See Table 1 in the attached paper for differentiation between Palmqvist and median cracks. The latter are also called radial-median cracks or half-penny cracks. Palmqvist cracks extend from the corner points of the Vickers indent outwards. Typically, Vickers indentation gives rise to four Palmqvist cracks. Median cracks extend both into the depth beneath the indentation and in the diagonal direction to beyond the corner points. Typically, Vickers indenation gives rise to two median cracks. The view from the top gives similar crack patterns, so it is not trivial to distinguish between both types of cracks. In the paper you will also find selected references that provide relations between fracture toughness and crack dimensions.

At least two types of cracks are produced by a Vickers indenter, i.e., the Palmqvist crack system with half-ellipse sub-structure and median or half-penny crack system (Figure in attachment). Generally, a tough material shows Palmqvist crack system. However, most materials will present both crack systems depending on the applied load. For number of brittle materials the Palmqvist cracks were formed in the low-load regime. On the other hand, Palmqvist cracks were formed only at high loads for reasonably tough materials, such as WC-Co composites with more than about 6 wt. % of Co. There are two ways to distinguish the crack system present in the analysed material. One way to define the crack system is the verification of c/a ratio (c is the crack length from the centre of the indent to the crack tip and a is the half value of the indentation diagonal, Figure in attachment). If it is less than 2.5, than the material shows Palmqvist crack system. The other way to differentiate between the two types of crack system is to polish the surface layers away. The median crack system will always remain connected to the inverted pyramid of the indent (ratio values of c/a is higher than 2.5), while the Palmqvist cracks will detach from the indentation, as shown in Figure in attachment.