Dynamic Fatigue Testing of Polyamide 6: The Determination of S-N Curves for an Anisotropic Glass Filled PA6

Short glass fiber reinforced Polyamides are of specific interest to replace metal parts since they have high strength to weight ratios, approach the strength of some die cast metals, and can be injection molded into complex shapes. However, the process of injection molding causes fiber orientation, which gives rise to anisotropic mechanical strength and failure. Engineers can use fatigue testing to predict the fatigue life at a given fiber orientation and make design improvements to compensate for the orientation effects. A full S-N Curve characterization was completed on a polyamide 6 filled with fiber glass and mineral to determine the fatigue resistance, under uniaxial stress, for over 1MM cycles. Special challenges and learnings are shared for the selection of fatigue apparatus, mold geometry, test bar cutting, orientation angles, capturing the unit cell properties, load frequency, temperature, and cycles to failure. A 25mm x 25mm x3mm mold with a full edge fan gate, was found to provide consistent fiber orientation at the selected unit cell position. Fiber orientations of 90, 45, and 0 degree were easily cut from the molded plaque by water jet, then returned to a dry state before testing. Hysteretic heating was avoided by keeping the test frequency below 10 Hertz. For the two selected temperatures, over 120 test bars, and over 2 months of testing time was required. The Endurance points (where failure no longer occurred) were identified on S-N Curves and shared. © 2023 The Authors.