Movie S2.

When a composite PDMS of large fiber/matrix modulus contrast (Ef/Em ∼ 12) is pulled to a stretch around 1.5, the crack blunts, but remains stable. As the stretch increases, the crack branches near the interface between the matrix and fiber. At a stretch of 1.85, the fibers at location far away from the crack front start to break, and the whole sample ruptures. A:B is 10:1 for the fibers, and is 30:1 for the matrix. We color the matrix in red to see the fibers.