FRACTURE MECHANICAL CHARACTERIZATION OF TEAR RESISTANCE OF SPUNBONDED FIBERWEBS (STRESS INTENSITY FACTOR, TOUGHNESS PARAMETER)

The conventional tear characterization of textile fabrics suffers from three limitations. First, the analyses of tear resistance of textile (woven) fabrics are based on qualitative description. Very little quantitative research has been reported. Second, for a given crack size, we cannot predict the critical load the structure can tolerate. Or conversely, for an expected service load, we do not know what the critical crack size is. Third, the conventional tear tests, do not test for in-plane shear mode of failure.; The techniques of fracture mechanics are aimed at resolving these problems. This dissertation briefly reviews the concepts of linear elastic fracture mechanics (LEFM). It then seeks to demonstrate the validity of a fracture toughness parameter, useful for predicting brittle fracture propagation in characterizing the onset of tear propagation in spunbonded fiberwebs.; Using a double edge-cut specimen configuration, the experimental evidence reveals that the stress intensity factor K(,1) (defined in the text) has a critical value for each of the fabrics investigated. In other words, it can be used to predict the failure stress for a given cut (tear) size; as such it defines the tear resistance (toughness) of the fabric.; In the second part, center-cut specimens, oriented at different bias angles ((theta)) relative to the cross machine direction (CD), are tested in tension. Keeping the cut orientation always along CD, it engenders symmetric and skew-symmetric loading of the cut simultaneously. The associated critical stress intensity factors K(,1) ((theta)) (for the opening mode) K(,2) ((theta)) (for the in-plane shear mode) are found to be material constants. By postulating a failure function for the mixed mode loading, the stress intensity factor for pure shear loading K(,2) is inferred. Comparing the value of K(,2) with that of K(,1), (for a given fabric) K(,2) is consistently found to be smaller; which implies that K(,2) may be sometimes more critical in determining the failure of fiberwebs.; In the third part of the investigation stress intensity factors (K(,1)) obtained from double-edge (DE) and center-cut (CC) specimens are compared. For sufficiently long samples the two values are very close to each other, although the CC value is always somewhat lower. This observation too is consistent with predictions of theoretical fracture mechanics. (Abstract shortened with permission of author.)...