The cause of fracture of two piston rods of hammers of a drop forge was determined. The first rod of 180 mm diam consisted of an unalloyed steel with 0.37% C and 0.67% Mn and had a strength of 56 kp/sq mm at 26% elongation. Fatigue fractures propagated from several points which could be recognized as flaky cracks already in the fracture, and which later were united. No material defects could be detected in the cross section parallel to the fracture plane except for these very short cracks. These comparatively insignificant defects were sufficient to cause the fracture during...