Fatigue crack characterization by image correlation.

The structural integrity of many materials from aircraft structures, pressure vessels
and offshore structures is closely related to the cracks existing in the component. The
stress intensity factor is widely recognized as a major parameter for assessing the
severity of a crack, and is extensively used in fatigue lifetime predictions. This work
presents a new methodology for experimental determination of the effective stress
intensity factor from image correlation data. In addition, the methodology allows the
reconstruction of the full stress field. Based on a multi-point over-deterministic
method, the full-field displacement information collected from the neighbourhood of
the crack tip is combined with an elastic model based on Muskhelishvili's complex
variable formulation to determine the opening and the sliding mode stress intensity
factors. Utilization was made of edge-finding routines for locating the crack tip
coordinates from the displacement fields. The technique was applied successfully to
study real fatigue cracks in different aluminium alloy specimens, subjected to static
and cyclic loads. Good agreement was also obtained for a crack emanating from a
hole, where complex passing stress fields exist. Different factors such as crack face
contact and rubbing effects, local mixed-mode arising from wedge opening or tilting
of the crack front are identified as being responsible of the differences between the
nominal and measured values. Finally the non-contacting tool was extended to the
analysis of crack face interaction and crack closure problems, and it was found to be
able to estimate the opening load.