. At the same time, with the increase of stress, matrix cracks begin to appear
in the low strength SiC region. As the fiber blocks the growth of microcracks in the
matrix, the cracks deflect along the interface, resulting in interface debonding, as
shown in Fig. 17.3a. At the same time, a small number of weak fibers break in the
fiber bundle.
The second stage: the number of high energy AE signals is increased rapidly, while
the matrix cracking events and interface debonding events are very few, indicating
that the stage of crack saturation has been entered. In two-dimensional plain weave
composites, due to the extrusion between the fiber bundles, the braided joints are easy
to produce hole defects, which is easy to cause matrix crack convergence and stress
concentration of the fiber bundles in braided joints. In the crack saturation region,
fiber and fiber bundle are the main bearing objects of tensile load. Strain energy is
released more through fiber and fiber bundle fracture. Some fibers begin to break at
the same time, and the AE energy is higher than that in the first stage (Fig. 17.8),
and the AE events of fiber bundle breakage begins to appear (Fig. 17.7). It can be
seen from Fig. 17.3c that the longitudinal fiber bundle breaks at the braided node,
and there is obvious fiber pullout. In addition, during this period, due to the effect
of fibers and fiber bundles on the matrix crack growth, cracks are generated in the
cross area of longitudinal and transverse fiber bundles, and begin to extend along
the longitudinal direction of fibers and fiber bundles, resulting in more interlaminar
delamination. Meanwhile, It can be seen from Fig. 17.3a that most of the interlaminar
delamination occurs in the cross area of fiber bundles. The interfacial slippage, fiber
bundle fracture and interlaminar delamination in the process of fiber bundle pullout
cause the elastic modulus of the material to decrease gradually at this stage, which
is consistent with the existing understanding of the accelerated damage stage of the
material.
The third stage: the fiber bundle breakage almost stopped, and other damage types
continued to occur. This is due to the large number of fractures of fibers and fiber
bundles as bearing parts in the saturated area of microcracks in the second stage; with
the increase of stress, the matrix part begin to be the bearing object in the fracture
area of fibers in this stage, and at the same time, the matrix cracks begin to form in the
rich matrix area, as shown in Fig. 17.3b. With the growth of long cracks in the matrix,
the interfacial debonding also increases continuously. The energy dissipation mainly