Micromechanical measurements

The experimental methods used for making micromechanical measurements are discussed in the context of the study of craze zones in glassy polymers. A transmission electron microscopy technique, which gives high spatial resolution, is used to measure craze profiles in thin polymeric films. Interference optics is used in the measurement of crack tip crazes in fracture mechanic specimens. The latter method is not restricted to crazes at the tips of stationary or slowly cracks under static loading, since the experimental difficulties associated with the investigation of high frequency fatigue and fast moving cracks have been overcome. Diffraction techniques are used to measure, indirectly, the distribution and size of craze fibrils. Fibril dimensions can also be estimated from scanning electron microscopy of fracture surfaces. The Dugdale model may be used to analyse craze micromechanics measurements but it is assumed that the stress in the zone is constant and it is necessary to use anothe r method if the craze stress distribution is required. Possible methods include a dislocation density stress analysis and the finite element method, the latter having the advantage that it is possible to input experimental boundary conditions. (IWM)