Plastic Deformation Pattern Research Articles

Physical and Mechanical Behavior of Polymer Glasses II. Poly(Methyl Methacrylate)

Physical and mechanical behavior of PMMA samples compressed in the press mold was characterized. The development of plastic deformation and accumulation of residual deformation were shown to take place even at temperatures far below glass transition temperature. The pressure dependence of glass transition temperature as well as the compression stress-strain diagram of PMMA samples showed well-pronounced inflection points at pressures of 0.09-0.1 and 0.38-0.4 GPa. Physical and mechanical behavior of PMMA was discussed in terms of structural pattern of plastic deformation concerning the gradual involvement of structural sublevels with different packing densities in deformation and relaxation.

The crack initiation and plastic work

The effect of the ratio a/W on the values of δ i , and i in a structural steel has been studied by measuring the crack opening displacement and J- integral at initiation of crack growth in three-point bend specimens with shallow and deep notches and by three-dimensional finite element method analysis. By studying the results of the experiment and finite element analysis, we conclude that the different plastic deformation pattern and the larger plastic work needed for shallow notches is the most significant factor in determining the difference in behaviour between shallow and deep notches.

On the Strength of Racks for Jack-up Units : 4th Report, Estimation of the Static Strength of the Racks, by Using Stnall Scale Specimens, for the Meshings in the Vicinity of Rack Tooth Tip and Fillet

The subject of paper discusses the results of fracture tests for racks of 11.55 mm in module (1/7 of that of the full scale rack) and the finite element elastic stress analysis. These results lead to the conclusion that the differential in the ultimate strength between the full scale and 1/7 scale racks is small and that the ultimate strength of the racks is well expressed, for the varying initial meshing points, in terms of the shear strength of the cross-section with which the pinion tooth tip interferes. The authors also elucidate the difference in the pattern of plastic deformation and fracture of the racks resulting from the difference in the initial meshing point.

Residual Stresses in Cold-Formed Workpieces

Every workpiece contains residual stresses which can affect its performance in service. Especially metal-forming processes leave high residual stresses in the workpiece because of the inherent nonuniform plastic deformation pattern. The knowledge of the magnitudes of these stresses in such workpieces is of prime importance for the industry in order to evaluate their influences and particularly to optimize the manufacturing process with regard to a convenient residual stress state. For this purpose, in this study several cold metal-forming processes have been analysed regarding the residual stresses left in the products, such as rod extrusion, containerless extrusion, tube extrusion and upsetting. The trend to evaluate convenient process parameters for a manufacturing process optimization amounts to numerical process simulations, because they become more and more economic compared to protracted experimental methods. Hence, this analysis has been basically conducted with the Finite-Element-Method. For the rod extrusion case also an experimental study has been performed in order to compare measurements with computations. The agreement is satisfactorily. As a result of the analysis, two ways of manipulating the residual stresses in extrusion-processes turn out to be as: Varying the cone-angle and/or varying the die-stiffness.

Predicting stress corrosion crack growth rates when linear elastic fracture mechanics conditions are not operative

The growth rate of a stress corrosion crack in situations where linear elastic fracture mechanics (LEFM) conditions are not operative is predicted on the basis that the crack tip opening angle (CTOA) is related to the growth rate dc/dt, the functional relation between the CTOA and dc/dt being obtained by coupling theoretical results for crack growth under small-scale yielding conditions in an inert environment, with the experimentally determined power law relation between the crack tip stress intensity K and dc/dt for environmentally-assisted crack growth under LEFM conditions. Then, by assuming that the same CTOA-dc/dt relation applies to non-LEFM conditions, and by determining the CTOA under these conditions, it is in principle possible to predict the stress corrosion crack growth rate under non-LEFM conditions. A specific model: the plane strain deformation of a solid with two symmetrically situated deep cracks, and with tension of the small remaining ligament, is analyzed in detail, and the effects of the extent of plastic deformation and loading pattern (i.e., displacement or load control), on the predicted stress corrosion crack growth rate, are examined in detail. The results are compared with those obtained via application of the K versus dc/dt relation, its conversion to a J versus dc/dt correlation and then the determination of J, and also with those obtained on the assumption that a K approach is derectly applicable. The extent to which these latter approaches give conservative or non-conservative growth rate predictions when compared with the present paper's predictions, is discussed in relation to the extent of plastic deformation and loading pattern.

ON THE THEORY OF GROWTH FAULTING*: GENESIS OF THE “UNIT”

A detailed and rigorous geomechanical analysis of the stability of overpressured, gently sloping, sediment layers is presented that underlies the Multi‐Unit Delta Model described in Part I (Crans, Mandl and Haremboure, Journ. Petrol. Geol., 2, 3, 1980). That delta model explains and permits quantitative reproduction of main features associated with growth faulting. Starting from the equilibrium equations, the Coulomb‐Mohr yield criterion and the proper initial and boundary conditions, the elastic and plastic stress fields in the sloping, overpressured layer are derived. The plastic stress field is calculated on the grid generated by the “characteristics” of the hyperbolic partial differential equation for the plastic stress state. These characteristics, being called in stress analyses “slip lines”, are potential faults. In the case considered, a parameter equation is derived for one set of slip lines, (potential growth faults), which may simplify into cycloids under special conditions. Once the plastic stress field has been generated, the plastic deformation of the layer can be calculated by introducing the proper boundary conditions to the flow rules or plastic “velocity equations” being discussed extensively. To complete the rheological description, the behavior of the sediment layer is described by attributing also thixotropic properties to the sediment. Although the case discussed is a very specific one, it illustrates how structural geological phenomena can be modeled on the computer in an appropriate geomechanical way. Such a numerical computer model shows the unique relation between plastic stress state and fault pattern, and the non‐unique relation between plastic stress state and deformation pattern, being typical for the theory of plasticity. Generally, the unique relation between stress state and fault pattern is of utmost importance for the seismic interpreter in judging the feasibility of fault patterns. Inversely, one may deduce the stress state in the formation from a convincing fault pattern obtained from structural seismic interpretation which is, among others, important for the evaluation of acoustic properties of the sedimentary sequence used for the probabilistic assessment of seismic amplitude anomalies in the search for hydrocarbons (Crans and Berkhout, 1979, 1980; Crans and Ausburn, 1980).

A note on fracture and deformation in cubical box structures due to impulsive loading

The purpose of the investigation was to examine and contrast the patterns of fracture and deformation of cubical box-like structures open on one side, when impulsively loaded. Part 1: Brittle cubical boxes of plaster of Paris, open on one side, were subjected to intense impulsive point loading. The resulting terminal fracture patterns are shown and discussed. Part 2: A small number of thin metal boxes were point loaded by the impact of a heavy slow moving mass. The resulting plastic deformation patterns are described and discussed. In the Appendix, energy absorption due to bending is estimated and related to the results of Part 2.

Crack-growth resistance in tantalum foil

Centrally slit foils of high-purity tantalum have been loaded to fracture using an instrumented straining stage within a scanning electron microscope. By recording the tests on videotape, experimental records of both load and crack length were obtained. The resulting crack-growth-resistance curves can be understood in terms of the precrack plastic-deformation patterns.

Plastic deformation patterns on cleavage surfaces of lithium fluoride

Abstract A detailed study of plastic deformation patterns on cleavage surfaces of lithium fluoride has been made by x-ray topography and optical microscopy. The nature of the deformation and some of the factors governing its generation are discussed. In particular, the presence of large cleavage steps is revealed as an important factor determining whether or not plastic relaxation will accompany cleavage. The precise manner in which cleavage proceeds is interpreted in the light of this evidence.

An Analysis of Plastic Deformation in Rolling Contact

When two metal cylinders roll together under a contact pressure sufficient to cause yielding, a surprising mode of plastic deformation occurs. The surface of each cylinder is progressively displaced in the forward direction of rotation relative to the core by plastic shearing in a thin subsurface layer. This phenomenon was first observed by Crook (6) in 1957 and the results of a more complete experimental investigation are reported by Hamilton (9) in an accompanying paper. In this paper an attempt is made to explain the mechanism of ‘forward flow’ by an approximate numerical analysis of the elastic-plastic stress cycles to which the material is subject in repeated rolling contact. On the basis of an idealized material which is elasticperfectly plastic and isotropic, it is shown that a forward displacement of the surface would be expected as a result of the complex cycle of stress and strain encountered in rolling. It is also shown that residual compressive stresses are introduced into the immediately subsurface layers during the first few cycles of the load. If the load does not exceed the elastic limiting load by more than 66 per cent further plastic deformation would then cease. At higher loads a steady pattern of plastic deformation is predicted which in its principal features is consistent with the observed behaviour.

The Pattern of Plastic Deformation in a Deeply Notched Bar With Semicircular Roots

Abstract The plastic deformation in a notched bar with deep semicircular roots pulled in plane strain is determined theoretically. The finite deformation is analyzed according to plastic-rigid theory. The motion is unsteady, and the velocity field at any instant is given in terms of the current geometry of the deformed free surface. A graphical step-by-step method is used to determine the deformation of a square grid scribed on the undeformed cross section. The deformed pattern details the regions of large plastic strain, and may be useful in considering the initiation of fracture cracks.