There have been many theoretical studies to predict the stress fields around the tip of a
growing fatigue crack. However, until recently the highly-localized, small scale nature of the
stresses has meant that direct measurement has not been possible. With the current generation of
synchrotron X-ray sources, sub-millimetre sampling dimensions are now possible, and it has
become possible to evaluate directly the stresses at the tip of a fatigue crack and to see how the
stresses evolve as the result of an overload, for example.
In this paper we present results of synchrotron X-ray diffraction analysis of the stress fields
around a fatigue crack in aluminium alloy 5091 (Al-Mg-Li-C-O); this is a dispersion-strengthened
alloy with a fine grain size, which makes it ideal for such experiments. Compact tension (CT)
specimens were prepared with constant amplitude fatigue loading. The energy dispersive X-ray
diffraction (EDXRD) technique was used for measuring strains around the crack tip along the mid
thickness of the specimen under in-situ loading. The measurement was carried out at the ESRF
(European Synchrotron Radiation Facility), Grenoble, France on the ID15A beam line. The
experimental crack tip stresses have been compared with the analytical fracture mechanics solution.
In situ through-thickness analysis of crack tip fields with synchrotron X-ray diffraction