In situ orientation of linear low density polyethylene films subjected to Mode I fracture load

Abstract

An in situ technique has been developed to study the deformation and orientation near a crack tip during Mode I fracture. The technique employs a pair of rotating crossed polar films positioned on either side of the sample. Images gained from transmitted light show kidney shaped process zones ahead of the crack tip that increase in size with deflection, primarily by isotropic expansion. Extinction bands, which are regions of either unoriented material or material oriented in the direction of one of the two polars, are observed as a function of the angular position of the polars with respect to the loading direction. A set of extinction bands at various crossed polar positions provides orientation direction information within the sample. Continuous flow of the molecules around the crack tip is observed at several stages of deformation and for all films tested. The orientation field around the crack tip evolves with increasing radius of curvature of the crack tip. A transition in orientation is observed when the cracks are coincident with the orientation direction of the film but not when they are normal to the orientation direction. The technique of rotating crossed polars is successfully used to determine the orientation direction in the vicinity of a crack tip for linear low density polyethylene during several stages of Mode I loading. The advantage of the technique is that in situ data may be collected quickly when compared to techniques such as X-ray scattering, which rely on data collection through scanning and not the parallel data collection utilized herein. The authors acknowledge that this technique is limited to material that transmits light.