The paper describes work done using synchrotron light to investigate the microstructure and how it behaves in 3D when a load is applied. Two different cast iron materials with different matrix structures and graphite morphologies were investigated; SiMo51, which is basically a spheroidal iron (SGI) alloyed with Si and Mo, and a lamellar graphite iron (LGI). The tensile test specimens were loaded in steps, at which x-ray tomography as well as 3DXRD measurements were made to characterize the microstructure. The result shows how the crack propagates and which path it takes through the materials. DVC was applied to analyze the strain fields. This work also shows how useful synchrotron experiments can be in the study of the mechanical behavior of cast iron.