Numerical failure analysis of steel sheets using a localization enhanced element and a stress based fracture criterion

Prediction of plane strain fracture of AHSS sheets with post-initiation softening

Effect of microstructure on the mechanical properties of Haversian cortical bone

Influence of crystallographic orientation on local strains in silicon: A combined high-resolution X-ray diffraction and finite element modelling investigation

This research work addresses the mechanical and metallurgical characterisation, as well as the failure features, of two types of lap-fillet autogenous laser-welded joints made of ultra-thin sheets by applying an appropriate welding technology for producing sound welds and flawless joints. Both welded samples, one made only of stainless steel (SS-SS) sheets, and the other made of stainless steel and carbon steel (SS-CS) plates, were subjected to tensile–shear loads that are representative of the in-service conditions. The experimental research was focused on determining, by the digital image correlation (VIC-2D) method, the strain field and the rotation angle of the welded joints that were developed during loading tests of the welded specimens. Comparing to the classical testing method applied to study the joint overall mechanical properties, the novelty of this research consists of local mechanical behaviour assessment of relevant zones from similar and dissimilar welded joints, by using the innovative technique VIC-2D. Based on the analysis of the experimental results, it was found that the maximum rotation angle is 2.5 times higher in the SS-SS similar welded joint, in comparison with the SS-CS dissimilar welded joint. Despite this finding, the SS-CS specimen failed in the CS base material, far from the weld, with the failure phenomenon being preceded by the material yielding and necking. This failure mode is consistent with the detected strength mismatch of the SS-CS joint, with respect to the CS base material. In contrast, the quasi-ductile fracture of the SS-SS welded joint occurred by plastic exhaustion at the boundary between the narrow Heat-Affected Zone (HAZ) of SS and the Fuzion Zone (FZ). These outcomes are consistent with the hardness profile, microstructural heterogeneities found in the lap-fillet welded joints, and the load versus elongation curves that are determined and discussed in this paper. This research provides new insight and original information on the materials’ response to the autogenous laser welding, which will contribute to improving the knowledge on the ultra-thin lap-fillet welded similar and dissimilar steels.

Modern advanced high-strength steel sheets for automotive applications are mostly zinc coated for corrosion resistance. However, the presence of zinc can—besides its positive effects—increase the material’s susceptibility to liquid metal embrittlement (LME) during resistance spot welding (RSW). Zinc and its eutectics are, due to their low melting point, present in liquid state during the welding process. This fact can, in combination with other factors like tensile strains or stresses, lead to the formation of brittle, intergranular cracks in the weld, and heat-affected zone. This phenomenon is commonly called liquid metal embrittlement. In order to understand the process from a practical perspective, one must learn what factors facilitate it. In this study, industry-relevant parameters are investigated regarding their influence on the occurrence of LME, embodied by the formation of surface cracks. It was found that electrode wear has less of an influence on the cracking susceptibility than welding current or tensile stresses. Finite element analysis is believed to provide a powerful tool in order to gain insights on the formation process. Modeling of the process shows promising initial results, revealing the underlying local stress and strain fields, unmeasurable with common techniques.

Oriented fracturing is suggested to address complex reservoir issues such as multiple fractures, near-wellbore tortuosity, maximum proppant concentration, vertical coverage, natural fractures, tortuosity and erosional effects. These issues have significant impact on reservoir performance and well deliverability. Net pressure matches and geomechanical data indicate that effective oriented fracture stimulation treatments can be implemented where other treatments have failed or unacceptable production changes occurred. Understanding of the near-wellbore stress distribution is critical to implementation of effective fracturing strategies. Reservoir quality variations and mixed lithologies with plastic behavior will alter the near-wellbore stress distribution, reducing the compressive stress of the rock. Thus, constant stress gradients (psi/ft) are misleading and rock mechanical properties measurements are of extreme importance. Based on a detailed geomechanical model, the technique can be selectively applied. It is useful in intelligent completions where zone determination and the design and placement of perforations and/or fractures are critical for optimum production performance. Another application is optimal placement of perforations for reservoir, productivity and stimulation considerations. This technique also efficiently places fracture stimulation treatments having optimized fracture geometry, vertical coverage, fracture length and treatment volumes. Several experimental1 and theoretical2 studies have investigated fracture initiation and propagation from vertical and deviated wellbores, providing insight into both the potential problems and the factors controlling near wellbore effects. The primary problems were identified to be: the near wellbore tortuosity, the occurrence of multiple competing fractures, micro-annulus pinch points etc., that lead to problems during job execution such as fracture initiation and premature screenouts. The primary controlling factors associated with these difficulties were identified as the orientation of wellbore and perforations in stress space, the effective shot density (number of "open holes"), perforation diameter, mechanical properties of the formation etc. Geomechanical models have found great use in drilling and completion. For the purpose of effective fracturing, our geomechanical modeling includes a detailed description of earth stresses (vertical, minimum and maximum horizontal), pore pressure (depletion, overpressured, variable pressures, layered systems), and rock mechanical properties. Perforating for fracturing methodology, as discussed in this work, uses the results of building a geomechanical model to 1) place the perforations oriented along the Preferred Fracture Plane (PFP); 2) account for stress contrasts, variable pressure effects and fracture initiation mechanics; and 3) optimize productivity. The methodology is based on the integration of a geomechanical reservoir description into fracturing design. This integration is discussed by Manrique et al.3,4 and Kordziel et al.5 as part of an integrated approach for stimulation treatment design and optimization. As demonstrated by field and production results, the improved efficiency of the perforating and/or fracturing strategies minimizes treatment failures and significantly impacts treatment implementation and production enhancement.

Local and global tensile deformation behavior of AA7075 sheet material at 673oK and different strain rates

Experimental quantification of the plastic blunting process for stage II fatigue crack growth in one-phase metallic materials

The application of the visioplasticity technique for derivation of the criterion for ductile rupture initiation in fully plastic notched bars

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SummaryIn this paper, we present an effective 6‐node triangular solid‐shell element (MITC‐S6), with particular attention on shear locking and thickness locking. To alleviate shear locking, the assumed transverse strain field of the MITC3+ shell element is used while modifying the bending enhancement mechanism. Thickness locking is treated using the assumed and enhanced strain methods for thickness strain. Two independent enhancements of strains are applied: The in‐plane and transverse shear strain fields are enhanced using the strain fields obtained from a bubble interpolation function for in‐plane translations, and the thickness strain field is enhanced for linear variation in the thickness direction. The general three‐dimensional material law is employed. The proposed element passes all the basic tests including zero‐energy mode, patch, and isotropy tests. Excellent performance is observed in various linear and nonlinear benchmark tests, wherein its performance is compared with that of existing 6‐node triangular and 8‐node quadrilateral solid‐shell elements. Copyright © 2016 John Wiley & Sons, Ltd.

Experimental and modeling study of cyclic plasticity and ductile fracture of thin structural steel sheets

Correlations between strain localisation evolution and acoustic emission characteristics of granite subjected to uniaxial compression

The accurate mechanical characterisation of fibres of micrometric length is a challenging task, especially in the case of organically-formed fibres that naturally exhibit considerable irregularities along the longitudinal fibre direction. The present paper proposes a novel experimental methodology for the evaluation of the local mechanical behaviour of organically-formed (aged and unaged) and regenerated cellulose fibres, which is based on in-situ micro-tensile testing combined with optical profilometry. In order to accurately determine the cross-sectional area profile of a cellulose fibre specimen, optical profilometry is performed both at the top and bottom surfaces of the fibre. The evolution of the local stress at specific fibre locations is next determined from the force value recorded during the tensile test and the local cross-sectional area. An accurate measurement of the corresponding local strain is obtained by using Global Digital Height Correlation (GDHC), thus resulting in multiple, local stress–strain curves per fibre, from which local tensile strengths, elastic moduli, and strains at fracture can be deduced. Since the variations in the geometrical and material properties within an individual fibre are comparable to those observed across fibres, the proposed methodology is able to attain statistically representative measurement data from just one, or a small number of fibre samples. This makes the experimental methodology very suitable for the mechanical analysis of fibres taken from valuable and historical objects, for which typically a limited number of samples is available. It is further demonstrated that the accuracy of the measurement data obtained by the present, local measuring technique may be significantly higher than for a common, global measuring technique, since possible errors induced by fibre slip at the grip surfaces are avoided.

Shells of revolution with local plasticity