Meaning
Stress state calculations at a given point within a deformed polymer dictate the transition between ductile stretching and brittle fracture. This stress metric, known as stress triaxiality, is defined as the ratio of the hydrostatic stress to the equivalent von Mises stress. It represents the degree to which a material is subjected to three-dimensional tension or compression rather than simple shear.
Failure Acceleration
High levels of this ratio promote the nucleation and growth of micro-voids within the polymer matrix. Under high stress triaxiality, the polymer cannot deform plastically to relieve the stress, which accelerates the transition to brittle fracture. This mechanism is responsible for sudden failures in areas such as the neck thread or the base of blow-moulded bottles when they are subjected to sudden loads.
Mitigating this risk requires reducing sharp internal corners.
Moulding Relevance
During the stretch blow moulding process, the material is drawn biaxially, creating a complex multi-axial stress field. If the preform is not heated evenly, regions of high stress triaxiality can develop in the cooler sections. These regions are more likely to fail by tearing during the high-pressure inflation phase.
Design Consideration
Engineers use finite element analysis to locate regions where this parameter is high under service loads. Redesigning these areas by increasing fillet radii or adjusting the wall thickness reduces the risk of in-service container failure. This step is particularly important for bottles designed to hold carbonated beverages.
