Prediction of load requirement and instabilities during end forming of friction stir processed AA 6063-T6 thin-walled tubes

PurposeThe current work aims to propose a finite element (FE) simulation methodology to predict the formability of friction stir processed (FSPed) tubes by end forming. Moreover, a strain mapping method is also presented to predict the end forming instabilities.Design/methodology/approachIn this work, FE simulation of end forming of raw tubes and FSPed AA6063-T6 tubes are done using Abaqus (explicit) incorporating anisotropic properties of the raw tube and FSPed zone. Actual thickness of the FSPed zone is also implemented. Expansion, reduction and beading are the end forming operations considered. Load requirement and instabilities are predicted. A new method “strain mapping method” is followed to predict the failure instabilities in expansion and beading, while during reduction, wrinkling is predicted by FE simulations. Lab scale experiments on FSP and end forming are done for validation at various rotational speeds.FindingsResults reveal that in the case of expansion and reduction of FSPed tubes, forming load predictions are accurate, while in beading, after initiation of bead, predictions are not accurate. Experimental observation on the type of instability is consistently predicted during numerical simulations. Prediction of displacement at failure by strain mapping method is encouraging in most of the cases including those that are FSPed. Hence, it is suggested that the method can be utilized to evaluate the onset of failure during tube expansion and beading.Originality/valueFE simulation methodology including anisotropic properties of raw tube and FSPed tubes is proposed, which is not attempted until now even for normal tubes. Strain mapping method is easy to implement for instability predictions, which is done usually by failure theories and forming limit diagram.

Review and Suggestion of Failure Theories in Voids Scenario for VARTM Processed Composite Materials

Fiber-reinforced composite structures are used in different applications due to their excellent strength to weight ratio. Due to cost and tool handling issues in conventional manufacturing processes, like resin transfer molding (RTM) and autoclave, vacuum-assisted resin transfer molding (VARTM) is the best choice among industries. VARTM is highly productive and cheap. However, the VARTM process produces complex, lightweight, and bulky structures, suitable for mass and cost-effective production, but the presence of voids and fiber misalignment in the final processed composite influences its strength. Voids are the primary defects, and they cannot be eliminated completely, so a design without considering void defects will entail unreliability. Many conventional failure theories were used for composite design but did not consider the effect of voids defects, thus creating misleading failure characteristics. Due to voids, stress and strain uncertainty affects failure mechanisms, such as microcrack, delamination, and fracture. That’s why a proper selection and understanding of failure theories is necessary. This review discusses previous conventional failure theories followed by work considering the void’s effect. Based on the review, a few prominent theories were suggested to estimate composite strength in the void scenario because they consider the effect of the voids through crack density, crack, or void modeling. These suggested theories were based on damage mechanics (discrete damage mechanics), fracture mechanics (virtual crack closure technique), and micromechanics (representative volume element). The suggested theories are well-established in finite element modeling (FEM), representing an effective time and money-saving tool in design strategy, with better early estimation to enhance current design practices’ effectiveness for composites. This paper gives an insight into choosing the failure theories for composites in the presence of voids, which are present in higher percentages in mass production and less-costly processes (VARTM).

Comprehensive Analysis of Borehole Stability with Temperature, Swelling, and Pore Pressure Change for Layered and Orthotropic Formations

Borehole stability depends on various parameters such as rock strength, rock deformations, in-situ stress, borehole trajectory, shale swelling, pore pressure change due to osmosis, overbalance mud weight and temperature. The objective of this work is to construct analytical and numerical equations to predict borehole failure including all these parameters, and to comprehensively propose a methodology to improve the borehole stability. Analytical solutions are developed for inclined wells with respect to in-situ stress, shale swelling, pore pressure change due to osmosis, overbalance mud weight and temperature. A numerical model is developed for 3D inclined wells with orthotropic formation and layered formation. Using the analytical and the numerical models, stress state around inclined wells are evaluated. The breakout angle is predicted based on Mohr-Coulomb, Mogi, Lade and Drucker-Prager failure theories. Polar diagrams of mud weights are compared to judge the effect of each parameter and the magnitude predicted by the different failure theories. Shale swelling and pore pressure change due to osmosis are the most difficult to estimate among above-mentioned parameters. The laboratory measured swelling of cores obtained from various formations showed that the magnitude to induce breakouts caused by swelling was the largest comparing with other parameters. Therefore, when shale stability problems occur, we need to estimate the magnitude of shale swelling and osmosis due to water potential difference. Then, to overcome the shale stability problem, we evaluated the sensitivity of human controllable parameters on borehole stability. The parameters which can be controlled by drilling engineers are overbalance, type of mud, borehole temperature and borehole trajectory. If the shale swelling is small, the borehole stability is improved by the mud weight. However, from the swelling tests from the cores of Nankai-Trough, we estimated unless we used a swelling inhibitor to reduce the swelling less than 0.1%, the well was not possible to drill through. Actually, the well was abandoned due to instability after trying side track several times. Unlike previous works, this paper uses all important parameters (swelling, temperature, pore pressure, orthotropic formation, layered formation) to estimate the stresses around inclined wells with the same formation conditions for quantitative analysis. Failure analysis include Mohr, Mogi, Lade and Drucker-Prager. Finally, the polar diagrams of critical mud weight are used to judge whether we can choose well trajectory, orientation with respect to bedding planes, mud weight, shale inhibitor, and temperature to stabilize the borehole.

Between donors and beneficiaries: towards a theory of dynamic two-sided markets

The non-profit sector has experienced significant changes and growth over the past few decades. Market failure theories aim to explain why traditional non-profit organisations (NPOs) exist but fall short in accounting for the diversity and complexity we observe in the third sector today. This article takes a first step in applying a two-sided market model to describe the evolution of the sector. It finds that purely donative NPOs may have once had the characteristics of a platform in a two-sided market featuring donors and beneficiaries linked by non-profit intermediaries. However, the transition of the sector from donative to one reliant on earned income requires an extension of the model of two-sided markets so that it has a less static approach. The demand and supply sides in the two-sided market model have become more complex. The article therefore suggests a dynamic model, in which consumers and financiers of NPO products and services can move from one side of the platform to the other and take on different and at times overlapping roles.

Fatigue-Life Prediction of Mechanical Element by Using the Weibull Distribution

Applying Goodman, Gerber, Soderberg and Elliptical failure theories does not make it possible to determine the span of failure times (cycles to failure-Ni) of a mechanical element, and so in this paper a fatigue-life/Weibull method to predict the span of the Ni values is formulated. The input’s method are: (1) the equivalent stress (σeq) value given by the used failure theory; (2) the expected Neq value determined by the Basquin equation; and (3) the Weibull shape β and scale η parameters that are fitted directly from the applied principal stress σ1 and σ2 values. The efficiency of the proposed method is based on the following facts: (1) the β and η parameters completely reproduce the applied σ1 and σ2 values. (2) The method allows us to determine the reliability index R(t), that corresponds to any applied σ1i value or observed Ni value. (3) The method can be applied to any mechanical element’s analysis where the corresponding σ1 and σ2, σeq and Neq values are known. In the performed application, the σ1 and σ2 values were determined by finite element analysis (FEA) and from the static stress analysis. Results of both approaches are compared. The steps to determine the expected Ni values by using the Weibull distribution are given.

Experimental and numerical study on stiffness and damage of glass/epoxy biaxial weft-knitted reinforced composites

This paper deals with investigating the tensile characteristics of biaxial weft-knitted reinforced composites in terms of stiffness, strength and failure mechanism. The biaxial weft-knitted fabric was produced on an electronic flat knitting machine by E-glass yarns and then was impregnated with epoxy resin. Using an accurate geometrical model, the composite unit cell was designed in Abaqus software’s environment. Tensile tests were simulated in different directions on the created unit cell and the stiffness was calculated. By applying the proper failure theories, the composite strength was predicted and then critical regions of the unit cell were determined. In the next step, a micromechanical approach was also applied to estimate the same tensile features. Failure theories were also applied to predict the strength and most susceptible areas for failure phenomenon in the composite unit cell. The tensile properties of the produced composites were measured and compared with outputs of the finite element and micromechanical approaches. The results showed that the meso-scale finite element analysis approach can well predict the composite strength. In contrast, the meso-scale analytical equation model was not able to predict it acceptably because this model ignores the strain concentration. Both meso-scale finite element analysis and meso-scale analytical equation approaches predicted the similar locations for the composite failure in wale and course directions.