Showing papers on "Necking published in 2002"

Biofilm material properties as related to shear-induced deformation and detachment phenomena.

Abstract: Biofilms of various Pseudomonas aeruginosa strains were grown in glass flow cells under laminar and turbulent flows. By relating the physical deformation of biofilms to variations in fluid shear, we found that the biofilms were viscoelastic fluids which behaved like elastic solids over periods of a few seconds but like linear viscous fluids over longer times. These data can be explained using concepts of associated polymeric systems, suggesting that the extracellular polymeric slime matrix determines the cohesive strength. Biofilms grown under high shear tended to form filamentous streamers while those grown under low shear formed an isotropic pattern of mound-shaped microcolonies. In some cases, sustained creep and necking in response to elevated shear resulted in a time-dependent fracture failure of the "tail" of the streamer from the attached upstream "head." In addition to structural differences, our data suggest that biofilms grown under higher shear were more strongly attached and were cohesively stronger than those grown under lower shears.

Micromechanics of coalescence in ductile fracture

Abstract: Significant progress has been recently made in modelling the onset of void coalescence by internal necking in ductile materials The aim of this paper is to develop a micro-mechanical framework for the whole coalescence regime, suitable for finite-element implementation The model is defined by a set of constitutive equations including a closed form of the yield surface along with appropriate evolution laws for void shape and ligament size Normality is still obeyed during coalescence The derivation of the evolution laws is carefully guided by coalescence phenomenology inferred from micromechanical unit-cell calculations The major implication of the model is that the stress carrying capacity of the elementary volume vanishes as a natural outcome of ligament size reduction Moreover, the drop in the macroscopic stress accompanying coalescence can be quantified for many initial microstructures provided that the microstructure state is known at incipient coalescence The second part of the paper addresses a more practical issue, that is the prediction of the acceleration rate δ in the Tvergaard–Needleman phenomenological approach to coalescence For that purpose, a Gurson-like model including void shape effects is used Results are presented and discussed in the limiting case of a non-hardening material for different initial microstructures and various stress states Predicted values of δ are extremely sensitive to stress triaxiality and initial spacing ratio The effect of initial porosity is significant at low triaxiality whereas the effect of initial void shape is emphasized at high triaxiality

Drop formation dynamics of constant low-viscosity, elastic fluids

Abstract: The dynamics of drop formation under gravity has been investigated as a function of elasticity using a set of low-viscosity, ideal elastic fluids and an equivalent Newtonian glycerol-water solution. All solutions had the same shear viscosity, equilibrium surface tension, and density, but differed greatly in elasticity. The minimum drop radius in the early stages of drop formation (necking) was found to scale as expected from potential flow theory, independent of the elasticity of the solutions. Thus, during this stage of drop formation when viscous force is still weak, the dynamics are controlled by a balance between inertial and capillary forces, and there is no contribution of elastic stresses of the polymer. However, upon formation of the pinch regions, there is a large variation in the drop development to break-off observed between the various solutions. The elastic solutions formed secondary fluid threads either side of a secondary drop from the necked region of fluid between the upper and lower pinches, which were sustained for increasing amounts of time. The break-off lengths and times increase with increasing elasticity of the solutions. Evolution of the filament, length is, however, identical in shape and form for all of the polymer solutions tested, regardless of differing elasticity. This de-coupling between filament growth rate and break-up time (or equivalently, final filament length at break-up) is rationalised. A modified force balance to that of Jones and Rees [48] is capable of correctly predicting the filament growth of these low-viscosity, elastic fluids in the absence of any elastic contributions due to polymer extension within the elongating filament. The elongation of the necked region of fluid (which becomes the filament) is dominated by the inertia of the drop, and is independent of the elasticity of the solution. However, elasticity does strongly influence the resistance of the pinch regions to break-off, with rapid necking resulting in extremely high rates of surface contraction on approach to the pinch point, initiating extension of the polymer chains within the pinch regions. This de-coupling phenomenon is peculiar to low-viscosity, elastic fluids as extension does not occur prior to the formation of the pinch points (i.e. just prior to break-up), as opposed to the high viscosity counterparts in which extension of polymers in solution may occur even during necking. Once steady-state extension of the polymers is achieved within the pinch at high extension rates, the thread undergoes elasto-capillary break-up as the capillarity again overcomes the viscoelastic forces. The final length at detachment and time-to-break-off (relative to the equivalent Newtonian fluid) is shown to be linearly proportional to the longest relaxation time of the fluid. (C) 2002 Elsevier Science B.V. All rights reserved.

Strain-Controlled Tensile Deformation Behavior of Isotactic Poly(1-butene) and Its Ethylene Copolymers

Abstract: The tensile deformation behavior of poly(1-butene) and two of its ethylene copoloymers was studied at room temperature. This was done by investigating true stress−strain curves at constant strain rates, elastic recovery properties, and in-situ WAXS patterns during the deformation process. All samples showed a common rubberlike deformation behavior without necking down. The differential compliance, the recovery properties, and the evolution of the crystallite texture changed simultaneously at well-defined points. The strains at which these points occurred along the true stress−strain remained constants for the different samples despite their different percentage crystallinities. The same strain-controlled deformation behavior was also shown by a series of semicrystalline polymers in previous studies that carried out in our group. The well-defined way that the different samples respond to external stresses complies with our views of a granular substructure of the crystalline lamellae in a semicrystalline po...

Synergistic effects of plastic anisotropy and void coalescence on fracture mode in plane strain

Abstract: The macroscopic fracture in plane strain is known to be shear-like in ductile materials. In most structural materials, fracture starts after diffuse necking, at the centre of the specimen, by micro-void coalescence giving rise afterwards to the macroscopic shear fracture mode. In this paper, the effect of coalescence on shear band development and on associated fracture mode in plane strain is analysed numerically. The calculations are performed using a recent elastic-viscoplastic Gurson-like model that accounts for void shape evolution, coalescence and post-coalescence micromechanics along with isotropic hardening and orthotropic plasticity for the matrix behaviour. The latter is introduced to represent the actual flow properties of hot-worked materials. No kinematic hardening or nucleation formulation is used in order to focus attention on coalescence effects and to discuss, with respect to experiments, published results based on kinematic hardening and nucleation effects. The most important finding is the synergistic effect of plastic anisotropy and post-coalescence yield surface curvature upon the onset of a shear band after the fracture sets in at the centre of the specimen.

Low-temperature superplastic behavior of a submicrometer-grained 5083 Al alloy fabricated by severe plastic deformation

Abstract: A submicrometer-grained structure was introduced in a commercial 5083 Al alloy by imposing an effective strain of ∼8 through equal channel angular pressing. In order to examine the low-temperature superplastic behavior, the as-equal channel angular pressed (as-ECAP) samples were tensile tested in the strain rate range of 10−5 to 10−2 s−1 at temperatures of 498 to 548 K corresponding to 0.58 to 0.65 Tm, where Tm is the incipient melting point. The mechanical data of the alloy at 498 and 548 K exhibited a sigmoidal behavior in a double logarithmic plot of the maximum true stress vs true strain rate. The strain rate sensitivity was 0.1 to 0.2 in the low- and high-strain rate regions and 0.4 in the intermediate-strain rate region, indicating the potential for superplasticity. At 523 K, instead of the sigmoidal behavior, a strain rate sensitivity of 0.4 was maintained to low strain rates. A maximum elongation of 315 pct was obtained at 548 K and 5×10−4 s−1. The activation energy for deformation in the intermediate-strain rate region was estimated as 63 kJ/mol. Low-temperature superplasticity of the ultrafine grained 5083 Al alloy was attributed to grain boundary sliding that is rate-controlled by grain boundary diffusion, with a low activation energy associated with nonequilibrium grain boundaries. Cavity stringers parallel to the tensile axis were developed during deformation, and the failure occurred in a quasi-brittle manner with moderately diffusive necking.

The influence of iron content on the plane strain fracture behaviour of AA 5754 Al–Mg sheet alloys

Abstract: The effects of impurity iron content and a change from uniaxial to plane strain tension on AA 5754 Al-Mg sheet alloys were investigated. This was done by studying the fracture evolution in each case. These alloys do not exhibit the progressive damage accumulation often found in ductile commercial metals, because of a relatively low second-phase iron intermetallic volume fraction and a propensity for shear localization at large plastic strains. Necking strains and the strains at which intense shear localization occurred were not influenced by iron content. Once void nucleation has occurred, an increase in iron content causes a reduction in the strain necessary to attain failure. In addition, an increase in iron content also changes the fracture mode from double cup fracture to void sheet fracture. A change from uniaxial to plane strain tension reduces the strain to fracture and creates conditions for shear localization to occur more readily. Finally, the negative strain rate sensitivity exhibited at room temperature further increases the tendency toward shear localization. (C) 2002 Elsevier Science B.V. All rights reserved.

Plane-stress fracture behavior of syndiotactic polypropylenes of various crystallinity as assessed by the essential work of fracture method

Abstract: The fracture behavior of 1-mm-thick syndiotactic polypropylene (sPP) sheets of various crystallinity (owing the various stereo- and regioregularity) was studied by the essential work of fracture (EWF) concept. The specific work of fracture parameters were determined on tensile-loaded deeply double-edge notched (DDEN-T) specimens at various deformation rates (v = 1, 10 and 100 mm/min) at ambient temperature. As the DDEN-T specimens showed full ligament yielding prior to subsequent necking, the energy partitioning method was adopted for data reduction. The specific essential work of fracture slightly increased as a function of crystallinity. Its yielding-related constituent increased with both crystallinity and deformation rate similarly to that of the yield strength and E-modulus. The specific plastic work parameters were less sensitive to crystallinity and deformation rate. The strain-induced crystalline phase transition from helical toward all-trans conformation was accompanied by voiding and crazing.

A microscopy study of the transition from yielding to crazing in polypropylene

Abstract: The deformation mechanisms in polypropylene as a function of strain rate have been studied by scanning electron microscopy. Injection molded, dogbone specimens were tested in tension at a temperature of 50°C and strain rates from 10-4 to 90 s-1. With increasing strain rate a gradual transition from cold drawing and necking to nearly homogeneous deformation was observed. The transition is characterized by a strong elastic recoil after fracture. At e = 90 s-1, the elastic recovery has its maximum value of 70% of the strain at break (e = 0.3). The morphology was studied post mortem at small strains outside the neck using permanganically etched samples. The transition in the macroscopic behavior was found to coincide with a change in the deformation mechanism. Whereas yielding is the dominant mechanism at low strain rates, homogeneous voiding is found at e = 90 s-1. A correlation is suggested between the failure of chain slip within the lamellae and the occurrence of voiding, leading to distinct voiding patterns in the spherulites.

Effect of temperature on fracture properties of an amorphous poly(ethylene terephthalate) (PET) film

Abstract: Fracture behaviour of an amorphous polyethylene terephthalate (PET) film with a glass transition temperature (Tg) of 72°C and a thickness of 0.21 mm was studied between 23 and 70°C using Double Edge Notched Tension (DENT) specimens. Within this temperature range, DENT specimens fractured by ductile tearing of the ligament region after ligament region had been fully yielded. The load-displacement curves obtained for different ligament lengths were geometrically similar to one another. On the basis of these, Essential Work of Fracture (EWF) methodology was used to determine fracture toughness of the PET film as a function of temperature. A linear relationship was obtained between the total specific work of fracture, wf, and ligament length, L, at temperatures under consideration. Results showed that specific essential work of fracture, we, is independent of temperature but the specific non-essential work of fracture (β wp) increases with increasing temperature and drops in value near the glass-transition temperature. A linear relationship was also found for yielding (wy) and necking/tearing (wnt) components of wf as a function of ligament length. The specific essential work components were found to be temperature dependent and whilst component wey decreased component went increased with increasing temperature. The contribution of went to we was substantially greater than that of wey at all temperatures.

Flow through a convergence : Part 1. Critical Conditions for Unstable Flow

Abstract: The critical conditions leading to fracture in elongation and different types of flow instabilities were examined in uniaxial elongation and in a capillary rheometer equipped with dies having different entry profiles. Either ductile or brittle fracture may be observed, ductile being related to necking of material. The critical stress approach was used to predict fracture in elongation. All linear polymers studied in this work exhibited ductile fracture in uniaxial elongation, but the transition to brittle fracture is discussed in relation to existing experiments with other materials. In a ductile fracture regime, critical stress and work both increase with an increasing rate of deformation, whereas in a brittle regime the critical values remain constant. The converging flow studies indicated that two types of flow instability that have been previously related to each other, namely, pressure oscillations and volume distortions, are of different origins. The critical flow rate for pressure oscillations is independent of entry profile, and the origin for this type of instability lies along the wall of the capillary. On the other hand, the critical flow rate for volume distortions increased with a decreasing entry angle, indicating that volume distortions are not a consequence of pressure oscillation, nor are their origin at the capillary wall. Numerical simulations were used to determine the stress profiles within the flow, and it was shown that the onset of volume distortions is directly related to the magnitude of elongational stress and work, and may therefore be considered to be caused by fracture in elongation. In dies with 90° entry profile, volume distortions were observed simultaneously with pressure oscillations, making it difficult to distinguish between the two phenomena.