Meaning
Planar shear strain localization concentrates plastic deformation into narrow planar bands during compressive or tensile loading of amorphous polymers. Developing shear banding acts as a primary plastic yield mechanism that absorbs mechanical energy before ultimate material failure. The mechanism dominates compressive yielding in amorphous resins like polystyrene, polymethyl methacrylate, and polycarbonate, but is absent in highly crystalline polymers that yield via necking.
Localization Dynamics
Applied compressive stress initiates narrow shear zones inclined at approximately thirty-eight degrees to the loading axis. Local strain softening inside these bands allows intense plastic slip while adjacent material blocks remain essentially elastic. Optical microscopy reveals dark intersecting bands crossing part sections under polar light.
Polymer molecular weight controls the width and density of shear bands formed before failure.
Yield Initiation
Intermolecular friction drops rapidly once shear localization begins within the polymer matrix. High local shear strain aligns polymer chains within the band, reducing further resistance to sliding. Molded-in residual tensile stresses lower the critical stress needed to trigger shear bands during mechanical loading.
Contaminants in regrind resin create stress concentration sites that produce chaotic, premature shear bands and lower overall toughness.
Strain Softening
Strain localization within narrow bands lowers post-yield load capacity.
