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Fracture toughness of particle reinforced polymers is strongly affected by the size of
particles. It can be improved or reduced depending on the materials used and the volume
fractions at which the values are compared. Dissipation mechanisms, as particle
debonding and subsequent yielding of the polymer, are responsible for the characteristic
behaviour. If the debonding energy per volume is considered it can be concluded that
smaller particles are favourable for this value. But the product of the specific debonding
energy with the dissipation volume is the decisive quantity. Depending on the used
debonding criterion, i. e. stress or energy, different conclusions can be drawn. The energy
criterion leads to the conclusion that the debonding process inducing fracture toughness
independent of particle size, however, the stress criterion at the interface supports the
conclusion that smaller particles increase facture toughness.