KINK BANDS AND SHEAR DEFORMATION IN POLYBENZOBISOXAZOLE FIBERS

Kink bands with a thickness ranging from 45 nm to 1.8 mu m were observ ed in polybenzobisoxazole (PBO) fibres examined in the transmission el ectron microscope. These bands are localized, and oriented at 69 degre es +/- 3 degrees with respect to the fibre axial direction. The averag e shear strain is about 0.7 along the band. Bands of intermediate thic knesses (e.g. 0.1-0.3 mu m) are wavy and relatively less uniform than the thin (45 nm) and thick (1.8 mu m) bands. Sometimes cracks or voids with a preferential orientation develop in the band, particularly in 0.1 mu m or thicker bands. When two bands intersect with each other, a mutual shearing effect seems to occur at the band intersection, where the fibrils or voids in an existing band are reoriented in a new dire ction, which seems to be determined by the resultant shear displacemen t of the two bands. Enhanced voiding occurs at the intersection. Under large deformation, buckling occurs at regions populated with bands an d band intersections. These observations suggest a shear yielding mech anism for incipient kink bands, and the possibility of a large-scale s hear motion under large compression. However, this large-scale shear m otion may be lessened if buckling occurs.