Showing papers on "Hydrostatic stress published in 1987"

A damage mechanics constitutive theory for the inelastic behaviour of concrete

Abstract: A rate-independent constitutive theory for the behaviour of concrete in the inelastic range is proposed. It is based on damage mechanics concepts previously applied to rock materials by the author. The inelasticity is provided by two basic damage mechanisms, namely, shear damage and hydrostatic tension damage. A scalar damage parameter is used to represent the degradation of the elastic properties. In addition to the damage mechanisms, a plasticity yield surface is included to bound the model in the hydrostatic compression sense. A simple calibration procedure is presented and the model is shown to predict reasonable behaviour for a variety of monotonic and reversed loading paths. The model is readily implemented in finite element codes and a number of representative boundary value problems is solved. Suggestions for future developments conclude the paper.

Effect of Polyaxial Stress States on Fracture Strength of Soda-Lime Glass

Abstract: Triaxial stress tests of soda-lime glass were carried out by four-point bending of plates under hydrostatic pressure and by concentric-ring loading of disks under hydrostatic pressure. The triaxial data for fracture stresses plotted in principal stress space were compared with the theoretical fracture predictions. The Weibull statistical theory could not provide a good description on the data, but underestimated the effect of hydrostatic stress on the increase of the fracture strength. The deviation of the experimental data from the Weibull statistical prediction could be explained by assuming that subcritical crack growth enhanced at the same surface flaws in the specimen tested at atmospheric pressure was restricted under hydrostatic pressure.

Dynamic mechanical and creep studies of PMMA in the α- and β-relaxation regions. Physical ageing effects and non-linear behaviour

Abstract: Dynamic mechanical results for PMMA illustrate that the secondary (β) relaxation interacts with both shear and hydrostatic stress fields and covers a very broad range of frequency which becomes narrower with increasing temperature. It is suggested that the β-process involves coupled local motions of side groups and main chain which are hindered by a broad distribution of potential barriers (activation energies) corresponding to a wide range of local environments. The variation of the tensile creep compliance over 14 decades of timescale for as-received PMMA demonstrates the dominating influence of the β-relaxation region on the room-temperature creep behaviour and the onset of the overlapping primary (α) region at around 102s. A method is proposed for extracting the α-contribution to the net creep compliance and is applied to linear data obtained as a function of ageing time at room temperature and to nonlinear results at different stress levels for as-received material. The onset of marked non-linear creep behaviour is ascribed largely to stress-induced deageing which preferentially decreases the α-relaxation times and increases the merging of the α- and β-regions. The increased mobility of the α-process is discussed by means of an adaptation of Robertson's theory of plasticity. This suggests that the shear stress increases the fraction of high-energy conformations, and hence the structural temperature, in local environments having relatively low glass transition temperatures. The magnitude of this effect may be influenced by the hydrostatic stress component in accordance with the yield investigations of Duckett, Rabinowitz and Ward.

Prediction of shear angle variation in orthogonal wave-removing processes

Abstract: An analytical model is presented for predicting the dynamic response of the shear plane during an orthogonal wave-removing process. The model is derived based on the work-hardening slip-line field theory in cutting mechanics. It takes into consideration the variations of the hydrostatic stress along the shear plane and the mean frictional coefficient on the tool-chip interface in response to the change in the cutting configuration. The results from a series of simulation studies show that the predictions from the model are in very good agreement with the existing experimental evidence for a wide range of cutting conditions.

Computer Simulation of Hydrostatic Co-Extrusion of Bimetallic Compounds.

Abstract: 2.2 .1 Analytical Approaches ............................................................... 50 2 .2 .1 .1 Energy Method .............................................................. 50 2 .2 .1 .2 Slab A n a l y s i s .............................................................. 53 2 .2 .1 .3 Lower Bound Analysis ............................................. 54 2 .2 .1 .4 Upper Bound Analysis ............................................. 54 2 .2 .2 Numerical Approaches .................................................................... 55 CHAPTER 3 FINITE ELEMENT ANALYSIS OF METAL FORMING PROCESSES . . 57 3.1 I n t r o d u c t i o n ................................................................................................ 57 3.2 T h e o r y ............................................................................................................ 59 3.2 .1 Basic M e c h a n i c s .......................................................................... . 60 3 .2 .1 .1 Displacements .............................................................. 60 3 .2 .1 .2 S t ra in Measures ............................................................ 62 3 .2 .1 .3 S t ra in Rates ................................................................... 67 3 .2 .1 .4 S t ress Measures and S t ra in Rates ...................... 68 3 .2 .2 Governing Equations fo r E la s to -p la s t i c D e f o r m a t i o n .................................................................................... 70 3 .2 .3 Const i tu t ive Equations fo r E la s to -P la s t i c D e f o r m a t i o n .................................................................................... 72 3.3 The F in i te Element Program ..................................................................... 74 3.4 Simulation Parameters ............................................................................... 78 CHAPTER 4 HYDROSTATIC EXTRUSION EXPERIMENTS ...................................... 81 4.1 I n t r o d u c t i o n ................................................................................................ 81 4.2 Hydrostatic Extrusion Experiments ...................................................... 81 4.2 .1 Experimental F a c i l i t i e s .......................................................... 81 4 .2 .2 Experimental Procedure .............................................................. 83 4.3 Residual S t ress Measurements ............................................................... 86 4 .3 .1 Residual S t resses ....................................................................... 86 4 .3 .2 Sachs' Boring-out Technique .................................................. 87 4 .3 .2 .1 Radial S t resses ........................................................... 87 4 .3 .2 .2 Tangential S t resses ................................................... 89 4 .3 .2 .3 Longitudinal S tresses ............................................... 91 4 .3 .3 Electro-chemical Machining ...................................................... 92 4.4 Experimental S t ress Measurements ...................................................... 96 4.4 .1 Experimental F a c i l i t i e s and Materia ls ............................. 96 4 .4 .2 Sample Preparat ion ....................................................................... 99 4 .4 .3 Cal ib ra t ion o f Electro-chemical Machining .................... 100 4 .4 .4 Experimental Procedure ............................................................... 101

A Computational Method to Estimate the Formability of Cold Forging Steels

Abstract: Summary The formability of four low alloyed cold forging steels has been studied. The deformation paths and the limit strains have been measured by testing small cylindrical specimens under uniaxial compression. Various height to diameter ratios and friction conditions were used. The stresses on the free surface of deformed specimens have been calculated on the basis of the deformation paths. It has been assumed that the ductile fracture occurs by rapid growth of voids and other inhomogeneities of material when hydrostatic stress component to shear strength ratio reaches a critical value as proposed by Oyane. The forming limit curves of the steels tested, have been predicted by using this criterion. In the present paper it has been demonstrated the decissive effect of the inclusion content on the forming limit curves of cold forging steels.

Modified maximum principal stress criterion for propellant liner bond failures

Abstract: The structural characterization of propellant liner insulation bond systems in solid rocket motors has been improved through the use of a modified maximum principal stress failure criterion. This failure criterion provides a consistent method for reducing laboratory test failure data and extrapolating the results to predict bondline failures at other stress states and environmental conditions. This improved method of engineering analysis evolved in conjunction with the development of a test specimen that can be used to analog the grain-to-case loading conditions. Another engineering approach currently used that also accurately models the experimental test data calculates the magnitudes of the maximum normal and maximum shear stress components along the bondline of the motor. The individual component values are then compared to the stress-at-break values in tensile and shear tests of bondline specimens. Bond test data, using specimens pulled at angles to give simultaneous tensile and shear stress components, show that the method can be improved by applying the appropriate stress concentration factors. These stress concentration factors were determined from finite-element analyses of the bond specimen. Using a similar approach, it was shown that the modified maximum principal stress failure criterion accurately models specimen bondline failures for all pull angles and pressures considered. The testing procedures and methods of analysis used to evaluate these, as well as several other failure criteria, are presented.

Deviatoric Constitutive Relationship for Anisotropic Materials

Abstract: : An anisotropic constitutive formulation is presented in which the deviatoric terms have been completely separated from the hydrostatic terms. In this way, a non-linear equation of state is readily employed. For code calculations employing an incremental strain approach, separating the hydrostatic terms from the deviatoric terms provides for a more accurate means of calculating pressure. This results because of the fact that the proposed deviatoric formulation allows evaluation of hydrostatic stress directly from dilatation. Such a scheme is less prone to cumulative integration error than existing formulations, which use stress increments as the basis for modifying hydrostatic pressure. The error is most severe when calculating pressure for materials with variable compressibility. Additionally, an error in the original formulation produces error in the pressure calculation when the material undergoes plastic flow. This flaw is eliminated in the present formulation.