Mode‐I Fracture Toughness, KIc, was measured in six shale materials using the double‐torsion technique. During loading, crack propagation was imaged both using twin optical cameras, and with fast X‐ray radiograph acquisition. Samples of Bowland, Haynesville, Kimmeridge, Mancos, Middlecliff, and Whitby shales were tested in a range of orientations. The measured fracture toughness values were found to be in good agreement with existing literature values. The two imaging techniques improve our understanding of local conditions around the fracture‐tip, through in situ correlation of mechanical data, inelastic zone size, and fracture‐tip velocity. The optical Digital Image Correlation technique proved useful as a means of determining the validity of individual experiments, by identifying experiments during which strains had developed in the two “rigid” specimen halves. Strain maps determined through Digital Image Correlation of the optical images suggest that the scale of the inelastic zone is an order of magnitude smaller than the classically used approximation suggests. This smaller damage region suggests a narrower region of enhanced permeability around artificially generated fractures in shales. The resolvable crack‐tip was tracked using radiograph data and found to travel at a velocity around 470 μm/s during failure, with little variation in speed between materials and orientations. Fracture pathways in the bedding parallel orientations were observed to deviate from linearity, commonly following layer boundaries. This suggests that while a fracture traveling parallel to bedding may travel at a similar speed to a bedding perpendicular fracture, it may have a more tortuous pathway, and therefore access a larger surface area.

Abstract. Two common problems of mechanical strain relaxation (MSR) residual stress measurement methods are investigated in this work: (1) assumption of stress uniformity and (2) the effect of plasticity at relaxation. A new MSR technique, designed specifically for highly non-uniform in-plane residual stress fields, is applied in this work to measure the residual stress field resulted from pure bending of an Al7075 alloy. The method involves introducing a straight cut across the whole part in a single increment, and collecting full field displacement fields from the side surface. Application of a 2D high resolution digital image correlation (DIC) method proved successful in this work. The reconstructed residual stress agrees well with that predicted by FE modelling. It is shown that the direction of the propagation of the slit has a major influence on plastic flow during relaxation. The major conclusion from this work is that it is possible to substantially reduce, or completely eliminate, plastic flow on relaxation by careful planning of the slit orientation and the cutting schedule. Mechanical stress relaxation basics revisitedMost, if not all, mechanical stress relaxation (MSR) methods rely on an elastic model to convert the measured relaxation strain or displacements into stresses. Examples of MSR techniques are: slitting and elastic crack solutions [1], hole drilling and a 2D elastic solution for a hole in a plane under a constant traction at infinity [2, 3], a hole boring and the Lamé's solution for a thick-walled cylinder [4] or an arbitrary (though usually planar) 3D cut with a corresponding elastic solution (the contour method) [5]. The relaxation can be measured with strain gauges, with non-contact optical methods, such as a laser speckle interferometry or a digital image correlation (DIC), which measure displacements, or with grid, shadow or photoelastic coating methods, which measure some functions of displacement or strain.Although this general scheme has been used with great success over many decades to measure residual stresses in a variety of components, its success and applicability critically depend on two key assumptions. The relaxation process is assumed to be purely elastic. This is known not to be true in many practical applications, in particular, when the residual stress state is triaxial or when the magnitude of residual stress is close to yield, i.e. the most important cases of residual stress analysis. Although various plasticity corrections have been proposed, the issue is far from being completely resolved [3, 6--8].Moreover, the vast majority of analytical models include assumptions on the residual stress state itself. Most models assume that the stress state is uniform in the immediate vicinity of the relaxation measurement location. For example, for the hole drilling method, the typical analytical model is a hole in a 2D plate under uniform far field stress. Or, in the case of a deep hole drilling method [3], stress is assumed constant in plane normal to the axis of the cut, but i...

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Abstract.A recently proposed Mechanical Strain Relaxation (MSR) technique for the measurement of residual stress in thin plates is presented. The measurement of residual stress involves making a single straight cut and collecting the relaxation displacement data from the side surface. In this work the method was applied to an Aluminium friction stir welded (FSW) specimen. The cut was introduced with a wire electrical discharge machining. The displacements were recorded with a 3D digital image correlation (DIC) method. The measured FSW residual stress profile agreed well with that measured by Energy Dispersive X-ray Diffraction (EDXRD). It was observed that the amount of plastic strain, caused by stress redistribution during the relaxation process, strongly depends on the direction of the propagation of the cut. In particular, if a cut is propagated along the thickness of a plate, then the effect of plastic flow on the measured residual stress is negligible. Another attractive feature of the method is that it is relatively insensitive to random experimental noise. IntroductionOver the last two decades a wide range of mechanical strain relaxation (MSR) techniques have been developed and used to measure residual stresses in a wide variety of engineering components. Well known examples of MSR techniques are: hole drilling [1], slitting [2] and contour method [3]. Recently, new technique [4] has been proposed and it follows the general scheme of Mechanical Strain Relaxation (MSR) techniques, i.e. measuring the deformation due to removal of some stressed material in a body and then that measured relaxation is converted to stress.The main target of the proposed method is that the shape of in-plane residual stress field is highly non-uniform, including discontinuities. Other popular techniques, such as e.g. hole drilling, cannot be used in such cases because they assume stress uniformity within hole diameter [1] and many holes are required to measure residual stress field in relatively large measurement area, leading to complexity of the experiment [5].During cutting residual stress is redistributed and this might cause plastic deformation where the magnitude of residual stress is close to yield [2]. However, it is shown that this plastic strain on relaxation can be mitigated by choosing the correct propagation of the cut [4]. For this is reason, the technique is attractive in practical application because the measurement can be conducted regardless of the propagation of the cut. It is not always possible for many techniques, for example the slitting method, the propagation of the cut is very important because non-uniform RS can only be measure along depth [2].In this work, RS field in 3 mm thick FSW plates have been measured by the proposed method. The application is ideal for the technique because the specimen is very thin and the stress field is non-uniform. The techniques involves a single cut at the midsection of the longitudinal direction of the plate, propagated either from the front to the rear surface, called '...