Failure Processes in Composites - Multiaxial Failure Criteria

Fracture mechanical analysis of the interface failure in fiber reinforced plastics

The macroscopical failure of fiber reinforced composites is a complex process, which usually is investigated experimentally on a macroscopical scale, delivering overall properties of the material. However, the global failure of composites is due to the accumulation of elementary failure processes like fiber breakage, matrix cracking and interface failure. In order to estimate the strength and long-term behaviour of fiber reinforced composites, e.g., in terms of damage mechanics, it is worth the effort to analyse the elementary failure processes on a microscopical scale. Essential parameters influencing the process are the critical energy release rate of the interface, the matrix as well as the interfacial friction after debonding. In addition, prestresses due to curing or cooling processes have a significant influence on the energy changes during failure.

Commonly, the failure process starts with a fiber break, developing at a surface defect. Some insight into the failure of composites can be gained by microscopical observations of model experiments, so-called fragmentation tests. These tests reveal that different crack patterns can arise, depending on the specific strengths of matrix and interface. In case of a rather brittle matrix and high bond strength large penny shaped matrix cracks arise while in case of rather ductile matrices large 'batman' cracks without debonding occur. In case of a low bond strength debonding with very small or even without matrix cracks can be observed.

In order to evaluate the probability of the occurrence of the specific crack patterns, the energy released by the respective cracks can be analysed by finite element analyses. To this end, two most likely failure scenarios were investigated:

• The fiber breakage takes place without matrix or interface cracks. After total fiber break, an interface crack develops up to a length of four fiber radii (20 µm)
• Simultaneously with the propagation of the fiber crack, penny shaped radial matrix cracks of different lengths up to one fiber radius are studied. After total fiber breakage an interface crack starts to propagate.

In order to compare the results for different crack patterns, the crack propagation is analysed under fixed load conditions.

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