Showing papers on "Hydrostatic stress published in 2007"

Development of stress-modified fracture strain for ductile failure of API X65 steel

Abstract: The present paper proposes ductile failure criteria in terms of true fracture strain (the equivalent strain to fracture) as a function of the stress triaxiality (defined by the ratio of the hydrostatic stress to the equivalent stress) for the API X65 steel. To determine the stress-modified fracture strain, smooth and notched tensile bars with four different notch radii are tested, from which true fracture strains are determined as a function of the notch radius. Then detailed elastic–plastic, large strain finite element analyses are performed to estimate variations of stress triaxiality in the tensile bars, which leads to true fracture strains as a function of the stress triaxiality, by combining them with experimental results. Two different failure criteria are proposed, one based on local stress and strain information at the site where failure initiation is likely to take place, and the other based on averaged stress and strain information over the ligament where ductile fracture is expected. As a case study, ligament failures of API X65 pipes with a gouge are predicted and compared with experimental data.

On the geometric character of stress in continuum mechanics

Abstract: This paper shows that the stress field in the classical theory of continuum mechanics may be taken to be a covector-valued differential two-form. The balance laws and other funda- mental laws of continuum mechanics may be neatly rewritten in terms of this geometric stress. A geometrically attractive and covariant derivation of the balance laws from the principle of energy balance in terms of this stress is presented.

Relative influence of microstructure and macroscopic triaxiality on cavitation damage in a semi-crystalline polymer

Abstract: The relative influence of microstructure on mechanical behaviour and cavitation in a semi-crystalline polymer is studied here and compared to the macroscopic triaxiality effect induced by the sample shape. Microstructure effects result from process or annealing. In the later case, DSC and DMA experiments show secondary crystallization, homogenization of primary crystals and decrease of the amorphous phase mobility. Samples shapes exhibiting more or less macroscopic triaxiality are tested in tension before and after annealing at high temperature. More cavitation is observed after annealing – or in slowly cooled down materials – in weakly triaxial structures (dumbbell specimens or diabolo specimens with a large curvature radius). In highly triaxial samples (diabolo specimens with a small curvature radius) no difference are observed between as-received and annealed samples: the macroscopic hydrostatic stress is then of major importance compared to the raise of local hydrostatic stress within the amorphous layer resulting from high-temperature annealing.

Experimental and numerical analysis of in-plane and out-of-plane crack tip constraint characterization by secondary fracture parameters

Abstract: In this study, several two-parameter- concepts are analyzed experimentally and numerically with respect to their capability of characterizing in-plane and out-of-plane crack tip constraint effects. Different approaches utilizing the second term Tstress of the linear-elastic crack tip stress field, a higher term A2 of the power-law hardening crack tip stress field, a hydrostatic correction term Q for a reference stress field or the local triaxiality parameter h are compared. Experimental results for a pressure vessel steel 22NiMoCr3-7 are investigated by means of the different approaches regarding their capability of constraint characterization for enhanced transferability. Theoretical aspects are investigated in a modified boundary layer analysis and in three-dimensional nonlinear elastic-plastic finite element analyses of the specimens. It is found that, with respect to their capability of quantifying combined in-plane and out-of-plane constraint effects, the investigated concepts differ significantly.

Discrete 3D model as complimentary numerical testing for anisotropic damage

Abstract: It is proposed to use a discrete particle model as a complimentary “numerical testing machine” to identify the hydrostatic elasticity-damage coupling and the corresponding sensitivity to hydrostatic stresses parameter. Experimental tri-axial tensile testing is difficult to perform on concrete material, and numerical testing proves then its efficiency. The discrete model used for this purpose is based on a Voronoi assembly that naturally takes into account heterogeneity. Tri-tension tests on a cube specimen, based on a damage growth control, are presented. A successful identification of the hydrostatic sensitivity function of a phenomenological anisotropic damage model is obtained.

Void growth by dislocation-loop emission

Abstract: Experimental results from spall tests on aluminum reveal the presence of a dense dislocation structure in an annulus around a void that grew under the tensile pulse when a shock wave was reflected at the free surface of the specimen. The proposition is that dislocation emission from the void surface under load is a viable mechanism for void growth. To understand void growth in the absence of diffusive effects, the interstitial-loop emission mechanism under tensile hydrostatic stress is investigated. First, the micromechanics of pile-up formation when interstitial loops are emitted from a void under applied macroscopic loading is reviewed. Demand for surface energy expenditure upon void-surface change is taken into consideration. It is demonstrated that in face-centered cubic metals loop emission from voids with a radius of ∼10 nm is indeed energetically possible in the hydrostatic stress environment generated by shock loading. On the other hand, the levels of hydrostatic stress prevalent in common structural applications are not sufficient to drive loops at equilibrium positions above a ∼10 nm void. However, for voids larger than about 100 nm, the energetics of loop emission are easily met as a necessary condition even under the low stress environment prevalent in structural applications.

Stress field induced by swift heavy ion irradiation in cubic yttria stabilized zirconia

Abstract: X-ray diffraction (XRD) was used to investigate the damage and the correlated stress induced by the slowing down of swift heavy ions in cubic zirconia polycrystals doped with 10 mol % Y2O3. Samples were irradiated at room temperature with 940 MeV Pb ions at fluences ranging from 5×1011 to 4×1013 cm−2. Changes of XRD profiles were examined at increasing fluences. Residual macroscopic stresses induced by irradiation were determined using XRD by the “sin2 ψ method.” The state of stress in the irradiated layer was described by a combination of: (i) a hydrostatic stress caused by the formation of damaged tracks leading to swelling and (ii) a biaxial stress imposed by the bulk undamaged material, which controls the lateral expansion of the surface damaged layer. The evolution of the stress as a function of irradiation fluence was also determined: the intensity of the hydrostatic stress increases from 80 to 460 MPa when the fluence is increased from 5×1011 to 4×1013 cm−2 and that of the biaxial stress increases ...

Cylindrical cavity expansion in compressible Mises and Tresca solids

Abstract: The elastoplastic field induced by quasi-static expansion in steady-state plane-strain conditions of a pressurized cylindrical cavity (cylindrical cavitation) is investigated. Material behavior is modeled by Mises and Tresca large strain flow theories formulated as hypoelastic. Both models account for elastic-compressibility and allow for arbitrary strain-hardening (or softening). For the Mises solid analysis centers on the axially-hydrostatic assumption (axial stress coincides with hydrostatic stress) in conjunction with a controlled error method. Introducing an error control parameter we arrive at a single-parameter-dependent quadrature expression for cavitation pressure. Available results are recovered with particular values of that parameter, and an optimal value is defined such that the cavitation pressure is predicted with high accuracy. For the Tresca solid we obtain an elegant solution with the standard model when no corner develops in the yield surface. Under certain conditions however a corner zone exists near the cavity and the solution is accordingly modified revealing a slight difference in cavitation pressure. Comparison with numerical solutions suggests that the present study establishes cylindrical cavitation analysis on equal footing with existing studies for spherical cavitation.

Mechanical Stresses in Soils Assessed from Bulk‐Density and Penetration‐Resistance Data Sets

Abstract: Measurement of soil strength with sophisticated parameters is tedious and expensive. Therefore, we developed two straightforward methods to determine this parameter down to about 80 cm, based on the classical measurements of bulk density and penetration resistance as a function of depth. They were applied to three profiles of arable Luvisols, all developed from glacial till. For each method, a procedure was worked out that allows expression of the results in terms of a normal (NC) or precompacted (PC) state. We defined the NC state as that observed in packing characteristics of virgin soils like forests and meadows, and the PC state as the packing characteristics that exist in the topsoil of agricultural soils and intensely grazed areas. Bulk density data were used to examine the packing characteristics and overburden pressures with the assumption that the horizon was in a NC state below 80 cm. For penetration resistance, we assumed a linear increase in penetration resistance with depth to represent the hydrostatic stress distribution in the NC state and deviations of measured values from this line as the PC state. The upper approximately 60 cm of all three soils were compacted, which is proofed both for the penetration resistance and for the bulk density data. For both approaches, the dimensionless coefficient of "stresses at rest," K 0 , was calculated following the line of thought used in engineering soil mechanics (K 0 = σ x /σ z , where σ x and σ are the horizontal and vertical stresses, respectively). The K 0 values are highest in the precompacted soil horizons and decrease with depth.

Continuum damage modelling of environmental degradation in joints bonded with E32 epoxy adhesive

Abstract: A mesh-independent continuum damage model has been proposed to predict the residual strength of adhesively-bonded joints by introducing a displacement-based damage parameter into the constitutive equation of the damaged materials. This approach was originally developed for EA9321-bonded single-lap joints and this paper extends it to butt joints bonded with a different ductile adhesive (E32). This involves not only a different adhesive and joint configuration but the high hydrostatic stress requires a more realistic yielding model. A dry and a partially saturated steel joint were used to calibrate the moisture dependent damage parameters of the adhesive E32, and then these parameters were used without further modification to predict the failure of the other environmentally degraded steel butt joints and the aluminum butt joints. The FEA package ABAQUS was used to implement the coupled mechanical-diffusion analyses required. A von Mises yield model was used initially. Then, a linear Drucker–Prager plasticit...

Analysis of micro void formation at grain boundary triple points in monotonically strained AA6022-T43 sheet metal

Abstract: This study reports on the micro voids formed in a 1 mm thick sheet of rolled aluminum alloy AA6022 subjected to monotonic tensile loading. In this research micro voids were found to occur at certain grain boundary triple point (GBTP) junctions. The orientations of the grains at various GBTP junctions were determined using an electron backscatter diffraction detector mounted in a field-emission scanning electron microscope. Because damage evolution and eventual final failure are driven by local tensile hydrostatic stresses arising from microhetereogenities, the hydrostatic stress state surrounding the GBTP junction was evaluated using a crystal plasticity material model implemented in a finite element analysis code. Grain orientations that result in a local hydrostatic state of tension surrounding the GBTP junction were found to have a tendency to form micro voids.

A comparison of models for ductile fracture prediction in forging processes

Abstract: The possibility of predicting ductile fracture plays an important role in the design of components by forging processes. Experimental observations showed that the nucleation, growth and coalescence of voids are the mechanisms that control the initiation and propagation of fracture and that these mechanisms are influenced in different ways by factors like the hydrostatic stress, the equivalent stress or by the maximal principal stress. Many ductile fracture indicators, based on some or all of those factors, are available and used in many practical situations in the design of those components. In this work a comparative work of many of those criteria was undertaken. Different criteria were chosen amongst the more popular ones and from different groups, in which they may be classified, namely those based on micromechanics and those based on the geometry of voids or their growth mechanisms. The criteria based on the Continuous Damage Mechanics, in which a coupling between plastic deformation and material degradation is taken into account and that include different damage evolution descriptions for traction or compressive stress states, give a more correct and clear localization for the fracture initiation site.

Stability of simple cubic crystals

Abstract: Although simple cubic (sc) crystals tend to be “inherently unstable” under ambient conditions, their potential stability is enhanced in states of pure hydrostatic tension, which can be approached locally near stress raisers such as cracks. Our molecular-dynamics simulations verify that a range of hydrostatic stress exists under which sc crystals are stable. Three distinct destabilization mechanisms are observed, depending on the bulk-modulus magnitude and the magnitude and algebraic sign of the shear modulus μ′, which in turn depend on the stress level; two of these mechanisms result in fracture, while the third yields a sc-to-hcp phase transformation by a unique two-stage shearing process.

Viscoelasticity and shear yielding onset in amorphous glassy polymers

Abstract: In the present work, the effect of viscoelasticity on the yield behaviour of a polycarbonate, PC, was studied and the identification of a yield criterion which takes into account the effects of the mechanical history on the onset of plastic strain, was attempted. The attention was focused on the shear yielding plastic deformation process and different loading histories were performed under uniaxial compression: constant strain rate at different rates, stress relaxation at different applied strain levels, creep under different stress levels. Some tests were also carried out under shear loading, in which the hydrostatic stress component is equal to zero and its effect on the yield onset can be considered. For the definition of a yield criterion, different quantities, some already proposed in an analogous work on a styrene-acrylonitrile copolymer (SAN), were considered and determined at yield onset for each of the applied loading histories. The results obtained in this work show that the relative ratios of the viscoelastic strain over the overall strain and of viscoelastic energy over the deformation work are fairly constant irrespective of both loading history and stress state. The re-elaboration of the data previously obtained on SAN is consistent with these results. Discussing the experimental data, differences between the mechanical behaviour of the two glassy polymers were pointed out and a more difficult activation of the plastic deformation process of PC than SAN was generally observed.

Three‐Dimensional Modeling of Void Growth in Friction Stir Welding of Stainless Steel

Abstract: The growth of internal voids in the process of friction stir welding of stainless steel was simulated using a damage model that considers both strain hardening and porosity evolution. In the void growth equations, the mean stress (hydrostatic stress) was scaled by the state variable for plastic flow resistance, i. e. strength. The damage model was coupled with the viscoplastic deformation and thermal processes using a steady‐state Eulerian formulation in a finite element scheme. The porosity and strength were calculated by integration of the evolution equations along streamlines of the flow field. The distributions of microvoids as well as the strength within the material were obtained. These distributions were used to model the effects of operational parameters such as the tool rotational and translational speeds as well as the pin threads on the growth of porosity.

Nanostructurization of metals cryodeformed at hydrostatic stress

Abstract: Roles of temperature and hydrostatic stress forces in severe plastic deformation of metal objects are considered. Methods and devices are described that allow the structural states of metal with high mechanical characteristics to be obtained upon plastic deformation at low temperatures under conditions of hydrostatic stress.

Hydrostatic Stress Effect on the Optical Performance and the Stress Sensitivity of Optical Nonlinear Waveguide

Abstract: Sensitivity of optical parameters is a significant topic in developing optoelectronic devices. The stress sensitivity of nonlinear optical waveguides is closely related to hydrostatic stress. The hydrostatic stress can cause anisotropic and inhomogeneous distribution of the refractive index. In this paper analytical and numerical calculations are performed to study the effect of hydrostatic stress on the sensitivity of nonlinear optical waveguide sensors. The optical performance of the waveguide sensors under various hydrostatic stress states is also investigated. Transverse magnetic modes (TM) are considered in addition to transverse electric modes (TE) to study anisotropy. It is found that the value of the hydrostatic stress can change the value of the cutoff thickness. These changes may induce multimode. Moreover, the hydrostatic stresses influence the values of the stress sensitivity of the waveguide sensors and present anisotropic behavior to the system.