This paper reports on the results of an experimental and analytical investigation on the mode I and mode II fracture mechanics behaviour of a high-strength steel. The aim was to identify the effect of crack tip geometry and loading system on the loading capacity of the material. Notched as well as pre-cracked single edge notched specimens were loaded in symmetric and anti-symmetric bending. The experimental results show that crack tip and loading parameters have a significant influence on the apparent critical fracture toughness values. The influence of a finite notch radius on the mode I and mode II fracture toughness is quantified by a J-integral analysis which indicates that for both loading systems crack initiation can be estimated by a local strain energy density criterion. A weight function analysis of the mode II loading shows that crack tip shielding due to contact between the crack faces becomes important if the distance from the first support roller to the crack plane is less than the thickness of the specimen. This result helps to explain the large scatter which is present in one of the experimental loading configurations.