Characterization of dilation failure of composites using fractography
Carole Autori and Alan Crosky
Efficient design of structures from fibre reinforced plastic composites requires an accurate knowledge of the load at which failure will occur. Several empirical theories, based on mathematical descriptions of observed mechanical behaviour, have been developed for this purpose. While each provides good prediction for particular ranges of situations (eg, fibre orientation, stacking sequence, etc,) none is universal in application.
Traditionally it has been considered that the matrix in a composite simply holds the fibres in the required orientations, however, failure is in fact initiated in the matrix when the loading direction is greater than about 10° to the fibre direction. Thus, except over the very narrow range of orientations where the load is within 10° of the fibre orientation, onset of failure is determined by the properties of the matrix. Recognising this, workers from Boeing have developed a physically-based theory, known as the Onset Theory, in which molecular dynamic modelling of the polymer matrix is used to predict the onset of failure. The theory considers that matrix failure occurs by either dilatation or distortion.
The work shown below was undertaken to evaluate the predictions of the Onset Theory for dilatation failure. This was done by examining the fractographic features in the vicinity of the tip of a growing crack in samples loaded normal to the fibres.
SEM images of the crack tip with the fibres perpendicular to the plane of section. Image on right shows sample on left tilted at 40°.
SEM images of the crack tip with the fibres parallel to the plane of section.
SEM observation provides a two dimensional understanding of the crack path, however, the failure mechanism is a three-dimensional phenomenon. Focused ion beam milling was therefore used to examine the fractographic features in the third dimension and this has provided a considerably enhanced understanding of the processes that occur.
SEM image of the crack tip with the fibres parallel to the plane of section. Image on right shows sample on left after focused ion beam milling
The study has confirmed that the fractographic features present in the specimens are consistent with dilatation failure. The observation of replicate samples suggests that these features are reproducible.