539.3Longitudinal-shear fracture toughness characteristics have been computationally and experimentally determined for cylindrical and prismatic specimens with a circumferential and edge cracks, respectively. It is shown that during the determination of the fracture toughness characteristic of a finite-thickness prismatic specimen under longitudinal shear along the crack front there occurs displacement both in the transverse direction (mode II) and longitudinal one (mode III). This should be taken into account when using specimens of this type.Keywords: fracture toughness, longitudinal and transverse shear, cylindrical and prismatic specimens, circumferential and edge cracks.Introduction. The majority of experimental and theoretical investigations address the opening mode (mode I) cracks. However, there is some experimental evidence that the critical values of fracture toughness characteristics in loading by modes II and III can be higher or lower than those in mode I loading, depending on mechanical characteristics of materials, their structural features, etc. [1,2]. Also, there is little data on the assessment of the influence of thermomechanical pre-loading (TP) of specimens with cracks on fracture toughness characteristics in mixed-mode loading. On the other hand, the available information suggests that this influence is ambiguous, i.e., for some materials fracture toughness grows due to TP, while for other materials, on the contrary, it goes down [3,4].Technological defects in material, microcracks, scratches, burrs that arise during the manufacture of a part are randomly oriented with respect to the loads applied to the part. Generally, the crack edge displacement occurs under loading by mixed modes I+II+III. Therefore, it is important to have reliable information about critical values of fracture toughness characteristics in loading by mode other than mode I and to possess reliable means for computing fracture toughness in these cases.Despite the availability of numerous reference data on relationships between the stress intensity factor (SIF) and the crack length (the K-calibration) for various specimen geometries [5], this information should be updated taking into account the type of loading and specimen geometry. Owing to the progress achieved in computer technology and numerical methods of calculation, the finite-element method (FEM) has received a wide acceptance for these applications. However, for numerical computation of SIF and J-integral researchers follow different procedures, outlining benefits and drawbacks of each procedure [6][7][8][9][10].Morozov et al.[9] provided a brief review of the SIF determination methods, in particular, the direct methods based on the use of isoparametric quadratic elements, which enables the root singularity of the crack-tip stresses to be modeled by shifting an element middle node by a quarter towards the crack tip, and the energy methods that use a relationship between SIF and J-integral.The J-integral has found a wide application in the load capacity analysis of...