Predicting brittle failure in rocks is critical for earthquake hazard assessment, geothermal energy production, and safe geological storage of CO₂ or hazardous wastes. Recent 4D X-ray microtomography (4DXCT) studies have revealed how microfractures nucleate and grow beyond yielding, but they cannot capture the local stress fields driving fracture initiation and propagation. Here, we will combine 4DXCT with scanning 3D X-ray diffraction (s3DXRD) to monitor in situ microfracture development and internal stress evolution in sandstone cores under uniaxial compression. This approach will allow us to directly test how stress heterogeneities control fracture nucleation and how long-range elastic interactions between fractures drive their coalescence into localized faults. The results will provide new micromechanical insight into brittle failure and deliver internal stress measurements to constrain data-driven models of rock failure.