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Highlights • Study of crack-tip propagation with macro-scale and micro-scale analyses. • Experimental estimation of the crack-tip position and the crack orientation. • Macro-scale analysis includes estimation of the SIF (continuum mechanics) from DIC. • Micro-scale investigation includes fractography analysis with SEM. • Full-field displacement information is combined with Williams' series development. Abstract This work presents a comprehensive study for characterising the crack-tip mechanics and fatigue crack propagation in an Aluminium 2024-T351 alloy. It combines information obtained from three different sources: full-field displacement information from digital image correlation, analytical modelling of the crack-tip field and SEM fractographies. The displacement data measured around the crack-tip are fitted to a Williams' series development in order to evaluate singular and non-singular terms of the crack-tip field. The procedures also allows rigid body motion to be corrected and the crack-tip coordinates and crack orientation to be estimated. Fatigue striations from the fracture surface were analysed with SEM in order to estimate the crack growth rate for different boundary conditions. Representation of all the results together with the Paris law data of the alloy allows the procedures to be cross-validated and to fit with a good agreement micro-scale measurements with continuum mechanics estimations. [ABSTRACT FROM AUTHOR]

Highlights • New methodology for investigating the interior of engineering materials. • Estimation of the stress intensity factor in the bulk of metal alloys. • Hybrid approach combining experimental data and an elastic model describing the strain field surrounding the crack tip. • Analytical model based on Williams' series development to describe the strain field around the crack tip. • Experimental data measured in the bulk of the material with synchrotron X-ray diffraction. Abstract A generalised approach for determining the stress intensity factor in the bulk of non-transparent materials is presented. The new approach combines experimental elastic strain data, measured with powerful synchrotron X-ray diffraction, with an elastic model based on Williams' series development. The stress intensity factor is calculated using a multi-point over-deterministic method where the number of experimental data points is higher than the number of unknowns describing the elastic field surrounding the crack-tip. The tool is tested on X-ray strain measurements collected on a bainitic steel. In contrast to surface techniques the approach provides insights into the crack tip mechanics deep within the sample and makes full use of the plastic zone data. Satisfactory results are obtained with 1 term in the series development and a smaller region of interest. By increasing the number of terms, it is possible to improve the accuracy in the SIF predictions but this requires a larger region of interest. [ABSTRACT FROM AUTHOR]

Highlights • An ultrafine finite element model of a cracked component allows visualising the plastic zone in great detail. • A bi-dimensional equivalent plastic zone shape is derived from the three-dimensional yielded volume. • A correction factor for the theoretical plastic zone at plane stress conditions is determined. • Its evolution with the specimen thickness is obtained. • The numerical model has been validated with full-field digital image correlation experiments. Abstract In this work, we present a detailed numerical analysis of the evolution of the plastic zone through the thickness of compact tension aluminium alloy specimens. An ultra-fine non-linear finite element model was used to characterise with great detail the size and shape of the plastic zone for different thicknesses. The numerical plastic zone was compared in size and shape with the plane strain and plane stress Dugdale plastic zones. In general, the maximum plastic zone was found to be in the interior but closer to the surface than to the mid plane. We also propose an equivalent bi-dimensional plastic zone for converting the complex 3D distribution into a 2D plastic zone. Finally, the numerical results were validated using experimental displacement data measured optically from the surface of the specimen. [ABSTRACT FROM AUTHOR]

In-line inspection (ILI) has become a routine procedure in the Oil and Gas industry for performing cost-effective pipeline integrity assessments, allowing continuing monitoring and providing a basis for informed decisions in terms of repair, maintenance or a change to the operating conditions. The amount of ILI data is however immense and dealing with these data from a fitness-for-service point of view poses a significant challenge to the industry. Thus, smart methods for using ILI data in the assessment of the integrity of oil and gas transmission pipelines are essential. The aim of this paper is to propose a screening approach for reducing the amount of ILI inspection data requiring detailed structural integrity assessment. The screening approach has two main stages: (1) a geometry based filter assessing the shape of the flaw and (2) an elastic stress based filter that uses the point method, as in the Theory of Critical Distances (TCD), to identify the most severe flaws. The methodology uses the outputs from ILI (dimensions of flaws, orientation and distance from starting point) to generate a visualisation of the pits within the pipeline, a ranking of pits in terms of sphericity (roundness) and depth, to evaluate pit density and generate the models for finite element analysis. The method was tested on actual ILI data, where the number of pits in a 12.75 in. riser of 11 km length was reduced significantly (i.e. two/three orders of magnitude), such reduction depending on the level of conservatism introduced by the analyst. The tool will allow Oil and Gas owners and operators to reduce the immense amount of data obtained during pigging to a much less time-consuming set for flaw assessment. [ABSTRACT FROM AUTHOR]

This work presents a new methodology for evaluating crack initiation under biaxial conditions. The methodology consists of evaluating a number of crack parameters automatically with digital processing of high-magnification images of the crack. Five different strain conditions were evaluated on a low carbon ferritic–pearlitic steel specimen with tubular shape. A hole of 150 μm diameter was drilled to enforce the crack to initiate at a particular spot. Different combinations of axial and torsional strains were analysed during the initiation stage of the crack. Fatigue crack propagation curves clearly showed oscillations due to microstructure. It was also observed that these oscillations decreased as the torsional component of the strain was increased. Driving force was also evaluated in crack opening and sliding direction through crack opening displacement and crack sliding displacement. The results demonstrate that crack opening displacement is sensitive to microstructural barriers and crack sliding displacement is sensitive to crack deviations caused by the microstructure. [ABSTRACT FROM AUTHOR]

• Study of the fatigue propagation from notches of AISI 18Ni300 maraging steel. • Analysis performed on the maraing steel fabricated via additive manufacturing process (laser-beam powder bed fusion). • The investigation is focused on cracks propagating from two different notch sizes and without notch under three different biaxial loading paths. • Combination of critical plane models with theory of critical distances allows effective characterisation. • The quality of the predictions is analysed both in terms of the fatigue life and the cracking orientation. The current work aims to characterise the fatigue behaviour of an additively manufactured maraging steel, AISI 18Ni300. This is a class of high-strength steel widely used in biomedical, aircraft, aerospace, offshore, and military industries thanks to its good performance in terms of strength, toughness, ductility, dimensional stability, and weldability. Laser-beam powder bed fusion (additive manufacturing) is used to fabricate this type of steel and endows it with properties that make it an excellent candidate for producing prosthetic parts, thereby facilitating a reduction in manufacturing material consumption, labour, and machining time. Given the wide range of loads biomedical components are often subjected to, the current study focused on the multiaxial behaviour of this type of steel. To this end, Fatemi–Socie (FS) and Smith–Watson–Topper (SWT) critical plane methods combined with the theory of critical distances were applied to predict the fatigue life and cracking orientation of this material, with and without notches, under three biaxial loading scenarios. Cylindrical specimens were used and these were fabricated on the base plate in the vertical orientation using a linear printing system equipped with a Nd:YAG fibre laser. The building strategy involved the deposition of 40 μm thick layers at a scan speed of 800 mm/s. The two critical plane models returned good results at low life cycle fatigue but the FS model obtained better results at high life cycle fatigue. Regarding the crack angles, SWT model produced the best predictions. [ABSTRACT FROM AUTHOR]

• The applicability of fatigue toughness under multiaxial load is explored. • A fatigue life prediction model based on fatigue toughness is introduced. • The proposed model is formulated within a linear-elastic framework. • The model showed better predictive capabilities than other well-known models. In this paper, the cumulative strain energy density, also called fatigue toughness, was used to predict the fatigue life of notched members subjected to multiaxial loading. The relationship between the total energy absorbed to fracture and the number of cycles to failure was established from uniaxial strain-controlled fatigue tests. Multiaxial fatigue tests were performed on notched round bars made of high-strength steel under proportional bending-torsion considering different normal stress to shear stress ratios and different loading orientations. The fatigue lives predicted with the proposed model were compared to those obtained with other stress-based and energy-based models. Overall, the proposed model led to good predictions, and showed better predictive capabilities than the other tested models. [ABSTRACT FROM AUTHOR]

• New multiaxial fatigue characterisation of 316 steel including high mean deformation and non-proportional load paths. • Compression induces a lower strain range thereby increasing the fatigue life. • Thorough and independent analysis of newly proposed critical plane method (Lei-Gan) indicates an acceptable performance. • The most accurate model for estimating the fatigue life of 316 steel is Liu II. • Fatemi-Socie yields the best estimations in terms of cracking angle. In this work, the fatigue behaviour of 316 stainless steel is studied with different critical plane models. Seven cylindrical samples were used for the study, being subjected to different complex loading paths, generating combined stresses along the axial and transversal sample directions, these being: individual axial stress, individual hoop stress, alternating axial and hoop stress, a proportional combination of axial and hoop stress, and a non-proportional combination of L-shaped and square-shaped axial and hoop stress. The fatigue analysis is performed using five critical plane models; named Fatemi-Socie, Varvani-Farahani, Gan-Wu-Zhong, Liu I and Liu II. The models were assessed based on their fatigue life and crack angle prediction capacity. The Gan-Wu-Zhong recently proposed critical plane model was examined and provided acceptable results for the multiaxial loads tested on 316 steel. Nevertheless, Fatemi-Socie produced the most accurate results in terms of cracking orientation and Liu II gave the best fatigue life predictions. [ABSTRACT FROM AUTHOR]

• The effect of kinematic parameters on FCG was predicted numerically using the plastic CTOD. • Increasing kinematic parameters reduces non-linearly fatigue crack growth. • The kinematic parameters have a limited effect on crack closure. • The effect of kinematic saturation stress, X Sat , is more relevant than the effect of kinematic saturation rate, C X. The parametric study of the effect of material properties on fatigue crack growth (FCG) rarely has been addressed in literature. The consideration of plastic CTOD as crack driving force opened the opportunity to predict FCG rate numerically and therefore to develop parametric studies focused on the effect of loading, geometrical and material parameters. The objective here is to study the effect of kinematic saturation stress, X Sat , and kinematic saturation rate, C X , on FCG using this numerical approach. The increase of the kinematic parameters reduced the plastic CTOD and therefore the FCG rate. The variation is non-linear and the rate of variation of δ p decreases with the increase of X Sat and C X. The effect of the kinematic saturation stress, X Sat , is more relevant than the effect of kinematic saturation rate, C X. On the other hand, a small effect of kinematic hardening parameters on crack closure was found. Finally, the increase of the number of load cycles between crack increments produced a great reduction of crack closure but no effect of plastic CTOD on models without contact of crack flanks. [ABSTRACT FROM AUTHOR]