Meaning
Structural endurance measured under alternating stress amplitudes where plastic deformation remains negligible describes the fatigue regime above ten thousand cycles. Under these elastic strain conditions, high-cycle fatigue leads to microcrack initiation at stress concentrations such as sharp radii, gates or knit lines in injection molded components. This fatigue regime ends where high stress levels induce widespread gross plastic deformation before microcracks form.
Crack Initiation
Cyclic loading below the macroscopic yield strength induces localized dislocation motion and shear band formation in amorphous polymers. Microscopic void formation accelerates under high-cycle fatigue when cyclic tension unseats glass fibers from the surrounding resin matrix. Datasheet fatigue limits derived from unnotched specimens tested in laboratory air overestimate actual part performance in the presence of surface scratches or mold parting lines.
Recyclate contamination introduces stress risers that lower the threshold for crack initiation.
Dynamic Heating
Internal friction generated during high-frequency mechanical cycling raises part core temperature in thick-walled sections. Under high-cycle fatigue, low thermal conductivity in polymers such as polyamide prevents heat dissipation, leading to thermal softening or localized thermal runaway. Moulders must balance cycle frequency during qualification testing to isolate mechanical failure from thermal degradation.
Dynamic mechanical analysis helps identify the transition temperature where viscous dissipation begins to dominate elastic strain energy.
Specimen Geometry
Molding conditions dictate local molecular orientation and skin thickness, directly governing fatigue life. Rapid cooling in thin sections creates a rigid outer skin with high residual compressive stress that inhibits surface crack growth under high-cycle fatigue. Conversely, core regions cooling slowly exhibit larger spherulite structures that provide lower resistance to cyclic microcrack propagation.
Gates placed near high-stress areas introduce flow orientation boundaries that drastically reduce cycle life before total part fracture.
