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The effect of welding residual stress on surface fatigue crack propagation and fracture mechanics parameters of intersecting orthogonal cracks is discussed. The effect of welding residual stresses on fatigue propagation of the internal surface crack is analysed in the case of welded joint of the pipeline. To predict fatigue crack propagation, the modified Forman law taking into account the nonsingular T‐stress as the crack‐tip constraint has been employed. It is shown by means of numerical simulation (the software ANSYS and MATLAB) that welding residual stress and crack‐tip constraint effects lead to the semielliptical form of final configuration of the surface fatigue crack. Fracture mechanics parameters for intersecting surface cracks, namely, circular and semielliptical cracks in welded joint of the pipeline are also analysed by numerical simulation. It was concluded that welding residual stress intensifies the effect of intersecting orthogonal surface cracks on fracture mechanics parameters including the crack‐tip constraint.

A combined experimental and computational method for determination of the singular and non‐singular stress terms along the front of the 3D surface crack is proposed. It is suggested to evaluate the terms by means of comprehensive comparison between deformation responses on the structure surface in the vicinity of the crack obtained experimentally and from numerical solutions of the corresponding boundary problem of solid mechanics. As the deformation response, a local displacement field caused by the formation of a small hole at the tip of the crack recorded by digital speckle pattern interferometry may be considered. The proposed approach allows defining such real parameters of the structure as the active load conditions in the crack region and crack sizes. These parameters are used to solve the direct problem and to determine the stress intensity factor KI and Txx‐, Tzz‐stresses along the surface crack front by means of an improved technique of their calculation. The approach accuracy and stability at different conditions have been proved by means of numerical simulation that examined half‐space with planar semi‐elliptical surface crack under biaxial loading. So, the potential applicability of the proposed method is demonstrated. The biaxiality effect on KI and Txx, Tzz is discussed.

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Abstract. Experimental and computational method for determining parameters of stress-strain state is proposed which is based on estimation of compliance between the data sets obtained experimentally and the results of numerical calculations of the boundary problems in formulation of which all distinctive features of area geometry, character of the loads being considered and deformation characteristics of materials are taken into account. The procedure proposed was checked at a number of practically important problems.The methods for investigation into stress-strain state (SSS) of structures based on interference optical techniques for registration of strain or displacement fields (photoelastic techniques, holographic interferometry, speckle photography, electronic digital speckle pattern interferometry techniques) provide the acquisition of uninterrupted information at the surface of the object being studied [1,2]. These methods being commonly used to solve the problems of mechanics of deformed solids, fracture mechanics, vibration-based diagnostics, structural dynamics etc., have distinct advantages over traditional measuring tools that allows only averaged (accumulated over known time base) values of strains or displacements in one direction and at the point given to be registered. The adequacy of the results of such measurements requires, as a rule, additional substantiation.In modern practice the photoelastic coatings technique and, in recent years, electronic digital speckle pattern interferometry (EDSI) became the most common among these methods; significant advantage of these techniques is the capability of creating compact and reliable measurement instrumentation which can be used effectively under full-scale conditions. Besides, the evolvement of modern computer technologies offers a considerable increase in accuracy of measurement results as well as development of conceptually new methodological approaches to solving mechanics problems.The determination of SSS parameters for the object being studied on the basis of processing of the experimental data obtainable by interference optical techniques includes three main stages:• registration of baseline information on the basis of one or another effect of interference optics (photoregistration of interference patterns, hologram recording, registration of images with speckle pattern etc.);• presentation of obtainable information on electronic media in digital format in terms of phase differences of light waves or (after primary processing) directly through components of threedimensional vector of body surface point displacements;