Showing papers on "Necking published in 2013"

Characterization of internal fracture process of double network hydrogels under uniaxial elongation

Abstract: Previously we revealed that the high toughness of double network hydrogels (DN gels) derives from the internal fracture of the brittle network during deformation, which dissipates energy as sacrificial bonds. In this study, we intend to elucidate the detailed internal fracture process of DN gels. We quantitatively analysed the tensile hysteresis and re-swelling behaviour of a DN gel that shows a well-defined necking and strain hardening, and obtained the following new findings: (1) fracture of the 1st network PAMPS starts far below the yielding strain, and 90% of the initially load-bearing PAMPS chains already break at the necking point. (2) The dominant internal fracture process occurs in the necking and hardening region, although the softening mainly occurs before necking. (3) The internal fracture efficiency is very high, 85% of the work is used for the internal fracture and 9% of all PAMPS chains break at sample failure. (4) The internal fracture is anisotropic, fracture occurs perpendicular to the tensile direction, in preference to the other two directions, but the fracture anisotropy decreases in the hardening region. Results (1) and (2) are in agreement with a hierarchical structural model of the PAMPS network. Based on these findings, we present a revised description of the fracture process of DN gels.

Mechanical properties and scaling laws of nanoporous gold

Abstract: Nanoporous metals are a class of novel nanomaterials with potential applications in many fields such as sensing, catalysis, and fuel cells. The present paper is aimed to investigate atomic mechanisms associated with the uniaxial tensile deformation behavior of nanoporous gold. A phase field method is adopted to generate the bicontinuous open-cell porous microstructure of the material. Molecular dynamics simulations then reveal that the uniaxial tensile deformation in such porous materials is accompanied by an accumulation of stacking faults in ligaments along the loading direction and their junctions with neighboring ligaments, as well as the formation of Lomer–Cottrell locks at such junctions. The tensile strain leads to progressive necking and rupture of some ligaments, ultimately resulting in failure of the material. The simulation results also suggest scaling laws for the effective Young's modulus, yield stress, and ultimate strength as functions of the relative mass density and average ligament size in the material.

Evolution of microstructure, macrotexture and mechanical properties of commercially pure Ti during ECAP-conform processing and drawing

Abstract: Long-length ultrafine-grained (UFG) Ti rods are produced by equal-channel angular pressing via the conform scheme (ECAP-C) at 200 °C, which is followed by drawing at 200 °C. The evolution of microstructure, macrotexture, and mechanical properties (yield strength, ultimate tensile strength, failure stress, uniform elongation, elongation to failure) of pure Ti during this thermo-mechanical processing is studied. Special attention is also paid to the effect of microstructure on the mechanical behavior of the material after macrolocalization of plastic flow. The number of ECAP-C passes varies in the range of 1–10. The microstructure is more refined with increasing number of ECAP-C passes. Formation of homogeneous microstructure with a grain/subgrain size of 200 nm and its saturation after 6 ECAP-C passes are observed. Strength properties increase with increasing number of ECAP passes and saturate after 6 ECAP-C passes to a yield strength of 973 MPa, an ultimate tensile strength of 1035 MPa, and a true failure stress of 1400 MPa (from 625, 750, and 1150 MPa in the as-received condition). The true strain at failure failure decreases after ECAP-C processing. The reduction of area and true strain to failure values do not decrease after ECAP-C processing. The sample after 6 ECAP-C passes is subjected to drawing at 200¯C resulting in reduction of a grain/subgrain size to 150 nm, formation of (10 1 ¯ 0) fiber texture with respect to the rod axis, and further increase of the yield strength up to 1190 MPa, the ultimate tensile strength up to 1230 MPa and the true failure stress up to 1600 MPa. It is demonstrated that UFG CP Ti has low resistance to macrolocalization of plastic deformation and high resistance to crack formation after necking.

Hydrodynamics of double emulsion droplet in shear flow

Abstract: Hydrodynamic behaviors of double emulsion droplets in shear flow, both deformation and breakup, are investigated numerically. We find that the inner drop is deformed in a uniform vortical flow interior to the outer drop under steady state deformation conditions and provides an additional deformation resistance for the integral droplet especially when its volume fraction is large. In particular, we report four types of breakup modes via three mechanisms (i.e., necking, end pinching, and capillary instability), respectively, and present the corresponding phase diagrams to describe the breakup criteria (critical capillary numbers) and morphologies.

Micro-tension behaviour of lath martensite structures of carbon steel

Abstract: A micro-tension testing technique was used to investigate the deformation behaviour of lath martensite structures with several types of boundaries in carbon steel. The martensite structures exhibited sufficient necking strains and ductile fractures, whereas the uniform strain was limited owing to a lack of strain-hardening ability despite the increased flow stress. The yield stress of the lath martensite structures strongly depended on the in-lath-plane orientation. The critical resolved shear stress of the in-lath-plane slip systems was considerably lower than that of the out-of-lath-plane slip systems. This finding suggests that the block boundaries are an effective grain boundary for impeding dislocation gliding. Plastic deformation transfer was restricted by the packet boundaries, which greatly rotated the crystallographic orientation of the in-lath-plane slip systems between neighbouring martensite variants.

The dynamics of multiple neck formation and fragmentation in high rate extension of ductile materials

Abstract: Dynamic necking bifurcation in rapidly extending cylindrical rods is investigated. It has been found that both short wavelength and long wavelength perturbations are suppressed by inertia and an intermediate wavelength is favored. The analysis predicts an increase in the number of necks and an increase in the bifurcation strain with increasing extension rate, in agreement with experimental observations. In terms of the number of necks formed as a function of extension rate, good agreement has been found between the experiments and the analysis. At any given aspect ratio, the model also predicts that beyond a critical extension rate, the mode number of the dominant perturbation increases rapidly and the perturbation begins to look more like a surface instability. This could lead to a fragmentation mechanism at high extension speeds which is different from multiple necking. Currently no experimental results are available to test this prediction. Numerical simulations have been conducted to simulate the fragmentation results, using Gurson’s constitutive law along with a porous failure criterion. Good agreement between the experimental observations and the numerical results has been obtained for the fragmentation statistics. However, the numerical results consistently overestimate the number of necks and the fracture strain, possibly due to uncertainty in the constitutive data used, especially at large strains.

Tensile deformation and fracture of Ti–5Al–5V–5Mo–3Cr–1.5Zr–0.5Fe alloy at room temperature

Abstract: A combination of transmission electron microscopy, scanning electron microscopy and x-ray diffractometer is used to analyse the deformation mechanism and the fracture behavior of bi-model structure Ti–5Al–5V–5Mo–3Cr–1.5Zr–0.5Fe alloy during uniaxial tensile test. In the uniform deformation region, the strain of the primary α particles is dominated by the planar slipping and the dislocation tangling, and the deformation of the β matrix is accompanied by the dislocation tangling. In the necking region, the additional deformation twinning is found in the primary α particle, and the β phase is still strained by the dislocation tangling. The local shearing occurs in the necking region and results in the micro-twinning dominated deformation in secondary α precipitation and the 〈110〉β fiber texture. The room temperature tensile fracture of Ti-55531 alloy is extremely sensitive to the formation of the voids, and the critical dimension of the crack is the diameter of the primary α particles.

Interpreting the ductility of nanocrystalline metals

Abstract: Nanocrystalline (NC) metals are known for having excellent strength but perceived to have poor ductility. Miniature tensile tests on NC Ni-Fe measured ultimate strengths of 2 GPa and elongations, by digital image correlation, of up to 10%. Detailed examination of the fracture surface revealed dimpled rupture and cross-section reduction up to 75%, suggesting an intrinsic ability for small grained Ni-Fe to accommodate plasticity. A survey of published studies on NC metals reveals that this behavior is quite common; despite low macroscopic elongation, NC metals often achieve extensive deformation suggesting good intrinsic ductility. Unfortunately, the common sheet-like configuration of NC tensile specimens muddies a simple evaluation of ductility based on elongation, since thin and wide geometries promote localized necking that expedites catastrophic failure. This paper presents a compact review of ductility concepts and literature to interpret the experimental ductility measurements of an electrodeposited nickel alloy.

Time Dependent FLC Determination Comparison of Different Algorithms to Detect the Onset of Unstable Necking before Fracture

Abstract: The ISO standard 12004-2:2008E for the determination of forming limit curves based on the section method was approved in 2008. About 4 years of measuring experience in different laboratories has shown advantages and weaknesses of the standard and is leading to some minor changes in the specification. In the years from the development of this standard until today a further technical development of the optical measuring devices occurred, so that it is now possible to determine forming limit curves using the time history of the test. This procedure of determination is referred to a time dependent technique and could be the basis of the ISO 12004 part 2 proposal worked out by the work group Erweiterung FLC ISO 12004 of the German group of the IDDRG. This publication recapitulates existing work which was carried out from the IDDRG work group regarding the determination of forming limit curves for sheet metal materials. On one hand known issues with the current section based approach are discussed and on the other hand it deals with a comparison of different algorithms to determine the FLC from the time history of the Nakajima test using strategies to identify the instant of onset of instable necking. The different time dependent algorithms [ utilised are automatically selecting the area where necking is leading to fracture and then analyze the time history of such points using the first or the second time derivative of the true major strain, or of the true thinning strain using methods like: correlation coefficient (modified method based on [2]), gliding correlation coefficient, linear best fit (modified method based on [3]) and gliding difference of mean to median. The resulting experimental FLC points are compared with the results from the section technique described in ISO 12004 part 2 and with the maximum strain values measured in each test. Further a large number of forming limit curves were determined and used for a comparison of these different methods to define the most promising time dependent algorithm, which was selected as a suggestion for the working group defining the new proposed ISO standard 12004 part 2.

Strain localization and damage mechanisms during bending of AA6016 sheet

Abstract: The bendability of AA6016 sheets is a critical parameter for many automotive applications. In this experimental study the origins of damage and its evolution are characterized using interrupted and in-situ bending tests to correlate microstructural evolution with damage development. Local strains were estimated by optical and scanning microscopy (EBSD). Together with the load-displacement plots, they provided a set of physical parameters characterizing crack initiation. In particular, it is shown that (1) crack initiation occurs at the maximum of the rigidity–displacement curve; (2) cracking is preceded by strain localization in the form of macro-shear bands which induce surface roughening. Local necking then occurs in some surface grains and leads to ductile intergranular crack propagation. The sequence of microscopic changes at the grain scale up to and beyond crack initiation have been characterized and quantified in terms of local grain strains, coarse intragranular slip and shear band evolution over several grains.

Necking and Failure of Constrained 3D Microtissues Induced by Cellular Tension

Abstract: In this paper we report a fundamental morphological instability of constrained 3D microtissues induced by positive chemomechanical feedback between actomyosin-driven contraction and the mechanical stresses arising from the constraints. Using a 3D model for mechanotransduction we find that perturbations in the shape of contractile tissues grow in an unstable manner leading to formation of “necks” that lead to the failure of the tissue by narrowing and subsequent elongation. The magnitude of the instability is shown to be determined by the level of active contractile strain, the stiffness of the extracellular matrix, and the components of the tissue that act in parallel with the active component and the stiffness of the boundaries that constrain the tissue. A phase diagram that demarcates stable and unstable behavior of 3D tissues as a function of these material parameters is derived. The predictions of our model are verified by analyzing the necking and failure of normal human fibroblast tissue constrained in a loop-ended dog-bone geometry and cardiac microtissues constrained between microcantilevers. By analyzing the time evolution of the morphology of the constrained tissues we have quantitatively determined the chemomechanical coupling parameters that characterize the generation of active stresses in these tissues. More generally, the analytical and numerical methods we have developed provide a quantitative framework to study how contractility can influence tissue morphology in complex 3D environments such as morphogenesis and organogenesis.

Work Hardening Behavior in Steel with Multiple TRIP Mechanisms

Abstract: Transformation-induced plasticity (TRIP) behavior was studied in steel with the composition Fe-0.07C-2.85Si-15.3Mn-2.4Al-0.017N that exhibited two TRIP mechanisms. The initial microstructure consisted of both e- and α-martensites with 27 pct retained austenite. TRIP behavior in the first 5 pct strain was predominately austenite transforming to e-martensite (Stage I), but upon saturation of Stage I, the e-martensite transformed to α-martensite (Stage II). Alloy segregation also affected the TRIP behavior with alloy-rich regions producing TRIP just prior to necking. This behavior was explained by first-principles calculations which revealed that aluminum significantly affected the stacking fault energy in Fe-Mn-Al-C steels by decreasing the unstable stacking fault energy and promoting easy nucleation of e-martensite. The addition of aluminum also raised the intrinsic stacking fault energy and caused the e-martensite to be unstable and transform to α-martensite under further deformation. The two-stage TRIP behavior produced a high strain hardening exponent of 1.4 and led to an ultimate tensile strength of 1165 MPa and elongation to failure of 35 pct.

Theoretical analysis of strain- and stress-based forming limit diagrams

Abstract: Forming limit diagram is extensively used in the analysis of sheet metal forming to define the limit of deformation of materials without necking or fracture. This article contributes in constructio...