The Journal of Strain Analysis for Engineering Design

Strain analysis is an essential step in sheet forming applications to understand the deformation behaviour of the material. The strain analysis is conventionally performed by Circle Grid Analysis (CGA). In CGA, a grid of circles with 2 mm–5 mm diameter is printed on a flat sheet and deformed into the required shape. The length of major and minor axes of the deformed ellipses are measured to estimate the strains at different locations of the formed component. The major and minor axes length of the deformed ellipses are measured either by using a manual method or automatic methods. The automated methods are faster and accurate compared to manual methods, but to perform strain measurement using commercially available automatic strain measurement system, is very expensive. Therefore, in this paper, an image processing based software has been developed with user-friendly Graphical User Interface (GUI) for strain measurement and analysis in sheet metal forming operations. The software has been tested on images obtained by printing the ellipses of known dimensions on the flat sheet by laser etching, electrochemical etching and screen printing, which are generally used for printing circular grids on flat sheets for strain analysis in sheet metal forming. It is observed that the edges of the laser engraved ellipse are very slim and easily detected by software compared to other grid printing methods. The developed software can measure the ellipse dimensions with a maximum absolute percentage error of 1.975%. Further, the results of CGA software has been compared with the commercial strain measurement system called Grid Pattern Analyzer (GPA). The mean absolute error in strain measurement using CGA and GPA was found to be 0.0106 and 0.0178, respectively. The statistical test results reveal that there is no evidence to support a claim that there is a difference in mean performance between the two methods.

Ultrafine grained and nanostructured materials have drawn significant attention due to their unusual mechanical behavior. The purpose of this research was to study the mechanical properties and microstructural evolution of nanocrystalline copper obtained through a bottom-up procedure of nanoparticle consolidation by equal channel angular pressing (ECAP). ECAP was approved to be a proper approach to produce comparatively big nanocrystalline consolidates including acceptable mechanical properties. The effects of ECAP pass numbers on consolidation achievement were also assessed. The quite large strains observed in the samples were associated with the bimodal distribution condition of grain sizes. The present work reveals that ECAP consolidation of nanoparticles presents a novel opportunity to investigate the mechanical response of nanostructured metals and alloys moreover it gives a chance to the production of unique kinds of bulk materials for useful purposes.

The state of stress present in an elastic half-plane contact problem, where one or both bodies is subject to remote tension has been investigated, both for conditions of full stick and partial slip. The state of stress present near the contact edges is studied for different loading scenarios in an asymptotic form. This is of practical relevance to the study of contacts experiencing fretting fatigue, and enables the environment in which cracks nucleate to be specified.

The discontinuous crack surface in a rock affects the stability of the whole rock system. The experiments in this paper were carried out by prefabricating rock-like specimens with different types of flaws, then the specimens were tested under uniaxial compression. Moreover, based on the theory of particle flow, PFC^2D software was used for numerical simulation, and the comparative analysis of the experimental and simulative results was carried out to obtain the crack initiation sequence, propagation phenomenon, and failure mode of rock specimens with different flaw types. The results indicated that the wing crack started at the tip of flaw and the form of crack assumed split failure, followed by shear failure caused by the secondary crack. The tensile failure degree decreases and the influence of shear failure increases with the increase of flaw angle. The wing crack and secondary initiation stress value is 35%–55% and 85%–95% of the peak stress value. Crack coalescence appeared in adjacent flaws of rock-like specimens with multiple parallel single flaws, single-cross flaws and double-cross flaws, and the coalescence phenomenon always occurs when the stress peak value is reached. With the number of flaws increasing, the splitting failure of rock-like specimens became more and more serious, the splitting failure of double cross-flaw specimen is the most serious. As for the specimen with single-cross flaw, the wing crack would be produced at the tip of the flaw with larger obliquity. The results of this paper may offer certain reference value for the study on the mechanism of rock crack.

This paper presents a new theoretical model for rotating elastic hub-blade assemblies, made of functionally graded (FG) graphene nanoplatelet (GPL) reinforced nanocomposites, and their free vibration characteristics are investigated. This model is the first attempt to include two elastic components simultaneously and consider the coupled effect. The Euler-Bernoulli beam theory and the Donnell’s shell theory are employed to establish the mathematic model of the blade and hub, respectively. The effective material properties, varying continuously along the thickness of the beam and cylindrical shell, are determined via the Halpin-Tsai micromechanics model and the rule of the mixture. The Lagrange’s equation is adopted to derive the equations of motion which are then solved by employing the substructure mode synthesis method and the Galerkin method. A parametric study is conducted to examine the effects of the rotating speed, graphene nanoplatelet distribution pattern, GPL weight fraction, length-to-thickness ratio and length-to-width ratio of graphene nanoplatelets (GPLs) and blade dimension on the natural frequencies of the nanocomposite rotor system, which will significantly benefit on the structural and material design of GPL reinforced hub-blade assembly.

This paper investigated the modeling and free vibration characteristics of a spinning graphene nanoplatelet (GPL) reinforced porous nanocomposite blade. The blade is made of porous foam metal matrix reinforced with graphene nanoplatelets (GPLs). Several different GPL distributions and porosity distributions in the blade are taken into account. The effective material properties of the blade, determined via the open-cell scheme, the Halpin-Tsai model, and the rule of mixture, are assumed to be varying continuously along the its thickness direction. According to the Kirchhoff’s plate theory, the governing equations of the spinning blade are derived by adopting the Hamilton principle. On the other hand, the non-uniform spinning blade is modeled by the finite element method which is compared with the theoretical method. The theoretical results match very well with the finite element ones obtained from ANSYS. Particular focus is given to the effects of the spinning speed, porosity coefficient, distribution pattern of GPLs and porosities, GPL weight fraction, length-to-thickness ratio and length-to-width ratio of GPLs, blade length, and spinning radius on the free vibration performance of the blade rotor.

Type 316 steels have been heavily utilized as the structural material in many construction equipment and infrastructures. This paper reports the characterization of degradation in 316 austenitic stainless steel during the plastic deformation. The in-situ EBSD results revealed that, with the increase of plastic strain, the band contrast (BC) value progressively decreased in both grain and grain boundaries, and the target surface becomes uneven after the plastic tensile, which indicates that the increase of surface roughness. Meanwhile, the KAM and ρ_GND values are low in the origin specimen but increased significantly after the in-situ tensile. The results indicated that the KAM and ρ_GND are closely related to the deformation degree of the materials, which can be used as the indicator for assessing the degradation of 316 steel. Besides, the re-orientation of grain occurred after the tensile deformation, which can be recognized from the lattice orientation and local orientation maps.

Forming limit diagram (FLD) is the most intuitive method to evaluate and analyze the forming performance of sheet metal, which is widely used in production. To examine the formability of AZ31 magnesium alloy and 7050 aluminum alloy, the simplified bulging models based on the Nakazima experiment are established by ABAQUS finite element (FE) software, and the maximum punch force criterion is adopted as the instability criterion. The forming limit diagrams of 7050 high-strength aluminum alloy at room temperature and AZ31 magnesium alloy at warm working conditions are obtained by extracting the in-plane strain of the adjacent element of the maximum strain element at the moment of instability. Compared with experimental observation shows that the Nakazima virtual model established in this paper can accurately predict FLD. In addition, the influences of lubrication conditions and virtual punching speeds on the bulging process of AZ31 and AA7050 sheet metals are also investigated. The results show that the better the lubrication environment, or the lower the punching speed, the better the formability of the sheet, and reducing the punching speed has a more significant improvement effect on the formability of AZ31 sheets.