Visualization of tensile stress induced material response at a crack tip in polymers under critical load by NMR imaging

NMR imaging is applied to study the crack growth resistance of polymers, being an important toughness parameter of which the understanding is still incomplete. A dedicated stretching device was developed to keep notched materials under load during the NMR measurements allowing to visualize the near crack tip damage behavior in polymers. Two polymers were investigated: ABS, a blend of poly(styrene-co-acrylonitrile) with 28 wt % polybutadiene, and a block copolymer poly(butylene terephthalate)/ poly(tetramethylene oxide) (PBT/PTMO). MRI investigations were performed on loaded specimens with a crack grown under critical conditions as well as on unloaded specimens. Numerous damage bands appear in the images of strained ABS which converge toward the crack tip and remain present upon unloading. Image contrast is demonstrated to arise from a reduced material density in these damage bands. The material density near the crack tip is reduced to about one-fourth of the normal density. Obviously severe crazing and rubber particle cavitation occurs at the crack tip. For the PBT/PTMO block copolymer a more continuous distribution of stress induced material response is observed that disappears almost completely upon unloading. Here image contrast mainly arises from a reduction of chain mobility in soft domains due to stress induced chain orientation.