FRACTURE-BEHAVIOR OF LIQUID-CRYSTAL EPOXY-RESIN SYSTEMS BASED ON DIGLYCIDYL ETHER OF 4,4'-DIHYDROXY-ALPHA-METHYLSTILBENE .2. EFFECT DUE TO BLENDING WITH TACTIX-ASTERISK-556 EPOXY-RESIN AND PHENOLIC MONOMERS

The mechanical behaviours of unoriented, poured resin castings based o n formulated blends containing the diglycidyl ether of 4,4'-dihydroxy- alpha-methylstilbene monomer are studied. It is found that the mechani cal and fracture behaviours of these liquid crystalline epoxy (LCE) bl ends vary significantly. In general, the LCE blends possess much highe r fracture toughness and fatigue crack resistance than conventional ep oxy resins. At low temperatures (-40 degrees C), the K-IC values of th e LCE blends are slightly higher than those measured at room temperatu re. The common fracture mechanisms observed in the ductile LCE blends are crack segmentation, crack branching, crack bridging and crack blun ting. The fracture surfaces of the tougher LCE blends only exhibit lim ited ductile drawing (furrow pattern) at the slow crack growth region; no signs of shear lips on the edges of the starter crack region are o bserved. The optical microscopy and transmission electron microscopy w ork suggests that orientation and/or transformation toughening may be the source for such high fracture toughness of the LCE blends. The pos sible cause(s) of the unusual fracture behaviour of the LCEs is discus sed. Approaches for making high performance LCE blends are also addres sed.