Meaning
Defects that occur during resin flow or consolidation where pockets of air or volatile gases become locked within the curing laminate degrade the mechanical performance of the composite. When void entrapment happens, the micro-voids reduce the shear and compressive strength of the matrix, creating points of high local stress that can lead to early failure under load. These voids typically form when the resin flows unevenly through the porous fiber architecture, bypassing dry areas.
Controlling this phenomenon is a major goal of laminate consolidation and autoclave pressure cycles. Automated inspection techniques like ultrasound are used to detect these sub-surface defects after the part is removed from the mold.
Defect Formation
Gas bubbles are locked into the composite structure as the liquid polymer matrix transitions into a solid network. During the early stages of the cure cycle, void entrapment occurs if the localized consolidation pressure is too low to dissolve the trapped gas into the resin. These pockets of gas can expand if the temperature rises too rapidly, leading to larger delamination zones.
Flow Front Kinetics
Inhomogeneous flow rates through different regions of the fiber bed cause the resin front to merge and trap pockets of air. This behavior is governed by the balance between the viscous forces driving the flow and the capillary forces pulling the resin into the dry bundles. If the flow front moves too quickly, it bypasses the fine spaces within the fiber bundles, trapping air inside them.
Process Optimization
Process variables like vacuum level and injection rate must be balanced to minimize the size and number of trapped voids. Applying a high vacuum before injecting the resin ensures that the air is evacuated from the fiber bed. This preparation reduces the initial quantity of gas available to be trapped, improving the consistency of the manufactured part.
