Form Of Strain Localization Research Articles

Effects of local strain on the plastic deformation and fracture mechanism of heterogeneous multilayered aluminum

Elucidating the effect of local strain on the mechanical properties is of great significance for the design of high-performance layered metals. For this purpose, we conceived the present study, featured by tailoring the local strain by layer thickness design, and simultaneous monitoring of local strain and geometrically necessary dislocations (GNDs) via coupling in-situ electron backscatter diffraction (EBSD) and high-resolution digital image correlation (DIC). In addition, synchrotron X-ray micro-computed tomography (μCT) was employed to analyze the microcracks that serve as another form of strain localization. Such detailed experimental studies revealed that the interfacial strain gradient was rapidly elevated, and the strain localization band was effectively dispersed as the layer thickness decreased. This leads to two typical transitions, from grain-boundary-related to layer-interface-related plastic deformation mode, and from macroscopic shear to zig-zag fracture mode. Their influences on the mechanical properties, as well as underlying mechanisms, were discussed based on the relationship among the layer thickness, strain gradient, strain localization, GND density, and microcracks. Our work not only contributes to the fundamental understanding of the mechanical behavior of multilayered metals but also offers guidance for the structural design of high-performance metals aimed at achieving superior strength-ductility combinations.

Determination of the formability limits for Grade 1 titanium sheet by means of ALSAD method

Stamping is one of the most widely used sheet metal forming processes. Esthetics and durability of the deformed products are limited by defects in the form of strain localization, grooves or cracks. Forming limits, e. g. in form of forming limit curve (FLC), help to avoid above-mentioned defects in the sheet metal during forming. Besides the common steel sheets for deep drawing processes, e.g. DC04, components can also be made of metal with specified strength and chemical resistance properties. One of such materials is titanium sheet. In this paper, FLC for GR1 titanium sheet will be presented. It was obtained by means of a new author’s method named as ALSAD (Analysis of Laser Speckle Activity Differences). So far, applied methods of FLC determination have been mainly based on the measurement of the strain distribution in the area of the created groove, deformation gradient and deformation speed. For this reason, detection of strain localization has been dependent on the strain measurement method. The ALSAD, based on laser speckle phenomenon allows detection of defects initiation on the sample surface independently of the strain distribution measurement and it allows, for example, to relate a moment of defect initiation directly to the sample height or forces occurring in the process. In this paper strain values for strain localization, grooves and cracks for GR1 titanium sheet will be shown. Obtained results show good formability of this material, which was verified by means of 3D digital microscope. Such results allow tracing precisely the formation of the defects starting from strain localization up to the crack appearance, which would be helpful for advanced numerical simulations.

Detection of defects in bulging processes by means of dynamic laser speckle

The method described in this paper has been used to determine the moment when defects appear in the form of strain localization and grooves in the Marciniak bulging tests. In this method based on dynamic laser speckle, the sheet is illuminated by laser. Interference after reflection from the specimen surface creates a characteristic image of bright and dark spots. Specimen surface changes related with deformations and movements cause changes in the shape and size of spots, which are presented by authors in the form of speckle activity image. In order to identify the type of defect, the additional 3D microscopy measurements were performed. This paper presents analysis of the changes in the speckle activity image during the bulging process which enabled the defects identification during the bulging process.

Limiting parameters of forming round steel sheet, locally loaded by combined profile punch

There are shown the results of application of an axisymmetric rigid-plastic finite element membrane model to the analysis of a circular steel sheet stretching process under action of a punch with profile, which consists of two circular arcs with small radii. The failure of the deformed shell is computationally revealed in the form of strain localization.

The Effect of Punch Radius on the Deformation of Ultra-High Strength Steel in Bending

Bendability is an important material property for ultra-high strength steel. The bendability of a certain material is expressed as the minimum bending radius Rmin of the inner surface of the bend and expressed in multiples of the sheet thickness. Bendability is limited by either cracking on the surface or the edges of the bend or by surface waviness that usually precedes cracking on the outer surface. Surface waviness is a form of strain localization in bending and the intensity of the phenomenon is dependent on e.g. the punch radius, the lower tool width and the sheet thickness. In this study the bendability of a 960MPa grade steel was investigated using optical strain measurements of three-point bending tests to determine the strain level and the bending angle when localization starts with different punch radii. The unbent samples were marked with a grid using laser marking and the deformation was measured with the GOM ARGUS strain analysis system after bending. The quality of the bend was also evaluated visually. In addition, tensile tests were performed and evaluated with the GOM ARAMIS deformation analysis system to investigate the local mechanical properties of the studied steel. The results of strain measurements and visual evaluation were then compared. It was found that beyond a certain angle the maximum strain across the bend did not significantly change with further increases in the bending angle when the punch radius was at least three times the sheet thickness. But with smaller punch radii the maximum strain increased almost linearly with increasing bending angle until fracture appeared. With the smaller punch radii deformation localizes and surface waviness begins to form in smaller bending angles because the deformation is concentrated in a narrow zone.

A mechanism-based model for deformation twinning in polycrystalline FCC steel

Deformation twinning, a common and important plastic deformation mechanism, is the key contributor to the excellent combination of strength and ductility in twinning-induced plasticity (TWIP) steel. In the open literature, a significant amount of research has been reported on the microstructural characteristics of deformation twinning and its influence on the overall deformation behavior of TWIP steel. In this study, we examine the feasibility of a mechanism-based crystal plasticity model in simulating the microstructural level deformation characteristics of TWIP steel. To this end, a model considering both double-slip and double-twin is developed to investigate the stress–strain behavior and local microstructural features related to the formation and growth of micro-twins in low stacking fault energy (SFE) TWIP steel. The twin systems are described as pseudo-slips that can be activated when their resolved shear stress reaches the corresponding critical value. A hardening law that accounts for the interaction among the slip and twin systems is also developed. Numerical simulations for different mesh sizes and single crystal patch tests under different loading modes are carried out to verify the modeling procedure. Our simulation results reveal that, despite its simple nature, the double-slip/double-twin model can capture the key deformation features of TWIP steel, including twin volume fraction evolution, continuous strain hardening, and the final fracture in the form of strain localization.

Predicting the influence of pore characteristics on ductility of thin-walled high pressure die casting magnesium

In this paper, a two-dimensional microstructure-based finite element modeling method is adopted to investigate the effects of porosity in thin-walled high pressure die casting Mg materials on their ductility. For this purpose, the cross-sections of AM50 and AM60 casting samples are first examined using optical microscope to obtain the overall information on the pore characteristics. The experimentally quantified pore characteristics are then used to generate a series of synthetic microstructures with different pore sizes, pore volume fractions and pore size distributions. Pores are explicitly represented in the synthetic microstructures and meshed out for the subsequent finite element analysis. In the finite element analysis, an intrinsic critical strain value is used for the Mg matrix material, beyond which work-hardening is no longer permissible. With no artificial failure criterion prescribed, ductility levels are predicted for the various microstructures in the form of strain localization. Mesh size effect study is also conducted, from which a mesh size dependent critical strain curve is determined. A concept of scalability of pore size effects is then presented and examined with the use of the mesh size dependent critical strain curve. The results in this study show that, for the regions with lower pore size and lower volume fraction, the ductility generally decreases as the pore size and pore volume fraction increase whereas, for the regions with larger pore size and larger pore volume fraction, other factors such as the mean distance between the pores begin to have some substantial influence on the ductility. The results also indicate that the pore size effects may be scalable for the models with good-representative pore shape and distribution with the use of the mesh size dependent critical strain curve.