Meaning
Material failures occurring under sustained, multi-directional stress states over an extended duration result from slow, progressive damage in the polymer microstructure. Structural engineers evaluate multi-axial creep rupture to predict the long-term survival of pressurized plastic pipes and storage tanks. This failure mechanism differs from uniaxial failure because the combined stresses restrict localized yielding, causing a more brittle transition.
The boundary for this behavior is defined by the stress state and temperature above which polymer chains permanently slip.
Stress Interaction
Combined tensile and shear loads accelerate the formation of microscopic crazes in the amorphous regions of the polymer. Analyzing multi-axial creep rupture requires multiaxial test rigs that apply concurrent pressure and axial tension to tubular specimens. These complex stress fields reduce the time to failure compared to simple uniaxial tensile creep.
Moulding Influence
Residual stresses locked in during the rapid cooling of an injection molded part add to the applied operational stresses. This internal stress state lowers the pressure required to initiate creep failure. Annealing the molded parts relieves these molded-in stresses, extending the product lifetime under load.
Material Selection
High-molecular-weight resins possess longer polymer chains that increase the number of tie molecules between crystalline lamellae, improving resistance to sustained multiaxial deformation. Regrind polymers exhibit shorter chains that speed up the initiation of creep voids. Specifying high-performance resins ensures structural integrity in demanding environments.
