Effect of vibration on the creep of tensile-test specimens of the aluminum alloy Al−Mg−Si (PA-4)

In this work are presented the results of research on vibrational creep at normal temperature of an aluminum alloy containing magnesium and silicon (designated PA-4). A uni-directional positive load is applied to a tensile-test specimen, such that the stress intensity in the specimen is of the type\(\sigma (t) = \sigma _m (1 + Asin\omega t)\) whereσ _m is the mean (static) stress intensity andA =σ _a /σ _m is the ratio of the vibratory-stress intensity to the mean intensity. The results are given in the form of families of curves of plastic (i.e., permanent) deformation for various values ofA, namely,A=0.0000, 0.0065, 0.05500, 0.1000 and 0.2000. Taking the creep limit for plastic strain as ε _p = 1.8 percent, equations for this creep limit were deduced from experimental data. The following conclusions are drawn from these investigations:

1. 1.
   Vibrations of small-stress-amplitude ratioA encourage creep, particularly with more lengthy tests.     
   
   

Viscoplasticity with creep and plasticity bounds

A viscoplastic theory is developed that reduces analytically to creep theory under steady-state conditions and becomes plasticity theory at its rate-independent bound. A viscoplastic model is then constructed by defining material functions that have close ties to the physics of inelasticity. As a consequence, this model is characterized easily — only steady-state creep data, saturated hysteresis loops, and monotonic stress/strain curves are required. The general applicability of the model is demonstrated by the characterization of three f.c.c. metals. A variety of validation experiments is also provided.     

Multiaxial creep and cyclic plasticity in nickel-base superalloy C263

Physically-based constitutive equations for uniaxial creep deformation in nickel alloy C263 [Acta Mater. 50 (2002) 2917] have been generalised for multiaxial stress states using conventional von Mises type assumptions. A range of biaxial creep tests have been carried out on nickel alloy C263 in order to investigate the stress state sensitivity of creep damage evolution. The sensitivity has been quantified in C263 and embodied within the creep constitutive equations for this material. The equations have been implemented into finite element code. The resulting computed creep behaviour for a range of stress state compares well with experimental results. Creep tests have been carried out on double notched bar specimens over a range of nominal stress. The effect of the notches is to introduce multiaxial stress states local to the notches which influences creep damage evolution. Finite element models of the double notch bar specimens have been developed and used to test the ability of the model to predict correctly, or otherwise, the creep rupture lifetimes of components in which multiaxial stress states exist. Reasonable comparisons with experimental results are achieved. The γ^′ solvus temperature of C263 is about 925 °C, so that thermo-mechanical fatigue (TMF) loading in which the temperature exceeds the solvus leads to the dissolution of the γ^′ precipitate, and a resulting solution treated material. The cyclic plasticity and creep behaviour of the solution treated material is quite different to that of the material with standard heat treatment. A time-independent cyclic plasticity model with kinematic and isotropic hardening has been developed for solution treated and standard heat treated nickel-base superalloy C263. It has been combined with the physically-based creep model to provide constitutive equations for TMF in C263 over the temperature range 20–950 °C, capable of predicting deformation and life in creep cavitation-dominated TMF failure.     

Boundary element approach to creep deformation of edge notched and cracked specimens

A direct formulation of the boundary element method using a complex variable numerical approach is presented for the time-dependent inelastic stresses in edge notched and cracked creeping metallic structural components subject to high temperature gradients. Particular attention is focused on the numerical evaluation of energy rate contour integrals in single edge cracked specimens in tension. The constitutive models used in the numerical calculation are internal state variable creep–plasticity or elastic power law creep model. Numerical results are compared with solution obtained from other methods for different loading rates.     

Finite-element analysis of slow incompressible viscous fluid motion

Summary The governing field equations and the constitutive relation are specialized to the boundary value formulation of incompressible viscous fluid motion excluding thermal effects. When choosing the velocities and the hydrostatic pressures as variables, the established non-linear matrix equation in terms of finite-element properties becomes valid for both two- and three-dimensional application. It is shown that a restricted class of problems readily fits within the scope of existing finite-element software designed for conventional structural mechanics analysis provided an effective Lagrange multiplier technique can be incorporated.Two curved triangular finite-element models are proposed for both two-dimensional and axisymmetric flow, based on a and order Lagrangian and 3rd order Hermitian interpolation set for the velocities. Some typical examples are attached including linear stationary and transient problems, as well as non-linear cavity flow at moderate Reynolds numbers.

---

Übersicht Die Feldgleichungen und das Stoffgesetz werden für die Formulierung der Randwertaufgabe für inkompressible zähe Strömungen spezialisiert, wobei thermische Effekte unberücksichtigt bleiben. Die entwickelte nichtlineare Matrizenbeziehung in Termen der Finite-Element-Schreibweise gilt sowohl für zweidimensionale als auch für dreidimensionale Anwendungen, sofern die Geschwindigkeitskomponenten und der hydrostatische Druck als Strömungsvariablen gewählt werden. Es wird gezeigt, daß eine eingeschränkte Klasse von Problemen direkt im Rahmen bereits bestehender Finite-Element-Programme gelöst werden kann, die für herkömmliche Aufgaben der Strukturmechanik entwickelt wurden, sofern ein effektiver Einbau der Methode der Lagrangeschen Multiplikatoren möglich ist.Für zweidimensionale und achsensymmetrische Strömungen werden zwei krummseitige dreiecksförmige Finite-Element-Modelle vorgeschlagen, deren Geschwindigkeitsfelder mit Hilfe einer Lagrangeschen Interpolation zweiter Ordnung bzw. eines Hermiteschen Ansatzes dritter Ordnung approximiert werden. Einige typische Beispiele behandeln lineare stationäre und instationäre Probleme, sowie eine nichtlineare Hohlraumströmung bei kleineren Reynolds-Zahlen.

---

---

Professor Dr. Ing. Dr. h. c. Eduard C. Pestel zu seinem 60. Geburtstag herzlichst gewidmet.     

Finite-element analysis of rectangular plates with rectangular inserts

The present investigation deals with the stress distribution in the vicinity of rectangular inserts in finite rectangular plates. This problem is more complex due to the singularities at the corners of the inserts. In this paper, the finite-element technique is used to determine the deformations and, subsequently, the stresses. The paper treats the problem in a generalized form in the sense that the size and orientation of the insert are taken as variables. The finite rectangular plate is subjected to a uniform axial tensile load. The material of the plate and that of the insert are considered to be different. Element selections are made which are optimal with regard to accuracy and computational effort. The local element stresses which generate considerable discontinuity at the element nodes are plotted. Averaging process for the local stress calculations is discussed and these are compared with the results available¹ which are obtained by experimental techniques.     

Finite-element analysis of indentation experiments in surface-coated materials

There are only a few methods suitable for a quantitative characterization of the mechanical properties of surface-coated materials; indentation testing is one of those methods. Within the last decade, a great deal of effort has been made to improve the indentation test and to gather information on the complete deformation process from experimental and numerical investigations. Following this line, this contribution concentrates on the numerical calculation of the elastic-plastic field of deformation in the specimen during the indentation process and the corresponding load-depth of indentation curves in dependence on the dominating parameters. The basic idea is to determine the influence of geometrical imperfections of the indenter on the—experimentally obtained—mechanical properties such as hardness and to provide methods which enable one to distinguish between properties of the system used for testing and the material investigated. Results obtained for uncoated and coated materials are compared. Paper was presented at the 1992 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 8–11.     

Fundamental experiments in plasticity and creep of aluminum—Extension of previous results

Paper presents combined stress experiments in plasticity and creep of aluminum 1100-0. The purpose of these experiments was to determine the motion of the yield surface in tension-torsion space for three complicated prestressing paths, to investigate the validity of the normality hypothesis, to investigate the development of creep strains after prestressing, and finally to investigate the validity of the constant volume hypothesis.It is shown that the law of hardening proposed by the author previously [3,5,6] is valid, except possibly when the prestress path intersects the yield surface at a small angle. It is also shown that the normality hypothesis is valid. After prestressing the creep strain vector has in the beginning the same direction as the plastic strain vector but later its direction may change. Finally it is shown that at the level of permanent strains less than 1% the plastic strains follow the constant volume hypothesis but the creep strains do so only when they begin to appear.     

Bifurcation buckling of shells of revolution including large deflections,plasticity and creep

A summary is first presented of the conceptual difficulties and paradoxes surrounding plastic bifurcation buckling analysis. Briefly discussed are nonconservativeness, loading rate during buckling, and the discrepancy of buckling predictions with use of J₂ flow theory vs J₂ deformation theory. The axisymmetric prebuckling analysis, including large deflections, elastic-plastic material behavior and creep is summarized. Details are given on the analysis of nonsymmetric bifurcation from the deformed axisymmetric state. Both J₂ flow theory and J₂ deformation theory are described. The treatment, based on the finite-difference energy method, applies to layered segmented and branched shells of arbitrary meridional shape composed of a number of different elastic-plastic materials. Numerical results generated with a computer program based on the analysis are presented for an externally pressurized cylinder with conical heads.     

Constitutive modeling of cyclic plasticity and creep,using an internal time concept

Using the concept of an internal time as related to plastic strains, a differntial stress-strain relation for elastoplasticity is rederived, such that (i) the concept of a yield-surface is retained; (ii) the definitions of elastic and plastic processes are analogous to those in classical plasticity theory; and (iii) its computational implementation, via a “tangent-stiffness” finite element method and a “generalized-midpoint-radial-return” stress-integration algorithm, is simple and efficient. Also, using the concept of an internal time, as related to both the inelastic strains as well as the Newtonian time, a constitutive model for creep-plasticity interaction, is discussed. The problem of modeling experimental data for plasticity and creep, by the present analytical relations, as accurately as desired, is discussed. Numerical examples which illustrate the validity of the present relations are presented for the cases of cyclic plasticity and creep.     

Precipitate coarsening-induced plasticity: Low temperature creep behaviour of tempered SAE 52100

Hardened and tempered SAE 52100 under uniaxial compressive loading deforms even at temperatures well below the tempering temperature and compressive loads well below the yield strength. The origin of the observed creep is associated with the coarsening of the nanosized precipitates (cementite, χ and η). The local volume changes that occur near dissolving and coarsening carbides in the presence of a uniaxial load cause macroscopic dimensional changes. For such process the kinetics is governed by the volume diffusion of C atoms. The coarsening of precipitates located near grain boundaries has been observed by in situ heating experiments with transmission electron microscopy. The observed logarithmic time dependent plastic strain can be described by the coarsening-induced plasticity model developed in this paper that combines concepts from transformation induced plasticity and precipitate coarsening models.     

Evaluation of multiaxial theories for room-temperature plasticity and elevated-temperature creep and relaxation of several metals

Based on the assumption that the material satisfies the condition of isotropic hardening for either a von Mises or a Tresca material, finite-strain theories are derived for solid circular torsion members for the conditions that the inelastic deformations are either time independent or time dependent. In the latter case, both creep and relaxation theories are derived. At room temperature the theories are evaluated for each of eight metals using finite-strain data from tension, compression and torsion members. Of the six metals that are found to satisfy the condition required for the isotropic-hardening model, two are von Mises, one is Tresca, and the other three are between von Mises and Tresca. At elevated temperatures, the theories are evaluated for each of five of the latter six metals, using data from tension and torsion members. Material properties obtained from the tension specimens are used to predict creep and relaxation curves for the torsion members. Contrary to the results at room temperature, creep curves for the torsion members do not all fall within the region bounded by von Mises and Tresca theories. In the case of relaxation, either excellent agreement is obtained between the von Mises strain-hardening theory and experimental data or the theory is conservative.     

Net-stress analysis in creep mechanics

Summary In the paper a net-stress tensor is obtained by a linear transformation from Cauchy's stress tensor, i.e. a fourth-order (not total symmetric) tensor is introduced the components of which can be calculated by the components of an anisotropic creep-damage tensor of rank two.We have to distinguish between anisotropy corresponding to a forming process, for instance rolling, and anisotropic damage growth. Then, constitutive equations and anisotropic damage growth equations are given by symmetric second-order tensor-valued tensor functions in three argument tensors: the Cauchy stress tensor, the anisotropic creep-damage tensor of rank two, and a fourth-order constitutive tensor characterizing the anisotropy from, e.g., rolling.The central problem is to find an irreducible set of tensor generators involving the mentioned argument tensors and to construct an integrity basis associated with the representation of the tensor response function.In finding simplified constitutive equations for more practical use some examples are discussed.

---

Untersuchung zum Net-Stress Tensor in der Kriechmechanik

Übersicht Im Aufsatz wird ein Tensor vierter Stufe eingeführt, der als linearer Operator einen net-stress Tensor mit Cauchys Spannungstensor verknüpft. Die Koordinaten dieses Tensors vierter Stufe lassen sich aus den Koordinaten eines Kriechschadentensors zweiter Stufe ermitteln.Es wird zwischen anisotroper Schadensentwicklung und ursprünglich im Werkstoff vorhandener Anisotropie unterschieden, die beispielsweise durch die Herstellung (etwa Walzvorgang) bedingt ist. Bei der Aufstellung von Stoffgleichungen sind somit drei Argumenttensoren zu berücksichtigen: Cauchys Spannungstensor, Schadenstensor zweiter Stufe und Tensor der Anfangsanisotropie vierter Stufe.Das Hauptproblem besteht darin, nicht reduzierbare Tensorgeneratoren zu finden und eine Integritätsbasis zu konstruieren, die der Darstellung angepaßt ist.Für den praktischen Gebrauch werden vereinfachte Darstellungsmöglichkeiten besprochen.

---

---

This paper was presented at the Second Symposium on Inelastic Solids and Structures held in Bad Honnef in September 1981     

Experimental analysis of transformation plasticity

Transformation plasticity is an irreversible strain observed when metallurgical transformation occurs under small external stress lower than the yield stress of the weaker phase. This paper is devoted to an experimental analysis of that phenomenon in ferrous alloys. The particular case of the steel which composes vessels in French nuclear reactors is considered. This steel is called 16 MND 5 and A 533 respectively in AFNOR and ASTM norms. The main results proposed in literature concerning transformation plasticity on this material were analyzed and perfectible aspects are pointed out. The results of several performed tests are analyzed. The transformation plasticity coefficient, the kinetics as well as the dependence on the norm and the direction of the applied stress are particularly studied. The experimental results are compared to the predictions of the main existing models.     

Strain gradient plasticity analysis of transformation induced plasticity in multiphase steels

“To what extent do plastic strain gradients affect the strengthening resulting from the transformation of small metastable inclusions into hard inclusions within a plastically deforming matrix?” is the central question addressed here. Though general in the approach, the focus is on the behavior of TRIP-assisted multiphase steels. A two-dimensional embedded cell model of a simplified microstructure composed of a single metastable austenitic inclusion surrounded by a soft ferritic matrix is considered. The cell is inserted in a large homogenized medium. The transformation of a fraction of the austenite into a hard martensite plate is simulated, accounting for a transformation strain, and leading to complex elastic and plastic accommodation. The size of a transforming plate in real multiphase steels is typically between 0.1 and 2 μm, a range of size in which plastic strain gradient effects are expected to play a major role. The single parameter version of the Fleck–Hutchinson strain gradient plasticity theory is used to describe the plasticity in the austenite, ferrite and martensite phases. The higher order boundary conditions imposed on the plastic flow have a large impact on the predicted strengthening. Using realistic values of the intrinsic length parameter setting the scale at which the gradients effects have an influence leads to a noticeable increase of the strengthening on top of the increase due to the transformation of a volume fraction of the retained austenite. The geometrical parameters such as the volume fraction of retained austenite and of the transforming zone also bring significant strengthening. Strain gradient effects also significantly affect the stress state inside the martensite plate during and after transformation with a potential impact on the damage resistance of these steels.     

Effect of constraint induced by crack depth on creep crack-tip stress field in CT specimens

In this paper, the effect of constraint induced by the crack depth on creep crack-tip stress field in compact tension (CT) specimens is examined by finite element analysis, and the effect of creep deformation and damage on the Hutchinson–Rice–Rosengren (HRR) singularity stress field are discussed. The results show the constraint induced by crack depth causes the difference in crack-tip opening stress distributions between the specimens with different crack depth at the same C*. The maximum opening stress appears at a distance from crack tips, and the stress singularity near the crack tips does not exist due to the crack-tip blunting caused by the large creep deformation in the vicinity of the crack tips. The actual stress calculated by the finite element method (FEM) in front of crack tip is significantly lower than that predicted by the HRR field. Based on the reference stress field in the deep crack CT specimen with high constraint, a new constraint parameter R is defined and the constraint effect in the shallow crack specimen is examined at different distances ahead of the crack tip from transient to steady-state creep conditions. During the early stages of creep constraint increases with time, and then approaches a steady state value as time increases. With increasing the distance from crack tips and applied load, the negative R increases and the constraint decreases.     

Large strain creep analysis of thick-walled cylinders

The finite-strain theory has been used to study the creep behaviour of a thick-walled cylinder under large strains. The analysis is divided into two parts. In part 1 the creep deformation of a thick-walled cylinder of an anisotropic material subjected to internal pressure has been discussed. The effect of the anisotropy has been depicted graphically. It is found that the anisotropy of the material has a significant effect on the axial stress, strain and strain rate. Part 2 of the paper deals with the creep analysis of cylinders of either isotropic or anisotropic materials subjected to combined internal and external pressures. The effect of the anisotropy is found to be similar to that found in part 1. It is seen, however, that the introduction of external pressure results in decreasing the strain rate and thus increasing the life of the cylinder.     

Strain-space plasticity analysis of fibrous composites

A strain-space formulation of plasticity theory for metal matrix fibrous composites is discussed. Specific results are obtained for a composite system in which the fibers are elastic until failure, while the matrix is of the Mises type, with kinematic hardening. The material model of the composite is based on the assumption that the fiber diameter is vanishingly small, and the fiber volume fraction is finite. Explicit expressions are obtained for instantaneous strain concentration factors in the phases, for the instantaneous overall stiffness, and for the overall loading surface and hardening rule during mechanical straining along a prescribed path.