Meaning
Permanent deformation of the helical ridges in a plastic hole when subjected to excessive axial or rotational force describes this mechanical failure. Thread flank yielding occurs when the stress applied by a fastener exceeds the compressive or shear strength of the polymer. Once the flank yields, the screw can no longer maintain clamping force and the joint fails.
Shear Stress
Force applied by the screw threads concentrates at the interface where the metal meets the plastic. If the load is too high, the plastic threads are sheared off or crushed.
Torque Influence
Proper assembly requires enough torque to keep the parts together without reaching the point of thread flank yielding. In soft resins like polyethylene, this window is very narrow, making the use of power drivers risky. High-performance polymers with high glass content increase the yield threshold, allowing for much higher assembly forces.
Specification of a maximum torque remains safely below the yield point to account for variations in material and tool wear.
Geometric Design
Increasing the thread engagement length or using a larger diameter screw distributes the load across more surface area. This reduction in local pressure prevents the material from reaching its yield point under the same total load. Boss designs must balance wall thickness with thread depth to ensure the surrounding plastic can support the internal pressure of the fastener.
