Meaning
Stress-induced void formation in the rubbery or amorphous domains of a polymer blend under load initiates shear yielding in the matrix. Through cavitation, local stress concentrations are relieved as the secondary phase deforms and creates microscopic vacuoles. This mechanism prevents brittle fracture by absorbing mechanical energy and distributing the stress across a broader volume.
Yielding Mechanism
Microvoid formation occurs primarily at the poles of elastomer particles or within the soft rubber domains. Once cavitation has occurred, the triaxial stress state in the matrix is converted to a uniaxial or biaxial stress state. This stress conversion allows the surrounding polymer to undergo plastic deformation, which raises the overall toughness of the moulded article.
Stress Influence
Elastic moduli and particle size dictate the onset of voiding during mechanical deformation. Below a specific particle size threshold, the energy required for cavitation exceeds the energy for craze initiation, resulting in a brittle failure mode. Conversely, larger elastomer particles initiate voids under lower applied stress, which triggers mass plastic flow before cracking can start.
Moulding Consequence
Inadequate cooling or excessive pressure drops during injection moulding can lead to thermal voiding resembling this phenomenon. When the melt in the centre of a thick wall cools and contracts, cavitation arises if the holding pressure is insufficient to pack the cavity. These internal voids weaken the structural integrity of the final part and lead to premature failure under normal operational loads, representing a serious defect in thick-wall moulding runs.
