Clinching is a mechanical joining technology with low cost and good mechanical performance, and offers advantages for joining high-strength aluminum alloys. Numerical models of such joints assist in understanding the underlying mechanisms and in optimizing the process, however their predictive capability requires accurate experimental validation. Conventional validation methods for mechanical joints cannot resolve internal deformation states and crack initiation during loading. In contrast, in-situ X-ray computed tomography~(in-situ CT) enables the quantitative measurement of both the global deformation and the initiation site, size, shape, and propagation of evolving cracks. This study presents the first application of in-situ CT to quantitatively validate a constitutive damage and failure model by capturing the entire deformation and failure process in a tensile shear test involving two aluminum alloy sheets joined by clinching. The numerical model is set up considering excessive plastic deformations, residual stresses, and complex contact interfaces in the initial state. The in-situ CT method accurately visualizes the sequence of failure mechanisms within the joint, including sheet pull-out, local necking, ductile fracture initiation, and subsequent macrocrack propagation through one of the sheets. Eventually, the sheet surfaces, the true crack initiation site and the crack surface are captured and used to validate the numerical predictions.