The integrity of structural materials is oftentimes defined by their resistance against catastrophic
failure through dissipative plastic processes at the crack tip, commonly quantified by the J-integral
concept. However, to date the experimental stress and strain fields necessary to quantify the J-integral
associated with local crack propagation in its original integral form were inaccessible. Here, we
present a multi-method nanoscale strain- and stress-mapping surrounding a growing crack tip in two
identical miniaturized fracture specimens made from a nanocrystalline FeCrMnNiCo high-entropy
alloy. The respective samples were tested in situ in a scanning electron microscope and a synchrotron
X-ray nanodiffraction setup, with detailed analyzes of loading states during elastic loading, crack tip
blunting and general yielding, corroborated by a detailed elastic-plastic finite element model. This
complementary in situ methodology uniquely enabled a detailed quantification of the J-integral along
different integration paths from experimental nanoscale stress and strain fields. We find that
conventional linear-elastic and elastic-plastic models, typically used to interpret fracture phenomena,
have limited applicability at micron to nanoscale distances from propagating cracks. This for the first
time unravels a limit to the path-independence of the J-integral, which has significant implications in
the development and assessment of modern damage-tolerant materials and microstructures.