AbstractThe notch sensitivity of aliphatic polyketone (PK) terpolymers was investigated in this article. The notch‐tip radius was varied between the size of an actual propagating crack tip of 1–2 μm and the largest notch tip of 1000 μm radius. The larger notch‐tip radii (1000–15 μm) were milled into the polymer. The smallest notch‐tip radii of 2 and 15 microns were made with a novel technique with the aid of an Eximer laser. The fracture behavior of the PK‐terpolymer was found to be sensitive toward changes in notch‐tip radius. The fracture energy decreased strongly with decreasing notch‐tip radius even when fractured ductile. Sharper notches were not as effectively blunted as were larger notches in the ductile region. The brittle‐to‐ductile transition temperature (Tbd) was increased with decreasing notch‐tip radius for the larger notches (1000–100 μm). For sharper notches (2 and 15 μm) the Tbd decreased again. This behavior could be explained by the decreasing amount of elastic energy release with sharper notches. The energy release proved to be very important for this material. A mixed‐mode fracture behavior was found to occur with larger notches because of the elastic energy present in the sample before fracture. The notch sensitivity was reduced considerably by the addition of a 10% core‐shell rubber phase. For the blend material the notch‐tip radius had almost no influence on the Tbd. The fracture energy on the other hand was still lowered with a sharper notch. This was attributed to the difference in the crack initiation process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2558–2575, 2004