KINK BANDS AND SHEAR DEFORMATION IN POLYBENZOBISOXAZOLE FIBERS

Kink bands with a thickness ranging from 45 nm to 1.8 mu m were observed in polybenzobisoxazole (PBO) fibres examined in the transmission electron microscope. These bands are localized, and oriented at 69 degrees +/- 3 degrees with respect to the fibre axial direction. The average shear strain is about 0.7 along the band. Bands of intermediate thicknesses (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 direction, which seems to be determined by the resultant shear displacement of the two bands. Enhanced voiding occurs at the intersection. Under large deformation, buckling occurs at regions populated with bands and band intersections. These observations suggest a shear yielding mechanism for incipient kink bands, and the possibility of a large-scale shear motion under large compression. However, this large-scale shear motion may be lessened if buckling occurs.